RESEARCH METHODS
IN
ECOLOGY
BY
FREDERIC EDWARD CLEMENTS, PH.D.
ASSOCIATE PROFESSOR OF PLANT PHYSIOLOGY IN THE UNIVERSITY OF NEBRASKA
_ILLUSTRATED_
LINCOLN, NEBRASKA
THE UNIVERSITY PUBLISHING COMPANY
1905
COPYRIGHT, 1905
BY FREDERIC E. CLEMENTS AND IRVING S. CUTTER
All rights reserved
=Press of Jacob North & Company=
LINCOLN, NEBRASKA
PREFACE
The present volume is intended as a handbook for investigators and for
advanced students of ecology, and not as a text-book of the subject. An
elementary text-book covering the same field, but adapted to the needs
of undergraduate students, is in preparation. The handbook is
essentially an account of the methods used by the author in his studies
of the last eight years, during which a serious attempt has been made to
discover and to correlate the fundamental points of view in the vast
field of vegetation. No endeavor is made to treat any portion of the
subject exhaustively, since a discussion of general methods and general
principles is of much greater value in the present condition of ecology.
The somewhat unequal treatment given the different subjects is due to
the fact that it has been found possible to develop some of these more
rapidly than others. Finally, it must be constantly kept in mind that
ecology is still in a very plastic condition, and in consequence,
methods, fundamental principles, and matters of nomenclature and
terminology must be approached without prejudice in order that the best
possible development of this field may be attained.
Grateful acknowledgment for criticisms and suggestions is made to
Professor Doctor Charles E. Bessey and Professor Doctor Roscoe Pound,
who have read the text. The author is under especial obligations to
Doctor Edith S. Clements for the drawings of leaf types, as well as for
reading and criticising the manuscript. Professor Goodwin D. Swezey,
Professor of Astronomy in the University of Nebraska, has kindly
furnished much material for the determination of the sun’s altitude, and
consequent light intensities, and has read the section devoted to light.
Mr. George A. Loveland, Director of the Nebraska Section of the U. S.
Weather Bureau, has contributed many helpful suggestions to the
discussion of meteorological instruments. To Nella Schlesinger, Alice
Venters, and George L. Fawcett, advanced students in experimental
ecology, the author is indebted for many experiments which have been
used in the discussion of adjustment and adaptation.
Acknowledgment is also made to the following for various cuts: Henry J.
Green, Brooklyn, New York; Julien P. Friez, Baltimore, Maryland; C. H.
Stoelting Co., Chicago, Illinois; Draper Manufacturing Co., New York
city; Gundlach-Manhattan Optical Co., Rochester, New York; Rochester
Optical Co., Rochester, New York; Bausch and Lomb Optical Co.,
Rochester, New York.
FREDERIC EDWARD CLEMENTS.
The University of Nebraska,
May, 1905.
CONTENTS
CHAPTER I. THE FOUNDATION OF ECOLOGY
THE NEED OF A SYSTEM
PAGE
1. The scope of ecology 1
2. Ecology and physiology 1
Historical Development
3. Geographical distribution 2
4. The plant formation 2
5. Plant succession 3
6. Ecological phytogeography 4
7. Experimental ecology 4
8. Ecology of the habitat 5
9. The evidence from historical development 6
Present Status of Ecology
10. The lack of special training 6
11. Descriptive ecology 7
12. The value of floristic 8
13. Reconnaissance and investigation 8
14. Resident investigation 9
15. The dangers of a restricted field 9
Applications of Ecology
16. The subjects touched by ecology 10
17. Physiology and pathology 11
18. Experimental evolution 11
19. Taxonomy 12
20. Forestry 14
21. Physiography 15
22. Soil physics 15
23. Zoogeography 15
24. Sociology 16
THE ESSENTIALS OF A SYSTEM
25. Cause and effect: habitat and plant 16
26. The place of function 17
CHAPTER II. THE HABITAT
CONCEPT AND ANALYSIS
27. Definition of the habitat 18
28. Factors 18
Classification of Factors
29. The nature of factors 19
30. The influence of factors 19
Determination of Factors
31. The need of exact measurement 20
32. The value of meteorological methods 20
33. Habitat determination 21
34. Determinable and efficient differences 21
Instrumentation
35. Methods 22
36. Method of simple instruments 22
37. Method of automatic instruments 23
38. Combined methods 23
CONSTRUCTION AND USE OF INSTRUMENTS
39. The selection of instruments 24
Water-content
40. Value of different instruments 25
Geotome methods
41. The geotome 25
42. Soil borers 26
43. Taking samples of soil 26
44. Weighing 27
45. Computation 28
46. Time and location of readings 28
47. Location of readings 29
48. Depth of samples 30
49. Check and control instruments 30
Physical and Physiological Water
50. The availability of soil water 30
51. Terms 31
52. Chresard determination under control 32
53. Chresard readings in the field 33
54. Chresard values of different soils 34
Records and Results
55. The field record 35
56. The permanent record 36
57. Sums and means 36
58. Curves 37
Humidity
59. Instruments 37
Psychrometers
60. Kinds 37
61. The sling psychrometer 38
62. Readings 39
63. Cog psychrometer 39
64. Construction and use 40
65. Hygrometers 40
Psychrographs
66. The Draper psychrograph 41
67. Placing the instrument 42
68. Regulating and operating the instrument 43
69. The weekly visit 44
Humidity Readings and Records
70. The time of readings 44
71. Place and height 45
72. Check instruments 45
73. Humidity tables 46
74. Sums, means, and curves 47
Conversion scale for temperatures
75. Records 48
Light
76. Methods 48
The Photometer
77. Construction 49
78. Filling the photometer 50
79. Making readings 50
80. The Dawson-Lander sun recorder 51
81. The selagraph 52
Standards
82. Use 53
83. Making a standard 53
84. Kinds of standards 54
Readings
85. Time 55
Chart for determining sun’s altitude 57
86. Table for determining apparent noon 58
87. Place 59
Table of intensity at various angles 60
Reflected and Absorbed Light
88. The fate of incident light 60
89. Methods of determination 61
90. Leaf and epidermis prints 62
Expression of Results
91. Light records 63
92. Light sums, means and curves 63
Temperature
93. 64
Thermometers
94. Air thermometers 64
95. Soil thermometers 64
96. Maximum-minimum thermometers 65
97. Radiation thermometers 67
98. Thermographs 67
Readings
99. Time 69
100. Place and height 70
Expression of Results
101. Temperature records 70
102. Temperature sums and means 70
103. Temperature curves 71
104. Plant temperatures 71
Precipitation
105. General relations 72
106. The rain gauge 73
107. Precipitation records 74
Wind
108. Value of readings 74
109. The anemometer 75
110. Records 76
Soil
111. Soil as a factor 76
112. The value of soil surveys 77
113. The origin of soils 77
114. The structure of soils 78
115. Mechanical analysis 79
116. Kinds of soils 79
117. The chemical nature of soils 80
Physiography
118. Factors 80
Altitude
119. Analysis into factors 81
120. The barometer 82
Slope
121. Concept 83
122. The clinometer 83
123. The trechometer 84
Exposure
124. Exposure 85
125. Surface 85
126. Record of physiographic factors 86
127. Topography 86
Biotic Factors
128. Influence and importance 86
129. Animals 87
130. Plants 87
METHODS OF HABITAT INVESTIGATION
131. 88
Method of Simple Instruments
132. Choice of stations 88
133. Time of readings 89
134. Details of the method 89
135. Records 91
Method of Ecograph Batteries
136. 92
Expression of Physical Factor Results
137. The form of results 94
Factor Records
138. 94
Factor Curves
139. Plotting 95
140. Kinds of curves 96
141. Combinations of curves 96
142. The amplitude of curves 98
Factor Means and Sums
143. 98
CHAPTER III. THE PLANT
STIMULUS AND RESPONSE
General Relations
144. The nature of stimuli 100
145. The kinds of stimuli 100
146. The nature of response 101
147. Adjustment and adaptation 102
148. The measurement of response 103
149. Plasticity and fixity 104
150. The law of extremes 105
151. The method of working hypotheses 106
Hydroharmose
Adjustment
152. Water as a stimulus 107
153. The influence of other factors upon water 107
154. Response 108
155. The measurement of absorption 109
156. The quantitative relation of absorption and
transpiration 111
157. Measurement of transpiration 113
158. Field methods 114
159. Expression of results 116
160. Coefficient of transpiration 117
Adaptation
161. Modifications due to water stimuli 118
162. Modifications due to a small water supply 118
163. The decrease of water loss 118
164. The increase of water supply 121
165. Modifications due to an excessive water supply 121
166. Plant types 122
167. Xerophytic types 122
168. Types of leaf xerophytes 123
169. Types of stem xerophytes 125
170. Bog plants 126
171. Hydrophytic types 127
Photoharmose
Adjustment
172. Light as a stimulus 129
173. The reception of light stimuli 131
174. The response of the chloroplast 132
175. Aeration and translocation 134
176. The measurement of responses to light 135
Adaptation
177. Influence of chloroplasts upon form and structure 138
178. Form of leaves and stems 139
179. Modification of the epidermis 140
180. The differentiation of the chlorenchym 142
181. Types of leaves 144
182. Heliophytes and sciophytes 144
EXPERIMENTAL EVOLUTION
183. Scope 145
184. Fundamental lines of inquiry 146
185. Ancestral form and structure 146
186. Variation and mutation 147
187. Methods 149
Method of Natural Experiment
188. Selection of species 149
189. Determination of factors 151
190. Method of record 152
Method of Habitat Cultures
191. Scope and advantages 153
192. Methods 153
193. Transfer 154
194. Modification of the habitat 156
Method of Control Cultures
195. Scope and procedure 157
196. Water-content series 158
197. Light series 160
CHAPTER IV. THE FORMATION
METHODS OF INVESTIGATION AND RECORD
198. The need of exact methods 161
Quadrats
199. Uses 161
200. Possible objections 163
Kinds of Quadrats and Their Use
201. Size and kinds 164
202. Tapes and stakes 164
203. Locating quadrats 165
The List Quadrat
204. Description 165
205. Manner of use 166
206. Table of abundance 166
The Chart Quadrat
207. Description and use 167
208. The chart 168
209. Mapping 168
210. Factors and photographs 170
The Permanent Quadrat
211. Description and uses 170
212. Manner of use 172
The Denuded Quadrat
213. Description 173
214. Methods of denuding and recording 174
215. Physical factors 175
Aquatic Quadrats
216. Scope 175
Transects
217. The transect 176
The Line Transect
218. Description and method 176
219. The location and size 177
The Belt Transect
220. Details 178
The Permanent Transect
221. Advantages 179
222. Details 179
The Denuded Transect
223. 180
The Layer Transect
224. 180
Ecotone Charts
225. 181
The Migration Circle
226. Purpose 182
227. Location and method 182
228. The denuded circle 183
229. Photographs 183
Cartography
230. Value of cartographic methods 183
231. Standard scale 184
232. Color scheme 184
233. Formation and vegetation maps 185
234. Continental maps 187
Photography
235. 188
236. The camera and its accessories 188
237. The choice of a camera 190
238. The use of the camera 191
239. The sequence of details 192
240. The time of exposure 193
241. Developing 194
242. Finishing 195
Formation and Succession Herbaria
243. Concept and purpose 196
244. Details of collecting 197
245. Arrangement 197
246. Succession herbaria 198
DEVELOPMENT AND STRUCTURE
247. Vegetation an organism 199
248. Vegetation essentially dynamic 199
249. Functions and structures 199
Association
250. Concept 200
251. Causes 201
252. Aggregation 203
Kinds of Association
253. Categories 204
254. Stratum association 204
255. Ground association 205
256. Species guild association 206
257. Light association 206
258. Water-content association 208
THE DEVELOPMENT OF THE FORMATION
259. 210
Invasion
260. 210
Migration
261. 210
262. Mobility 211
263. Organs for dissemination 211
264. Contrivances for dissemination 212
265. Position of disseminules 214
266. Seed production 215
267. Agents of migration 216
268. The direction of migration 219
Ecesis
269. Concept 220
270. Germination of the seed 221
271. Adjustment to the habitat 223
Barriers
272. Concept 224
273. Physical barriers 225
274. Biological barriers 225
275. Influence of barriers 226
Endemism
276. Concept 227
277. Causes 228
278. Significance 228
Polyphylesis and Polygenesis
279. Concept 230
280. Proofs of polygenesis 231
281. Origin by polyphylesis 232
Kinds of Invasion
282. Continuous and intermittent invasion 234
283. Complete and partial invasion 235
284. Permanent and temporary invasion 235
Manner of Invasion
285. Entrance into the habitat 236
286. Influence of levels 238
Investigation of Invasion
287. 239
Succession
288. Concept 239
289. Kinds of succession 240
Primary Successions
290. 241
291. Succession through elevation 241
292. Succession through volcanic action 242
293. Weathering 243
294. Succession in residuary soils 243
295. Succession in colluvial soils 244
296. Succession in alluvial soils 245
297. Succession in aeolian soils 246
298. Succession in glacial soils 247
Secondary Successions
299. 247
300. Succession in eroded soils 247
301. Succession in flooded soils 248
302. Succession by subsidence 249
303. Succession in land slips 249
304. Succession in drained or dried soils 249
305. Succession by animal agency 250
306. Succession by human agency 250
307. Succession in burned areas 251
308. Succession in lumbered areas 252
309. Succession by cultivation 253
310. Succession by drainage 253
311. Succession by irrigation 253
312. Anomalous successions 254
313. Perfect and imperfect successions 254
314. Stabilization 255
Causes and Reactions
315. 256
316. Succession by preventing weathering 257
317. Succession by binding aeolian soils 258
318. Succession by reducing run-off and erosion 259
319. Succession by filling with silt and plant remains 260
320. Succession by enriching the soil 261
321. Succession by exhausting the soil 262
322. Succession by the accumulation of humus 263
323. Succession by modifying atmospheric factors 264
Laws of Succession
324. 264
Classification and Nomenclature
325. Basis 267
326. Nomenclature 267
327. Illustrations 270
Investigation of Succession
328. General rules 270
329. Method of alternating stages 271
330. The relict method 272
THE STRUCTURE OF THE FORMATION
331. 274
Zonation
332. Concept 274
Causes of Zonation
333. Growth 275
334. Reactions 276
335. Physical factors 276
336. Physiographic symmetry 278
Kinds of Zonation
337. 279
338. Radial zonation 280
339. Bilateral zonation 280
340. Vertical zonation 280
341. Vegetation zones 281
Alternation
342. Concept 283
343. Causes 284
344. Competition 285
345. Kinds of alternation 289
The Formation in Detail
346. The rank of the formation 292
347. The parts of a formation 295
348. Nomenclature of the divisions 299
349. The investigation of a particular formation 299
Classification and Relationship
350. Basis 300
351. Habitat classification 301
352. Nomenclature 302
353. Developmental classification 304
354. Regional classification 304
355. Mixed formations 304
EXPERIMENTAL VEGETATION
356. Scope and methods 306
Method of Natural Habitats
357. Natural experiments 307
Method of Artificial Habitats
358. Modification of habitat 307
359. Denuding 308
360. Modification of the formation by transfer 309
Method of Control Habitats
361. Competition cultures 310
362. Details of culture methods 311
GLOSSARY 314
BIBLIOGRAPHY 324
RESEARCH METHODS IN ECOLOGY
CHAPTER I. THE FOUNDATION OF ECOLOGY
THE NEED OF A SYSTEM
=1. The scope of ecology.= The clue to the field of ecology is found in
the Greek word, οἲκος, home. The point of view in the following treatise
is constantly that which is inherent in the term itself. Ecology is
therefore considered the dominant theme in the study of plants, indeed,
as the central and vital part of botany. This statement may at first
appear startling, if not unfounded, but mature reflection will show that
all the questions of botanical science lead sooner or later to the two
ultimate facts: plant and habitat. The essential truth of this has been
much obscured by detached methods of study in physiology, morphology,
and histology, which are too often treated as independent fields. These
have suffered incomplete and unsymmetric development in consequence of
extreme specialistic tendencies. Analytic methods have dominated
research to the exclusion of synthetic ones, which, in a greatly
diversified field, must be final, if botanical knowledge is something to
be systematized and not merely catalogued. Physiology in particular has
suffered at the hands of detached specialists. Originally conceived as
an inquiry into the origin and nature of plants, it has been developed
strictly as a study of plant activities. It all but ignores the physical
factors that control function, and the organs and tissues that reflect
it.
=2. Ecology and physiology.= There can be little question in regard to
the essential identity of physiology and ecology. This is evident when
it is clearly seen that the present difference between the two fields is
superficial. Ecology has been largely the descriptive study of
vegetation; physiology has concerned itself with function; but, when
carefully analyzed, both are seen to rest upon the same foundation. In
each, the development is incomplete: ecology has so far been merely
superficial, physiology too highly specialized. The one is chaotic and
unsystematized, the other too often a minute study of function under
abnormal circumstances. The greatest need of the former is the
introduction of method and system, of the latter, a broadening of scope
and new objectives. The growing recognition of the identity of the two
makes it desirable to anticipate their final merging, and to formulate a
system that will combine the good in each, and at the same time
eliminate superficial and extreme tendencies. In this connection, it
becomes necessary to point out to ecologist and physiologist alike that,
while they have been working on the confines of the same great field,
each must familiarize himself with the work and methods of the other,
before his preparation is complete. Both must broaden their horizons,
and rearrange their views. The ecologist is sadly in need of the more
intimate and exact methods of the physiologist: the latter must take his
experiments into the field, and must recognize more fully that function
is but the middleman between habitat and plant. It seems probable that
the final name for the whole field will be physiology, although the term
ecology has distinct advantages of brevity and of meaning. In this
event, however, it should be clearly recognized that, although the name
remains the same, the field has become greatly broadened by new
viewpoints and new methods.
HISTORICAL DEVELOPMENT
=3. Geographical distribution.= The systematic analysis of the great
field of ecology is essential to its proper development in the future. A
glance at its history shows that, while a number of essential points of
attack have been discovered, only one or two of these have been
organized, and that there is still an almost entire lack of correlation
and coordination between these. The earliest and simplest development of
the subject was concerned with the distribution of plants. This was at
first merely an offshoot of taxonomy, and, in spite of the work of
Humboldt and Schouw, has persisted in much of its primitive form to the
present time, where it is represented by innumerable lists and
catalogues. Geographical distribution was grounded upon the species, a
fact which early caused it to become stereotyped as a statistical study
of little value. This tendency was emphasized by the general practice of
determining distribution for more or less artificial areas, and of
instituting comparisons between regions or continents too little known
or too widely remote. The fixed character of the subject is conclusively
shown by the fact that it still persists in almost the original form
more than a half century after Grisebach pointed out that the formation
was the real unit of vegetation, and hence of distribution.
=4. The plant formation.= The corner-stone of ecology was laid by
Grisebach in 1838 by his recognition of the plant formation as the
fundamental feature of vegetation. Earlier writers, notably Linné (1737,
1751), Biberg (1749), and Hedenberg (1754), had perceived this relation
more or less clearly, but failed to reduce it to a definite guiding
principle. This was a natural result of the dominance of descriptive
botany in the 18th century, by virtue of which all other lines of
botanical inquiry languished. This tendency had spent itself to a
certain degree by the opening of the 19th century, and both plant
distribution and plant physiology began to take form. The stimulus given
the former by Humboldt (1807) turned the attention of botanists more
critically to the study of vegetation as a field in itself, and the
growing feeling for structure in the latter led to Grisebach’s concept
of the formation, which he defined as follows: “I would term a group of
plants which bears a definite physiognomic character, such as a meadow,
a forest, etc., a phytogeographic formation. The latter may be
characterized by a single social species, by a complex of dominant
species belonging to one family, or, finally, it may show an aggregate
of species, which, though of various taxonomic character, have a common
peculiarity; thus, the alpine meadows consist almost exclusively of
perennial herbs.” The acceptance of the formation as the unit of
vegetation took place slowly, but as a result of the work of Kerner
(1863), Grisebach (1872), Engler (1879), Hult (1881, 1885), Goeze
(1882), Beck (1884), Drude (1889), and Warming (1889), this point of
view came to be more and more prevalent. It was not, however, until the
appearance of three works of great importance, Warming (1895), Drude
(1896), and Schimper (1898), that the concept of the formation became
generally predominant. With the growing recognition of the formation
during the last decade has appeared the inevitable tendency to
stereotype the subject of ecology in this stage. The present need, in
consequence, is to show very clearly that the idea of the formation is a
fundamental, and not an ultimate one, and that the proper superstructure
of ecology is yet to be reared upon this as the foundation.
=5. Plant succession.= The fact that formations arise and disappear was
perceived by Biberg as early as 1749, but it received slight attention
until Steenstrup’s study of the succession in the forests of Zealand
(1844 prox.). In the development of formations, as well as in their
recognition, nearly all workers have confined themselves to the
investigation of particular changes. Berg (1844), Vaupell (1851),
Hoffmann (1856), Middendorff (1864), Hult (1881), Senft (1888), Warming
(1890), and others have added much to our detailed knowledge of
formational development. Notwithstanding the lapse of more than a half
century, the study of plant successions is by no means a general
practice among ecologists. This is a ready explanation of the fact that
the vast field has so far yielded but few generalizations. Warming
(1895) was the first to compile the few general principles of
development clearly indicated up to this time. The first critical
attempt to systematize the investigation of succession was made by
Clements (1904), though this can be considered as little more than a
beginning on account of the small number of successions so far studied.
Future progress in this field will be conditioned not only by the more
frequent study of developmental problems by working ecologists, but
also, and most especially, by the application of known principles of
succession, and by the working out of new ones.
=6. Ecological phytogeography.= Until recent years, the almost universal
tendency was to give attention to formations from the standpoint of
vegetation alone. While the habitat was touched here and there by
isolated workers, and plant functions were being studied intensively by
physiologists, both were practically ignored by ecologists as a class.
The appearance of Warming’s _Lehrbuch der oecologischen
Pflanzengeographie_ (1896) and of Schimper’s _Pflanzengeographie auf
physiologischer Grundlage_ (1898) remedied this condition in a measure
by a general discussion of the habitat, and by emphasizing the
importance of the ecological or physiological point of view. Despite
their frank recognition of the unique value of the habitat, the major
part of both books was necessarily given to what may be termed the
general description of formations. For this reason, and for others
arising out of an almost complete dearth of methods of investigation,
ecology is still almost entirely a floristic study in practice, although
there is a universal recognition of the much greater value of the
viewpoint which rests upon the relation between the formation and its
habitat.
=7. Experimental ecology.= Properly speaking, the experimental study of
ecology dates from Bonnier[1] (1890, 1895), though it is well understood
that experimental adjustment of plants to certain physical factors had
been the subject of investigation before this time. The chief merit of
Bonnier’s work, however, lies in the fact that it was done out of doors,
under natural conditions, and for these reasons it should be regarded as
the real beginning of this subject. Bonnier’s experiments were made for
the purpose of determining the effect of altitude. Culture plots of
certain species were located in the Alps and the Pyrenees, and the
results were compared with control cultures made in the lowlands about
Paris. In 1894 he also made a comparative study of certain polydemic
species common to the arctic islands, Jan Meyen and Spitzenberg, and to
the Alps. Both of these methods are fundamental to field experiment, but
the results are inconclusive, inasmuch as altitude is a complex of
factors. As no careful study was made of the latter, it was manifestly
impossible to refer changes and differences of structure to the definite
cause. In a paper that has just appeared, E. S. Clements (1905) has
applied the method of polydemic comparison to nearly a hundred species
of the Rocky mountains. In this work, the all-important advance has been
made of determining accurately the decisive differences between the two
or more habitats of the same species in terms of direct factors,
water-content, humidity, and light. In his own investigations of
Colorado mountain vegetation, the author has applied the method of field
cultures by planting seeds of somewhat plastic species in habitats of
measured value, and has thought to initiate a new line of research by
applying experimental methods to the study of vegetation as an organism.
In connection with this, there has also been developed a method of
control experiment in the plant house under definitely measured
differences of water and light.
=8. Ecology of the habitat.= Since the time of Humboldt, there have been
desultory attempts to determine the physical factors of habitats with
some degree of accuracy. The first real achievement in this line was in
the measurement of light values by Wiesner in 1896. In 1898 the writer
first began to study the structure of habitats by the determination of
water-content, light, humidity, temperature, wind, etc., by means of
instruments. These methods were used by one of his pupils, Thornber
(1901), in the study of a particular formation, and by another, Hedgcock
(1902), in a critical investigation of water-content. Two years later,
similar methods of measuring physical factors were put into operation in
connection with experimental evolution under control in the plant house.
E. S. Clements (1905), as already indicated, has made the use of factor
instruments the foundation of a detailed study of polydemic species, i.
e., those which grow in two or more habitats, and which are, indeed, the
most perfect of all experiments in the production of new forms. In a
volume in preparation upon the mountain vegetation of Colorado, the
writer has brought the use of physical factor instruments to a logical
conclusion, and has made the study of the habitat the basis of the whole
work. Out of this investigation has come a new concept of vegetation
(Clements 1904), namely, that it is to be regarded as a complex organism
with structures and with functions susceptible of exact methods of
study.
=9. The evidence from historical development.= This extremely brief
resume of what has been accomplished in the several lines of ecological
research makes evident the almost complete absence of correlation and of
system. The whole field not merely lacks system, but it also demands a
much keener perception of the relative value of the different tendencies
already developed. It is inevitable from the great number of tyros, and
of dilettante students of ecology in comparison with the few
specialists, that the surface of the field should have received all of
the attention. It is, however, both unfortunate and unscientific that
great lines of development should be entirely unknown to all but a few.
There is no other department of botany in which the superficial study of
more than half a century ago still prevails to the exclusion of better
methods, many of which have been known for a decade or more. It is
clear, then, that the imperative need of ecology is the proper
coordination of its various points of view, and the working out of a
definite system which will make possible a ready recognition of that
which is fundamental and of that which is merely collateral. The
historical development, as is well understood, can furnish but a slight
clue to this. It is a fact of common knowledge that the first
development of any subject is general, and usually superficial also.
True values come out clearly only after the whole field has been
surveyed. For these reasons, as will be pointed out in detail later, the
newer viewpoints are regarded as either the most important or the most
fundamental. Experimental ecology will throw a flood of light upon plant
structure and function, while exact methods of studying the habitat are
practically certain of universal application in the future.
PRESENT STATUS OF ECOLOGY
=10. The lack of special training.= The bane of the recent development
popularly known as ecology has been a widespread feeling that anyone can
do ecological work, regardless of preparation. There is nothing in
modern botany more erroneous than this feeling. The whole task of
ecology is to find out what the living plant and the living formation
are doing and have done in response to definite complexes of factors, i.
e., habitats. In this sense, ecology is practically coextensive with
botany, and the student of a local flora who knows a few hundred species
is no more competent to do ecological work than he is to reconstruct the
phylogeny of the vegetable kingdom, or to explain the transmission of
ancestral qualities. The comprehensive and fundamental character of the
subject makes a broad special training even more requisite than in more
restricted lines of botanical inquiry. The ecologist must first of all
be a botanist, not a mere cataloguer of plants, and he must also possess
a particular training in the special methods of ecological research. He
must be familiar with the various points of attack in this field, and he
must know the history of his subject thoroughly. Ecology affords the
most striking example of the prevalent evil of American botanical study,
i. e., an indifference to, or an ignorance of the literature of the
subject. The trouble is much aggravated here, however, by the breadth of
the field, and the common assumption that a special training is
unnecessary, if not, indeed, superfluous. Ignorance of the important
ecological literature has been a most fertile source of crude and
superficial studies, a condition that will become more apparent as these
fields are worked again by carefully trained investigators.
=11. Descriptive ecology.= The stage of development of the subject at
the present time may be designated as _descriptive ecology_, for
purposes of discussion merely. This is concerned with the superficial
description of vegetation in general terms, and results from the fact
that the development has begun on the surface, and has scarcely
penetrated beneath it. The organic connection between ecology and
floristic has produced an erroneous impression as to the relative value
of the two. Floristic has required little knowledge, and less
preparation: it lends itself with insidious ease to chance journeys or
to vacation trips, the fruits of which are found in vague descriptive
articles, and in the multiplication of fictitious formations. While
there is good reason that a record should be left of any serious
reconnaissance, even though it be of a few weeks’ duration, the
resulting lists and descriptive articles can have only the most
rudimentary value, and it is absurd to regard them as ecological
contributions at all. No statement admits of stronger emphasis, and
there is none that should be taken more closely to heart by botanists
who have supposed that they were doing ecological work. An almost
equally fertile source of valueless work is the independent treatment of
a restricted local area. The great readiness with which floristic lists
and descriptions can be made has led many a botanist, working in a small
area, or passing hurriedly through an extended region, to try his hand
at formation making. From this practice have resulted scores of
so-called formations, which are mere patches, consocies, or stages in
development, or which have, indeed, no existence other than in the minds
of their discoverers. The misleading definiteness which a photograph
seems to give a bit of vegetation has been responsible for a surplus of
photographic formations, which have no counterparts in nature.
Indispensable as the photograph is to any systematic record of
vegetation, its use up to the present time has but too often served to
bring it into disrepute. There has been a marked tendency to apply the
current methods of descriptive botany to vegetation, and to regard every
slightly different piece of the floral covering as a formation. No
method can yield results further from the truth. It is evident that the
recognition and limitation of formations should be left absolutely to
the broadly trained specialist, who has a thorough preparation by virtue
of having acquainted himself carefully with the development and
structure of typical formations over large areas.
=12. The value of floristic.= In what has been said above, there is no
intent to decry the value of floristic. The skilled workman can spare
the material which he is fashioning as readily as the ecologist can work
without an accurate knowledge of the genera and species which make up a
particular vegetation. Some botanists whose knowledge of ecology is that
of the study or the laboratory have maintained that it is possible to
investigate vegetation without knowing the plants which compose it.
Ecology is to be wrought out in the field, however, and the field
ecologist—none other, indeed, should bear the name—understands that
floristic alone can furnish the crude material which takes form under
his hands. It is the absolute need of a thorough acquaintance with the
flora of a region which makes it impossible for a traveler to obtain
anything of real ecological value in his first journey through a
country. As the very first step, he must gain at least a fair knowledge
of the floristic, which will alone take the major part of one or more
growing seasons. This information the student of a local flora already
has at the tip of his tongue; in itself it can not constitute a
contribution to ecology, but merely the basis for one. In this
connection, moreover, it can not be used independently, but becomes of
value only after an acquaintance with a wide field. Floristic study and
floristic lists, then, are indispensable, but to be of real value their
proper function must be clearly recognized. They do not constitute
ecology.
=13. Reconnaissance and investigation.= In striving to indicate the true
value and worth of ecological study, it becomes necessary to draw a
definite line between what we may term _reconnaissance_ and
_investigation_. By the former is understood the preliminary survey of a
region, extending over one or two years. The objects of such a survey
are to obtain a comprehensive general knowledge of the topography and
vegetation of the region, and of its relation to the other regions about
it. The chief purpose, however, is to gain a good working acquaintance
with the flora: a reconnaissance to be of value must do this at all
events. Certain general facts will inevitably appear during this
process, but they will invariably need the confirmation of future study.
It would be an advantage to real ecology if reconnaissance were to
confine itself entirely to the matter of making a careful floristic
survey. Investigation begins when the inquiry is directed to the
habitat, or to the development and structure of the formation which it
bears, i. e., when it takes up the manifold problems of the οἶκος. Such
a study must be based upon floristic, but the latter becomes a part of
investigation only in so far as it leads to it. Standing by itself, it
is not ecological research: it is the preparation for it. This
distinction deserves careful thought. The numerous recruits to ecology
have turned their attention to what lay nearest to hand, with little
question as to its value, or to where it might lead. The result has been
to make reconnaissance far outweigh investigation in amount, and to give
it a value which properly belongs to the latter. Furthermore, this
mistaken conception has in many cases, without doubt, prevented its
leading to valuable research work.
=14. Resident investigation.= Obviously, if reconnaissance is a
superficial survey, and investigation thorough extensive study, an
important distinction between them is in the time required. While one
may well be the result of a journey of some duration, the other is
essentially dependent upon residence. In the past the great disparity
between the size of the field and the number of workers has made
resident study too often an ideal, but in the future it will be
increasingly the case that a particular region will be worked by a
trained ecologist resident in it. This may never be altogether true of
inaccessible and sterile portions of the globe. It may be pointed out,
however, that, between the tropics and the poles, residence during the
summer or growing period is in essence continuous residence. In the
ultimate analysis, winter conditions have of course some influence upon
the development of vegetation during the summer, but the important
problems which a vegetation presents must be worked out during the
period of development. For temperate, arctic, and alpine regions, then,
repeated study during the growing period for a term of years has
practically all the advantages of continuous residence. For all
practical purposes, it is resident study.
=15. The dangers of a restricted field.= In the resident study of a
particular region, the temptation to make an intensive investigation of
a circumscribed area is very strong. The limits imposed by distance are
alone a sufficient explanation of this, but it is greatly increased by
the inclination toward detailed study for which a small field offers
opportunity. This temptation can be overcome only by a general
preliminary study of the larger region in which the particular field is
located. The broader outlook gained in this way will throw needed light
upon many obscure facts of the latter, and at the same time it will act
as a necessary corrective of the tendency to consider the problems of
the local field in a detached manner, and to magnify the value of the
distinctions made and the results obtained. Such a general survey has
the purpose and value of a reconnaissance, and is always the first step
in the accurate and detailed investigation of the local area or
formation. Each corrects the extreme tendency of the other, and thorough
comprehensive work can be done only by combining the two methods. When
the field of inquiry is a large area or covers a whole region, the
procedure should be essentially the same. A third stage must be added,
however, in which a more careful survey is made of the entire field in
the light of the thorough study of the local area. The writer’s methods
in the investigation of the Colorado vegetation illustrate this
procedure. The summers of 1896, 1897, 1898 were devoted to
reconnaissance; those of 1899–1904 were given to detailed and
comprehensive study by instrument and quadrat of a highly diversified,
representative area less than 20 miles square, while the work of the
final summer will be the application of the results obtained in this
localized area to the region traversed from 1896–98. This is practically
the application of methods of precision to an area of more than 100,000
square miles. It also serves to call attention to another point not
properly appreciated as yet by those who would do ecological work. This
is the need of taking up field problems as a result of serious
forethought, and not as a matter of accident or mere propinquity. A
carefully matured plan of attack which contemplates an expenditure of
time and energy for a number of years will yield results of value, no
matter how much attention an area may have received. On the other hand,
an aimless or hurried excursion into the least known or richest of
regions will lead to nothing but a waste of time, especially upon the
part of the ecologist, who must read the articles which result, if only
for the purpose of making sure that there is nothing in them.
APPLICATIONS OF ECOLOGY
=16. The subjects touched by ecology.= The applications of ecological
methods and results to other departments of botany, and to other fields
of research are numerous. Many of these are both intimate and
fundamental, and give promise of affording new and extremely fruitful
points of view. It has already been indicated that ecology bears the
closest of relations to morphology and histology on the one side, and to
physiology on the other—that it is, indeed, nothing but a rational field
physiology, which regards form and function as inseparable phenomena.
The closeness with which it touches plant pathology follows directly
from this, as pathology is nothing more than abnormal form and
functioning. Experimental work in ecology is purely a study of
_evolution_, and the facts of the latter are the materials with which
_taxonomy_ deals. _Forestry_ has already been termed “applied ecology”
and in its scientific aspects, which are its foundation, it is precisely
the ecology of woody plants, and of the vegetation which they
constitute. Apart from botany, the physical side of ecology is largely a
question of _soil physics_, and of _physiography_. On the other hand,
vegetation is coming more and more to be regarded as a fundamental
factor in _zoogeography_ and in _sociology_. Furthermore, with respect
to the latter, it will be pointed out below that the principles of
association which have been determined for plants, viz., invasion,
succession, zonation, and alternation, apply with almost equal force to
man.
=17. Physiology and pathology.= The effect of ecology in emphasizing the
intrinsically close connection between physiology and morphology has
already been mentioned. Its influence in normalizing the former by
forcing it into the field as the place for experiment, and by directing
the chief attention to the plant as an organism rather than a complex of
organs, is also rapidly coming to be felt. Ecology will doubtless exert
a corrective influence upon pathology in the near future. This is
inevitable as the latter ceases to be the merely formal study of
specific pathogenic organisms, and turns its attention to the cause of
all abnormality, which is to be found in the habitat, whether this be
physical, as when the water-content is low, or biotic, when a parasitic
fungus is present. The relative ease with which specific diseases can be
studied has helped to obscure the essential fact that the approach to
pathology must be through physiology. Much indeed of the observational
physiology of the laboratories has been pathology, and it will be
impossible to draw a clear line between them until precise experiment in
the habitat has come into vogue.
=18. Experimental evolution.= As a result of the extremely fragmentary
character of the geological record, nothing is more absolute than that
there can be no positive knowledge of the exact origin of a form or
species, except in those rare cases of the present day, where the whole
process has taken place under the eye of a trained observer. The origin
of the plant forms known at present must forever lie without the domain
of direct knowledge. If it were possible, by a marvel of ingenuity and
patience to develop experimentally _Myosurus_ from _Selaginella_, this
would not be absolutely conclusive proof that _Myosurus_ was first
derived in this way. When all is said, however, this would be the very
best of presumptive evidence. It must also be recognized that this is
the nearest to complete proof that we shall ever attain, and with this
in mind it becomes apparent at once that evidence from experiment is of
paramount importance in the study of evolution (the origin of species).
The phase of experimental ecology which has to do with the plant has
well been called experimental evolution. While this is a field almost
wholly without development at present, there can be little question that
it is to be one of the most fertile and important in the future.
Attention will be directed first to those forms which are undergoing
modification at the present time. The cause and direction of change will
be ascertained, and its amount and rapidity measured by biometrical
methods. The next step will be to actually change the habitat of
representative types, and to determine for each the general trend of
adaptation, as well as the exact details. By means of the methods used
and the results obtained in these investigations, it will be possible to
attack the much more difficult problem of retracing the development of
species already definitely constituted. This will be accomplished by the
study of the derived and the supposed ancestral form, but owing to the
great preponderance of evolution over reversion, the study of the
ancestral form will yield much more valuable results.
The general application of the methods of experimental ecology will mark
a new era in the study of evolution. There has been a surplus of
literary investigation, but altogether too little actual experiment. The
great value of De Vries’ work lies not in the importance of the results
obtained, but in calling attention to the unique importance of
experimental methods in contributing to a knowledge of evolution. The
development of the latter has been greatly hindered by the dearth of
actual facts, and by a marked tendency to compensate for this by
verbiage and dogmatism. This is well illustrated by the present position
of the “mutation theory,” which, so far as the evidence available is
concerned, is merely a working hypothesis. An incredible amount of bias
and looseness of thought have characterized the discussion of evolution.
It is earnestly to be hoped that the future will bring more work and
less argument, and that the literary evolutionists will become less and
less reluctant to leave the relative merits of variation and mutation to
experiment.
=19. Taxonomy.= Taxonomy is classified evolution. It is distinct from
descriptive botany, which is merely a cataloguing of all known forms,
with little regard to development and relationship. The consideration of
the latter is peculiarly the problem of taxonomy, but the solution must
be sought through experimental evolution. The first task of the latter
is to determine the course of modification in related forms, and the
relationships existing between them. With this information, taxonomy can
group forms according to their rank, i. e., their descent. The same
method is applicable to the species of a genus, and, in a less degree,
perhaps, to the genera which constitute a family. The use to which it
may be put in indicating family relationships will depend largely upon
the gap existing between the families concerned. While interpretation
will always play a part in taxonomy, the general use of experiment will
leave much less opportunity for the personal equation than is at present
the case. Taxonomy, like descriptive botany, is based upon the species,
but, while there may exist a passable kind of descriptive botany, there
can be no real taxonomy as long as the sole criterion of a species is
the difference which any observer thinks he sees between one plant and
another. The so-called species of to-day range in value from mere
variations to true species which are groups of great constancy and
definiteness. The reasons for this are obvious when one recalls that
“species” are still the product of the herbarium, not of the field, and
that the more intensive the study, the greater the output in “species.”
It would seem that careful field study of a form for several seasons
would be the first requisite for the making of a species, but it is a
precaution which is entirely ignored in the vast majority of cases. The
thought of subjecting forms presumed to be species to conclusive test by
experiment has apparently not even occurred to descriptive botanists as
yet. Notwithstanding, there can be no serious doubt that the existing
practice of re-splitting hairs must come to an end sooner or later. The
remedy will come from without through the application of experimental
methods in the hands of the ecologist, and the cataloguing of slight and
unrelated differences will yield to an ordered taxonomy.
Experimental evolution will solve a taxonomic problem as yet untouched,
namely, the effect of recent environment upon the production of species.
It is well understood that some species grow in nature in various
habitats without suffering material change, while others are modified to
constitute a new form in each habitat. It is at once clear that these
forms (or ecads) are of more recent descent than the species, i. e., of
lower rank. It must also be recognized that a constant group and a
highly plastic one are essentially different. If constancy is made a
necessary quality of a species, one is a species, the other is not. If
both are species, then two different kinds must be distinguished. Among
the species of our manuals are found many ecads, alongside of constant
and inconstant species. These can be distinguished only by field
experiment, and their proper coordination is possible only after this
has been done. Indeed, the whole question of the ability or the
inability of environmental variation to produce constant species is one
that must be referred to repeated and long-continued experiment in the
field.
A minor service of considerable value can be rendered taxonomy by
working over the diagnosis from the ecological standpoint. Many
ecological facts are of real diagnostic value, while others are at least
of much interest, and serve to direct attention to the plant as a living
thing. The loose use of terms denoting abundance, which prevails in
lists and manuals, should be replaced by the exact usage which the
quadrat method has made possible for vegetation. The designation of
habitats could be made much more exact, and the formation, as well as
the habitat form or ecad, and the vegetation form or phyad, should be
indicated in addition. The general terms drawn from pollination,
seed-production, and dissemination might also be included to advantage.
=20. Forestry=, if the purely commercial aspects be disregarded, is the
ecology of a particular kind of vegetation, the forest. Therefore, in
pointing out the connection between them, it is only necessary to say
that whatever contributes to the ecology of the forest is a contribution
to forestry. There are, however, certain lines of inquiry which are of
fundamental importance. First among these, and of primary interest from
the practical point of view, are the questions pertaining to the
distribution of forests and their structure. Of even greater
significance are the problems of forest development, movement, and of
reforestation, which are comprised in succession. The gradual invasion
of the plains and prairies by the forest belt of the east and north is
in full conformity with the laws of invasion, and the ecological methods
to be employed here serve not merely to determine the actual conditions
at present, but also to forecast them with a great deal of accuracy. The
slow but certain development of forests on new soils, and their more
rapid reestablishment where the woody vegetation has been destroyed by
burning or lumbering, are ordinary phenomena of succession, for which
the ecologist has already worked out the laws, and determined the
methods of investigation. Having once ascertained the original and
adjacent vegetation and the character of the habitat, the ecologist can
indicate with accuracy not only the character of the new forest that
will appear, but also the nature of the antecedent formations. A full
knowledge of the character and laws of succession will prove of the
greatest value to the forester in all studies of forestation and
reforestation. Forests which now seem entirely unrelated will be seen to
possess the most intimate developmental connection, and the fuller
insight into the life history gained in this way will have a direct
bearing upon methods of conservation, etc. It will further show that the
forester must know other vegetations as well, since grassland and
thicket formations have an intimate influence upon the course of the
succession, as well as upon the advance of a forest frontier.
One of the greatest aids which modern ecology can furnish forestry,
however, is the method of determining the physical nature of the
habitat. So far, foresters have been obliged to content themselves with
a more or less superficial study of the structure of forest formations,
without being able to do more than guess at the physical causes which
control both structure and development. This handicap is especially
noticeable in the case of forest plantings in non-forested regions,
where it has been impossible to estimate the chances of success, or to
determine the most favorable areas except by actual plantations.
Equipped with the proper instruments for measuring water-content,
humidity, light and temperature, the ecologist is able to determine the
precise conditions under which reproduction is occurring, and to
ascertain what non-forested areas offer the most nearly similar
conditions. A knowledge of habitats and the means of measuring them
enables the forester to discover the causes which control the vegetation
with which he is already familiar, and to forecast results otherwise
hidden. Furthermore, it makes it possible for him to enter a new region
and to determine its nature and capabilities at a minimum of time and
energy.
=21. Physiography.= Physiographic features play an important part in
determining the quantity of certain direct factors of the habitat.
Perhaps a more important connection between physiography and ecology is
to be found in succession. The beginning of all primary, and of many
secondary successions is to be sought in the physiographic processes
which produce new habitats, or modify old ones. On the other hand, most
of the reactions which continue successions exert a direct influence
upon the form of the land. The most pronounced influence of terrestrial
successions is found in the stabilization which their ultimate stages
exert upon land forms, even where these are highly immature. The chief
effect of aquatic successions is to be found in the “silting up” and the
formation of new land, which result from the action of vegetation upon
silt-bearing waters. The closeness of the relation between succession
and the forms of the land has led to the application of the term
“physiographic ecology” to that part of the subject which deals with the
development of vegetation, i. e., succession.
=22. Soil physics.= This subject is as much a part of ecology as is
forestry. It is intrinsically that subdivision of ecology which deals
with the edaphic factors of the habitat, and their relation to the
plant. Since the basis is physics, there has been a general tendency to
overvalue the determination of soil properties, and to ignore the fact
that these are decisive only when considered with reference to the
living plant. As the soil contains the water which is the factor of
greatest importance to plants, soil physics is an especially important
part of ecology. Its methods are discussed under the habitat.
=23. Zoogeography.= Since animals are free for the most part, and hence
not confined so strictly to one spot as plants, their dependence upon
the habitat is not so evident. The relation is further obscured by the
fact that no physical factor has the direct effect upon them which water
or light exerts upon the plant. Vegetation, indeed, as the source of
food and protection, plays a more obvious, if not a more important part.
This is especially true of anthophilous insects, but it also holds for
all herbivorous animals, and, through them, for carnivorous ones. The
animal ecology of a particular region can only be properly investigated
after the habitats and plant formations have been carefully studied.
Here, as in floristics, a great deal can be done in the way of listing
the fauna, or studying the life habits of its species, without any
knowledge of plant ecology; but an adequate study must be based upon a
knowledge of the vegetation. Although animal formations are often poorly
defined, there can be no doubt of their existence. Frequently they
coincide with plant formations, and then have very definite limits. They
exhibit both development and structure, and are subject to the laws of
invasion, succession, zonation, and alternation, though these are not
altogether similar to those known for plants, a fact readily explained
by the motility of animals. Considered from the above point of view,
zoogeography is a virgin field, and it promises great things to the
student who approaches it with the proper training.
=24. Sociology.= In its fundamental aspects, sociology is the ecology of
a particular species of animal, and has in consequence, a similar close
connection with plant ecology. The widespread migration of man and his
social nature have resulted in the production of groups or communities
which have much more in common with plant formations than do formations
of other animals. The laws of association apply with especial force to
the family, tribe, community, etc., while the laws of succession are
essentially the same for both plants and man. At first thought it might
seem that man’s ability to change his dwelling-place and to modify his
environment exempts him in large measure from the influence of the
habitat. The exemption, however, is only apparent, as the control
exerted by climate, soil, and physiography is all but absolute,
particularly when man’s dependence upon vegetation, both natural and
cultural, is called to mind.
THE ESSENTIALS OF A SYSTEM
=25. Cause and effect: habitat and plant.= In seeking to lay the
foundation for a broad and thorough system of ecological research, it is
necessary to scan the whole field, and to discriminate carefully between
what is fundamental and what is merely collateral. The chief task is to
discover, if possible, such a guiding principle as will furnish a basis
for a permanent and logical superstructure. In ecology, the one relation
which is precedent to all others is the one that exists between the
habitat and the plant. This relation has long been known, but its full
value has yet to be appreciated. It is precisely the relation that
exists between cause and effect, and its fundamental importance lies in
the fact that all questions concerning the plant lead back to it
ultimately. Other relations are important, but no other is paramount, or
able to serve as the basis of ecology. Ecology sums up this relation of
cause and effect in a single word, and it may be that this advantage
will finally cause its general acceptance as the proper name for this
great field.
In the further analysis of the connection between the habitat and the
plant, it is evident that the causes or factors of the habitat act
directly upon the plant as an individual, and at the same time upon
plants as groups of individuals. The latter in no wise decreases the
importance of the plant as the primary effect of the habitat, but it
gives form to research by making it possible to consider two great
natural groups of phenomena, each characterized by very different
categories of effects. Ecology thus falls naturally into three great
fundamental fields of inquiry: habitat, plant, and formation (or
vegetation). To be sure, the last can be approached only through the
plant, but as the latter is not an individual, but the unit of a complex
from the formational standpoint, the formation itself may be regarded as
a sort of multiple organism, which is in many ways at least a direct
effect of the habitat. In emphasizing this fundamental relation of
habitat and vegetation, it is imperative not to ignore the fact that
neither plant nor formation is altogether the effect of its present
habitat. A third element must always be considered, namely, the
historical fact, by which is meant the ancestral structure. Upon
analysis, however, this is in its turn found to be the product of
antecedent habitats, and in consequence the essential connection between
the habitat and the plant is seen to be absolute.
=26. The place of function.= In the foregoing it is understood that the
immediate effect of the physical factors of the habitat is to be found
in the functions of the plant, and that these determine the plant
structure. Function has so long been the especial theme of plant
physiology that methods of investigation are numerous and well known,
and it is unnecessary here to consider it further than to indicate its
general bearing. The essential sequence in ecological research, then, is
the one already indicated, viz., habitat, plant, and formation, and this
will constitute the order of treatment in the following pages. That
portion of floristic which is not mere descriptive botany belongs to the
consideration of the formation, and in consequence there will be no
special treatment of floristic as a subdivision of ecology.