Gleasonian pattern of mammalian distribution at a macrogeographical scale in Texas

11 Pages·2001·8.2 MB·English

by  James G Owen

Occasional Papers
Museum of Texas Tech University
Number 207 20 March 2001
Gleasonian Pattern of Mammalian Distribution at a
Macrogeographical Scale in Texas
James G. Owen
Gleasonian assemblages (Gleason, 1926; whole, in which members are significantly interdepen¬
Whittaker, 1975) are populations of species that chance dent upon each other. Such species occur together
and environmental conditions have brought together necessarily, or at least at a considerably higher fre¬
at the same time and place. Biotic exchange among quency than one would expect by chance alone.
members of such assemblages is facultative rather than Clemensian communities, in the same region, may dif¬
obligatory. Membership in a suite of such species may fer from each other because of temporal asynchr onies
change, as environmental conditions change. The type during the early stages of their development. Never¬
of community structure predicted by Gleason’s thesis theless, all pass through a series of changes leading to
may be interpreted as a consequence of the convergence upon a final state, that has been likened
Hutchinsonian n-dimensional niche (Hutchinson, 1958; to a super-organism (Clements, 1916; Brown and
Brown, 1995), where spatially autocorrelated resource Lomolino, 1998). Many investigators have challenged
levels and distributional centroids change continuously the Clemensian view and it has been slowly super-
across the landscape (Brown, 1984). This pattern ceded by Gleason’s interpretation (Brown, 1984;
focuses on the idiosyncratic, individualistic properties Whittaker, 1975).
of communities (Brown, 1995). It is very pervasive
in nature and has been demonstrated over a wide range Both of these views may be applied to patterns
of scales, in contemporary (Brown and Kurzius, 1987; of species importance along environmental gradients
Gleason, 1926; Grossman et al., 1982; Mac Arthur, and lead to testable hypotheses. Clements’ view pre¬
1972; Whittaker, 1975) and fossil communities, and dicts discrete associations of species, separated by
vertebrate (Wolfe, 1981; Graham, at al., 1996) and relatively short ecotones; Gleason’s predicts that spe¬
invertebrate groups (Buzas and Culver, 1994). cies importance values will be distributed along a gradu¬
ally changing continuum, without sharp steps. Mea¬
Cements1 (1916) concept of community organi¬ surements along an environmental gradient should serve
zation is the polar antithesis of Gleason’s. Clements’ to test the hypothesis of Clements versus Gleason.
(1916) view focuses on emergent properties, com¬ The question is which pattern comes closest to repre¬
mon to the members of a community (Brown, 1995), senting nature? As with many strongly opposing mod¬
It holds that biotic assemblages comprise an integrated els both contain elements of truth. Importantly, the
2 Occasional Papers, Museum of Texas Tech University
conclusions to which one is led may be methodology species, and the orders Chiroptera n = 31, Rodentia
and scale dependent (Brown and Lomolino, 1998), n - 62, and Carnivora n = 29.
In this paper I explore patterns of geographical While there is taxonomic unity within these cat¬
association, within the conceptual framework of the egories, they embrace great ecological variation. Eco¬
Clemensian and Gleasonian models, of mammalian logical interactions among populations of some of the
species on a regional scale. A high degree of geo¬ species subsumed within these taxonomic categories
graphical separation, into associated groups, of mem¬ probably do not occur enough to be significant. This
bers of a taxon would suggest a Clemensian pattern. study focuses on the distributional patterns, at a re¬
A high degree of geographical mixing among the ele¬ gional scale, that would be predicted by the models of
ments of a taxon would suggest a Gleasonian pattern. Clements and Gleason. It does not imply species in¬
The area treated is the state of Texas and the mamma¬ teractions, at a local scale, as an etiological basis for
lian species treated are the class Mammalia n = 141 such patterns.
Methods and Materials
The state of Texas occupies about 69.2 x 104 specimens as a guide to map preparation. All maps
km2 in the south-central region of the U.S.A. Since consisted of a system of quadrats, each representing
the states' size, shape, and orientation were determined 63.9 km on a side (Owen, 1990). I recorded a species
by political considerations, it may be considered to be as either present or absent within each quadrat. Such
an arbitrary sample for the study of mammalian distri¬ maps have often been used as a basis for making in¬
butions. I prepared maps of the geographical distribu¬ ferences about the biology of the distributions of mam¬
tions, within Texas, of 141 species of mammals native malian species (Simpson, 1964; Pagle, et ah 1991).
to the state. I used all known records of marginal These data are functionally equivalent to a full census.
Results
Distribution of species among quadrats with 1 fitted the data to Poisson distributions. These
respect to sympatric species distributions are used as null models that represent the
number of quadrats that would be inhabited by differ¬
In this paper, sympatric means geographical sym- ent numbers of species, expected on the basis of a
patry on a regional scale. A necessary and sufficient random distribution of species among quadrats. The
condition for sympatry, between a given suite of spe¬ modes of the Poisson curves are located to the right,
cies, is that they occur together in the same quadrat, with respect to the empirical data for Mammalia,
for at least one quadrat, somewhere within their col¬ Chiroptera, and Rodentia; each of their modal values
lective ranges. It does imply close proximity at the is low. These taxa have considerably more area allo¬
local community level, although this may often be the cated to species-poor classes than one would expect
case for many suites of species. This variable is sensi¬ on the basis of chance alone. The Poisson curve for
tive to the size, shape, and orientation of ranges with Carnivora is located a little to the left of the data; its
respect of each other. Figure 1 shows the number of modal value is very low and the entire curve is rela¬
quadrats, as a function of the number of species per tively flat. Carnivores have much more area allocated
quadrat. This plot represents the correspondence be¬ to species-rich classes than one would predict on the
tween area and species richness. basis of chance. Not surprisingly, none of the Pois¬
son curves were significant using a chi-square test of
goodness-of-fit.
Owen— Gleasonian Pattern of Mammalian Distribution in Texas 3
Number of species
Figure 1. Number of quadrats, as a function of the number of species per quadrat Dashed cutvcs are
Poisson distributions fitted to the data. Solid curve for Carnivora is a positive binomial distribution.
Because the data resemble lognormal distribu¬ give little confidence in failure to reject the hypothesis
tions, I also fitted this model; none was significant. of a slope equal to zero.
The empirical data have higher, more peaked modes,
than a two-parameter lognormal distribution. The data
for Carnivora, but not the other taxa, had a significant Combinations of geographically
fit to a positive binomial distribution 16.22 =A3S . sympatric species
The fit to a positive binomial suggests that carnivores
may have a regular or uniform distribution, a condi¬ I use the word combination to mean the set of
tion that is unusual in field or geographical ecology species that is present within a given quadrat. Distinct
(Elliott, 1977; Ludwig and Reynolds, 1988) combinations of sympatric species can differ from each
other with respect to either the identity of their con¬
Figure 2 presents patterns exhibited by the num¬ stituent species or with respect to the number of spe¬
ber of species when they are plotted as a function of cies of which a combination is comprised, or both. If
the number of species, with which they are sympat- two combinations differ in size, then they necessarily
ric. Many species are geographically sympatric with differ in species composition, but the converse is not
a wide range of other species, varying from few to necessarily true,
many. The data have much spread but there is an
overall tendency for many species to be geographi¬ A plot of the number of species against the num¬
cally sympatric with many other species. The straight ber of different combinations, with which said spe¬
lines represent regressions, none of which were sig¬ cies are combination members, is illustrated in figure
nificant. The power of these tests was so low as to 3. Each of these regressions was significant (P < 0,05),
4
Occasional Papers, Museum of Texas Tech University
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Cl
to
Number of sympatric species
Figure 2, Number of species, as a function of the total number of species with which
they are geographically sympatric.
except the one for carnivores* The power was low species exist as a part of many different, but often
for each test, except Rodentia. There is a general ten¬ small (except Carnivora), species combinations.
dency for species of Mammalia, Chiroptera, and Ro¬
dentia to be members of combinations of small size. Figure 4 shows the frequency distribution of the
In the plot for Carnivora there is a slight increase from number of combinations of species as a function of
left to right in the regression line, due to the influence the number of times each combination occurred. The
of the large datum on the extreme right. In sum, many data for Chiroptera, Rodentia, and Carnivora are fitted
Owen— Gleasonian Pattern of Mammalian Distribution in Texas 5
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Mammalia
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Chiroptera
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Number of different species combinations
Figure 3. Frequency distribution of the number of species, as a function of the number
of different combinations in which they occurred.
to power functions; each curve is highly significant Mammalia had 184 different combinations, which
(P <0.001). The lowest possible value for this vari¬ is not significantly different from the maximum of 189
able is one. This value would be the case if all quad¬ (binomial test, P = 0.72), Ninety seven percent of
rats were completely homogeneous i.e., if every spe¬ these combinations were observed only once. Ro¬
cies were sympatric with every other species through¬ dents had 164 combinations, which also was not sig¬
out the entire state. Because each quadrat is charac¬ nificantly different from the maximum possible value
terized by one and only one combination, the maxi¬ of 189 (P = 0.069). Most of these combinations
mum possible number of combinations is 189. This (89.6%) also occurred only once. Bats had 73 combi¬
value would obtain if each of the quadrats had a unique nations. This value was significantly less than the
assemblage.
6 Occasional Papers, Museum of Texas Tech University

0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16
Number of times species combination was observed
Figure 4. Frequency distribution of the number of combinations of species, as a function of the number of times each
combination was observed.
maximum number possible (P < 0*001). Of these illustrated as a function of the number of species per
53.4% occurred once* Lastly carnivores occurred in quadrat. I contrasted these data with a positive bino¬
82 species combinations, which also is significantly mial distribution having its probability of “success”
less than the maximum possible (P < 0.001). Of these set equal to P = 0.5. This distribution is symmetrical
59.8% occurred only once. It is noteworthy that bats, by construction and approaches a normal distribution,
which can disperse by flying, had a lower number of as sample size increases without bound. This approach
combinations than any other taxon. The upshot of amounts to the null hypothesis that different combina¬
this figure is that most combinations of sympatric spe¬ tions are equally likely. The procedure includes some
cies occur only once and that most of the others oc¬ combinations that were not observed because one or
cur only two or three times. more of the component species do not have overlap¬
ping geographical ranges. This model is neutral in the
The frequency distribution (Fig. 5) of the num¬ sense that it assumes that no species interactions or
ber of species belonging to different combinations is extrinsic factors influence distribution. As such I did
7
Owen— Gleasonian Pattern of Mammalian Distribution in Texas

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Number of species per quadrat
Figure 5. Frequency distribution of the number of species pertaining to different combinations, as a function of the
number of species per quadrat (black dots). Circles represent the expected number of species pertaining to different
combinations, based on the null hypothesis that each different combination is equally likely.
not expect the data to conform to this model, but rather of their corresponding null modes. Their distributions
was interested in the manner and degree in which lack have strong positive skew. They are characterized by
of conformity might be expressed. an abundance of species-poor classes, i.e. classes that
are composed of a relatively low number of species.
I used a two-sided normal approximation to the
Mann-Whitney test to assess the null hypothesis that Interestingly, the distribution of carnivores dif¬
the empirical and theoretical data belong to the same fers from the other groups in several ways. Carni¬
underlying population. The alternative is that they dif¬ vores are not positive skewed but rather seem to be
fer in location. This test was highly significant (P < slightly negatively skewed (Fig. 5). They have more
0.001) for each taxon. Lack of fit seems to be attrib¬ area allocated to high species richness classes, with
utable to both the locations of the modes and to their respect to the other three taxa, and more species com¬
degree of skewness. The empirical modes of all mam¬ binations that are composed of a larger number of spe¬
mals, rodents, and bats are strongly shifted to the left cies. Overall they are more nearly symmetrical, their
8
Occasional Papers, Museum of Texas Tech University
mode is displaced a little to the right of the null mode Impressionistically, carnivore assembly seems to be
and, except for a single datum point, the empirical dis¬ closer to the null model than to that of the other three
tribution for carnivores is completely enclosed within taxa.
the density of its corresponding null distribution.
Discussion
The data presented in this paper, together with (Fig. 1) and; (2) the lack of fit to binomial curves (Fig.
findings referenced from the literature, suggest that a 5). This means that one or more of the basic postu¬
mosaic-like pattern is characteristic of biotic assem¬ lates of these distributions is seriously viol ated. A two-
blages across a large range of different taxonomic, dimensional Poisson or binomial process assumes,
temporal, and spatial scales. The spatial scales are among other things, that each quadrat or sampling unit
known to range from local, homogeneous habitat has equal probability of being inhabited by a given spe¬
patches to subcontinental regions. The taxonomic cies. In particular, this postulate seems likely to have
scales range from feeding guilds of mice to an entire been violated by the Texas data. The state is not ho¬
class and the temporal scales span millions of years. mogeneous with respect to its resource content for
mammals. Some areas offer more favorable condi¬
Two exceptional patterns are emergent among tions and so accumulate more species than others. This
Carnivora: (1) High species-richness is distributed over is consonant with the widely varying qualitative fea¬
a greater area for carnivores than it is for all mam¬ tures of different habitat types in Texas (Gould, 1975).
mals, rodents, or bats; (2) geographically, carnivores Limit relationships between the Poisson and binomial
are grouped together in such a way as to produce com¬ distributions imply that, for large sample sizes, and
binations of species that are species-rich relative to small probability of success for each species pres¬
the other taxa. Both of these phenomena may be ex¬ ence-absence event, it makes little practical difference
plained as consequences of the larger geographical which of the two distributions one uses (Hogg and
ranges of carnivores. Larger geographical ranges Craig, 1978; Boswell, et al., 1979).
should overlap more with each other, yielding numeri¬
cally higher combinations of species. This effect is Each of the patterns in the data, suggests a
reinforced by the presence of high carnivore richness patchy, mosaic-like geographical assemblage with little
in central Texas (Owen, 1988). The accumulation of internal homogeneity. At a regional scale species num¬
carnivore species in central Texas and its gradual de¬ ber, degree of sympatry, and combinational size, oc¬
cline outwards to the east and west, gives carnivores cur in constantly changing spatial configurations. This
more opportunity to acquire numerically high combi¬ change is probably produced by responses to environ¬
nations, while remaining within the study area. This mental conditions that are species-specific, rather than
exceptional pattern for carnivores does not obviate their assembly-specific. The pattern corresponds to a
inclusion within a Gleasonian frame of reference. The Gleasonian interpretation of mammalian distribution in
same pattern and its explanation would seem probable Texas at a statewide scale. Quadrats cannot (except
on a continental scale. North American carnivores arbitrarily) be organized to form internally cohesive
have considerably larger ranges than several other or¬ biogeographical assemblages. This is the case in spite
ders of North American mammals (Pagle, et al., 1991). of the ease with which certain areas of the state can
be superficially distinguished on the basis of their veg¬
The Gleasonian pattern of spatial association etation physiognomy. Knowledge of the species com¬
found in this study is not completely antithetical to position of one quadrat does not permit the prediction
non-randomness. Two non-random patterns are iden¬ of the species composition of other quadrats, at the
tified: (1) The lack of fit of the data to Poisson curves ordinal and class levels of taxonomy.
Owen— Gleasonian Pattern of Mammalian Distribution in Texas 9
Fossil faunas have demonstrated great age of
dant data suggest that the pattern is real. Such long
conditions that fit the Gleasonian hypothesis of spe¬
expanses of time suggest that the Gleasonian pattern
cies associations. Buzas and Culver (1994) studied
is not an epi-phenomenon, but rather an integral part
foramini feral communities, from Cenozoic shelf de¬ of the ecological milieu through geological time.
posits of the North American Atlantic Coastal Plain.
These communities exhibited little unity, during nearly That a mosaic-like pattern holds up for taxa at
55 million years of successive oceanic transgressions the class and ordinal level is perhaps to be expected.
and regressions. Their results indicated a lack of local Members of the same class or order exhibit a wide
community unity. Graham et ah, (1996) analyzed fos¬ range of form and function. The rub is that Brown
sil mammalian communities at 2,945 localities in the and Kurzius (1987) found that patterns, qualitatively
United States. They documented geographical range very similar to the ones described in this paper, char¬
shifts of individual species, at different times, in dif¬ acterize members of a well defined desert, rodent feed¬
ferent directions, and at different rates, in response to ing guild, where perhaps it was not to be expected.
late Quaternary environmental fluctuations. Such abun¬
Acknowledgments
The data analyses and interpretations used in this (1987). I thank James Brown for comments on an
paper were inspired by a similar study, at a different early draft of this manuscript. I also thank two anony¬
scale and taxonomic level, by Brown and Kurzius mous reviews for comments.
10
Occasional Papers, Museum of Texas Tech University
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