How come similar lizards coexist? Towards a functional understanding of species coexistence

Anamarija Žagar, Miguel A. Carretero, Al Vrezec, Katarina Drašler, Antigoni Kaliontzopoulou

We examined how is it possible that two lizard species that are at first sight remarkably similar in their outer appearance can coexist and live together in the same geographical area. Usually, morphologically and ecologically similar species in areas of geographical overlap (sympatry) come into competition that could result in segregation or differentiation – one species either moves away or changes to use a different part of the ecological niche. In our particular study system, there is no definite pattern of spatial segregation or differentiation in morphology and species share the study area, occurring either in single-species or two-species populations that are mostly in the dispersal ability range of both species. We were interested how functional morphology and whole-organism performance plays a role in promoting this observed pattern of coexistence. Thus we measured morphological traits, the strength of bite and the speed of running and climbing of males and females of both species. The biggest difference between species was found in two functional traits, bite force and climbing speed, which were also linked with corresponding morphological traits. The species with larger and taller heads exhibited stronger bite forces and females had longer trunks that reduced climbing speed. Stronger bite forces and larger heads may potentially promote segregation between species in dietary preferences since the lizard with stronger bite could eat harder prey. On the other hand, the lizard with a flatter head could use narrower crevices, hence, have a better chance to escape predators that search for them inside shelters. Stronger bites and larger heads also provide one species with a dominant position in male-male combats that lizards engage in to defend their preferred area for e.g. basking, moving, seeking females etc. Moreover, lower climbing speeds in females with longer trunks may lower their anti-predator escape abilities, but could on the other hand positively influence reproductive effort. Our results exemplify how important it is to focus on the functionality of traits when we are trying to understand the observed community structure in nature. It is not only the variation in outer appearance, but the functional diversification that is responsible for the complexity of community structure via coexistence.

Image caption: Difference in how strong are the bites (bite force) and how fast can they climb (climbing speed) for two lizard species that look remarkably similar on the outer appearance but with subtle differences in functional traits found ways to coexist in a geographical area. Symbols for females are circles and for males are squares. Photo credits: Miha Krofel.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

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