Different open-canopy vegetation types affect matrix permeability for a dispersing forest amphibian.
Abstract
Population viability often depends on conserving functional connectivity in fragmented landscapes. For pool-breeding amphibians, population connectivity is largely maintained through juvenile dispersal, often through various vegetation types that may differ as filters or conduits to movement. We quantified the relative permeability of different types of open-canopy vegetation to juvenile wood frogs Lithobates sylvaticus to determine whether this influences functional connectivity during dispersal. We conducted experimental releases of juveniles (n=561) in ten runways representing five treatments: hayfield, moderate-cover lawn (45-85% cover), open lawn (0% cover), row crop (forage-corn) and recent clear-cut. Runways consisted of 35×2.5 m enclosures, located perpendicular to a forest edge and extending into treatment areas with tracking stations at 10, 20 and 30 m. As indices of permeability, we measured the number of animals traversing each station, the proportion changing direction, movement timing and movement rates. Based on an index that compounds four metrics and scales them relative to mature forest as a control, permeability varied between open-canopy cover types in the following order: row crop < hayfield < clear-cut < open lawn < moderate-cover lawn. The highest proportions of individuals changed direction (towards forest) in the hayfield, moderate-cover lawn and clear-cut, suggesting that juveniles may make forays into the open and subsequently assess habitat. Nonetheless, individuals could eventually transit entire runways, indicated by overall recaptures at 30 m (e.g. hayfield, 29%; moderate-cover lawn, 24%; and clear-cut, 20%) at the end of our six-week experiment. Synthesis and applications. We provide quantitative evidence that open-canopy cover types may act as differential ecological filters to ranging movements, and ultimately dispersal. Differences in the willingness of animals to enter treatments, coupled with motility and residency times, support the differing roles of open-canopy vegetation as both filters and conduits to movement. Thus, it may be overly simplistic to estimate matrix permeability as uniformly low in models that predict movement in fragmented landscapes. To promote functional connectivity, modification of vegetation composition and configuration may provide an underutilized tool for conservation practitioners to reduce the effective isolation of habitat patches for post-metamorphic amphibians.