David A. Patrick

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

Harvesting timber is a common form of land use that has the potential to cause declines in amphibian populations. It is essential to understand the behavior and fate of individuals and the resulting consequences for vital rates (birth, death, immigration, emigration) under different forest management conditions. We report on experimental studies conducted in three regions of the United States to identify mechanisms of responses by pond-breeding amphibians to timber harvest treatments. Our studies demonstrate that life stages related to oviposition and larval performance in the aquatic stage are sometimes affected positively by clearcutting, whereas effects on juvenile and adult terrestrial stages are mostly negative. Partial harvest treatments produced both positive and weaker negative responses than clearcut treatments. Mitigating the detrimental effects of canopy removal, higher surface temperature, and loss of soil-litter moisture in terrestrial habitats surrounding breeding ponds is critical to maintaining viable amphibian populations in managed forested landscapes.

Terrestrial habitat selection and strong density-dependent mortality in recently metamorphosed amphibians.

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.

Looking beyond research in doctoral education

Doctoral education in the natural sciences has traditionally focused on training students as researchers. However, this is no longer sufficient to provide students with the range of skills needed to succeed in academic and non-academic employment. Based on a review of national studies and current literature, we recommend a shift from the current model, with its focus on dissertation research, to a broader conception of doctoral education that includes training and mentoring that will be relevant to future careers. We also highlight some of the national initiatives that have made progress in these areas. Doctoral education programs cannot remain static if they are to continue to create marketable graduates. Instead, partnerships should be formed with hiring institutions, so that doctoral programs can evolve in parallel with the dynamics of the job market.

Orientation of Juvenile Wood Frogs, Rana Sylvatica, Leaving Experimental Ponds

Abstract Some amphibians are able to orient toward habitat features, but it is not always clear whether (1) these animals can directly detect the habitat toward which they are moving (e.g., scenting water from a wetland); or (2) they are detecting an indirect cue that is consistently correlated with the location of suitable habitat (e.g., the location of sunrise). In 2004, we translocated 400 Rana sylvatica tadpoles from an isolated population on Sears Island in Maine, where emerging metamorphs travel northeast towards a forested wetland. We placed study animals in arrays consisting of a central artificial pool, with a circular drift fence at 0.2 m from the pools edge to assess orientation of metamorphs at emergence and a similar fence at 5 m to assess orientation postemergence. Arrays were placed at 10 m and 50 m from a forested wetland, with the wetland cue to the southwest (i.e., the opposite direction of the wetland at Sears Island). Rana sylvatica exhibited significant orientation toward the northeast at the 0.2 m fence, indicating that emerging metamorphs retained the same directionality as at the site where they were hatched. A significant result at the 5 m fence indicated that animals continued to head toward the northeast. These results suggest that the population of Rana sylvatica on Sears Island may rely on indirect cues for orientation. Relying on indirect cues offers less adaptability to changes in habitat such as breeding site loss or road construction, and thus could lead frogs into ecological traps.

Effective Culvert Placement and Design to Facilitate Passage of Amphibians across Roads

Abstract Efficient deployment of culverts to mitigate mortality of amphibians on roadways requires identification of locations within road networks where animals cross (hotspots), points within identified hotspots for culvert placement, and attributes of culverts that make them behaviorally palatable to migrating individuals. In this study, we assessed road crossing frequency of Spotted Salamanders, Ambystoma maculatum, and American Toads, Anaxyrus americanus, along a 700-m transect within a known crossing hotspot, and related these distributions to habitat variables within the hotspot including the presence of existing culverts. We also placed experimental arrays of culverts of varying attributes in the path of migrating Spotted Salamanders to examine culvert preference by salamanders under typical movement environments and appropriate animal behavioral states. Our studies of patterns of road occurrence demonstrated that both species avoided crossing where there was a wetland within 15 m of the downslope of the road and that neither species showed a strong preference for crossing near existing culverts. When considering the choice for experimental culverts by Spotted Salamanders, we found no preference for culverts of varying aperture size, length, or substrate. Our results indicate that patterns of occurrences of the two species of amphibian within a crossing hotspot may be linked to the physical attributes at the site. For Spotted Salamanders in particular, predicting where they will cross within a hotspot may not be easy. Spotted Salamanders showed little preference for culverts of different design, indicating that a variety of culvert designs can suffice for mitigation if placed in appropriate locations.

Impact of forestry practices at a landscape scale on the dynamics of amphibian populations

Forest loss is a primary cause of worldwide amphibian decline. Timber harvesting in the United States has caused dramatic changes in quality and extent of forest ecosystems, and intensive forest management still occurs. Although numerous studies have documented substantial reductions in amphibian densities related to timber harvest, subsequent extinctions are rare. To better understand the population dynamics that have allowed so many amphibian species to persist in the face of widespread forest disturbance, we developed spatially explicit metapopulation models for four forest-dependent amphibian species (Lithobates sylvaticus, Ambystoma opacum, A. talpoideum, and A. maculatum) that incorporated demographic and habitat selection data derived from experiments conducted as part of the Land Use Effects on Amphibian Populations Project (LEAP). We projected local and landscape-scale population persistence under 108 different forestry practice scenarios, varying treatment (partial cut, clear-cut with coarse woody debris [CWD] removed, and clearcut with CWD retained), cut patch size (1, 10, or 50 ha), total area cut (10, 20, or 30%), and initial amphibian population size (5, 50, or 500 adult females per local breeding population). Under these scenarios, landscape-scale extinction was highly unlikely, occurring in < 1% of model runs and for only 2 of the 4 species, because landscape-scale populations were able to persist via dispersal even despite frequent local extinctions. Yet for all species, population sizes were reduced to -50% in all clear-cut scenarios, regardless of the size of harvested patches. These findings suggest that debate over timber harvesting on pool-breeding amphibian populations in the United States should focus not on questions of landscape-scale extinction but on the ecological consequences of dramatic reductions in amphibian biomass, including changes in trophic interactions, nutrient cycling, and energy transfer. Additionally, we conclude that amphibian declines and extinctions are far more likely to occur as a result of permanent habitat loss resulting from development than from the temporary degradation of habitat caused by current forestry practices.

Snake Occurrences in Grassland Associated with Road versus Forest Edges

Abstract Roads cause direct mortality of animals, but less is known about how this mortality translates into changes in populations. This is particularly true for snakes, which have been subject to little research at the population level compared to other taxa. We studied the effects of proximity to a heavily traveled road on snake populations in three old-field sites at Cicero Swamp Wildlife Management Area in Cicero, New York. We conducted 26 surveys of snakes from June to August 2006 on a regular grid of cover boards at different distances from the road and nearest forest edge and measured the microclimate at each cover board. Adult Common Gartersnakes, Thamnophis sirtalis, and Northern Brown Snakes, Storeria dekayi dekayi, occurred independently of distance from the road but were more abundant farther from the forest edge. The lack of a “road effect” was surprising, given that both species of snakes have been shown to be prone to road-mortality in previous studies and could be caused by either snakes avoiding crossing roads or mortality events being rare. Increased abundance of snakes farther from the forest edge suggests that snakes prefer sites that receive direct sunlight for longer periods of the day. Our study indicates that snake abundance is not necessarily influenced by proximity to roads and that the location of other habitat edges such as a forest-grassland interface may be a more important driver of patterns of spatial distribution.