Within the last decade, Southern California has experienced increasingly harsh climatic conditions, with a combination of record low rainfall and temperature highs resulting in extended drought events (Mann and Gleick, 2015). With increasing anthropogenic changes, animal species have become dependent on both inter- and intraspecific interactions to reduce potential adverse effects (Dangles et al., 2018; Rahman and Candolin, 2022). To reduce harsh abiotic conditions, animals will associate with foundational species, defined as species that play a strong role in structuring an ecological community and define an ecosystem through physically modifying the environment and creating habitats (Westphal et al., 2018; Ellison, 2019). Foundational species often exhibit facilitative associations with animals, where one interacting species benefits while the other is unaffected (Callaway and D’Antonio, 1991; Noble et al., 2016; Lortie et al., 2016, Lortie et al., 2018; Dangles et al., 2018; Ellison, 2019; Lucero et al., 2020, Lucero et al., 2022). Foundational species define ecosystems by shaping the biodiversity of associating species and manage ecosystem processes through erosion control, biodiversity support, and microclimatic regulation (Ellison, 2019; Lortie et al., 2021). Within Southern California deserts, shrubs act as foundational species, providing benefits to both plant and animal communities (Lortie et al., 2016; Zuliani et al., 2021). Vertebrate species are reliant on foundational shrub species (Pugnaire et al., 1996; Braun et al., 2021; Zuliani et al., 2023a) as they are used to escape predation (Filazzola et al., 2017; Salido and Vicente, 2019), thermoregulate (Ivey et al., 2020; Gaudenti et al., 2021; Zuliani et al., 2023b), and as a food source (Lortie et al., 2020). Further understanding the facilitative associations between foundational shrubs and vertebrate species can provide key insight into how environmental resources are used both by vertebrate individuals and communities.

Given that foundational shrubs positively influence local animal communities, increasing the number of shrubs available should provide more opportunities for facilitative associations. In terms of plant-animal interactions, the density of shrubs can influence the net outcome of animal interactions while also influencing the local community composition (Springer et al., 2003; Zuliani et al., 2021). As shrub density increases within an ecosystem, vertebrate species are given more opportunities to benefit from the facilitative associations, such as having more areas to thermoregulate (Milling et al., 2018; Zuliani et al., 2023b). For instance, the federally endangered species Gambelia sila, the Blunt-Nosed Leopard Lizard, uses shrubs within the desert of Southern California to thermoregulate, and has been predicted to have higher abundances as shrub densities increase (Ivey et al., 2020, Ivey et al., 2022; Zuliani et al., 2023b). While shrub cover is more commonly measured when assessing plant communities (Lortie et al., 2020), it can be used in tandem with shrub density to predict animal abundances (Van Auken, 2009; Zuliani et al., 2023a).

While foundational shrubs can positively influence animal communities, desert and grassland ecosystems globally are experiencing shrub encroachment, a phenomenon where woody shrub species increase in density, contributing to significant changes in vegetation cover (Van Auken, 2009; Losapio et al., 2024). Shrub encroachment has complex and sometimes contradictory effects on ecosystem health and biodiversity with traditional perspectives viewing shrub encroachment negatively, as it can increase desertification and reduce diversity in desert and grassland ecosystems (Van Auken, 2009; McCleery et al., 2018). For example, high densities of woody shrubs can negatively affect avian species distribution and diversity (Andersen and Steidl, 2019). However, recent studies have revealed that increasing shrub cover can also benefit wildlife, though these effects are highly context and species dependent (Stanton et al., 2021). In North American arid ecosystems, studies have documented positive relationships between increasing shrub densities and associated species (Stanton et al., 2018; De Souza et al., 2022). Vertebrate species that associate with foundational shrubs generally benefit from encroachment through increased abundance and richness (Zuliani et al., 2023a; Owen et al., 2024). Some Southern California ecosystems, such as the Carrizo Plain National Monument, exemplify these benefits as encroachment can reconvert arid grasslands back into a shrubland (Browning et al., 2008). Increases in shrub density and cover, particularly of foundational species, can promote facilitative associations that positively influence vertebrate abundance and richness (Whitford, 1997; Eldridge and Soliveres, 2014; Schooley et al., 2018; Zuliani et al., 2024). Understanding how changing shrub densities influence animal community provides valuable insights for assessing community composition and informing conservation practices.

Desert ecosystems in Southern California are experiencing higher frequencies of mega-droughts (Reynolds et al., 1999; Kogan and Guo, 2015; Gols et al., 2021). As temperatures increase within deserts, vertebrate associations change to promote activities that are more suitable for thermoregulation and reduce abiotic stressors (Moore et al., 2018). High near-surface air temperatures (NSAT) will influence species at both an individual and community level (Newbold, 2018). These increasing temperatures will cause a significant decline in vertebrate communities by reducing seasonal migration patterns and behavior (Newbold, 2018; Riddell et al., 2021). However, the effects increasing temperatures have on vertebrates are species dependent and can be mediated through ecosystem resources, such as the presence of burrows and shrubs (Pike and Mitchell, 2013; Riddell et al., 2021; Zuliani et al., 2021). Within desert environments, high temperatures are associated with reduction in both humidity and precipitation (Walker and Landau, 2018). Under extreme abiotic conditions, such as high temperatures and significantly low precipitation, the regions within southern California are classified as arid or semi-arid (Abella et al., 2012; Marengo and Bernasconi, 2015). As global temperatures increase, vertebrate species will continue to depend on ecosystem resources, including shrubs, to mitigate harsh abiotic conditions.

As the frequency of these mega-drought events increase in Southern California, ecosystems will become more arid (Germano et al., 2011; Abella et al., 2012; Marengo and Bernasconi, 2015). Increasing aridification of dryland ecosystems is causing a dynamic shift in the structure of ecosystems as plant and animal communities adjust to new conditions (Hacker and Gaines, 1997; James and Tallis, 2019). Across Southern California, there is an increasing aridity gradient in dryland ecosystems, suggesting that local species utilize ecosystem resources differently to reduce harsh abiotic conditions (Welles and Funk, 2021). As these ecosystems become more arid, the negative impacts on plant and animal communities become more prevalent (Lucero et al., 2020). Within animal communities, aridity can have negative impacts on their associations, becoming more reliant on ecosystem resources, such as shrubs, which are more readily available at higher densities (Tews et al., 2004; Zuliani et al., 2021). Furthering the knowledge of how variations in aridity influence local animal species, and how these species utilize ecosystem resources to ameliorate these stressors, can increase our understanding of how these conditions alter environmental processes and community associations.

Determining the effects of shrub density on vertebrate community composition can further the current understanding of facilitative associations and provide insight on how this ecosystem resource is used in dryland ecosystems. Here, we conducted a 2-year study at six different sites across Southern California. We tested the hypothesis that increasing shrub density will positively influence vertebrate communities within Southern California. In this study, we tested the following predictions:

In this study, we examined the effects shrub densities on the vertebrate community composition, individual observations, species richness, and evenness across ecosystems within Southern California. We found support for the hypothesis that increasing shrub density will positively influence vertebrate communities across ecosystems within Southern California. Shrub density positively influenced the observations and richness of vertebrate species. Fine-scale temperatures had a significant negative impact on individual observations and species richness in 2022 and a negative influence on individual observations and evenness in 2023. Sites were significantly more arid in 2022 than in 2023. Community compositions significantly varied between shrubs and open microsites, suggesting that shrubs promote vertebrate communities. Individual vertebrate observations and species richness decreased at sites with lower aridity scores in 2022, while only individual observations decreased at more arid sites in 2023.

In this study, we found that shrub density positively influences animal vertebrate communities. Structural resources, both artificial and natural, are critical in studies that not only observe individual associations, but also measure key structural components of an ecosystem. Globally, foundational shrubs act as benefactor species (Ruttan et al., 2021) through temperature amelioration (Ivey et al., 2020; Zuliani et al., 2023a), production of seeds and other food sources (Lortie et al., 2020), and refuge from predation (Filazzola et al., 2017). However, the effects these foundational shrubs have on vertebrate communities may not have the same influence and can be context-dependent (Stanton et al., 2018). Ephedra californica provides these facilitative effects that are necessary functions for vertebrate species (Westphal et al., 2018; Zuliani et al., 2021). Smaller vertebrate species including Dipodomys heermanni, Gambelia sila, and Ammospermophilus leucurus, utilize these shrubs to reduce predator-prey interactions (Longland and Dimitri, 2021). Species that are reliant on these positive interactions tend to be in locations where ecological resources, including shrub density and cover, are readily accessible and abundant (Zuliani et al., 2023a). For instance, Gambelia sila, the Blunt-nosed Leopard Lizard, is known to use areas of high shrub density and cover to aid in thermoregulation (Ivey et al., 2020; Lortie et al., 2021; Zuliani et al., 2023b). Other desert species, such as Dipodomys ingens (the Giant Kangaroo Rat) and Lepus californicus (the Black-Tailed Jack Rabbit), utilize these shrubs not only for cooling but for foraging and protection from predation (Johnson and Anderson, 1984; Prugh and Brashares, 2010). However, as shrub densities increase, the effects of shrub encroachment may become more prevalent. This increase in shrub density will reconvert these ecosystems into shrublands (Browning 2008), while also enhancing ecosystem functioning through the reduction of desertification and microclimatic buffering (Eldridge and Soliveres, 2014; Filazzola et al., 2017). However, the effects these shrubs have are ecosystem specific, and have varying impacts on local animal communities. Understanding the importance of the direct interaction between these shrubs and local animal communities, across an increasing stress gradient, can provide insight into the local animal community composition and utilization of these shrubs.

The composition of these vertebrate communities, while not different between shrub and open microsites, did differ across sites in Southern California. This suggests that while our study sites have similar community compositions, the variation in their shrub density and temperature directly influences individual associations. Vertebrate species, such as Lepus californicus, while found at all tested sites, were much more frequently observed at Cuyama and the Carrizo Plain National Monument than in Tecopa. This suggests that while some species are able to inhabit multiple ecosystems, sites with more favorable conditions are likely to be more suitable and preferred (Vale and Brito, 2015). Several factors can cause this variation in community composition including elevation, temperature gradient, available vegetation, aridity, shrub cover and density, and water availability (Marengo and Bernasconi, 2015; Welles and Funk, 2021; Ivey et al., 2022; Hillier-Weltman et al., 2025). Previous studies found that kangaroo rat species like Dipodomys heermanni forage more actively in open areas than in woody shrub habitats, which initially appears to contrast with our findings that shrub density increases vertebrate observations (Daly et al., 1992; Upham and Hafner, 2013). However, this difference likely reflects the distinction between foraging behavior and overall habitat use, as our camera traps captured all vertebrate activities including sheltering and thermoregulation rather than just active foraging. Additionally, moonlight can significantly alter the activity and frequency of observations of these nocturnal rodent species (Upham and Hafner, 2013). The presence of moonlight may influence the foraging behavior and observations of kangaroo rat species as they shift their activity from open areas to those with more cover (White and Geluso, 2007). While our study did not measure moonlight, future studies could take this into consideration when assessing nocturnal animal community composition. Directly analyzing the differences in these communities, in combination with multiple factors that can influence community composition, could further explain not only the associations these vertebrate species have with foundational shrubs, but how the increasing aridity of Southern California deserts is altering these ecosystems.

Anthropogenically driven climatic events, specifically drought and temperature extremes, will continue to increase the aridity of ecosystems across Southern California, influencing both vertebrate community measures and composition. In this study, we found that increasing near-surface air temperatures reduced observations, richness, and evenness of vertebrate species. As these high temperatures become more prevalent and extreme, shelter via shrub canopies may be a crucial resource to mitigate these abiotic conditions (Westphal et al., 2018; Ivey et al., 2020; Gaudenti et al., 2021). Previous studies conducted within the Carrizo National Monument showed a higher frequency of shrub-animal association during peak hours of the day when temperatures reached their maximums (Gaudenti et al., 2021; Zuliani et al., 2021; Ivey et al., 2022), suggesting that temperature amelioration is a direct benefit to local vertebrate communities. These shrub canopies generate shade, reducing both near-surface air and soil temperatures within their microclimate, minimizing the risks of overheating (Huxman et al., 2004). The area underneath these shrubs provides more hospitable microclimates for vertebrate communities in an increasingly arid ecosystem. However, these climate extremes are increasing in intensity and are particularly driven by global warming, which is amplifying aridity through rising temperatures and altered precipitation patterns (Dai, 2013). These increasing temperatures and reduced water availability directly contribute to the increased frequency and severity of mega-drought events (Kogan and Guo, 2015; Mann and Gleick, 2015). Furthering the current understanding of how these increasingly harsh climatic events impact vertebrate community measurements and composition can provide substantial insight into both the management of these ecosystems as well as the preservation of these vertebrate communities.

Our study demonstrates that shrub density strongly and consistently influences vertebrate observations and richness across Southern California desert ecosystems, providing compelling evidence for facilitative interactions between foundational shrubs and vertebrate communities. Microsite-level shrub density emerged as a robust predictor of vertebrate activity, with positive effects across both study years despite varying climatic conditions. Conversely, increasing near-surface air temperatures consistently reduced vertebrate observations and richness, emphasizing the critical role of shrub-mediated thermal refugia in these arid systems. These findings have significant implications for desert conservation under accelerating climate change. As temperatures continue to rise and drought events intensify in Southern California, protecting shrub densities will be essential for maintaining vertebrate diversity and community stability. Our results suggest that conservation strategies should prioritize the preservation of foundational shrub species like E. californica, as they serve as key facilitators supporting vertebrate communities through multiple mechanisms including thermal buffering, predator refuge, and resource provisioning during increasingly harsh climatic conditions.