Low-intensity monitoring of small-mammal habitat associations and species interactions in an urban forest-preserve network

Abstract

ContextAnthropogenic landscape modification and fragmentation result in loss of species and can alter ecosystem function. Assessment of the ecological value of urban reserve networks requires baseline and continued monitoring. However, depending on the desired indicators and parameters, effective monitoring can involve extensive sampling that is often financially or logistically infeasible. AimsWe employed a low-intensity, mixed-detector survey design to monitor the small-mammal community across a network of 53 fragmented forest preserves (225 sites) in a highly urbanised landscape in the Chicago metropolitan area from August to October, 2009–2012. MethodsWe used a sequential process to fit single-season occupancy and pairwise co-occurrence models for six common small mammal species to evaluate habitat associations and interspecific interactions. Key resultsShrew species and meadow voles occurred more often in open canopy-associated habitats, whereas occupancy was greater for eastern chipmunks, grey squirrels and white-footed mice in closed-canopy habitats. Habitat associations were complicated by negative pairwise interactions, resulting in reduced occurrence of meadow voles when predatory short-tailed shrews were present and lower occupancy rates of white-footed mouse when chipmunk competitors where present. White-footed mice co-occurred with short-tailed shrews, but detection of white-footed mice was lower when either eastern chipmunks or short-tailed shrews were present, suggesting that densities of these species could be inversely related. ConclusionsWe found evidence for both habitat segregation and interspecific interactions among small mammal species, by using low-intensity sampling across the reserve network. Thus, our sampling and analysis approach allowed for adequate assessment of the habitat associations and species interactions within a small-mammal community. ImplicationsOur findings demonstrated the utility of this monitoring strategy and community as bioindicators for urban-reserve networks. The approach described holds promise for efficient monitoring of reserve networks in fragmented landscapes, critical as human population densities and urbanisation increase, and we discuss how adaptive sampling methods could be incorporated to further benefit conservation efforts.

Using Long‐Term Data to Compare Two Sizes of Sherman Trap

“…We monitored preserves (and all points within) for 4 nights (5 days) over an 8‐week period from late August through early November. These rapid biodiversity surveys have been shown to adequately assess species‐habitat associations and species interactions (Cassel et al 2020). During each monitoring session, we set 2 small Sherman traps (5.1 × 6.4 × 16.5 cm) and 2 large Sherman traps (7.6 × 8.9 × 22.9 cm) in a 2 × 2 square grid around a metal t‐post marking each permanent plot.…”

“…Bait and bedding were replaced as needed, and traps with captures were rebaited and reset. For more details on the small mammal trapping program see Cassel (2014) and Cassel et al (2020).…”

The impact of urbanisation on chipmunks, arboreal and flying squirrels: a global systematic review

“…Responses of two squirrel species, Eastern grey squirrel and fox squirrel, changed from negative to positive once the proportion of green space within a city was >~20%. Similarly, a low-intensity, mixed-detector survey (different trap models to target different-sized species, from shrews to squirrels) across 53 fragmented forests preserved in the highly urbanised Chicago metropolitan area, showed that occupancy of Eastern chipmunks and Eastern grey squirrels was highest in closed-canopy habitat (Cassel et al 2020). Authors argued that small mammal communities can be used as bioindicators for urban-reserve networks.…”

Scales of Effect for Urban Mammals Vary Among Species and Environmental Attributes

“…We created 50, 100, 500, and 1000 m-radius buffer zones around study sites in a geographic information system (ArcGIS version 10.6; ESRI 2017), as well as buffers approximating male and female urban home range extents for each of the three species included in this analysis (Table 1). We selected two extents (50 m, 100 m) to represent small or 'local' scales as both are commonly used to characterize local environments in species habitat modeling (e.g., Parsons et al 2018, Cassel et al 2020. We selected a 500 m buffer, an extent often used to estimate environmental covariates in species habitat modeling with urban mammals (e.g., Magle et al 2016, Gallo et al 2017, to represent an intermediate spatial extent.…”