Amphibian Distributions in a Landscape of Forests and Agriculture: an Examination of Landscape Composition and Configuration

Landscapes can be described by two essential features: the composition and spatial arrangement of patches. We considered the roles of these basic landscape descriptors by examining how the occurrence of nine amphibian species in breeding ponds was associated with the area of forested habitat and the proximity of ponds to forested habitat. We used visual and call surveys to compare the composition of amphibian assemblages in 116 ponds adjacent to or separated from forest and surrounded by different amounts of forested land. The area of forest and pond adjacency to forest were not associated ( t 0.13, n isolated 64, n connected 52, p 0.21), which means these factors can manifest their effects separately. We used logistic regression to test predictions about associations between each species and forest area and to test for associations with pondforest adjacency. Seven of nine species were associated with forest area. Wood frogs ( Rana sylvatica ), green frogs ( Rana clamitans ), eastern newts ( Notopthalmus viridescens ), spotted salamanders ( Ambystoma maculatum ), and salamanders of the blue-spotted/Jefferson’s complex ( Ambystoma laterale/A. jeffersonianum ) were more likely to occupy ponds in more forested areas, whereas leopard frogs ( Rana pipiens ) and American toads ( Bufo americanus ) were negatively associated with forest area. Three species were associated with pondforest adjacency. Spotted salamanders and salamanders of the blue-spotted/Jefferson’s complex were more likely to occupy ponds that were adjacent to forest. In areas with little forest, leopard frogs were more likely to occur in adjacent ponds, but the reverse was true for areas with extensive forests. Our results suggest that the composition of the landscape surrounding breeding ponds is associated with the likelihood of occurrence of most of the species examined and that landscape configuration is also important for a smaller subset of species. Distribución de Anfibios en un Paisaje de Bosques y Agricultura: un Análisis de la Composición y Configuración del Paisaje Resumen: Los paisajes pueden ser descritos por dos características esenciales: la composición y la disposición de fragmentos. Consideramos la función de estos descriptores básicos de paisaje analizando como se asoció la presencia de nueve especies de anfibios en estanques de reproducción con el área de hábitat boscoso y la cercanía de estanques a hábitat boscoso. Utilizamos registros visuales y auditivos para comparar la composición de comunidades de anfibios en 116 estanques adyacentes a un bosque o separados del mismo y rodeados de diferentes cantidades de terreno boscoso. El área de bosque y la distancia de los estanques del al bosque no estuvieron asociados ( t 0.13, n aislado 64, n conectado 52, p 0.21) lo que significa que estos factores pueden manifestar sus efectos por separado. Utilizamos regresión logística para probar predicciones referentes a asociaciones entre cada especie y el área de bosque y para comprobar si las asociaciones si relacionaban con la adyacencia de los estanques al bosque. asociaciones con la adyacencia de estanques bosque. Siete de nueve especies se asociaron con el área de bosque. Ranas de bosque ( Rana sylvatica ), ranas verdes ( Rana clamitans ), tritones del este ( Notopthalmus viridescens ), salamandras manchadas ( Ambystoma maculatum ) y salamandras del complejo manchas azules/Jefferson ( Ambystoma laterale/A. jeffersonianum ) ocuparon más estanques en áreas más boscosas, mientras que ranas leopardo ( Rana pipi* Current address: Department of Zoology, 3029 Cordley Hall, Oregon State University, Corvallis, OR 97331–2914, U.S.A., email [email protected] Paper submitted December 20, 2000; revised manuscript accepted July 25, 2001. 746 Habitat Area and Adjacency Guerry & Hunter Conservation Biology Volume 16, No. 3, June 2002 Introduction Landscapes can be characterized by both the area and types of patches they contain and by the spatial arrangement of those patches ( Turner 1989; Dunning et al. 1992). Landscape composition reflects only the types and amounts of landscape components without reference to their spatial arrangement, whereas measures of landscape configuration are spatially explicit but lack information about patch type and extent. Both composition and configuration can variously affect the individuals, populations, and communities that inhabit a landscape (see review by Kareiva 1990). Understanding the separate influences of each of these landscape features on the ecology of certain taxa may inform conservation planning. For example, if the effects of landscape composition predominate, conservation efforts should focus on protecting as much suitable habitat as possible, but if the effects of configuration are also important, attention to the spatial arrangement of habitat patches is also necessary. Disentangling the roles of composition and configuration has proven difficult because the two are often correlated. For example, habitat loss (a composition feature) and habitat isolation (a configuration feature) are often confounded ( Lynch & Whigham 1984; Fahrig 1997; Hargis et al. 1997; Bowers & Dooley 1999). Organisms that require two different habitat types to complete their life cycles offer a unique situation in which to separately examine the roles of these essential landscape features. For such organisms, Dunning et al. (1992) defined landscape complementation as the process by which the proximity of two critical habitat patches of different types complements occupancy, abundance, or persistence in each patch. Therefore, landscape complementation is a result of landscape configuration. Landscape composition can be examined independently from configuration by looking at the effects of variation in the amount of habitat of one type in the vicinity of a focal patch of the other type. It may prove particularly important to understand the response of amphibians to landscape composition and configuration. Most pond-breeding amphibians depend on two types of habitat: wetlands for reproduction and upland forests for foraging, hibernating, and/or traveling (Zug 1993; Stebbins & Cohen 1995). Consequently, both the proximity of breeding habitat and terrestrial habitat and the area of terrestrial habitat may play a key role in the determination of occupancy of a focal patch ( Pope et al. 2000). At the individual level, most amphibians are likely to respond to landscape complementation because they are small and slow moving (Stebbins & Cohen 1995), have limited dispersal capabilities (Sinsch 1990) and small home ranges (Stebbins & Cohen 1995), and are tied to moist microclimates because of their highly permeable skin that must remain cool and moist for efficient respiration ( Feder 1983; Larson et al. 1984). At the population level, if amphibians exhibit metapopulation structure (Gill 1978; Sinsch 1992; Gulve 1994; Marsh & Trenham 2001), then reduced immigration and emigration rates resulting from the disconnection of required habitat patches could have severe repercussions. Laan and Verboom (1990) demonstrated a negative association between amphibian occupancy of breeding ponds and the distance from the ponds to the nearest patch of forest. Similarly, research on adult amphibians in terrestrial habitats has shown that forest patch area and isolation of forest patches from one another are important influences on occupancy ( Marsh & Pearman 1997; Gibbs 1998 a ; Kolozsvary & Swihart 1999). The area of forest surrounding breeding ponds can also influence amphibian assemblages ( Laan & Verboom 1990; Vos & Stumpel 1995; Findlay & Houlahan 1997; Hecnar & M’Closkey 1996, 1998; Lehtinen et al. 1999). To our knowledge, however, the influences of upland habitat extent (an aspect of landscape composition) and pondforest adjacency (an aspect of landscape configuration) on amphibian distributions have not been separated. Our objective was to examine landscape composition and configuration separately by looking at associations between amphibian species occurrence and forest extent and pond-forest adjacency in a landscape of forests and agriculture in northeastern Maine. In this landscape, a relatively inhospitable agricultural matrix that separates many ponds from the surrounding uplands has replaced much of the forest that once dominated the area. We looked at landscape composition by examining amphibian species occurrence in 116 breeding ponds surens ) y sapos americanos ( Bufo americanus ) se asociaron negativamente con el área de bosque. Tres especies se asociaron con la adyacencia al bosque. Salamandras manchadas y salamandras del complejo manchas azules/Jefferson tenian mayor probabilidad de ocupar estanques adyacentes a bosque. En áreas con poco bosque, ranas leopardo ocurrieron en estanques adyacentes, pero lo contrario sucedió en áreas con bosques extensos. Nuestros resultados sugieren que la composición del paisaje que rodea a los estanques de reproducción se asocia con la probabilidad de ocurrencia de la mayoría de las especies analizadas y que la configuración del paisaje también es importante para un subconjunto de especies más pequeño.