Phenotypic diversity facilitates niche partitioning in a sky island assemblage of spiny lizards

Abstract

If closely related species should be more similar and therefore more likely to compete for resources, why do we see so many examples of species-rich congeneric communities in nature? To avoid competition, many species possess suites of traits associated with differential resource use – or ecomorphologies – that promote niche differentiation with co-occurring species. Yet, the axes through which niche partitioning evolves and the traits involved are still poorly understood in most systems. Island systems, in which species interactions can form strong forces of selection due to limited resources, provide valuable insights into how ecomorphological diversity contributes to coexistence. Here, we examined axes of resource use and morphological traits that facilitate niche partitioning in a community of spiny lizards (genus Sceloporus) in a sky island system, the Chiricahua Mountains of southeastern Arizona, USA. By quantifying structural, temporal and thermal niche use for over 300 co-occurring lizards from four species over three field seasons, we show that sympatric species diverge primarily in perch height and type, but also in thermal and temporal resource use. Our results further demonstrate interspecific divergence in a suite of phenotypic traits known to covary with ecology in other lizard radiations, including body size, scale size, hindlimb length, foot length and dorsal coloration. Studies of niche partitioning and ecomorphology, especially of closely related species, deepen our understanding of how diverse communities assemble and how morphological diversity accumulates across the tree of life, and our findings highlight the importance of examining multiple axes of resource use to better understand these processes.

Dietary niche partitioning of three Sky Island Sceloporus lizards as revealed through DNA metabarcoding

“…These two species are more generalist in their habitat as they occupy perches from the ground level up into trees and rocks, perhaps providing more opportunities for the two species to overlap in foraging areas. However, the overlap between S. slevini and S. virgatus was similar; these two species have very similar spatial niches (Westeen et al, in press), and their dietary niche overlap is consistent with this. This similarity may limit their ability to co‐exist, and as such they are narrowly allotopic in this system.…”

“…The differences in diet we uncovered between the three Sceloporus species match what is known about niche partitioning more generally in this system. We previously showed that perch height and type were significantly different between species in this system (Westeen et al, in press). Given the nature of these species as sit‐and‐wait predators, it follows that these microhabitat differences lead to different availability of prey, which in turn contribute to the dietary differences we observed.…”

“…For S. jarrovii the two sites we sampled are in close geographic proximity (3.3 km straight‐line distance), yet population‐level diet overlap (Pianka overlap = 0.396) is similar to that of S. slevini (Pianka overlap = 0.338), which had substantially more distance between populations. We previously uncovered differences in microhabitat use by S. jarrovii between these sites (Westeen et al, in press), which may contribute to the dietary divergence between populations. Previous works report Hymenopterans, especially ants, as major diet items (Barbault et al, 1985; Goldberg & Bursey, 1990; Watters, 2008).…”

“…Slevin's bunchgrass lizard, S. slevini , is also small‐bodied, though more elongate, has reduced limbs compared to S. virgatus , and is almost exclusively grass‐dwelling (Ballinger & Congdon, 1981). These two species are narrowly allotopic in this system but overlap in spatial niche and ecomorphological space (Westeen et al, in press). Yarrow's spiny lizard, S. jarrovii is a medium‐to‐large lizard that is strongly saxicolous and occasionally arboreal (Simon & Middendorf, 1976).…”

“…Yarrow's spiny lizard, S. jarrovii is a medium‐to‐large lizard that is strongly saxicolous and occasionally arboreal (Simon & Middendorf, 1976). It is syntopic with the two smaller species but retains a distinct microhabitat and temporal niche from S. slevini ; it overlaps somewhat spatially and temporally with S. virgatus (Westeen et al, in press). Sceloporus virgatus and S. jarrovii are sit‐and‐wait predators (Watters, 2009; Weiss, 2001); foraging habits for S. slevini have not been recorded but likely also conform to sit‐and‐wait predation given their shy nature and affinity for bunchgrass clusters (EPW, personal observation).…”