deep alluvial soil, the ground litter of leaves and decaying logs, the
low bushes and small trees, and finally the tall trees of the forest.
Each of these provides certain types of shelter for amphibians. The
moist soil and litter on the forest floor is an important microhabitat
for fossorial and strictly terrestrial species. The dense foliage of
the trees, tree holes, and bromeliads growing on the trees provide
shelter for arboreal species. Arboreal and terrestrial bromeliads and
the terrestrial elephant-ear plants (_Xanthosoma_) contain water in the
axils of their leaves throughout the year and thus provide an important
habitat for amphibians. The low, spiny, deciduous trees of the scrub
forest and the grasses and scattered trees in the savannas provide
little shelter. In the savannas there are depressions, some of which
contain water throughout the year; these are often surrounded by trees
providing refugia for amphibians during the dry season. In the scrub
forest many species congregate along streams and in moist stream beds
during the dry season.
Now that the important ecological factors of the major environments
have been outlined, we may examine the local distribution of amphibians
in each of these. Beginning with the rainforest, we find only one
fossorial species, _Gymnopis mexicanus_. A large number of species are
found on the forest floor; characteristic inhabitants of the leaf
litter are: _Bufo valliceps_, _Eleutherodactylus rhodopis_,
_Microbatrachylus pygmaeus_, and _Syrrhophus leprus_. Other terrestrial
amphibians usually are not scattered throughout the rainforest, as are
those named immediately above, but instead inhabit areas of forest
adjacent to ponds or streams; these species include: _Bufo marinus_,
_Eleutherodactylus natator_, _Eleutherodactylus rugulosus_,
_Leptodactylus labialis_, _Leptodactylus melanonotus_, _Rana palmipes_
and _Rana pipiens_. The most striking ecological assemblage of
amphibians in the rainforest is the arboreal group of species,
including:
_Bolitoglossa occidentalis_
_Bolitoglossa platydactyla_
_Eleutherodactylus alfredi_
_Hyla baudini_
_Hyla ebraccata_
_Hyla loquax_
_Hyla microcephala martini_
_Hyla picta_
_Phrynohyas modesta_
_Phrynohyas spilomma_
_Phyllomedusa callidryas taylori_
In the savannas _Rhinophrynus dorsalis_, _Engystomops pustulosus_, and
_Gastrophryne usta_ are fossorial species. _Bufo marinus_,
_Leptodactylus melanonotus_, _Leptodactylus labialis_, _Rana palmipes_,
and _Rana pipiens_ are found in the vicinity of permanent water in the
savannas. Although the savanna habitat does not provide the ecological
conditions for the existence of an arboreal fauna, many arboreal
species from the surrounding rainforest utilize the extensive marshes
and ponds in the savannas for breeding purposes. Thus, _Hyla baudini_,
_Hyla microcephala martini_, _Hyla picta_, and _Phrynohyas spilomma_
have been found breeding in savannas. In parts of savannas where clumps
of trees surround depressions containing water throughout the year,
individuals of the species named above, together with _Hyla loquax_ and
_Phyllomedusa callidryas taylori_, may not only breed, but remain
throughout the year.
In the semi-arid scrub forest the same fossorial species as exist in
the savannas are found. Likewise, _Bufo marinus_, _Leptodactylus
labialis_, _Leptodactylus melanonotus_, and _Rana pipiens_ are found
near permanent water. Terrestrial species in this semi-arid environment
include _Bufo canaliferus_, _Bufo coccifer_, _Bufo marmoreus_,
_Syrrhophus pipilans_, and _Diaglena reticulata_. Of these, _Syrrhophus
pipilans_ sometimes inhabits low trees and bushes; the others may be
fossorial. The arboreal species in the scrub forest include _Hyla
baudini_, _Hyla robertmertensi_, _Hyla staufferi_, and _Phyllomedusa
dacnicolor_.
_Eleutherodactylus rugulosus_ and _Hylella sumichrasti_ live along
streams in the scrub forest. _Hylella sumichrasti_ lays its eggs in
these streams.
In comparing the ecological differences in the amphibian assemblages in
the three major habitats, the most obvious difference is the great
percentage of arboreal species in the rainforest as compared with
savanna and scrub forest. Only four arboreal species are found in the
scrub forest, none in the savannas, but eleven in the rainforest.
Likewise, there is an absence of ground-dwelling forms in the arid
habitats; in the latter the only terrestrial species are those that
are found near water. A possible exception is _Syrrhophus pipilans_.
From the above analysis of ecological distribution we may see that the
rainforest provides a variety of habitats for amphibians and that these
habitats are suitable for amphibian life throughout the year. On the
other hand, the savannas and scrub forests are characterized by extreme
conditions of desiccation, a factor of considerable importance in
limiting the ecological distribution of amphibians. However, there
still is a diversity of amphibians in these semi-arid environments.
Obviously, these species are adapted in various ways for survival
during the dry season, at which time environmental conditions are such
that the animals cannot carry on their normal activities.
Although there is not an abundance of data concerning the seasonal
activity of the fauna, what is available shows some interesting
correlations with the environments. During the dry season in the scrub
forest there is essentially no amphibian activity; an occasional _Rana
pipiens_ may be seen along a river, or a _Bufo marinus_ may be seen at
night. In the rainforest the terrestrial-breeding amphibians are active
during the dry season. _Eleutherodactylus rugulosus_ is found at night
or by day along streams. _Eleutherodactylus rhodopis_,
_Microbatrachylus pygmaeus_, and _Bufo valliceps_ are active during the
day; these plus _Bolitoglossa occidentalis_, _Bolitoglossa
platydactyla_, _Eleutherodactylus alfredi_, _Eleutherodactylus
natator_, and an occasional _Hyla_ are active at night.
With the onset of the heavy summer rains and the subsequent formation
of breeding ponds, amphibian activity reaches a peak. This is
especially noticeable in the semi-arid environments, where during the
dry season there is little activity.
Among the anurans in the isthmus the four species of
_Eleutherodactylus_, the two species of _Syrrhophus_, and the one
species of _Microbatrachylus_ are either known, or presumed, to lay
eggs on the ground; these develop directly into small frogs. All of the
other anurans deposit their eggs in water or attach them to objects
over water (_Phyllomedusa_); these hatch into tadpoles, which later
metamorphose into frogs. _Hylella sumichrasti_ is known to breed only
in streams. All of the other species breed in ponds, but at times some
species deposit their eggs in streams; in this last group are _Bufo
valliceps_, _Bufo marmoreus_, _Phyllomedusa callidryas taylori_, and
_Rana pipiens_.
Although the ecological data are incomplete, they do show that
ecological conditions differ greatly in the three major environments,
different species of amphibians inhabit these environments, and that
the fauna is ecologically diversified in each environment.
_Distribution of the Fauna_
Plotting the distributions of species of amphibians known to live in
the lowlands of the Isthmus of Tehuantepec results in an array of
geographic patterns. These may be analyzed with respect to those
species that are restricted either to the Gulf lowlands or the Pacific
lowlands, or those that occur on both the Gulf and Pacific lowlands.
Furthermore, the distributions may be analyzed with respect to those
species whose ranges extend from Mexico across the Isthmus of
Tehuantepec into Central America, those that reach the isthmus from
Central America but do not extend into Mexico proper, and those that
reach the isthmus from Mexico but do not extend into Central America.
It should be kept in mind that the following analysis is of the lowland
inhabitants only. Species inhabiting the foothills and mountains will
be discussed later.
1. SPECIES RESTRICTED TO THE GULF LOWLANDS. Of the 36 species of
amphibians recorded from the Isthmus of Tehuantepec, nine (25 per cent)
are in this group. Four of these (_Eleutherodactylus alfredi_,
_Syrrhophus leprus_, _Hyla loquax_, and _Hyla picta_) live in the Gulf
lowlands to the east and to the west of the isthmus. Three others
(_Hyla ebraccata_, _Hyla microcephala martini_ and _Phyllomedusa
callidryas taylori_) are primarily Central American in their
distribution and reach the northwestern limits of their ranges in the
Gulf lowlands of the isthmus, whereas _Bolitoglossa platydactyla_ and
_Eleutherodactylus natator_ reach the southern limits of their
distributions in the isthmus.
2. SPECIES RESTRICTED TO THE PACIFIC LOWLANDS. This group includes six
species, or 17 per cent of the amphibian fauna of the isthmus. Two of
these (_Bufo coccifer_ and _Syrrhophus pipilans_) range to the east and
to the west of the isthmus on the Pacific lowlands. Two others (_Bufo
canaliferus_ and _Hyla robertmertensi_) range from the isthmus into
Central America, and _Diaglena reticulata_ and _Phyllomedusa
dacnicolor_ range on the Pacific lowlands of Mexico southeastward to
the isthmus.
3. SPECIES THAT OCCUR ON THE PACIFIC AND GULF LOWLANDS. This group
includes 19 species, or 53 per cent of the total amphibian fauna. Of
these, nine species (25 per cent of the entire amphibian fauna) are
widespread throughout the lowlands of Mexico and Central America; these
are:
_Gymnopis mexicanus_
_Rhinophrynus dorsalis_
_Bufo marinus_
_Engystomops pustulosus_
_Leptodactylus labialis_
_Leptodactylus melanonotus_
_Hyla baudini_
_Hyla staufferi_
_Rana pipiens_
Four species occur on the Gulf lowlands to the east and to the west of
the isthmus, but on the Pacific lowlands they occur only to the east;
this group includes _Bufo valliceps_, _Eleutherodactylus rhodopis_,
_Phrynohyas modesta_, and _Phrynohyas spilomma_. Three species live to
the east and to the west of the isthmus on the Pacific lowlands, but
only to the west on the Gulf lowlands; these include _Eleutherodactylus
rugulosus_, _Microbatrachylus pygmaeus_, and _Gastrophryne usta_.
Six species that cross the isthmus live on the humid Gulf lowlands and
on the humid lowlands of Chiapas and Guatemala, but not on the
semi-arid Plains of Tehuantepec; these include _Bolitoglossa
occidentalis_, _Eleutherodactylus rhodopis_, _Microbatrachylus
pygmaeus_, _Phrynohyas modesta_, _Phrynohyas spilomma_, and _Rana
palmipes_. Of these, _Microbatrachylus pygmaeus_ also occurs in
scattered humid environments to the west of the isthmus on the Pacific
lowlands.
Two species are endemic to the isthmian region. _Bolitoglossa
veracrucis_ is known only from the humid northern slopes of the
isthmus. _Hylella sumichrasti_ occurs on the Pacific slopes of the
isthmus and extends to the east into western Chiapas.
In analyzing the distribution of the amphibians with respect to those
that are restricted to either the Pacific or Gulf lowlands or those
that cross the continental divide in the isthmus, we find that 25 per
cent of the species are restricted to the Gulf lowlands, 17 per cent
are restricted to the Pacific lowlands, and 53 per cent cross the
isthmus. In analyzing the distribution patterns with respect to those
that extend across the isthmus of Tehuantepec from east to west, we
find that 14 per cent of the species do not extend east of the isthmus
into Central America and that 19 per cent do not range west of the
isthmus into Mexico proper; 61 per cent of the species range to the
east and to the west of the isthmus. Of the 36 species of amphibians
inhabiting the isthmus only nine species (25 per cent) range across the
isthmus, that is, occur on the Gulf and Pacific lowlands, and also
range to the east and to the west of the isthmus. To these wide-ranging
species the diversified environments of the isthmus do not present a
barrier to distribution. The other 27 species (75 per cent) either do
not cross the isthmus from east to west or from north to south; thus,
probably in one way or another the isthmus presents a barrier to their
distribution.
THE AMPHIBIAN FAUNA OF THE FOOTHILLS AND ADJACENT HIGHLANDS
To amphibians inhabiting the foothills and mountains of southern Mexico
and northern Central America, the isthmus presents a great barrier to
dispersal. For example, salamanders of the genus _Thorius_, the
_mexicanus_ and _augusti_ groups of the genus _Eleutherodactylus_, the
_bistincta_ group of the genus _Hyla_, and the genus _Tomodactylus_
occur on the Mexican Plateau and southward into the mountains of
Oaxaca. Nevertheless, no members of these groups are present in the
Guatemalan-Chiapan Highlands. The genera _Chiropterotriton_,
_Magnadigita_, _Pseudoeurycea_, and _Ptychohyla_, as well as the
_eximia_ group of _Hyla_ are represented by different species in the
Guatemalan-Chiapan Highlands than in the mountains of Mexico on the
other side of the isthmus. Several species of _Plectrohyla_ occur in
the Guatemalan-Chiapan Highlands, but none is known from the Mexican
Highlands, although one species occurs in the Tuxtlas.
Living in the humid forests of the foothills are salamanders of the
genus _Lineatriton_, frogs of the _spatulatus_ group of
_Eleutherodactylus_, _Anotheca coronata_, _Hyla miotympanum_, and
_Phyllomedusa moreleti_. All of these occur in the foothills of the
Sierra Madre Oriental in eastern Mexico and in Los Tuxtlas.
_Lineatriton_, _Hyla miotympanum_, and the _spatulatus_ group of
_Eleutherodactylus_ do not occur in the foothills of the
Guatemalan-Chiapan Highlands; those amphibians reach the end of their
ranges at the isthmus. _Phyllomedusa moreleti_ and _Anotheca coronata_
are found in the northern foothills of the Guatemalan-Chiapan
Highlands, and _Phyllomedusa moreleti_ is found in the foothills on the
Pacific slopes of the Chiapan Highlands.
Although the above analysis is not so detailed as that of the lowland
inhabitants, it does show that all of the genera and species of
amphibians known to inhabit the foothills and highlands adjacent to the
isthmus, only two species of amphibians cross the isthmus from one
highland mass to the other. Thus, it is evident that the Isthmus of
Tehuantepec presents a great barrier to dispersal of these groups of
amphibians.
ESTABLISHMENT OF PRESENT PATTERNS OF DISTRIBUTION
From the foregoing analysis of geographical and ecological distribution
in the Isthmus of Tehuantepec we may strive for an interpretation of
the events that led to the establishment of patterns of distribution
displayed not only by the amphibians, but other terrestrial vertebrates
as well. The thesis that I am proposing below is based on the premise
that in southern Mexico and northern Central America climatic
fluctuation during the Pleistocene was of sufficient magnitude to cause
vegetational shifts, both vertically and latitudinally, resulting in
the establishment of alternating continuous and discontinuous lowland
and highland environments, although this climatic fluctuation was not
so great as to eliminate tropical lowland environments from the region.
I feel that the present patterns of distribution of the amphibians in
the Isthmus of Tehuantepec may be explained on this premise.
Many authors dealing with the herpetofauna of Middle America have
followed Schuchert's (1935) suggestion of a seaway in the isthmus
during the Cenozoic. Thus, Burt (1931), Duellman (1956, 1958a), Gloyd
(1940), Oliver (1948), Smith and Laufe (1946), and Stuart (1941)
employed the presence of a seaway to explain distribution and
speciation in various genera. Durham, Arellano, and Peck (1952), Olson
and McGrew (1941), and Stirton (1954) have provided geological evidence
that there probably was no Cenozoic seaway in the Isthmus of
Tehuantepec. Even if there were a seaway in the Pliocene or Miocene
(the dating of this possible seaway is open to question), its presence
is not necessary to explain the present patterns of distribution in the
isthmus.
In recent years the study of natural biotic environments, palynology,
and Pleistocene chronology in Middle America has produced a wealth of
data, which although still fragmentary begins to form a picture of past
climatic events in that part of the world. Sedimentary studies by
Hutchinson, Patrick, and Deevey (1956) and Sears, Foreman, and Clisby
(1955) have provided evidence of drastic climatic shifts in Mexico
during the Pleistocene. Further evidence of bioclimatic fluctuation is
provided by Martin and Harrell (1957) and Martin (1958); the latter has
suggested that there was a displacement of the tropical zones in
southern Mexico and northern Central America by as much as 3000 feet
during the glacial maximum. Much of the evidence of such drastic
vertical shifts in environments is based on the presence of
Pleistocene montane glaciers on Mexican volcanoes (White, 1956) and
Chirripo in Costa Rica (Weyl, 1955). Dorf (1959) supports this idea of
drastic climatic change.
In his studies of the avifauna of Mexico and Guatemala Griscom (1932
and 1950) made an important issue of the continuity of the bird fauna
in what he called the Subtropical Life-zone, which essentially consists
of cloud forest, a widespread, but discontinuous, habitat on the Gulf
(windward) slopes of the Mexican and Central American highlands at
elevations between 1000 and 2000 meters. To account for this apparent
uniformity in the avifauna Griscom hypothesized a continuity of cloud
forest environment in the Pleistocene; this would result in the
depression of cloud forests to the coastal lowlands and the
displacement of tropical lowland environments far to the south in
Central America. Stuart (1951) objected to this displacement of lowland
tropical rainforest; he stated that a descent to sea level of a
subtropical zone would have brought about either widespread
extermination of the tropical fauna or acclimatization of that fauna to
subtropical conditions.
Although palynological studies and some faunal studies of subtropical
and temperate animals suggest a drastic climatic fluctuation that might
have eliminated tropical environments in southern Mexico and northern
Central America, there is much biological evidence indicating the
existence of tropical environments in this region even during the
glacial maximum. Especially significant is the diversity of species
inhabiting the present tropical environments; many of these have
differentiated from related taxa to the south.
In the Pleistocene, climate fluctuated and vegetation shifted
correspondingly in southern Mexico and northern Central America. Most
of the palynological studies and many studies of Pleistocene chronology
deal with montane regions, either the Mexican Plateau or the mountains
rising from the plateau. No such studies have been made in lowland
tropical environments. During glacial advances the tropical lowland
environments in Mexico probably were not eliminated, for the great
diversity of animals in these environments supports the hypothesis that
they have been in existence for some time, although periodically they
may have been discontinuous.
In order to understand the nature of bioclimatological events in the
Pleistocene in lowland tropical environments of southern Mexico,
certain factors that are of little importance in the interpretation of
Pleistocene chronology in the highlands must be considered. These
factors are: 1) climatic moderation by oceans, 2) fluctuation in sea
level, and 3) fluctuation in level of the water table as affected by
sea level.
It is well-known that large bodies of water moderate the temperature on
adjacent land. Furthermore, it is known that faunas of marine
invertebrates shifted latitudinally in the Pleistocene; Trask, Phleger,
and Stetson (1947) recorded cold-water Foraminifera then as far south
as the Sigsbee Deep in the middle of the Gulf of Mexico. Large bodies
of warm water, such as the Gulf of Mexico, Caribbean Sea, and Pacific
Ocean of today, probably were not sufficiently cooled at the time of
glacial advance to affect greatly the temperature of the winds blowing
across them. Even if these bodies of water were somewhat cooler than
now, the prevailing winds blowing from them onto the lowlands of Mexico
and northern Central America would have aided in maintaining relatively
high temperatures there. These warm winds probably counteracted the
cooling effect of glaciation in the lowlands and thereby maintained
tropical conditions near the seas.
Although no adequate studies of Pleistocene beach lines have been made
in southern Mexico, such information is available for peninsular
Florida on the other side of the Gulf of Mexico (Cooke, 1945).
Fluctuation in sea level in the Pleistocene has been used by Hubbell
(1954), Goin (1958), and Duellman and Schwartz (1958) to explain
present patterns of distribution of animals in Florida. If Cooke's
interpretations can be applied to the western side of the Gulf of
Mexico, even generally, it would be supposed that sea level varied from
about 300 feet lower than at present during the Illinoian Glacial
Period to about 275 feet higher than at present during the Aftonian
Interglacial Period. Lowering of sea level would expand the lowlands in
the isthmus; rising sea level would restrict them, leaving only the
central ridges and many islands in the isthmus, but never forming a
seaway between the Gulf of Mexico and the Pacific Ocean.
Probably the level of the water table in the coastal lowlands and the
gradients of the streams in the lowlands and foothills was closely
correlated with fluctuation in sea level. If sea level fluctuated as
much as 575 feet in the Pleistocene, changes in the level of the water
table must have been of considerable magnitude.
During times of glacial advances the lowlands of the isthmus probably
were more extensive and had more semi-arid tropical environments than
at present, with patches of rainforest existing in sheltered valleys
along the major streams. In the course of bio-climatic fluctuation the
semi-arid environments (scrub forest and/or savanna) were continuous at
times from the Pacific lowlands across the isthmus to the Gulf
lowlands. At those times such typical inhabitants of the semi-arid
environments as _Rhinophrynus dorsalis_, _Engystomops pustulosus_, and
_Hyla staufferi_ could have made their way across the isthmus. At times
of most extensive glaciation, such as the Illinoian, temperatures in
the isthmus probably were low enough to permit the growth of pine-oak
forest and cloud forest continuously across the central ridges from the
Mexican to the Chiapan-Guatemalan highlands. At those times such
highland members of the fauna as _Chiropterotriton_, _Pseudoeurycea_,
_Magnadigita_, and the _eximia_ group of _Hyla_ could have crossed the
isthmus. During Wisconsin time, climate probably fluctuated less than
during previous glaciations; probably no montane environments, except
cloud forest, were represented in the isthmus during the Wisconsin.
Even at this relatively late date such animals as _Lineatriton
lineola_, _Anotheca coronata_, and _Phyllomedusa moreleti_ could have
crossed the isthmus.
During the interglacial periods, which in the isthmian region were
characterized by warmer temperatures, higher sea level and consequently
more restricted areas of lowlands, and possibly more rainfall than in
the glacial periods, the continuity of pine-oak forest and cloud forest
from east to west across the isthmus was interrupted. Probably, too,
the semi-arid environments were restricted, and the rainforests were
more widespread. At those times animals now inhabiting the rainforests
of the Gulf lowlands and those inhabiting the Pacific lowlands of
Chiapas and Guatemala could have crossed the isthmus. In this group are
species such as _Bolitoglossa occidentalis_, _Eleutherodactylus
rhodopis_, _Microbatrachylus pygmaeus_, and _Rana palmipes_.