Source Pond Research Articles

Management effects on fungal assemblages in irrigation ponds: are biodiversity conservation and the control of phytopathogens compatible?

The study of fungi and fungus-like organisms in irrigation ponds has applied interest since waterborne phytopathogens can disperse through the irrigation system to the crop roots. We tested the hypothesis that manag- ing irrigation ponds to maintain high levels of biodiversity could entail agronomic benefits through the control of phytopathogens. The structure of assemblages of fungi and fungus-like taxa were characterized and compared us- ing samples taken from two strata depths (superficial and deep). This was done in four management/water source pond groups of southeastern Spain: groundwater-fed ponds with submerged aquatic vegetation stands (untreated SAV ponds); groundwater-fed ponds submitted to low and high doses of copper sulfate, and ponds fed with recy- cled urban wastewater bearing intermediate doses of copper sulfate. Higher density and diversity of fungi, besides a singular assemblage composition, were found in untreated SAV ponds compared to treated ponds, particularly for samples taken in the deep stratum. Phytopathogenicity of the main phytopathogens detected (Pythium isolates) was generally low and rather unaffected by the type of pond management. However, much evidence from poten- tial unsuitable hosts (submerged macrophyte species) and fungal composition (related to the presence of potential pathogen antagonists) suggests that managing ponds to preserve submerged vegetation might prove useful for controlling root rot diseases.

A novel approach to parasite population genetics: Experimental infection reveals geographic differentiation, recombination and host‐mediated population structure in Pasteuria ramosa, a bacterial parasite of Daphnia

The population structure of parasites is central to the ecology and evolution of host-parasite systems. Here, we investigate the population genetics of Pasteuria ramosa, a bacterial parasite of Daphnia. We used natural P.ramosa spore banks from the sediments of two geographically well-separated ponds to experimentally infect a panel of Daphnia magna host clones whose resistance phenotypes were previously known. In this way, we were able to assess the population structure of P.ramosa based on geography, host resistance phenotype and host genotype. Overall, genetic diversity of P.ramosa was high, and nearly all infected D.magna hosted more than one parasite haplotype. On the basis of the observation of recombinant haplotypes and relatively low levels of linkage disequilibrium, we conclude that P.ramosa engages in substantial recombination. Isolates were strongly differentiated by pond, indicating that gene flow is spatially restricted. Pasteuria ramosa isolates within one pond were segregated completely based on the resistance phenotype of the host-a result that, to our knowledge, has not been previously reported for a nonhuman parasite. To assess the comparability of experimental infections with natural P.ramosa isolates, we examined the population structure of naturally infected D.magna native to one of the two source ponds. We found that experimental and natural infections of the same host resistance phenotype from the same source pond were indistinguishable, indicating that experimental infections provide a means to representatively sample the diversity of P.ramosa while reducing the sampling bias often associated with studies of parasite epidemics. These results expand our knowledge of this model parasite, provide important context for the large existing body of research on this system and will guide the design of future studies of this host-parasite system.

Bacterial Survival in Fractured Shale-Gas Wells of the Horn River Basin

Summary Traditional ideology on the upper temperature limits of life suggest that extreme temperatures such as those observed in the Horn River Basin shale-gas formation (i.e., up to 175°C) should alleviate microbiologically influenced corrosion and souring concerns associated with hydraulic-fracturing procedures. The present case history investigates the accuracy of this theory. Horn River basin wells currently in the flowback or production stage and the source water used to fracture these wells were studied for the presence and identity of viable bacteria. The effectiveness of two biocides (one glutaraldehyde-based and the other cocodiamine-based) in eradicating these bacteria in both the field and the laboratory was also studied. The results show that source ponds are highly contaminated with bacteria. Furthermore, the high-temperature, high-pressure downhole conditions in this region are not sufficient for eradicating bacteria introduced during the fracturing process. These bacteria survive and establish viable, proliferating communities. Biocide, applied continuously "on the fly" during fracturing, was effective at mitigating the downhole bacteria concern. Laboratory studies further showed that more-established bacterial communities were much more resistant to biocidal treatment. Therefore, early mitigation strategies will likely be key in the prevention of microbiologically induced corrosion and souring.

Assessing Stress Levels of Captive-Reared Amphibians with Hematological Data: Implications for Conservation Initiatives

Larval amphibians are increasingly being reared for conservation initiatives to bolster declining populations. Few researchers, however, have asked whether reared individuals are functionally equivalent to their wild counterparts. Compared with those in the wild, amphibians reared in captivity may develop in relatively stress-free environments, because they are usually fed ad libitum, raised in the absence of predators and pathogens and in controlled environments. Thus, with few challenges throughout development, would their resting stress levels or reactions to acute stressors be normal? We addressed this question by rearing Litobates sphenocephalus and Ambystoma opacum from eggs and 10-day-old larvae through to late larval stages in artificial pond environments and by determining their ratios of neutrophils (N) to lymphocytes (L) (two leukocytes that covary with stress hormones) before and after a standardized stressor. We obtained similar samples from wild-caught larvae of equivalent developmental stages and from the same source pond. Baseline and stress-induced N∶L ratios of reared L. sphenocephalus were statistically similar to those of wild individuals. In contrast, baseline N∶L ratios of reared A. opacum were slightly higher than those of wild individuals. In general, the magnitude of the leukocyte response to stress for both species (a 3-fold increase in N∶L over baseline), was similar to that of wild individuals, suggesting that captive-reared amphibians are capable of mounting a normal physiological stress response. Although this last point provides support for the use of captive-rearing for conservation and research purposes, the unusually high baseline N∶L ratios of reared salamanders will require additional research to determine the functional meaning.

Production of Gulf Killifish in the Black Belt Region of Alabama Using Saline Groundwater

AbstractThe Gulf killifish Fundulus grandis, commonly called the bull minnow, mudminnow, or cacahoe, is a popular live bait along the Gulf of Mexico coast in the United States. Although culture techniques for Gulf killifish have been established for more than 30 years, this species is not commonly produced. Suitable coastal areas are thought to be one of the limitations. Low‐salinity groundwater occurs in much of the United States, including portions of the Alabama Black Belt region; however, the relative abundance of minerals in the water varies. Spawning success was compared among Gulf killifish held in saline groundwater (salinity = 4.2–11.1‰) from four inland sites and in 5‰ synthetic seawater. Gulf killifish production (fry to subadult) was compared between two sites of similar salinity but with different mineral profiles (site A had 4.0‰ salinity and received water from a fertilized source pond; site B had 3.2‰ salinity and received water with no fertilizer additions); initial stocking densities in 1.15‐m2 outdoor tanks were 25, 50, and 100 fry/m2. Gulf killifish spawned successfully under all conditions tested. Mean (±SD) fry production of 1.7 ± 0.88 fry/g of female was obtained. The yields and mean weights obtained differed by site and stocking density. After 104 d of production, the highest yield was 133.3 ± 40.4 g/m2 (1,333 ± 404 kg/ha) at site A for fish stocked at 100 fry/m2. The yield at site B for fish at the same stocking density was 62.6 ± 18.8 g/m2. Market‐sized fish (total length ≥ 6.35 cm; weight = 3.3 g) were obtained at site A for the stocking density of 25 fry/m2, but market‐sized fish were not obtained for any stocking density at site B. Although the mineral profiles differ between saline groundwater and natural seawater, saline groundwater is suitable for the reproduction and growth of this species. Egg production and fish growth were similar to levels expected for Gulf killifish culture in brackish‐water ponds along the Gulf of Mexico coast.

INTEGRATION OF AQUACULTURE AND ARID LANDS AGRICULTURE FOR WATER REUSE AND REDUCED FERTILIZER DEPENDENCY

SUMMARYField irrigation is costly in arid regions, and over-fertilization of farmland can lead to high groundwater nitrate levels and significant environmental challenges. Integrative aquaculture and agriculture (IAA) systems allow the reuse of water and nutrients to offset production costs while promoting greater sustainability. The aim of this study was to test the effectiveness of an IAA system using treatments formed from one water source, groundwater (GRND) or fish pond effluent (EFF), and one chemical fertilizer regime, eliminated (ELIM) or historical (HIST). Treatments were applied to field plots of barley or cotton. There were typically positive effects of EFF applications on crop growth and yield relative to GRND applications under identical fertilizer regimes. However, GRND-HIST almost always outperformed EFF-ELIM, suggesting that substituting effluent irrigations for a historical fertilization regime without pond biosolid or reduced fertilizer applications could be detrimental to crop production.

Dissolved oxygen requirements for hatching success of two ambystomatid salamanders in restored ephemeral ponds

To assess feasibility of reintroduction of extirpated spotted salamanders (Ambystoma maculatum) in restored flatwoods wetlands, hatching rates were monitored using pond enclosures.Ambystoma maculatum hatching success was compared to that of conspecifics in source ponds and to blue-spotted salamanders (Ambystoma laterale) that had persisted in restored ponds despite habitat degradation. Restored ephemeral ponds with hypoxic conditions had consistent hatching failure forA. maculatum. To isolate effects of dissolved oxygen (DO), laboratory gradients were used to identify levels of DO necessary forA. maculatum andA. laterale hatching success. DO treatments included 0, 2.0, 4.0, 5.0, 6.0, 7.0, and 8.0 mg/l forA. maculatum and 2.0, 4.0, and 6.0 mg/l forA. laterale. Ambystoma laterale hatched across all treatments.Ambystoma maculatum hatching was successful in treatments >4.0 mg/l. Prescribed burns of dried ponds and selective girdling reduced leaf litter and increasedin situ photosynthesis resulting in greater DO.Ambystoma laterale may have persisted in degraded ponds because of differences fromA. maculatum in egg structure, and thus oxygen delivery. Land use changes contributing to hypoxia, including changes in forest composition and fire regime, may help explain the loss ofA. maculatum from regional assemblages.

Comparative landscape dynamics of two anuran species: climate‐driven interaction of local and regional processes

The dynamics of ecological communities emerge from the interplay of local and regional processes, but there are few long‐term data on multiple interacting species across multiple sites to evaluate these processes. We report the population dynamics of two species of treefrogs, the spring peeper (Pseudacris crucifer) and the chorus frog (P. triseriata), in 37 ponds over 11 years. We examined the relation between larval traits of these species and responses to environmental change (a severe drought) at the local (pond) level, and the consequences to regional dynamics of the species. Consistent with experimental evidence, predators limited abundances of both species, competitive effects were undetectable, and pond hydroperiod and forest canopy cover affected species' presences and population sizes. The drought caused strong reductions in pond hydroperiods and predator densities, which led to increases in colonization probability and decreases in extinction probability for the chorus frog. These habitat dynamics resulted in the colonization of 15 new ponds and exponential growth in regional population size of the chorus frog. Colonization probability was positively related to pond connectivity. Pond occupancy rates for the spring peeper were relatively constant, but important source ponds on the landscape changed with the drought. Ponds with extensive forest canopy cover were sink habitats for both species, and high pond connectivity appeared to be necessary to maintain species' presences in these habitats. Landscape dynamics were responsible for changes in the fundamental spatial structure of species' populations, e.g., the chorus frog transitioned between approximations of a classic metapopulation and a patchy population over the course of the study. Many of these results were driven by the interplay of climate variation and spatial food web structure, indicating the importance of incorporating this interaction into metacommunity theory. Our results further indicate the crucial role of habitat (pond) heterogeneity to species persistence, i.e., moderating the potential Moran effects of spatially correlated weather conditions. Overall this study illustrates: (1) how natural systems constantly transition between metapopulation/metacommunity states typically treated as static in the literature, (2) the extensive interconnection of phenomena at different scales in ecology, and (3) the importance of habitat heterogeneity and landscape context for amphibian conservation strategies.

Measuring and modeling dispersal of adult zooplankton

Habitat fragmentation poses an inherent problem for metacommunity dynamics, as dispersal among communities is hindered by increasing isolation and the loss of patches. Wetlands are one such system that have undergone excessive destruction and fragmentation in recent years. Zooplankton within these communities have historically been considered frequent and widespread dispersers, but direct, quantitative measures of zooplankton dispersal are rare. In this study, I performed two experiments to quantify zooplankton dispersal and to identify the primary dispersal vectors. I first set up an array of traps at 10, 30, 60, 120 and 180 m around an isolated pond to collect dispersing individuals. Nearly 1,500 adult zooplankton were captured in traps up to 180 m from the pond, with approximately 60% of dispersers being captured in traps at 10 m from the pond. A second experiment using open and animal-excluded traps suggested that large animals were the primary dispersal vector for these zooplankton. Using a subset of these data, I fit four models to describe the shape and magnitude of adult cladoceran dispersal at this site. All models showed the majority of cladocerans were deposited very close to the source pond, with three models suggesting that the trapping area encompassed 67% or more of the dispersal distances. These results suggest that adult zooplankton movement among ponds may be significant in areas where aquatic habitats are plentiful. Yet, in recent years climate change and anthropogenic disturbances have reduced the number and size of aquatic habitats in many regions of the world, likely curtailing effective transport of individuals in many cases. As a result, fragmented zooplankton metacommunities may experience increased dispersal limitation, stronger priority effects, higher levels of inbreeding and selection against traits engendering high dispersability.

Lotic dispersal of lentic macroinvertebrates

Little is known on dispersal of lentic macroinvertebrates. We quantified dispersal of lentic macroinvertebrates through pond connections in a highly connected pond system, and investigated how dispersal rates were affected by connection properties and time of the day (day, night). Furthermore, by comparing the composition of assemblages of dispersing macroinvertebrates with the macroinvertebrate assemblages of source ponds, we tested whether dispersal was neutral or a taxon‐specific process. We found that many taxa dispersed through the pond connections. Taxa richness of the dispersing macroinvertebrate assemblage was proportional to taxa richness in the source ponds. The number of individuals that dispersed, however, was not related to source pond densities, possibly because of the highly patchy distribution of lentic macroinvertebrates within ponds. Elevated dispersal rates were recorded for Baetidae, Chironomidae and Physidae, indicating a taxon‐specific use of pond connections as dispersal pathway. None of the physical properties of the connections affected dispersal. Macroinvertebrates dispersed more during the night than during daytime. Of seven tested families, Chaoboridae and Chironomidae showed higher dispersal during the night, probably resulting from diel vertical migration, since pond connections mainly transported near‐surface water. Flying and non‐flying invertebrates dispersed equally frequent through pond connections. For non‐flying invertebrates that disperse infrequently over land, dispersal through pond connections may provide an important additional means of dispersal.

TROUT PREDATORS AND DEMOGRAPHIC SOURCES AND SINKS IN A MAYFLY METAPOPULATION

Habitat patches often differ in environmental conditions, and the degree of environmental heterogeneity among patches is thought to influence population dynamics at the regional (metapopulation) scale. In a previous study, I found evidence of a source– sink dynamic in a metapopulation of the mayfly Callibaetis ferrugineus hageni occupying beaver ponds with or without brook trout predators, where adult females frequently dispersed among ponds, and frequently oviposited in ponds with trout. Here, I tested the hypothesis that trout create habitat sinks by manipulating trout densities and estimating local population growth rates (λ) of Callibaetis. In addition, Callibaetis emergence rates were estimated in the mayfly generation before the manipulation. Among ponds, λ ranged from 0.062 to 2.631, and nine Monte Carlo simulations scenarios revealed that four ponds had λ significantly below replacement (sinks) while the presence of source ponds was less conclusive. The manipulation of trout did not result in a detectable treatment effect, perhaps because of low statistical power resulting from high within-treatment variation in trout densities. However, point estimates of λ and emergence rates were negatively and exponentially related to trout density, such that nearly all ponds with more than 1 trout/100 m2 were sinks. Further, multiple regression analyses were consistent with the hypothesis that trout reduced λ and emergence rates, while other biotic and abiotic factors did not provide explanatory power. The results support the hypothesis that source–sink population dynamics may be common in freshwater habitats, and that prey metapopulation dynamics may be predicted from the distribution of predators, prey movement and habitat selection behaviors, and factors influencing the outcome of predator–prey interactions.

Habitat specialization and adaptive phenotypic divergence of anuran populations

We tested for adaptive population structure in the frog Rana temporaria by rearing tadpoles from 23 populations in a common garden experiment, with and without larval dragonfly predators. The goal was to compare tadpole phenotypes with the habitats of their source ponds. The choice of traits and habitat variables was guided by prior information about phenotypic function. There were large differences among populations in life history, behaviour, morphological shape, and the predator-induced plasticities in most of these. Body size and behaviour were correlated with predation risk in the source pond, in agreement with adaptive population divergence. Tadpoles from large sunny ponds were morphologically distinct from those inhabiting small woodland ponds, although here an adaptive explanation was unclear. There was no evidence that plasticity evolves in populations exposed to more variable environments. Much among-population variation in phenotype and plasticity was not associated with habitat, perhaps reflecting rapid changes in wetland habitats.

Impacts on microbial communities and cultivable isolates from groundwater contaminated with high levels of nitric acid–uranium waste

Microbial communities were characterized at contaminated sites that had elevated levels of nitrate, nickel, aluminum, and uranium (up to 690 mM, 310 μM, 42 mM, and 30 μM, respectively). The bacterial community structure based upon clonal libraries of the SSU rRNA genes (screened clones = 876) was diverse at the background site, but the three acidic samples had decreased diversity and the majority of clones were closely related to Azoarcus and Pseudomonas species. Arthrobacter and Novosphingobium sequences were recovered from the background samples but not the acidic sites, and similar pseudomonad populations were present at the background and acidic sites albeit at different relative abundances. Heterologous sequence coverage analyses indicated the microbial communities at the contaminated sites were very similar ( p = 0.001) but different from the background site. Bacterial isolates ( n = 67) classified as β-or γ- Proteobacteria, high G+C Gram-positive or low G+C Gram-positive were obtained from the background and one contaminated sample, and some of the isolates had less than 95% sequence identity with previously observed microorganisms. Despite variations in nitrate and heavy metal levels and different proximities to the source ponds, the three acidic samples had similar microbial populations. However, the least contaminated site (lowest nitrate and aluminum) had increased diversity compared to the other acidic samples. The results suggested that the combined contamination has decreased the microbial diversity, and Azoarcus populations were observed at a drastically increased frequency compared to the background site that had a more even distribution of multiple taxa.

Response of amphibians to restoration of a southern Appalachian wetland: A long-term analysis of community dynamics

Although amphibians are increasingly being used to assess ecosystem function of compensatory wetlands, there are almost no long-term studies of responses to ecological restoration. Consequently, much uncertainty exists about the appropriate timeframes and best criteria for evaluating responses to wetland restoration. We studied aspects of pond colonization and long-term community dynamics in ponds created at a mitigation site in western North Carolina. We examined whether landscape variables influenced the initial colonization of 22 constructed ponds and conducted a long-term study of changes in species richness and community composition in ten constructed and ten reference ponds over seven breeding seasons. During the first year of pond filling, species richness and the number of egg masses of the wood frog (Rana sylvatica) and spotted salamander (Ambystoma maculatum) were positively correlated with pond size, depth, and hydroperiod but independent of distance to the nearest forest, paved road, or source pond. The ten constructed ponds in the long-term study first filled in 1996 and were larger, deeper, warmer, more oxygen-rich, and of longer seasonal hydroperiod than reference ponds. Seven species bred in the constructed ponds during the first year of filling, and species richness reached equilibrium within two years of initial pond filling. Most species colonized constructed ponds rapidly, but frequency of use by eastern newts (Notophthalmus viridescens) increased slowly over five years. Constructed ponds supported significantly more species than reference ponds, and the annual turnover rate of breeding populations was approximately 25% for both pond types. Our data suggest that post-restoration monitoring for 2–3 years may be sufficient to characterize species and communities that will utilize ponds for the first decade or so after pond creation.

Scale‐dependence and mechanisms of dispersal in freshwater zooplankton

Communities of organisms form as a result of both interspecific and abiotic interactions within local habitat patches and dispersal among patches in a region. Local processes are expected to play a dominant role when dispersal occurs much more often than extinction. We performed two field experiments to examine rates and mechanisms of dispersal in freshwater pond zooplankton communities. First, we tested the effect of distance from a source on the rate of colonization of artificial habitat by placing wading pools at 5, 10, 30 and 60 m from two natural fishless ponds and observing the succession of zooplankton. Seventy‐eight percent of the species in the source ponds that were capable of living in the pools colonized at least once during the experiment. A new species was found in the pools on average once every four days, suggesting that colonization events occur on the order of days to weeks for many species. Colonization rates declined further from the source at one pond but not the other, and the effect of distance was relatively weak at both ponds. This suggests that many species disperse broadly over short distances. The second experiment tested the role of animal vectors for zooplankton dispersal by restricting access to the pools. Eight treatments were imposed that excluded potential animal vectors along a body size gradient from large mammals to small insects. While the treatments affected zooplankton colonization, many species invaded even when all animals larger than 1 mm were excluded. Animal vectors may therefore be less important for dispersal than wind. Our results suggest that zooplankton are highly effective dispersers over short distances, and can disperse via several mechanisms. Local interactions should therefore play a dominant role in structuring these communities at small regional scales.

Growth and Development of Larval Rana temporaria: Local Variation and Countergradient Selection

I raised tadpoles of the Common Frog (Rana temporaria) from populations in eight source ponds in southern Sweden, in a common garden experiment at two densities. Tadpoles from different populations differed in development rate; those from source ponds with high tadpole densities developed faster than those from less crowded ponds. Thus, differences among ponds in tadpole performance, which were documented in previous field studies, must have a genetic or maternal component. This result of source pond crowding likely resulted from microevolution and is an example of countergradient selection. In contrast, I found no significant effect of source pond hydroperiod; tadpoles from temporary ponds grew and developed at a rate similar to those from permanent ponds. Tadpoles of R. temporaria can respond plastically to pond drying by increasing development rate. I suggest adaptive plasticity in development rate decreased selection by pond drying in natural ponds.

Population dynamics in a Danish metapopulation of spadefoot toads Pelobates fuscus

If an amphibian species has a high degree of site fidelity to its natal pond, subpopulations do not mix, and this may have implications: the subpopulations may get different because of genetic drift, and their status may be affected due to stochastic events. During a four‐year period (1994–1997), demographic parameters of a Danish metapopulation consisting of ca 1000 adult Pelobates fuscus in five subpopulations were investigated with the aim of quantifying exchange of individuals in subpopulations and potential differences in demographic parameters, and to clarify population regulating mechanisms. This was done through the use of drift fences surrounding the five breeding ponds, individual marking (PIT‐tagging) and weight recordings of the adult females. Metamorphs were recorded when leaving the pond. There was no significant differences between ponds in sex ratio, eggs laid per female, fraction of females breeding and survival of eggs to froglets and of adults. However, there was a significant difference in juvenile survival (52% in one pond and 31% in the others). Sex ratio was 1.70 males:1 female. Mean number of eggs laid per female in the pond was 1762. The number of eggs laid was age‐specific, fecundity peaking at age five. Larval survival rate was density dependent: decreasing in an overcompensating fashion at high densities. Exchange rate between ponds was low; 1.09% of the marked and recaptured adult population was found to change pond during the study period. The low exchange rate was confirmed in a simple translocation experiment. This population shows features of a source‐sink metapopulation where the identity of the source pond changes due to succession. It is presumably regulated either in the larval stage by density dependent mechanisms (in the subpopulations with many adults) or in the adult stage by density independent mechanisms (in the subpopulations with few adults).

Early metamorphosis in common frog Rana temporaria tadpoles at risk of drying: an experimental demonstration

AbstractDoes the development rate of common frog tadpoles accelerate if their habitat dries? To study this, the water level in experimental tanks was reduced shortly before time of metamorphosis. Water level remained high in control tanks. The experiment was performed at two different tadpole densities and replicated four times, with tadpoles from different source ponds. The experimental treatment, simulating a drying pond, resulted in earlier metamorphosis while no significant difference in size at metamorphosis was found. Resources per capita decreased as a result of the decreased water level so the increase in development rate was not an effect of feeding conditions. Temperatures in the tanks were such that it is unlikely that the increased development rate was due to temperature effects. I interpret the advancement of metamorphosis as an adaptive response to the threat of drying. This response has been documented for several other anuran species. All those breed in temporary water bodies, supporting the hypothesis that the trait is an evolved adaptation for breeding in such waters.

Plasticity or fixed adaptive traits? Strategies for predation avoidance in Rana arvalis tadpoles.

Tadpoles of Rana arvalis originating from seven island populations were tested for responses to non-lethal predator presence. In general, tadpole growth was reduced and the relative tail depth was increased at predator presence. There was no effect of predator presence on the predicted size at metamorphosis. The differentiation rate, translating as length of the larval period, was lower at predator presence, but this seems to be merely an effect of the reduced growth. Although populations differed with respect to growth, relative tail length, relative tail depth, differentiation rate and predicted size at metamorphosis, no obvious differences were found in their responses to predator presence. Data on predator occurrences in the source ponds show that tadpoles originating from ponds with a high predation pressure have a higher differentiation rate, i.e. they will metamorphose at an earlier date than those from "safe" ponds (if raised under the same conditions). Moreover, they are also predicted to metamorphose at a smaller size, which is in accordance with theoretical models. Despite the fact that populations differed in growth, no correlation was found between growth and predation risk in the source ponds.

Predicting the Persistence of Amphibian Populations with the Help of a Spatial Model

We have used a stochastic population model with immigration to calculate extinction rates for two amphibian species: the common toad, Bufo bufo, and the crested newt, Triturus cristatus. Population models for amphibians are difficult to parameterize. It was not possible to measure all the relevant parameters for the model. The other parameters were obtained from other studies or estimated on the basis of similar species. Contour maps for persistence were generated using the model. The persistence of populations associated with a pond is affected both by the pond's internal dynamics and its proximity to a source pond. On a 20-generation time scale, toad ponds that were initially occupied were relatively unaffected by the proximity of a source. Persistence was almost certain ( > 95%) when the average carrying capacity of a pond was greater than 30 adult females. Initially unoccupied toad-ponds were equally likely to persist if they lay within 4 km of a source and could support more than 50 adult females. Initially occupied newt ponds were likely to persist if they supported more than 40 females or lay within 0.5 km of a typical source pond. Initially unoccupied newt ponds were mainly dependent upon immigration rates. Very small ponds (N k > 10) only needed to be within 1.5 km of a source pond. This model allows us to use the data available, to make predictions about the criteria which must be met by a landscape to ensure the survival of amphibian populations. It offers the possibility of better predictions when the data is refined. The model also suggests directions for further research such as the statistical study of environmental variation, the nature of minimum viable populations for toads and the statistics of long-range dispersal.