Introduction

Understanding the processes that enable species coexistence is a key theme of ecology with important implications for interpreting diversity patterns and predicting how systems respond to global change (Valladares, Bastias, Godoy, Granda, & Escudero, 2015). Interspecific competition is thought to have a major influence on community structure for many taxonomic groups. Niche theory (Chase & Leibold, 2003; P. Chesson, 2000; Letten, Ke, & Fukami, 2017) asserts that species coexistence is promoted through differential use of resources driven by functional differences between species, which results in communities that tend to be assembled by functionally dissimilar species (Schoener, 1974). This has been shown in numerous cases, including fish (Ross, 1986), shorebirds (Bocher et al., 2014) and rodent communities (Codron et al., 2015). Alternatively, community structure and coexistence, primarily in sessile organisms, has been often explained through neutral processes, such as dispersal or stochasticity (The neutral theory of biodiversity and biogeography; Hubbell, 2001). This framework has been often used as a null model to evaluate whether observed patterns deviate from neutral expectations (Alonso, Etienne, & McKane, 2006; McGill, Maurer, & Weiser, 2006). Yet, some studies of mobile organisms have failed to identify evidence of resource partitioning (e.g. Luiselli, 2008), suggesting that in some cases biotic interactions only play a minor role in governing community assembly, perhaps because resources are not limiting (Salinas‐Ramos, Ancillotto, Bosso, Sánchez‐Cordero, & Russo, 2020), and therefore neutral processes likely play a more important role.
Morphologically similar species pose a challenge for understanding mechanisms of coexistence from a niche theory perspective because they are more likely to be functionally similar, and therefore less likely to be able to use resources in a different way, a pre-requisite for resource partitioning (Weiher & Keddy, 1999). Consequently, considerable attention has been given to understanding resource partitioning among morphologically identical (cryptic) or similar co-occurring species (e.g. Gabaldón, Montero-Pau, Serra, & Carmona, 2013; Jiang, Feng, Sun, & Wang, 2008; Razgour et al., 2011). Many studies have focused on the trophic dimension, an important aspect of species’ ecological niche (Schoener, 1974). DNA metabarcoding and High Throughput Sequencing (molecular diet analysis) approaches helped overcome many of the limitations of traditional morphological methods (Sousa, Silva, & Xavier, 2019), opening the door to new opportunities for studying mechanisms of species coexistence (Arrizabalaga-Escudero et al., 2018; Krüger et al., 2014; Razgour et al., 2011). However, the majority of coexistence studies focus on only sympatric populations, preventing an evaluation of how the presence of a competitor may change resource use, thus limiting the power of inferences (Salinas‐Ramos et al., 2020). Moreover, most studies also focus on diet only, disregarding prey selection relative to prey availability or resource limitation (Salinas-Ramos et al. 2019). Accounting for prey selection (e.g. Rytkönen et al., 2019) can provide a more complete picture of consumer trophic preferences (Lawlor, 1980).
The processes that govern community assembly, including coexistence mechanisms, vary with spatial scale (Lewis, Bailey, Vandewoude, & Crooks, 2015; Snyder & Chesson, 2004; Viana & Chase, 2019), yet spatial scale is rarely considered in coexistence studies (Hart, Usinowicz, & Levine, 2017). A better understanding of the scale of coexistence mechanisms and how different processes interact is important for both basic and applied ecology (Peixoto, Braga, & Mendes, 2018).
This study aims to identify whether trophic ecology enables morphologically similar species to coexist across spatial scales. We focus on two recently described, morphologically nearly identical, insectivorous bat species, whose trophic ecology has not been studied to date, Myotis crypticus and Myotis escalerai. These bats are restricted to the Western Mediterranean Basin, where they overlap across the north of the Iberian Peninsula, but at the fine-scale are known to co-occur only in a few locations (Juste, Ruedi, Puechmaille, Salicini, & Ibáñez, 2018). Phylogeographic analysis and species distribution modelling suggest that their ranges have been shaped by competition (Razgour, Salicini, Ibáñez, Randi, & Juste, 2015). These bats therefore provide an excellent case study for understanding mechanisms of coexistence among morphologically similar species. We use DNA metabarcoding and High Throughput Sequencing to characterise the trophic ecology of M. crypticus and M. escalerai by analysing their taxonomic and functional diets and their prey selection relative to prey availability in sympatric versus allopatric populations at both fine and broad spatial scales. Given their near identical morphology and echolocation calls, the overall trophic niches of the two bats are expected to be similar and niche overlap should be high. We hypothesise that if resource partitioning is the main process facilitating coexistence, competing species will diverge in their use of resources in sympatry compared to allopatry (e.g. Klawinski, Vaughan, Saenz, & Godwin, 1994). We test the predictions that 1) trophic niche overlap and diet similarity are higher in allopatric than sympatric locations; and 2) differences in trophic niche overlap are most pronounced at the fine spatial scale where individuals of the two species share the same foraging areas.