Role of microhabitat and temporal activity in facilitating coexistence of endemic carnivores on the California Channel Islands

Abstract

Most extinctions worldwide have occurred on islands, primarily due to interactions with exotic species, but rarely due to interactions among endemic species. This potential exists on two of the California Channel Islands, Santa Cruz and Santa Rosa, as endemic island spotted skunks (Spilogale gracilis amphiala) appear to have rapidly declined to rarity, possibly due to interference competition with endemic island foxes (Urocyon littoralis). Niche partitioning is expected in response to interference competition; hence, it is surprising that skunks and foxes show extensive overlap in macrohabitat use and circadian activity. However, the role of microhabitat associations and fine-scale temporal activity in facilitating coexistence of the two species has not been explored. We evaluated microhabitat associations of island spotted skunks and island foxes across both islands using data from live traps and wildlife cameras collected during 2015–2017, and we analyzed fine-scale temporal activity using camera data from 2016 and 2017. On both islands, skunks had a positive response to ground-level cover provided by rugged topography or woody vegetation such as shrubs or trees, whereas foxes had a weak or negative response, suggesting partitioning of these microhabitat characteristics. Additionally, on both islands the peak in timing of skunk activity offset the peak for foxes, which implies that skunks use fine-scale adjustments in activity to avoid foxes. Past grazing by exotic herbivores likely reduced habitat refuges for skunks; however, as vegetation on both islands recovers, regrowth of shrubs and trees may provide cover that will improve prospects for coexistence of island spotted skunks and island foxes.

The gut microbiomes of Channel Island foxes and island spotted skunks exhibit fine‐scale differentiation across host species and island populations

“…However, genus‐level analyses consistently returned two genera within Actinobacteria ( Arcanobacterium and Mobiluncus ) as significantly more abundant in island spotted skunks compared to island foxes across both islands. As Actinobacteria is one of the most abundant bacterial taxa in soils (Lewin et al., 2016 ), this may reflect habitat use by island spotted skunks, which spend more time in ravines or covered places while foraging for soil‐dwelling prey (Bolas, Sollmann, et al., 2022 ; Crooks & Van Vuren, 1995 ). We therefore posit that the increased presence of certain bacterial classes in island foxes and island spotted skunks may derive from eco‐evolutionary differences in their population histories, diets, and preferred habitat types, as well.…”

“…Yet, island spotted skunks tend to be more specialized in their habitat use, while island foxes utilize all areas across the island (Crooks & Van Vuren, 1995 ). There may be further differences in microhabitat use, with island spotted skunks often found in areas with ground‐level cover in the form of rugged terrain or dense vegetation (Bolas, Sollmann, et al., 2022 ; Gagorik, 2020 ). Regarding temporal dynamics, island spotted skunks are largely nocturnal, whereas island foxes are cathemeral (active during both the day and night; Bolas, Sollmann, et al., 2022 ; Zhang et al., 2022 ), with differing degrees of seasonality.…”

“…There may be further differences in microhabitat use, with island spotted skunks often found in areas with ground‐level cover in the form of rugged terrain or dense vegetation (Bolas, Sollmann, et al., 2022 ; Gagorik, 2020 ). Regarding temporal dynamics, island spotted skunks are largely nocturnal, whereas island foxes are cathemeral (active during both the day and night; Bolas, Sollmann, et al., 2022 ; Zhang et al., 2022 ), with differing degrees of seasonality. Finally, island spotted skunks are strictly carnivorous while island foxes are largely omnivorous (Crooks & Van Vuren, 1995 ; Cypher et al., 2014 ).…”

Support for the size‐mediated sensitivity hypothesis within a diverse carnivore community

“…In our study system, because spotted skunks uniquely showed sensitivity to each of the four hypothesized drivers of carnivore occurrence, we suggest that spotted skunks are likely the ideal sentinel species in this study area, and likely more broadly across their range. Indeed, recent studies on spotted skunks have shown their utility in advancing our understanding of disease outbreaks (Harris et al, 2021), historical climate and land use change (Cheeseman et al, 2021a(Cheeseman et al, , 2021b, changing predator community dynamics (Bolas et al, 2022), and food web linkages between aquatic, terrestrial and arboreal systems (Tosa et al, 2023). Such sensitivities by small carnivores are not unique to our system and are likely represented in small carnivores in ecosystems globally (Do Linh San et al, 2022;Marneweck, Allen, et al, 2022).…”

Channel Island Foxes (Urocyon littoralis) are Viable Seed Dispersal Agents of Toyon (Heteromeles arbutifolia)

“…Since the island fox is the most abundant (and often the only) midsized mammalian omnivore on the Channel Islands, it may often be the sole mammalian agent of seed dispersal. However, much of the research on the potential ecological roles of the island fox has focused on predation and competition (Roemer et al 2002, Phillips et al 2007, Coonan et al 2010a, 2010b, Orrock 2010, Orrock and Fletcher 2014, Bolas et al 2022. Despite the potential for island foxes to provide important seed dispersal services, it is still unclear whether island foxes disperse the seeds of island plants.…”