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Acta Ecologica
Ecol. Gener., 1982, Vol. 3, no 3, p. 323-330.
Spatial organization and habitat partitioning
in a mountain lizard community of Mexico
Alfredo Ortega, Ma. Eugenia Maury
Instituto de Ecología,
Nuevo Bosque de Chapultepec A. P. 18-845 México 18, D. F. Mexique.
Robert Barbault
Laboraroire de Zoologie,
École Normale Supérieure, 46, rue d'Ulnz, 75230 Paris Cedex 05 (Frunce).
SUMMARY
The spatial structure of a mountain lizard community located in the Sierra Madre Occidental,
N. E. Mkxico, was studied from May 1979 to October 1980. The community is composed of seven
species, including four Sceloporus. Analysis of microhabitat and substrate utilization by al1 the species
in two main habitats shows that species are not specialized according to habitat but are specialized
to microhabitat-substrate. Spatial niche-overlap values are very low for al1 paired species. The determining role of microhabitat selection in organizing this lizard community and its general ecological
meanings are discussed.
Habitut partitioning - Subsrrore selecrion - Community
KEY-WORDS:
Mé.xicu - Sierra Madre Occiden~ol-Lizords - Sceloporus.
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L'organisation spatiale d'un peuplement de lkzards caractkristique de la foret d'altitude de
chenes et de pins de la Sierra Madre Occidentale a ktk ktudike de mai 1979 a octobre 1980 au Mexique.
Le peuplement comprend sept especes, dont quatre Scelupurus. L'analyse de la distribution spatiale
de ces especes entre deux grands types d'habitat et onze types de substrat ou de microhabitat a mis
en relief I'ktroite spkcialisation de ces lkzards vis-a-vis, non de l'habitat, mais du micrahabitat ou du
substrat. Le r6le d'une telle spécialisation dans l'organisation de la communautk et sa signification
kcologique plus gknkrale sont discutks.
MOTS-CLÉS: Partage de l'habitat - Sélection du subsrrui - Peuplement
Mexique - Sierra Madre occidentale - Lézards - Sceloporus.
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The suggestion that spatial segregation may be one of the most important
factors minimizing actual or potential competition between species and thus contribute to their stable coexistence was emphasized, from a theoretical point of view,
(1966) and supported by much empirical data reviewed
by MACARTHUR& PTANKA
by SCHOENER
(1974). In the particular case of lizard communities, although differentia1 food utilization may contribute to minimize ecological overlap between species
et al., 1978;
(PIANKA,1973, 1975; FUENTES,1976; HUEY& PIANKA,1977; BARBAULT
PIANKAet al., 1979; MAURY,1981) severa1 workers demonstrated how important
habitat partitioning may be (PIANKA,1966; JENSSEN,1973; MELLADOet al., 1975;
Acta Eculogicu/Ecolugia Generalis, 0243-766X/1982/323/$ 5.0010 Gauthier-Villars
324
A . ORTEGA, M . E. MAURY, R . BARBAULT
In this study we analyse the spatial structure of a lizard community representative of the American mountain pine-oak forest. This work must be taken as a
first step towards a more complete study of the ecological organization of this community. The determinant role of microhabitat selection in organizing lizard communities is discussed.
MATEEUALS AND METHBDS
The study was carried o u t at the Biosphere Reserve of La Michilia o n the eastern slopes of the
Sierra Madre Occidental, between 23O20' and 23"30' north, and between 104"20' and 104°07' west.
The elevation of the area varies between 2,350 and 2,850 m. The mean annual teinperature is between
17.4O C and 20.4" C ; the annual precipitation between 525 to 609 mm, strongly concentrated
in the suininer (Sune to September). Geologically the area lies o n igneous rocks of tertiary extrusions.
The plant community of the area is an oak-pine forest but the vegetation is highly diversified,
& SALDIVAR,
1978). However the principal types
including 1 1 distinct units of vegetation (MARTINEZ
of vegetations are the oak-pine forest (fig. 1) and the grassland (fig. 2).
The lizard community oF the area consists of seven species: Sceloporrrs grnininicirs Wieginann,
1828; tlie niost common reptile of the area, Scelopor~rsscnlnris Wicgmann, 1828; Scelopor~c:p o i ~ i s e í t i
Baird & Girard, 1854; Scelopor~rsjnrroei Cope, 1875; Plirynosornn orbicrrlcrre (Linnaeus, 1789);
B n r i ~ i ni t ? l b r i c ~ (Wiegmann,
~t~
1834) and E~rn7ece.shieuirosíris (Gunther, 1860).
The oak-pins forest and grassland were systematically covered in the search for lizards, between
May of 1979 t o October of 1980. For each lizard observid we recorded: species, sex, hour, habitat,
microhabitat o r substrate. Eleven Icinds of microhabitat-s~ibstrate were recognized: fallen trunks,
fallen branclies, trees, stumps, shrubs, rocks, steep rock faces, ground, ground with grasses, ground
Fic. 1. - Aspect of the oak-pine forest (on the ground note one fallen tree trunk).
A c l n Ecologica/CEcologin Geiierolis
FIG.2.
- Aspect
of the grassland (in the background note the oak-pine forest).
with leaf litter, ground with fallen tree trunks. Sampling included al1 day-light hours during the four
seasons of the year, with a total of 1,650 records.
The x2 goodness-of-fit test was employed to determine the statistical significance of the differences observed between distribution of species.
For the calculation of habitat niche breadth values Simpson's diversity index was used (LEVINS,
1968) in a standardized form (varying between O and 1):
where p i is the proportion of individuals found on the i t h substrate and N the number of classes
of substrate ( 1 1).
Microhabitat niche overlap values were measured using PIANKA'Sindex (1973):
where p i . and pik are the frequency of utilization of the iLhmicrohabitat by the jth and kIh species
respectively. T h e values obtained using this formula vary between O and l .
RESULTS
Distrib~ctionof species between habitats
If we exclude the rarely seen species, Barisia imbricata and Phrynosoma orbiculare, for which there was too little data to be conclusive, al1 the species coexist
in each habitat (table 1).
Vol. 3, no 3
-
1982
A . ORTEGA, M. E. MAURY, R. BARBAULT
Spectrum of species frequency in the two kinds of habitat:
uak-pine forest (1) and grassland (2)
SceluNumbers
purus
observed grammicus
Scleupurus
scalaris
Scleupurus
puinsetti
Scleupurus
jarruvi
Phrynosoma
orbiculare
Eumeces
brevirostris
Barisia
imbricata
The spectrum of species frequency appeared slightly different between the two
kinds of habitat ( X 2 significant at p S 0.05) mainly because Scleoporus jarrovi was
scarcer in grassland than in oak-pine forest. This is because this lizard is a rock and
wall dwelling species and this micro-habitat is not often found in the grassland habitat.
Distribution of species in microhabitats or substrates
Comparing microhabitat utilization by the various species, much can be learned
about the qualitative nature of ecological differences among species (table 11).
Sceloporus grammicus is a fallen trunk dweller since most individuals were seen
on fallen trees (59.6 %) and fallen branches (15.2 %). Sceloporus scalaris is terrestrial
(89.1 % on ground), as are the three scarce species, Phrynosoma orbiculare (93.9 %),
Eumeces brevirostris (100 %) and Barisia imbricata (77.7 %). Sceloporus poinsetti
is mainly observed on rocks (43.6 %) but also on fallen trunks (27.7 %) and stumps
(21.8 %) while the other rockdwelling species, Sceloporus jarrovi is generally found
on steep rock faces (67.7 %) and also on big rocks (27.4 %).
Al1 differences between paired species are statistically significant ( p < 0.01)
except between Phrynosoma orbiculare and Barisia imbricata, where there is no difference ( P > .05).
The very low microhabitat niche breadth values found for al1 species ( S .22,
table 111) indicate how very specialized these lizards are in microhabitat or substrate
selection.
Overlapping of microhabitat niche
The overlapping of microhabitat niche values between the species studied are
low (table IV). If we exclude rare species such as Phrynosoma orbiculare and Barisia
imbricata, the only high overlap value is between Sceloporus poinsetti and Sceloporus
grummicus (0.564). However if we consider that Sc. poinsetti is not found on trunks
with a diameter smaller than 1.0 m it is likely that the true ecological overlap value
with Sc. grammicus is lower.
DISCUSSION AND CONCLUSIONS
The distribution and frequency of occurrence of lizard species in the study
area are mainly the result of the spatial distribution and frequency of occurrence of
some of the microhabitats or substrates to which species are closely restricted.
Acta O3culogica/O3culogia Generalis
SPATIAL ORGANIZATION IN A LlZARD COMMUNITY
TABLE
11
Distribufion qf species b y microhnbitat (relative occlrrrence).
Numbers
Fallen
trunks
Fallen
branches
Trees
Stumps
Shrubs
(in)
Rocks
Steep
rock
faces
Ground
within
grasses
Ground
within
leaf litter
Ground
within
wood
Ground
(other)
S c grnmrnicrc.~. . . . . . . . . .
Sc. scnlaris.. . . . . . . . . . . .
Sc. poinsetti. . . . . . . . . . . .
Sc. jurroai . . . . . . . . . . . . .
Plz. orbicularc . . . . . . . . . .
E. breairostris (l). .......
Numbers.
........
Sc. grurnrnicus . . .
Sc. scalaris. . . . . .
Sc. poinsetti . . . . .
Sc. .jnrroi)i . . . . . .
Ph. orbicrllr~rc . . .
E . breairostris (l) .
B. i~nbricata.. . . .
Numbers.
(l)
.
Near stones, holes or under lops.
TABLE
111
Microhabitat niche brerrdth or diilersity of substrnte utilized b y the species.
Sc.
grammicus
(l)
Sc.
scalaris
Sc.
poinsetti
Sc.
jarrovi
Ph.
E.
orbiculnra (l) brevirostris
Since these samples were small these measures are only roughly indicative.
p.-p
Vol. 3, no 3
- 1982
B.
imbricata (1)
328
A. ORTEGA,
M. E. MAURY,
R.
BARBAULT
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In other words, these lizard species do not exhibit habitat specificity but they do exhibit
substrate specificity. Within the area they may be found in any habitat identified by
MART~NEZ
& SALDIVAR
(1978), provided that the substrate to which they are limited
is present there. Thus the ecological organization of this lizard community may be
clearly depicted by a dendrogram (fig. 3) summarizing the data on similarity indices
of microhabitat use between species (table IV)
A? orbiculare
Ilttar dw.1l.i
I
E. brevirostris
S gremmicus
fallen trunk dweller
r
S. poinsett;
and tres dwsllsr
1
1
O
FIG.3.
ñ1
.
S.jerrovi
rock tare(wal1) dwaller
I
1
0.2
0.3
- Graphical
I
0.4
0.5
1
0.6
0.7
I
0.0
lndlces ot
d
0.9
1
rimiiaiity
representation of the ecological structure of the lizard community.
TABLEIV
Microhabitat niclie ouerlap ualues betkveen species.
Sc.
grammicus
Sr.
Sc.
scalaris poinsetti
Sc.
jarroui
Ph.
orbiculare
E.
breuirostris
B.
imbricata
Sr. grnmmicus . . .
Sc. scnlnris. . . . .
Sc. poinsetti. . . . .
Sc. jarroui.. . . . .
Ph. orbiculare. . .
E. breuirostris. . .
B. imbricata . . . .
Ojk
. . . . . . . . . ..
The community appears to be composed of species specialized for substrate or
microhabitat, each pertaining to a general ecological type of lizard which can also be
recognized in other areas and even on other continents (MELLADOet al., 1975;
Acta ~cologica/íEcologiaGeneralis
SPATIAL ORGANIZATION I N A LIZARD COMMUNITY
329
SAGE, 1973): there is a litter dweller species (Eumeces brezlirostris), three ground
dwelling species (a small one, Sceloporus scalaris, a large one Barisia imbricata (l),
and the ant-eater Phrynosoma orbiculare), a tree trunk dwelling species (Sceloporus
grammicus) and two rock dwelling species (Sceloporus poinsetti and Sceloporus
jarrovi).
Although the spatial niche overlap values between most paired species are very
low, according to common niche theory, this does not prove that interspecific competition was the structuring force which produced the observed pattern, nor that competitive interactions are presently excluded by space segregation.
Firstly, substrate or microhabitat specialization is not necessarily, nor likely
a competitively induced phenomenon. Tn such a case lizards may depend strongly
upon close substrate adaptation to avoid predators, to thermoregulate efficiently
and to be succesful in mating and defending territory. Thus, substrate selection and
specialization which they exhibit may be a response evolved to cope with a complex
combination of various selective pressures, and not only with the pressure exerted
by interspecific competition.
Secondly, when the coexisting populations are limited with respect to food it
remains to be seen whether they prey on the same species in the same places, thus
competing for food. Thus for instance, although remaining most of the time on tree
trunks, Sceloporus grammicus may gather prey on the ground, as does the same sized
species Sceloporus scalaris, and compete with it.
Although this study does not allow us to draw definite conclusions about the
ecological forces involved here, it does show the jmportance of habitat partitioning
by means of microhabitat specialization in the structuring of this mountain lizard
community.
ACKNOWLEDGMENTS
This work was supported jointly by the Institute of Ecology of Mexico-City, by the CONACYT
and by the program MAB of UNESCO.
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Acta Ecologica/Ecologia General,