Mountain caves of the central region of Veracruz: A vertebrate biodiversity reservoir in a Neotropical hotspot

inhabit these subterranean ecosystems [1]. Cave vertebrates have been poorly studied on a global scale and studies have mainly focused on bats and fish [2]. Despite this, vertebrates of all classes (i.e., mammals, birds, reptiles, amphibians, and fish) use caves and find in them suitable habitat conditions to survive [3]. The distribution of vertebrate biodiversity in cave systems, particularly in relation to the floor and altitudinal levels inside caves, has been attempted to be explained on the basis of dif- ferent theories such as resource availability [4], the theory of microclimatic gradients [5,6], the theory of trophic dynamics through food networks and trophic niches [7], and the theory of ecological adaptations and restrictions [8]. Given that they are complex systems, not all theo- ries can be applied, and thus many biodiversity studies in caves are only descriptive. Three environments are generally distinguished in caves: euphotic, which is characterized by variable sunlight conditions; disphotic, which is also known as the twilight zone; and apho- tic, which comprises spaces in total darkness and high humidity conditions [9]. The fauna in caves is classified according to how long the organisms stay in these environments. Therefore, fauna that occurs facultatively or occasionally in these environments is known as trogloxene, the fauna that uses these sites seasonally is known as troglophile, and the fauna that completely develops inside caves is known as troglobiont [10–12].

The characteristics of the surrounding landscape may have an effect on the biodiversity of vertebrates in caves. For example, cave entrances are described as transition zones between the epigean and hypogean environments, where there is lower environmental stability because food resources are limited under twilight conditions [4]. Thus, based on the specific and unique conditions of each cave, both of karst and volcanic origin, it is possible to hypothesize about the influence of the light conditions on the capacity of vertebrates to use these environ- ments permanently or temporally [7,13]. In these ecosystems, each cave has specific ecological limits and imposes environmental restrictions on vertebrates in such a way that only the spe- cies with a greater capacity to adapt to these extreme environments are able to use them, which can lead to endemism [14]. Therefore, even though a lower biodiversity of vertebrates has been recorded in environments related to volcanic caves, endemic species have also been found in karst caves [15]. In this study, we analyzed vertebrate communities to highlight how the abundance and diversity of native mammals, birds, amphibians, reptiles, and fish are influenced by cave origin (karst or volcanic). The use of biodiversity profiles based on Hill numbers [16] allows for a complete characterization of the biodiversity of a community [17] because they combine infor- mation on species richness, species rarity, and species dominance. We therefore used beta-

diversity metrics to assess how vertebrate species composition varies between karst and volca- nic caves, between altitudinal zones, and between environments inside the caves. Considering the specific geological features of karst and volcanic caves [1] and given that biodiversity in caves is influenced by the characteristics of the surrounding landscape [4], karst caves would be expected to contain a higher biodiversity of vertebrates (mammals, birds, amphibians, reptiles, and fish) compared to volcanic caves. Moreover, according to altitudinal zonation, terrestrial biodiversity is highest at mid-altitudes, and thus the highest biodiversity of vertebrates would be expected to occur in caves located in the middle zone of the altitudinal gradient. Furthermore, because light conditions influence biodiversity in the internal environ- ment of caves, the euphotic zone would be expected to have a higher richness of trogloxene birds and reptiles, the disphotic zone would be expected to have a lower richness of all verte- brate classes, and the aphotic zone would be expected to have a higher biodiversity of some tro- globiont mammals (particularly bats) and a lower richness of amphibians and fish. Examining beta diversity in karst and volcanic caves along an altitudinal gradient allows the identification of species turnover patterns [18]. Different caves at varying altitudes may harbor PLOS ONE Vertebrates in caves of Neotropical hotspot PLOS ONE | https://doi.org/10.1371/journal.pone.0306105 August 9, 2024 2 / 19