Aram J. K. Calhoun

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.

Conservation Planning for Amphibian Species with Complex Habitat Requirements: A Case Study Using Movements and Habitat Selection of the Wood Frog Rana sylvatica

Abstract Conservation of fauna breeding in vernal pools is challenging given their complex life histories. Many species, including the widespread North American Wood Frog (Rana sylvatica), require both aquatic and terrestrial habitat, yet insufficient information exists about movements between these environments, nor fine-scale selection patterns within them. To inform conservation planning, we conducted a radio-telemetry study of seasonal patterns of Wood Frog movements and habitat selection in southern Maine. Forty-three frogs were tracked an average of 25.6 days each, April to November 2003. In early spring, Wood Frogs generally selected damp leaf litter retreats on the margins of breeding pools. Following breeding, frogs selected forested wetlands (9.3% of the landscape) over forested uplands (90.7% of the landscape) in 75.3% of radio locations (N  =  544). Postbreeding movements from breeding pools to nearby, closed-canopy, forested wetlands ranged from 102–340 m (median 169m, N  =  8) and included stopovers in upland forest floors ranging from one to 17 days (median two days, N  =  7). Summer refugia were characterized by shady, moist (nonaquatic), and sphagnum-dominated microhabitats. In urbanizing areas, we recommend a shift from a core-habitat conservation model to a spatially explicit approach that considers pool-breeding amphibian habitat as a network of migration-connected habitat elements (e.g., breeding pools, upland forest, nearby forested wetlands). In our study, this approach reduced the amount of land potentially requiring protection by > 2/3 from that of core habitat models. With the rapid dissemination of GIS technology, spatially explicit planning for pool-breeding amphibians is increasingly feasible.

EVALUATING VERNAL POOLS AS A BASIS FOR CONSERVATION STRATEGIES: A MAINE CASE STUDY

Vernal pools in northeastern North America are typically seasonal woodland pools that support breeding populations of amphibians and invertebrates dependent upon fishless environments for successful reproduction. A survey of 304 vernal pools in southern, central, and northern Maine, USA was conducted to assess pool physical characteristics, landscape setting, and presence of pool-breeding amphibians for the purpose of guiding potential pool conservation strategies. In particular, information on reproductive effort by pool-breeding amphibians was used to assess the statewide applicability of the Maine Natural Resources Protection Act’s proposed definition of Significant Vernal Pool, a category of Significant Wildlife Habitats that allows closer environmental review of proposed impacts to vernal pools. The results of our study show regional differences in pool characteristics and amphibian usage. Defining “significance” based on number of egg masses and diversity of vernal pool indicator species is a useful tool but should be considered in the context of such landscape characteristics as availability of suitable terrestrial habitat and distribution of other breeding habitats and wetlands.

Do geographically isolated wetlands influence landscape functions

Geographically isolated wetlands (GIWs), those surrounded by uplands, exchange materials, energy, and organisms with other elements in hydrological and habitat networks, contributing to landscape functions, such as flow generation, nutrient and sediment retention, and biodiversity support. GIWs constitute most of the wetlands in many North American landscapes, provide a disproportionately large fraction of wetland edges where many functions are enhanced, and form complexes with other water bodies to create spatial and temporal heterogeneity in the timing, flow paths, and magnitude of network connectivity. These attributes signal a critical role for GIWs in sustaining a portfolio of landscape functions, but legal protections remain weak despite preferential loss from many landscapes. GIWs lack persistent surface water connections, but this condition does not imply the absence of hydrological, biogeochemical, and biological exchanges with nearby and downstream waters. Although hydrological and biogeochemical connectivity is often episodic or slow (e.g., via groundwater), hydrologic continuity and limited evaporative solute enrichment suggest both flow generation and solute and sediment retention. Similarly, whereas biological connectivity usually requires overland dispersal, numerous organisms, including many rare or threatened species, use both GIWs and downstream waters at different times or life stages, suggesting that GIWs are critical elements of landscape habitat mosaics. Indeed, weaker hydrologic connectivity with downstream waters and constrained biological connectivity with other landscape elements are precisely what enhances some GIW functions and enables others. Based on analysis of wetland geography and synthesis of wetland functions, we argue that sustaining landscape functions requires conserving the entire continuum of wetland connectivity, including GIWs.

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.

Geographically isolated wetlands: Rethinking a misnomer

We explore the category “geographically isolated wetlands” (GIWs; i.e., wetlands completely surrounded by uplands at the local scale) as used in the wetland sciences. As currently used, the GIW category (1) hampers scientific efforts by obscuring important hydrological and ecological differences among multiple wetland functional types, (2) aggregates wetlands in a manner not reflective of regulatory and management information needs, (3) implies wetlands so described are in some way “isolated,” an often incorrect implication, (4) is inconsistent with more broadly used and accepted concepts of “geographic isolation,” and (5) has injected unnecessary confusion into scientific investigations and discussions. Instead, we suggest other wetland classification systems offer more informative alternatives. For example, hydrogeomorphic (HGM) classes based on well-established scientific definitions account for wetland functional diversity thereby facilitating explorations into questions of connectivity without an a priori designation of “isolation.” Additionally, an HGM-type approach could be used in combination with terms reflective of current regulatory or policymaking needs. For those rare cases in which the condition of being surrounded by uplands is the relevant distinguishing characteristic, use of terminology that does not unnecessarily imply isolation (e.g., “upland embedded wetlands”) would help alleviate much confusion caused by the “geographically isolated wetlands” misnomer.

Pond-Breeding Amphibian Species Richness and Habitat Selection in a Beaver-Modified Landscape

Abstract Beaver (Castor canadensis) activity creates wetland habitats with varying hydroperiods important in maintaining habitat diversity for pond-breeding amphibians with significantly different breeding habitat requirements. We documented pond-breeding amphibian assemblages in 71 freshwater wetlands in Acadia National Park, Maine, USA. Using 15 variables describing local pond conditions and wetland landscape characteristics, we developed a priori models to predict sites with high amphibian species richness and used model selection with Akaikes Information Criterion to judge the strength of evidence supporting each model. We developed single-species models to predict wood frog (Rana sylvatica), bullfrog (R. catesbeiana), and pickerel frog (R. palustris) breeding site selection. Sites with high species richness were best predicted by 1) connectivity of wetlands in the landscape through stream corridors and 2) wetland modification by beaver. Wood frog breeding habitat was best predicted by temporary hydroperiod, lack of fish, and absence of current beaver activity. Wood frog breeding was present in abandoned beaver wetlands nearly as often as in nonbeaver wetlands. Bullfrog breeding was limited to active beaver wetlands with fish and permanent water. Pickerel frog breeding sites were best predicted by connectivity through stream corridors within the landscape. As beavers have recolonized areas of their former range in North America, they have increased the number and diversity of available breeding sites in the landscape for pond-breeding amphibians. The resulting mosaic of active and abandoned beaver wetlands both supports rich amphibian assemblages and provides suitable breeding habitat for species with differing habitat requirements. Land managers should consider the potential benefits of minimal management of beavers in promoting and conserving amphibian and wetland diversity at a landscape scale.

Monitoring created seasonal pools for functional success: A six-year case study of amphibian responses, Sears Island, Maine, USA

Assessing the success of created seasonal pools as mitigation for the loss of wooded wetlands is generally based on two or three years of monitoring. We monitored a wetland mitigation site in midcoast Maine, USA, from 1999 to 2004 to track populations of wood frogs (Rana sylvatica) and spotted salamanders (Ambystoma maculatum) in three created seasonal pools. Our goal was to study breeding patterns for six years to assess if presence of breeding animals in the first three years corresponded with long-term reproductive success and to track changes in vegetation development and hydrology in the mitigated pools. Breeding effort (egg masses/female) and reproductive success (juveniles/egg mass) were assessed using full enclosure drift fence/pitfall trap arrays and egg mass counts. Breeding effort was similar in all three pools for both species. However, for wood frogs, reproductive success was inversely related to hydroperiod and highest in the single pool that successfully replicated the desired seasonal water regime. The other two pools developed permanent and semi-permanent hydroperiods, thereby supporting populations of green frogs (Rana clamitans). Green frog tadpoles within these pools preyed upon wood frog eggs and embryos, causing almost complete losses in the last four years of the study. While spotted salamander reproductive success was also reduced in the permanent pool, spotted salamanders were more successful at producing metamorphs in the presence of green frogs. Over the study period, common cattail (Typha latifolia) dominated the pools, and although wetland vegetation did develop, it was not typical of forested seasonal pools. Our work shows that the presence of breeding amphibians characteristic of seasonal pools in the first three years is not sufficient to determine reproductive success of target species in created pools. Hydrology and canopy cover are elements of critical concern if created pools are to support native amphibian communities associated with seasonal pools.

DEVELOPING VERNAL POOL CONSERVATION PLANS AT THE LOCAL LEVEL USING CITIZEN-SCIENTISTS

Use of citizen-scientists to collect data on natural resources is gaining credibility globally and is now considered a valuable tool in the conservation tool box. We conducted town-wide vernal pool inventories using citizen-scientists in four New England towns (USA) using voluntary best development practices (BDPs) for vernal pools. We tested the efficacy of using citizen-scientists to collect field data on vernal pools using published BDP guidelines. Steps included pool mapping and inventory, training of citizen-scientists, vernal pool field assessments, and guiding town development of local conservation strategies using data provided by the project. Potential vernal pools were remotely identified and photointerpreted. Partnerships among the University of Maine, the towns, and non-governmental organizations were forged to implement the project in spring 2003. Local coordinators in each town recruited volunteer citizen-scientists to conduct ground assessments. Volunteer training sessions were held prior to and during the field season. Fifty-two citizen-scientists surveyed and assessed 262 vernal pools. Quality control tests in the field confirmed that citizen-scientist data on amphibian egg mass counts were not significantly different from data gathered by biologists. Each pool was given a conservation priority rating based on the BDP assessment. Data were entered into a Geographic Information System database and delivered to each town. All towns initiated conservation plans and are developing conservation mechanisms to protect pools recognized as having conservation priority. Town strategies ranged from amending existing ordinances to improve wetland protection to incorporation of vernal pool resources into larger biodiversity mapping and planning projects. These four case studies illustrate that vernal pool conservation initiatives can be developed in local communities using the skills of trained citizen-scientists to collect accurate data. Communities are then better able to incorporate pool conservation strategies into the local planning and regulatory processes.

Patterns of Beaver Colonization and Wetland Change in Acadia National Park

Abstract The return of Castor canadensis (beaver) to areas of their former range has restored a natural disturbance regime to wetland landscapes in North America. We used aerial photographs to study wetland creation and modification by beaver in Acadia National Park, ME, during a period of beaver population expansion (1944–1997). We quantified the change in the number of available ponded wetlands in the landscape during the study period and documented an 89% increase in ponded wetlands between 1944 and 1997. Spatial and temporal patterns of beaver colonization and changes in wetland vegetation and hydrology were recorded at six time periods (1944, 1953, 1970, 1979, 1985, and 1997) for 33 beaver-created wetlands for which we had current amphibian assemblage data. Beaver colonization generally converted forested wetlands and riparian areas to open water and emergent wetlands, resulting in significant increases in the percentage of open water and emergent wetland habitat and a decrease in the percentage of forested wetland area at the study sites. Temporal colonization of beaver wetlands initially favored large sites occurring lower in the watersheds; sites that were impounded later were generally smaller, higher in the watershed, and more likely to be abandoned by the end of our study. Our results suggest that beaver have not only increased the number of available breeding sites in the landscape for pond-breeding amphibians, but also the resulting mosaic of active and abandoned beaver wetlands is likely to provide suitable breeding habitat for a diversity of species.