Nearby Source Populations Research Articles

Using multiple metrics to assess the effects of forest succession on population status: A comparative study of two terrestrial salamanders in the US Pacific Northwest

Investigations to determine stable or source-sink animal population dynamics are challenging and often infeasible for most species due to the time and expense of mark-recapture studies and the challenge of life histories attributes that result in low detectability and low recapture probabilities. Often, managers rely solely on occupancy or relative abundance patterns to assess a species’ sensitivity to environmental changes. Greater insight into population-level responses to environmental change can be gained by consideration of a combination of readily obtainable metrics, including occupancy, relative abundance, demographic structure and body condition. We examined how these metrics can improve our understanding of population-level responses to forest disturbance, using datasets for two exemplar species of terrestrial salamanders resident to the Pacific Northwestern USA. We compared population metrics for the Del Norte salamander (Plethodon elongatus) and the Siskiyou Mountains salamander (Plethodon stormi) across the seral continuum represented by four forest age classes: pre-canopy, young, mature, and old-growth. We compared these data with those collected from reference stands in mature (P. stormi) or old-growth (P. elongatus) forest containing robust populations. P. elongatus was twice as common as P. stormi. Both occupancy and salamander counts were lowest at pre-canopy sites for both species. Although there were numerous P. elongatus detections in young forests, higher proportions of these individuals were juveniles and sub-adults when compared to populations in late-seral forests. We found a negative relationship between the proportion of immature animals and total counts at a site, indicating that the high proportion of young animals in young forest stands is likely due to dispersal of young salamanders from nearby source populations and/or low survival of adult animals in young forests. We also found reduced body condition of P. stormi populations in young forests. Our results suggest that there are costs to populations occupying early seral forests, such as skewed age class structure and reduced body condition that are indicative of sink populations. Consideration of population-level metrics beyond occupancy and relative abundance can provide important insights when assessing a species’ sustainability in managed forest landscapes.

Zooplankton Successions in Neighboring Lakes with Contrasting Impacts of Amphibian and Fish Predators

Two pairs of neighboring subalpine lakes located in the Northern Calcareous Alps of Austria were investigated. Each pair comprised a deeper lake containing European minnows (Phoxinus phoxinus ), and a corresponding shallower lake harboring Alpine newts (Triturus alpestris ) as top predators. Plankton successions within fish and amphibian lakes differed markedly from each other. Throughout the year rotifers numerically dominated within the minnow lakes, while pigmented copepods (Genera Heterocope, Acanthodiaptomus , Arctodiaptomus , Mixodiaptomus ) and Daphnia were prominent in the amphibian lakes, at least early during the ice-free period. We argue that size-selective predation by minnows was the ultimate reason for this predominance of smaller zooplankton. While one of the minnow lakes was characterized by a succession of spatially and temporally segregated rotifer species, the other minnow lake permitted the development of populations of small-sized Bosmina and Ceriodaphnia during summer, probably due to the existence of a strong oxycline allowing zooplankton crustaceans to avoid predation from shore-based shoals of minnows. Once trout were introduced into this lake, minnows were visibly reduced in abundance. Bosmina and Ceriodaphnia disappeared and Daphnia together with a predacious copepod (Heterocope ) emerged either from egg banks or arrived from nearby source populations. We argue that the crustacean communities within the fishless lakes were adapted to the comparatively weak predation rates of Alpine newts. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Response of a Brook Trout Population and Instream Habitat to a Catastrophic Flood and Debris Flow

In June 1995, a massive flood and debris flow impacted fish and habitat along the lower 1.9 km of the Staunton River, a headwater stream located in Shenandoah National Park, Virginia. In the area affected by debris flow, the stream bed was scoured and new substrate materials were deposited, trees were removed from a 30-m-wide band in the riparian area, and all fish were eliminated. In the area that was unaffected by debris flow, habitat was moderately altered by the flood and fish populations persisted at decreased densities. Basinwide fish population and habitat surveys provided data to compare (1) the pre- and postevent population densities of brook trout Salvelinus fontinalis and instream habitat conditions and (2) postevent population density, brook trout growth, and instream habitat in the debris-flow-affected and unaffected areas. By June 1998, brook trout had recolonized the entire debris-flow-affected area, and population density exceeded preevent levels. Brook trout growth was significantly greater in the debris-flow-affected area than in the unaffected area through fall 1998, but it was not significantly greater in 1999. Population density appeared to have a negative influence on fish growth. A 1995 postevent habitat survey revealed increases in the number of pools and riffles and substrate size and decreases in pool and riffle surface area and depth. By 1999, the total number, surface area, and depth of pools and riffles had returned to near preevent levels and substrate size had decreased. Between 1995 and 1999, the amount of large woody debris increased in the debris-flow-unaffected area, where riparian trees had remained intact, and decreased in the affected area, where riparian trees had been eliminated. A number of factors, including a relatively intact watershed and nearby source populations, allowed the Staunton River to quickly respond to this dramatic natural event. Given the proper conditions for recovery, such events are less catastrophic than activities that lead to chronic stream degradation.

RESISTANCE AND RESILIENCE OF ALPINE LAKE FAUNA TO FISH INTRODUCTIONS

This paper reports on the response by amphibians, benthic macroinvertebrates, and zooplankton in naturally fishless alpine lakes to fish introductions and subsequent fish disappearance. We assessed resistance (the degree to which a system is altered when the environment changes) by comparing faunal distribution and abundance in lakes that have never been stocked with fish vs. the distribution and abundance in lakes that have been stocked and still contain fish. We assessed resilience (the degree and rate of a system's return to its previous configuration once the perturbation is removed) by comparing faunal distribution and abundance in lakes that were stocked at one time but have since reverted to a fishless condition (stocked-now-fishless lakes) vs. the distribution and abundance in lakes that have never been stocked. We quantified recovery rates and trajectories by comparing faunal assemblages of stocked-now-fishless lakes that had been fishless for 5–10, 11–20, and >20 yr. Faunal assemblages in the study lakes had low resistance to fish introductions, but in general showed high resilience. The mountain yellow-legged frog (Rana muscosa), conspicuous benthic macroinvertebrates, and large crustacean zooplankton (>1 mm) were dramatically reduced in distribution and abundance by fish introductions but generally recovered to predisturbance levels after fish disappearance. Inconspicuous benthic invertebrate taxa, small crustacean zooplankton (<1 mm), and rotiferan zooplankton (<0.2 mm) were either unaffected by fish or increased in the presence of fish. For both the benthic macroinvertebrate community and the zooplankton community as a whole, fish disappearance was followed by a steady change away from the configuration characteristic of fish-containing lakes and toward that of lakes that had never been stocked. Both communities remained markedly different from those in never-stocked lakes 5–10 yr after fish disappearance and converged on the configuration of never-stocked lakes only 11–20 yr after fish disappearance. Recovery was likely facilitated by the winged adult stages of many benthic macroinvertebrates, resting eggs of zooplankton, and nearby source populations of frogs. However, many frog populations have disappeared since the time that lakes in this study reverted to a fishless condition, and the viability of zooplankton egg banks should decline in fish-containing lakes over time. As a result, faunal resilience may be lower in lakes that revert to a fishless condition today than is suggested by the results of our study. These findings have important implications for the restoration of alpine lake ecosystems.

Principles for Management of Aquatic-Breeding Amphibians

Coordinated efforts by ecologists and natural resource managers are necessary to balance the conservation of biological diversity with the potential for sustained economic development. Because some amphibians have suffered world-wide declines during the last 20 years, it is important to consider biologically based management strategies that will preserve local and regional populations. This paper provides a brief overview of potential threats to local and regional populations, the state of knowledge on population and landscape processes, and the critical elements needed for an effective management plan for amphibians. Local population dynamics and ecological connectivity of amphibian metapopulations must be considered in effective management plans. There are 3 critical factors to consider in a management plan (1) the number or density of individuals dispersing from individual wetlands, (2) the diversity of wetlands with regard to hydroperiod, and (3) the probability of dispersal among adjacent wetlands or the rescue and recolonization of local populations. Wetland losses reduce the total number of sites where pond-breeding amphibians can reproduce and recruit juveniles into the breeding population. Loss of small, temporary wetlands (<4.0 ha) may be especially harmful to amphibians because of their abundance and high species diversity. Alteration of wetlands, particularly hydrologic cycles, can severely impair completion of larval metamorphosis through either early pond drying (if hydroperiod is shortened) or through increased predation (if hydroperiod is lengthened or connections made with fish-infested lakes, rivers, or canals). Wetland loss also increases the distance between neigh-boring wetlands that is critical to metapopulation source-sink processes. Reduction in wetland density reduces the probablity that populations will be rescued from extinction by nearby source populations. Local populations cannot be considered independent of source-sink processes that connect wetlands at the landscape or regional level. Further, the fragmentation of natural habitats from timber harvesting, agriculture, roads, drainage canals, or urban development impedes or prevents dispersal and decreases the probability of wetland recolonization. If our goal is to maintain or enhance present levels of amphibian diversity, then resource managers must incorporate critical elements into plans that protect population and landscape processes thereby maintaining viable populations and communities of amphibians.

Colonization of secondary woodlands by Anemone nemorosa

Migration of the herb species Anemone nemorosa from older woodlands into adjacent, recently established deciduous woodlands on former arable land was studied. The wood anemone had colonized part of all studied recent woods, varying in age between 30 and 75 years. Cover of A. nemorosa in the recent woods decreased with increasing distance from the older woodland at all sites but one, indicating dispersal limitation during colonization. The advancing edge of most populations was characterized by negative logarithmic decrease in cover with establishment of isolated pioneer individuals and later gradual infill between pioneers. Migration rates were calculated by three methods. The mean migration rate of A. nemorosa based on observed maximum cover in the recent woods was 0.20 m year1, 0.40 m year‐1 based on half maximum cover and 0.85 m year1 based on the individual found farthest from the former woodland border. The calculated migration rates were consistently higher than the rate of possible rhizome growth. Seed dispersal and establishment is thus very important for colonization of new woodlands. Migration rates increased with tree canopy cover–especially cover of broad‐leaved species with quickly decomposing litter–and with soil pH. Migration rates decreased with increasing grass cover in the field layer. These differences in migration rates may be due to increased micro‐site availability for establishment at high canopy cover, low grass cover and high soil pH. Our results show that the wood anemone generally colonizes recent woods from nearby source populations. However, colonization proceeds relatively slowly and is limited by both seed dispersal and availability of suitable micro‐sites. Gradients in abundance of A. nemorosa within secondary woods may be detectable for long periods of time and indicate the recent origin of a woodland.

Migration rates of grassland plants along corridors in fragmented landscapes assessed with a cellular automation model

This study investigated the efficacy of linear landscape elements in fragmented landscapes as corridors for perennial grassland species with short-range seed dispersal. Corridors are assumed to be essential for the persistence of metapopulations in fragmented landscapes, but it is unclear to what extent linear landscape elements such as ditch banks and road verges can function as corridors for those species. The principal factors that determine the rate of migration through corridors include the width and habitat quality of patches within a corridor (expressed as the population growth rate h) and the dispersal capacity of plants (expressed as the slope a of the relationship between seed number and log-distance). A cellular automation model was used to simulate the effects of the principal factors on the rate of migration. Simulations with different levels of the principal factors showed highly significant and positive main effects of dispersal capacity, habitat quality and width of corridors on the migration rate. Significant interactions existed between dispersal capacity x width and dispersal capacity x habitat quality (p < O.OOOl), indicating that the effects of width and habitat quality depended on the dispersal capacity. In narrow corridors most of the dispersed seeds were deposited outside the corridor, which significantly reduced migration rates, especially for species with long-range dispersal (a = -0.4). In wide corridors (up to 20 m), seed losses were much smaller and migration rates approximated those of continuous habitats. The contribution of the few long-range seeds to the rate of migration was significant when habitat quality was high (population growth rates up to 2.5). However, in all simulations migration rates were very low, i.e. < 5 dyr. It is concluded that linear landscape elements are not effective corridors in fragmented landscapes for plants with short-range seed dispersal, because migration rates are low (< 5 m/yr), landscape elements vary in the percentage of high quality patches, and refugia and suitable habitat patches are frequently several kilometres apart, making a cohesive infrastructure of corridors for plants elusive. It is argued that the best way to conserve endangered plant species that encounter dispersal barriers is to harvest seeds from nearby source populations and introduce them as suitable habitats.

The Effects of Management Systems on Ground‐Foraging Ant Diversity in Costa Rica

The impacts of four different management systems on diversity of ground—foraging ants were examined to assess potential for conservation of biological diversity in human—influenced agricultural or agroforestry systems. Using a variety of indices, diversity was measured along a gradient of anthropogenically disturbed habitats. The gradient, in terms of increasing levels of disturbance, ranged from primary rain forest, abandoned cacao plantations, productive cacao plantations, to banana plantations. The primary forest and abandoned cacao plantations were situated in a reserve and the productive cacao plantations, and banana plantations were located on privately owned land within 35 km of the reserve in the Sarapiqui Valley of Costa Rica. To sample ground—foraging ants, we placed tuna baits along the ground in each habitat. Although tuna baits attract a generalist assemblage of ants that may be better able to adapt to more disturbed sites, diversity of this community was significantly reduced with increased disturbance. Ant diversity was not different for forest and abandoned cacao sites except in terms of evenness, with the forest having a more even distribution of species. Forest and abandoned cacao sites were significantly more diverse than both productive cacao and banana plantations. The latter two did not differ significantly from each other in terms of diversity, but there was considerable variability within each of these habitat types. The results of this study suggest that conservation of diversity may be possible in a "mosaic" of different land uses, but further studies, such as those addressing the importance of scale and nearby source populations, are necessary before management practices can be implemented.