Habitat edge responses of generalist predators are predicted by prey and structural resources

Abstract

Generalist predators are thought to be less vulnerable to habitat fragmentation because they use diverse resources across larger spatial scales than specialist predators. Thus, it has been suggested that generalist predators may respond positively to habitat edges or demonstrate no edge response, because they can potentially use prey resources equally well on both sides of the habitat edge. However, most predictions about generalist predator responses to the habitat edge are based solely on prey resources, without consideration of other potential drivers. For instance, structural resources are essential for some species to build webs to capture prey or to avoid intraguild predation and cannibalism. In this study, we used both prey and structural resources to predict the response of four predator functional groups (hunting spiders, web‐building spiders, aerial predators, and epigeic predators that feed on the detrital/algal food web) to a habitat edge between two salt‐marsh grasses (Spartina alterniflora and Spartina patens). We found that generalist predators largely demonstrated negative responses to the habitat edge and had distinct habitat associations. Positive edge responses were only observed in one functional group (hunting spiders), and this pattern was driven by the two most abundant species. Negative responses to the habitat edge were more common among taxa and were better explained by structural resources rather than prey resources in the two habitats. Although it is generally acknowledged that specialists decline in fragmented habitats, generalists are thought to be more resilient. However, our research demonstrates that even generalists have habitat structural or food resource requirements that may limit their resilience to habitat loss and fragmentation.

Habitat edge effects decrease litter accumulation and increase litter decomposition in coastal salt marshes

“…This has been particularly true for coastal and peat-forming wetlands (Rytter et al 1989;Williams and Silcock 1997;Williams et al 1999;Mitchell et al 2003;Locky and Bayley 2007;Ward et al 2013). The few studies that directly examined the effects of habitat area or habitat edges in salt marshes have predominantly focused on invertebrate community composition (Denno 1983;Martinson et al 2012;Wimp et al 2011Wimp et al , 2019 or planting for restoration (Gittman et al 2018). Until now, there have been no direct analyses of how ecosystem functioning in the shrinking monocultures of the foundation species Spartina patens may be affected by habitat area or habitat edge effects.…”

“…dominates lower elevations. Since both Spartina species exist in monocultures that border one-another with distinct edges, salt marshes provide a useful ecosystem to examine edge effects without the confounding effect of changing plant community composition typical to terrestrial ecosystems (Wimp et al 2011(Wimp et al , 2019. Currently, the sizes of S. patens patches are declining, mainly from nutrient eutrophication and relative sea level rise which induce competitive interactions that favor S. alterniflora over S. patens (Warren and Niering 1993;Donnelly and Bertness 2001;Carey et al 2017).…”

“…Currently, the sizes of S. patens patches are declining, mainly from nutrient eutrophication and relative sea level rise which induce competitive interactions that favor S. alterniflora over S. patens (Warren and Niering 1993;Donnelly and Bertness 2001;Carey et al 2017). The transition to pure S. alterniflora monocultures is of critical importance for the animal and microbial communities endemic to S. patens, as well as the species that need both grasses in different life cycle stages (Raghukumar 2017;Johnson and Williams 2017;Wimp et al 2019). Further, S. patens has considerably lower decomposition rates than S. alterniflora, leading to an accumulation of litter (Frasco and Good 1982;Foote and Reynolds 1997;Elsey-Quirk et al 2011).…”

“…The increased accumulation of litter in S. patens patches leads to a thick litter layer just above the soil that reduces salinity, increases humidity, reduces summer temperatures, and provides refuge from flooding for multiple animal species (DiQuinzio et al 2002;Gedan and Bertness 2010;Johnson and Williams 2017). Considering S. patens is more likely to experience changes in elevation, salinity, tidal inundation, and biological communities at the edge of its habitat (Denno 1983;Bertness 1991;Johnson and Williams 2017;Wimp et al 2019), one may expect S. patens litter decomposition to be altered by smaller habitat areas and increased habitat edge exposure.…”

Ignoring uncertainty in predictor variables leads to false confidence in results: a case study of duck habitat use

“…For example, in a study attempting to identify predictors of bird abundance among montane meadows, vegetation samples were taken within each site and the site‐level means were used as covariates in regression models (Saveraid et al 2001). Using site‐level means as covariates seems to be the standard way information derived from samples is included as covariates in analyses (Kalies et al 2012, Pilliod et al 2013, Wimp et al 2019, Lindstrom et al 2020). Under this scenario, no bias is suspected because the covariate is based on multiple samples from each site.…”