Quantifying functional connectivity: experimental assessment of boundary permeability for the natterjack toad (Bufo calamita)

Abstract

Like other pond-breeding amphibians, the natterjack toad (Bufo calamita) typically presents a patchy distribution. Because the species experiences high probabilities of local population extinction, its persistence within landscapes relies on both local and landscape-scale processes [dispersal allowing the (re)colonization of habitat patches]. However, the structure and composition of the matrix surrounding local populations can alter the dispersal rates between populations. As shown previously (Landscape Ecol 19:829-842, 2004), the locomotor performances of individuals at the dispersal stage depend on the nature of the component crossed: some landscape components offer high resistance to movement (high resistance or high viscosity components) whereas others allow high efficiency of movement (low resistance components). We now examine the ability of individuals to discriminate between landscape components and select low-resistance components. Our experimental study investigates the ways in which young natterjack toads choose from among landscape components common to southern Belgium. Toadlets (the dispersal stage) were experimentally confronted with boundaries between surrogates of sandy soils, roads, forests, agricultural fields and intensive pastures. Our results show: 1 the ability of toadlets to react to boundaries between landscape components, 2 differences in permeability among boundaries, and 3 our inability to predict correctly the permeability of the boundaries from the patch-specific resistance assessed previously. Toadlets showed a preference for bare environments and forests, whereas they avoided the use of agricultural environments. This pattern could not be explained in terms of patch-specific resistance only, and is discussed in terms of mortality risks and resource availability in the various landscape components, with particular attention to repercussions on conservation strategies.

Biodiversity, Species Interactions and Ecological Networks in a Fragmented World

“…Furthermore, recruitment and predation of seeds in the forest interior might decrease relative to edges (Baldissera and Ganade, 2005;Fleury and Galetti, 2006;Jules and Rathcke, 1999;Vargas, 1999, but see Cunningham, 2000;Guimarães and Cogni, 2002). Besides the capability of a species to perceive suitable habitat fragments and the connectivity of the landscape, its persistence in a fragmented landscape depends on its ability to cross the edge between fragment and matrix (Morris, 1997;Stamps et al, 1987a;Stevens et al, 2006). Habitat edges can be characterised as 'hard' or 'soft' according to their permeability.…”

Landscape structure and boundary effects determine the fate of mutations occurring during range expansions

“…For example, a large proportion of plant seeds are dispersed by wind (for example, Tackenberg et al, 2003), whereas many species of spiders, spider mites and moth larvae utilize a high-risk dispersal strategy using aerial balloons (for example, Bonte et al, 2003;Bell et al, 2005). These passively dispersed species will show little response to habitat boundaries, whereas those species that exert more control over their dispersal, for example, most birds, mammals and amphibia and many insects, will exhibit very different movement behaviours close to boundaries than they typically do away from them (for example, Stevens et al, 2006;Chapman et al, 2007). Spatial models are a useful tool for determining how behaviour at these boundaries might influence population or community dynamics.…”

A stochastic movement simulator improves estimates of landscape connectivity

“…This map was converted to a raster grid at a 3-m cell size. As for Greenbuls, we hypothesized that landscape effects on connectivity are driven by nonrandom selection of landscape elements (i.e., preference) (Stevens et al 2006a). Cost values reflecting those preferences were derived from experimental releases of toadlets: individuals were released in a Yshaped experimental arena in which the two branches of the Y mimicked two different types of landscape element.…”

“…Cost values reflecting those preferences were derived from experimental releases of toadlets: individuals were released in a Yshaped experimental arena in which the two branches of the Y mimicked two different types of landscape element. Preference values were determined as the relative permeability of boundaries between the different types of landscape elements (¼100 À mean (over the different types of landscape elements) percentage of toadlets that enter a landscape element when starting in another one) (Stevens et al 2006a). Built-up areas were considered absolute barriers, and rivers and ponds were assigned intermediate values (Stevens et al 2006b).…”