Terrestrial Habitat Selection and Strong Density‐dependent Mortality in Recently Metamorphosed Amphibians

Abstract

To predict the effects of terrestrial habitat change on amphibian populations, we need to know how amphibians respond to habitat heterogeneity, and whether habitat choice remains consistent throughout the life-history cycle. We conducted four experiments to evaluate how the spatial distribution of juvenile wood frogs, Rana sylvatica (including both overall abundance and localized density), was influenced by habitat choice and habitat structure, and how this relationship changed with spatial scale and behavioral phase. The four experiments included (1) habitat manipulation on replicated 10-ha landscapes surrounding breeding pools; (2) short-term experiments with individual frogs emigrating through a manipulated landscape of 1 m wide hexagonal patches; and habitat manipulations in (3) small (4-m2); and (4) large (100-m2) enclosures with multiple individuals to compare behavior both during and following emigration. The spatial distribution of juvenile wood frogs following emigration resulted from differences in the scale at which juvenile amphibians responded to habitat heterogeneity during active vs. settled behavioral phases. During emigration, juvenile wood frogs responded to coarse-scale variation in habitat (selection between 2.2-ha forest treatments) but not to fine-scale variation. After settling, however, animals showed habitat selection at much smaller scales (2-4 m2). This resulted in high densities of animals in small patches of suitable habitat where they experienced rapid mortality. No evidence of density-dependent habitat selection was seen, with juveniles typically choosing to remain at extremely high densities in high-quality habitat, rather than occupying low-quality habitat. These experiments demonstrate how prediction of the terrestrial distribution of juvenile amphibians requires understanding of the complex behavioral responses to habitat heterogeneity. Understanding these patterns is important, given that human alterations to amphibian habitats may generate extremely high densities of animals, resulting in high density-dependent mortality.

Effects of Timber Harvest on Amphibian Populations: Understanding Mechanisms from Forest Experiments

“…We also showed that such microhabitat conditions lead to rapid water loss and high mortality in small-scale caging studies (Rothermel and Luhring 2005) and most likely provide the direct mechanism for reduced abundance demonstrated in many previous studies. Behavioral studies show that both juvenile and adult amphibians often avoid entering clearcuts when given a choice (Rittenhouse and Semlitsch 2006, Patrick et al 2008, Todd et al 2009; but see Graeter et al 2008). Using smaller-scale mechanistic studies, we found that retention of CWD helped reduce water loss and increase the survival of juveniles over short periods, especially in slash piles .…”

“…However, the willingness to travel through clearcuts differed by region and species or stage. In Maine, both juvenile and adult amphibians traveled through clearcuts, and individuals settled for short periods (Patrick et al 2006(Patrick et al , 2008. In Missouri, adults traveled through clearcuts, but virtually no metamorphs traveled successfully through clearcuts.…”

“…In Missouri, for example, we found that juveniles constrained by cages in control quadrants on south-facing slopes with thin soil had lower survival than juveniles constrained in clearcuts on north-facing slopes. Further, radiotelemetry monitoring revealed that temporal variation in habitat quality also affected amphibian habitat choice across large-scale arrays (Patrick et al 2008), and survival rates ). In dry years, individuals suffered higher mortality, but they were more likely to use relatively shady and cool sites that were less common and more isolated.…”

Conservation biology of the great crested newt in managed boreal forests in Finland

“…However, a forest is not a homogenous environment for the newt. The quality of a forest as a migration route through terrestrial habitats varies due to natural variations in vegetation cover or as a result of forest management activities (Patrick et al 2008). …”

Design guidelines for integrating amphibian habitat into golf course landscapes

“…Patrick, Harper, Malcolm, Hunter, and Calhoun (2008) found that juvenile wood frogs (R. sylvatica) will choose to stay in high quality habitat at extremely high densities rather than shift to low quality habitat and as a result will experience high density-dependent mortality. Quality habitat must be provided to ensure individuals have sufficient environments for foraging, refuge, overwintering, dispersal between habitats (Guerry & Hunter, 2002;Semlitsch & Bodie, 2003) and to shift habitats in forested areas which have experienced disturbances ).…”