Elizabeth B. Harper

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.

Demographic Consequences of Terrestrial Habitat Loss for Pool‐Breeding Amphibians: Predicting Extinction Risks Associated with Inadequate Size of Buffer Zones

Much of the biodiversity associated with isolated wetlands requires aquatic and terrestrial habitat to maintain viable populations. Current federal wetland regulations in the United States do not protect isolated wetlands or extend protection to surrounding terrestrial habitat. Consequently, some land managers, city planners, and policy makers at the state and local levels are making an effort to protect these wetland and neighboring upland habitats. Balancing human land-use and habitat conservation is challenging, and well-informed land-use policy is hindered by a lack of knowledge of the specific risks of varying amounts of habitat loss. Using projections of wood frog (Rana sylvatica) and spotted salamander (Ambystoma maculatum) populations, we related the amount of high-quality terrestrial habitat surrounding isolated wetlands to the decline and risk of extinction of local amphibian populations. These simulations showed that current state-level wetland regulations protecting 30 m or less of surrounding terrestrial habitat are inadequate to support viable populations of pool-breeding amphibians. We also found that species with different life-history strategies responded differently to the loss and degradation of terrestrial habitat. The wood frog, with a short life span and high fecundity, was most sensitive to habitat loss and isolation, whereas the longer-lived spotted salamander with lower fecundity was most sensitive to habitat degradation that lowered adult survival rates. Our model results demonstrate that a high probability of local amphibian population persistence requires sufficient terrestrial habitat, the maintenance of habitat quality, and connectivity among local populations. Our results emphasize the essential role of adequate terrestrial habitat to the maintenance of wetland biodiversity and ecosystem function and offer a means of quantifying the risks associated with terrestrial habitat loss and degradation.

The Role of Microhabitats in the Desiccation and Survival of Anurans in Recently Harvested Oak–Hickory Forest

Abstract Juvenile survival is an important life history feature, because recent modeling efforts suggest that modest changes in juvenile survival rates due to habitat change may greatly affect population growth rates. We compared water loss and survival rates of recently metamorphosed American Toad (Anaxyrus americanus), Green Frog (Lithobates clamitans), and Wood Frog (Lithobates sylvaticus) juveniles contained within four microhabitats, two of which occur in uncut control forest (i.e., forest ridgetop, forest drainage) and two within recently harvested forest (i.e., clearcut open, clearcut brushpile). Survival was higher in forest drainage than forest ridgetop, indicating that microhabitats within continuous forest are not equally suitable. Brushpiles of coarse woody debris reduced desiccation risks in clearcuts as indicated by survival differences between clearcut open and clearcut brushpile and survival in clearcut open was very low for all species in both years. We found species differences in survival as well as a species by microhabitat interaction in water loss rates. These results are best explained by observed behavioral differences as opposed to physiological differences among species. We conclude that desiccation can be a major source of mortality for juveniles entering terrestrial habitats, especially habitat altered by anthropogenic land-use. Desiccation risks are greatest in areas with low soil moisture conditions, which for our study included microhabitats within clearcuts without coarse woody debris, forested ridgetops, and years with below average rainfall.

EFFECTS OF TIMBER HARVESTING ON POND-BREEDING AMPHIBIAN PERSISTENCE: TESTING THE EVACUATION HYPOTHESIS

Numerous studies have documented the decline of amphibians following timber harvest. However, direct evidence concerning the mechanisms of population decline is lacking and hinders attempts to develop conservation or recovery plans and solutions for forest species. We summarized the mechanisms by which abundance of amphibians may initially decline following timber harvest into three testable hypotheses: (1) mortality, (2) retreat, and (3) evacuation. Here, we tested the evacuation hypothesis within a large-scale, replicated experiment. We used drift fences with pitfall traps to capture pond-breeding amphibians moving out of experimental clearcut quadrants and into control quadrants at four replicate arrays located within the Daniel Boone Conservation Area on the upper Ozark Plateau in Warren County, Missouri, USA. During the preharvest year of 2004, only 51.6% of the 312 individuals captured were moving out of pre-clearcut quadrants, and movement did not differ from random. In contrast, during both postharvest years of 2005 and 2006, the number of captures along the quadrant edge increased, and a higher proportion of individuals (59.9% and 56.6%, respectively, by year) were moving out of clearcut quadrants than entering. Salamanders moved out of clearcuts in large percentages (Ambystoma annulatum, 78.2% in 2005, 78.2% in 2006; A. maculatum, 64.0% in 2005, 57.1% in 2006). Frogs and toads also moved out of clearcut quadrants, but in lower percentages (Bufo americanus, 59.6% in 2005, 53.3% in 2006; Rana clamitans, 52.7% in 2006). Salamanders moved out of clearcuts with low-wood treatments more than out of clearcuts with high-wood treatments. Movement of salamanders out of clearcuts was independent of sex. Estimated movement out of clearcuts represented between 8.7% and 35.0% of the total breeding adults captured for two species of salamanders. Although we recognize that some portion of the amphibian population may retreat underground for short periods and others may not survive the effects of timber harvest, these data are the first direct evidence showing that individuals are capable of leaving clearcuts and shifting habitat use.

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.

Breeding and Recruitment Phenology of Amphibians in Missouri Oak-Hickory Forests

ABSTRACT Knowing the phenologies of species in a region helps guide management and conservation activities in breeding ponds and surrounding terrestrial habitats. We examined the phenology of pond-breeding amphibians in central Missouri oak-hickory forests. Two ponds were monitored for 4 y from 2000–2003 and five other ponds were monitored for 1 y during 2004 using drift fences with pitfall traps. We found 15 species of pond-breeding amphibians, nine of which we captured in sufficient abundance to evaluate breeding phenology. Among the nine species, breeding migrations occur from Feb. to Nov., while subsequent metamorph emigration occurred primarily from May to Oct.. Our ponds were nearly permanent, resulting in salamander-dominated communities in these oak-hickory forests. Pond-use was partitioned by species that differed in their timing of reproduction and post-metamorphic emergence. For example, breeding in the fall gives larval ringed salamanders a potential size advantage over the spring-breeding spotted salamander larvae. However, the fall breeding strategy requires ponds with long hydroperiods that hold water continuously from Aug. through May. Green frogs and central newts also required long hydroperiods for their larval stage (>160 d). American toads, however, are adapted to exploit ponds with shorter, less predictable hydroperiods as they only required ponds to hold water for as little as 60 d. Management for multiple species of pond-breeding amphibians in a landscape benefits from the inclusion of wetlands with a variety of hydroperiods.

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.

Pond-Breeding Amphibian Community Composition in Missouri

Abstract We examined pond-breeding amphibian community composition at 210 ponds in Missouri between 2002 and 2012 using drift fence, dipnet, and funnel trap data. We encountered a total of 20 pond-breeding amphibian species in the combined surveys. We also examined whether the presence of American Bullfrogs, Lithobates catesbeianus, and fish influenced these patterns of diversity. Our results indicate the presence of American Bullfrogs, fish, and their interaction influenced the community composition of amphibians at these sites but in opposite patterns. American Bullfrogs often had a positive relationship with the total number of species, total caudate species, and total anuran species, whereas fish presence was negatively associated overall with species diversity, and the presence of both American Bullfrogs and fish was negatively associated with anuran species diversity. It is important to have baseline community species composition data from wide geographical ranges so spatiotemporal changes in community structure can be noted and assessed.