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.