Predicting responses to climate change is a global priority as many species are threatened by the projected increases in global temperatures. Reptile and amphibian species are disproportionately threatened by anthropogenic climate change due to their need to thermoregulate (Aragón et al., 2010; Sunday et al., 2014; Vicente Liz et al., 2019; Gaudenti et al., 2021). Responses of ectotherms to climate change require an understanding of the evolutionary, ecological, behavioral, and physiological factors that drive where a species can occur or inhabit (Diaz & Cabezas-Diaz, 2004; Ferreira et al., 2016; Soares & Brito, 2006; Stewart et al., 2018; Sunday et al., 2014). Most reptiles and amphibians that occur in desert ecosystems face the primary thermal challenge of keeping cool at extremely high temperatures (Gaudenti et al., 2021; Sunday et al., 2014; Zuliani et al., 2023b). The ability of ectotherms to alter their seasonal activity and reproduction is a key adaptation (Ivey et al., 2022; Zuliani et al., 2023b). Nonetheless, availability of fine-scale habitat such as that provided by vegetation for cover, influences the capacity of these species to take advantage of shade and thermal refuges (Stewart et al., 2019; Gaudenti et al., 2021). These habitats are also important for foraging and other behaviors in deserts (Sinervo et al., 2010; Westphal et al., 2018; Ivey et al., 2021). Hence, vegetation can be a key factor and mediate the extent that climate warming affects thermoregulating ectotherms (Kearney et al., 2009; Westphal et al., 2018). Links between the ecology and associations between reptiles and amphibians and vegetation are thus needed to advance ecological theory (Lortie et al., 2015; Filazzola et al., 2017; Zuliani et al., 2023a) including facilitation (i.e., positive interactions) (Bertness & Callaway, 1994; Schemske & Horvitz 1988; Żywiec et al., 2019) in order to support future conservation practices.
Temperature is a key factor for many animal species in deserts. Heliothermic (sun-basking) reptiles and amphibians that live in desert ecosystems are constrained by the extreme temperatures of their environment and are vulnerable to climate warming because of direct exposure to extreme heat (Mosauer, 1936; Sinervo et al., 2010). In high-stress environments such as deserts, shelter is critical for these species (Fisher 2007; Westphal et al., 2018). Shrubs and vegetation are important contributors to habitat heterogeneity by providing reptiles and amphibians with a mosaic of microclimates for effective thermoregulation (Filazzola et al., 2017; Lortie et al., 2022b; Westphal et al., 2018; Ivey et al., 2022; Zuliani et al., 2023). A foundation species is defined as a species that supports ecological processes that shape community assembly and diversity patterns, despite not necessarily being common within a region (Ellison et al., 2005; Ellison, 2019; Lortie et al., 2018; Schöb et al., 2012). In order to mitigate the effects of climate change, foundation species, such as shrubs, are needed as a buffer from extreme temperatures and aridity (Basson et al., 2017; Barrows, 2011). Refuges and shelter from climate can also provide protection from predation (Dutra et al., 2011; Westphal et al., 2018) in addition to providing foraging and mating habitats (Basson et al., 2017; Goller et al., 2014, Lortie et al., 2022b). However, we also need to deepen the extent that site-level variation in the foundation species, such as shrubs, collectively influence animals, such as reptiles and amphibians. Single or fewer shrubs at a site can provide some of the benefits needed to support a diverse animal population; however, if individual animals need to forage widely, defend territories, or seek mates, then the density and general availability of shrubs within a site relevant to animals can be a key measure as well (Ivey et al., 2021; Zuliani et al., 2023b). The benefits of foundation species are well established, but the extent that population level density estimates of desert shrubs support animals warrants additional research, likely at local-to-regional scales (Filazzola et al., 2017; Lortie et al., 2015; Noble et al., 2016; Zuliani et al., 2023b).
Shrubs are often the dominant vegetation in deserts and semi-arid mixed grasslands. In the central deserts of California, Mormon tea (Ephedra californica) and creosote bush (Larrea tridentata) are two widely distributed species of native shrubs (Braun et al., 2021; Lortie et al., 2017; Schafer et al., 2012). Foundation species can create complex structures that provide diverse microhabitat availability, and if this fine-scale heterogeneity is relevant to reptiles and amphibians in deserts, then increases in shrub densities across sites within the region should increase the likelihood that these animal populations are more abundant and the communities more diverse (Zuliani et al., 2021 and 2023a). Increases in the density of native shrubs have been shown to increase the likelihood of relocations in telemetry studies for the endangered species Gambelia sila in this region (Ivey et al., 2021; Zuliani et al., 2023b). Here, we extend this prediction that increases in the resident population density native shrubs benefits many reptiles and amphibians within the region. First, benefits can be positive and linear as increasing shrub density increases shelter and refuge access within a site. However, if a site is relatively large for the animal species it supports or if the site overlaps with the home range and activity use patterns of the species (Ivey et al., 2021; Gaudenti et al., 2021), then it is likely that the benefits of shrub density will reach an asymptote as the diversity of the animal communities plateau when an adequate number of shrubs to support species is reached (Zuliani et al., 2023b). Relatively high densities of shrubs or shrub encroachment with changing climate (Loera et al., 2012) can also have negative effects on other plants and animals by decreasing diversity of resources provided by other plant species (Germano et al., 1994; Sinervo et al., 2010) or in increasing predation of reptiles and amphibians through challenges to mobility (Westphal et al., 2016; Zuliani et al., 2023b). Facilitating the growth of native annual plants does increase habitat heterogeneity and species diversity across different sites (Holzapfel & Mahall, 1999; Lucero et al., 2020) provided there is a mosaic of light and temperature at sites (Lucero et al., 2021; Pulsford et al., 2017). Reptiles and amphibians rely on E. californica and L. tridentata for shelter in Central California deserts (German et al., 2019; Ivey et al., 2020; Westphal 2018; Zuliani et al., 2023b), for protection from predation (Germano et al., 2019; Ivey et al., 2022; Zuliani et al., 2021), and for direct micro-environmental amelioration (Lortie et al., 2017; Ivey et al. 2020). There is also significant variation in shrub densities regionally (Filazzola et al., 2020) and the region supports a diversity of reptile and amphibian species (Stewart et al., 2019).
Global change is occurring at large scales. Testing for changes in both patterns in biodiversity regionally and key ecological interactions can preserve species at large scales within ecological regions. Here, we hypothesize that foundational shrub species E. californica and L. tridentata positively influence reptile and amphibian communities at different sites across an aridity gradient in the drylands of central California. The following predictions were tested:
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Increasing shrub density at sites increases the relative abundance and diversity of reptiles and amphibians.
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The evenness and community-level similarity in reptile and amphibian communities between sites is increased by increasing shrub densities because shrubs provide both increased micro-environmental heterogeneity and more stability in the system both environmentally and ecologically (Wilby and Shachak, 2004)
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Increasing aridity at sites increases the benefits provided by shrubs because of the importance of thermal refuges for reptiles and amphibians within the region (Lortie et al., 2022b).