NSF Collaborative Project:
'LEAP' - Land-Use Effects on Amphibian Populations
Over the past 200 years, conversion of land for agricultural use, industrial development, and urban sprawl has been considered the most serious threat to biological diversity. The loss of habitat divides or fragments once-continuous natural habitats into smaller pieces that are often separated by areas unsuitable to sustain viable species populations. A central tenet of conservation biology is that connectivity among fragments of habitat and interactions with adjacent habitats are the keys to understanding the impacts of fragmentation on population dynamics, risk of extinction, and maintenance of biodiversity. The LEAP project uses large-scale field experiments and spatially-structured model simulations to increase our understanding of these problems in forested ecosystems using amphibians. Specifically, our research will provide new insights into local demographic processes and metapopulation dynamics in disturbed habitats. Further, our research will provide pragmatic information to aid in efforts to balance conservation with sustainable land-use practices.
The focus of our research is to understand how important population processes in a model system of pond-breeding amphibians are altered by land-use practices that degrade and fragment natural habitats. Further, because our field experiments do not allow us to explicitly measure connectivity, we will use spatially-structured simulation models to examine whether local effects of disturbance will influence large-scale distributions and abundances of amphibians. Extensive field studies and experiments strongly indicate that two processes are critical for persistence of pond-breeding amphibian populations: local population and metapopulation dynamics. Thus, LEAP research focuses on field experiments and model simulations that measure responses associated with three processes:
1) Local population dynamics - the ecological and demographic processes related to larval metamorphosis and recruitment of juveniles into the breeding population, and therefore local population size and those available for dispersal among ponds
2) Dispersal and migration - the behavioral process of individual movement through terrestrial habitats for reproduction at home ponds or for dispersal to nearby ponds
3) Connectivity and recolonization - the landscape-level process by which individuals disperse, and rescue or recolonize nearby ponds, thereby maintaining metapopulation structure
Experimental Arrays and General Design
We have established four (n=4) replicate experimental arrays at the Daniel Boone Conservation Area, Warren County, Missouri . Each experimental array is oriented around an existing forested pond with four delineated terrestrial quadrants (~2.5 hectares each; 164 m X 250 m) surrounding it.
Size of the quadrants was based on biological criteria for terrestrial habitat needs of salamander populations (i.e., 164 m for retention of 95% of the population (Semlitsch 1998). Each quadrant was randomly assigned a different forest management treatment: 1) complete clearing with coarse woody debris (CWD) removed, 2) clearing with coarse woody debris retained, 3) partial cutting, and 4) uncut forest control.
Study Organisms in Missouri
To better understand how forestry practices affect pond-breeding amphibians we have chosen both forest-dependent species and habitat generalists as well as species with differing life history strategies. The spotted salamander, Ambystoma maculatum, is a long-lived forest dependent species that reaches maturity in 2 – 7 years. Breeding occurs in early spring and is explosive, often lasting only a few days. Following the breeding season adults in Missouri may migrate up to 172 m from the pond, but most remain within a 100 m radius of the breeding site. The American toad, Bufo americanus, is a habitat generalist that is negatively associated with forests in some regions. Sexual maturity is reached in 3 years and clutch sizes can contain as many as 20,000 eggs. In the genus Bufo, migration distances can range from 23 – 1600 m. The wood frog, Rana sylvatica, is a forest dependent species that breeds in ephemeral and semi-permanent ponds under closed canopy. Wood frogs are explosive early spring breeders and lay clutches of 304 – 874 eggs. Males reach sexual maturity in one year and females after two years. Most individuals breed only once or twice in their lifetime and live a maximum of 4 – 5 years. Outside of the breeding season wood frogs disperse an average of 1,140 m from breeding ponds. These three species, spotted salamander, American toad, and wood frog, exhibit a range of life-history strategies which will allow us to predict how habitat modification is likely to affect other species with similar life histories and habitat requirements.
SIGNIFICANCE AND IMPLICATIONS
Basic ecological processes serve as the foundation for maintaining local populations as well as the connectivity among them. These processes are also essential to understanding the mechanisms for the abundance and distribution of species, a major paradigm in ecology. Results from our research will allow us to predict whether alteration of terrestrial habitat can affect important demographic parameters, the ability of local populations to persist, and mechanisms for the distribution of amphibians in natural and disturbed landscapes. Understanding basic mechanisms that link habitat alteration to population persistence is essential to affect conservation solutions.