The response of tree squirrels to fragmentation: a review and synthesis

Abstract

Habitat fragmentation is often considered a major threat to biodiversity; however, our understanding of how fragmentation impacts populations is poor. Identifying appropriate models for such studies is difficult. Tree squirrels are dependent on mature forests for food, cover and nests; these are habitats that are being fragmented rapidly and that are easily defined by humans. Squirrels represent excellent models for study of fragmentation. The literature on tree squirrels was reviewed to glean data on density and home-range size in forest fragments. Sufficient data were available on four species (Sciurus carolinensis, S. niger, S. vulgaris, Tamiasciurus hudsonicus). Density was negatively related to fragment size for S. carolinensis and S. niger and marginally so for T. hudsonicus. Sciurus vulgaris did not exhibit this relationship. Home-range size was analysed for three species of Sciurus and was positively related to forest fragment size for S. carolinensis and S. niger. Again, only S. vulgaris did not to show this relationship. Sciurus vulgaris is rarely found in small forest fragments and is believed to be especially sensitive to fragmentation; other tree squirrels appear to be sensitive to fragmentation in more subtle ways. Home range compaction provides a mechanism by which densities may increase in small fragments. The demographic consequences resultant from the high densities of squirrels found in small woodlots are not known but may explain the forest damage, avian nest predation and reduced diversity often cited to occur in woodland fragments.

Response of the dominant rodent species to close-to-nature logging practices in a temperate mixed forest

“…This is typically represented by reduced vegetation heterogeneity and change in the amount of dead wood (Christensen et al 2005). As a reaction to such regional or local scale changes, animals may modify their behaviour and habitat preferences (Telleria & Santos 1995, Koprowski 2005, Suchomel et al 2009). …”

Melding kin structure and demography to elucidate source and sink habitats in fragmented landscapes

“…Moreover, the loss of habitats to agricultural conversion is a pervasive threat to wildlife populations across the globe and thus there is a pressing need to better understand the effects of habitat loss and fragmentation on population viability. For instance, within fragmented ecosystems, individual habitat patches commonly vary in attributes such as size, composition, and configuration, presumably resulting in spatiallyexplicit variance in demography and vital rates among local populations of species (Nupp and Swihart 1996, Koprowski 2005, Beasley et al 2011. It has been hypothesized that increases in variance of demographic parameters associated with underlying patterns of resource dispersion have been a primary causal mechanism in the extirpation of many wildlife species from human-dominated landscapes such as agricultural ecosystems (Donovan and Thompson 2001).…”

Patch occupancy by squirrels in fragmented deciduous forest: effects of behavior

“…For example, habitat patch area is negatively related to population density and positively related to home range size of cursorial squirrels (Koprowski 2005;Reunanen and Grubb 2005) and can affect the age structure and sex ratio of small generalist rodents (Wilder and Meikle 2006). Further, reducing habitat isolation increased gene flow among populations of the European red squirrel (Sciurus vulgaris) in Britain (Hale et al 2001).…”

“…We tested these hypotheses on an assemblage of sciurid rodents in an agricultural and exurban region of southern Ontario. Squirrels are forestdependent mammals that often respond negatively to fragmented landscapes that are dominated by agriculture (Taulman and Smith 2004;Koprowski 2005). We examined patterns of patch occupancy of five local squirrel species: the eastern chipmunk (Tamias striatus), the eastern gray squirrel (Sciurus carolinensis), the North American red squirrel (Tamiasciurus hudsonicus), the northern flying squirrel (Glaucomys sabrinus), and the southern flying squirrel (Glaucomys volans).…”