Brittany H. Ousterhout

Intermediate Pond Sizes Contain the Highest Density, Richness, and Diversity of Pond-Breeding Amphibians

We present data on amphibian density, species richness, and diversity from a 7140-ha area consisting of 200 ponds in the Midwestern U.S. that represents most of the possible lentic aquatic breeding habitats common in this region. Our study includes all possible breeding sites with natural and anthropogenic disturbance processes that can be missing from studies where sampling intensity is low, sample area is small, or partial disturbance gradients are sampled. We tested whether pond area was a significant predictor of density, species richness, and diversity of amphibians and if values peaked at intermediate pond areas. We found that in all cases a quadratic model fit our data significantly better than a linear model. Because small ponds have a high probability of pond drying and large ponds have a high probability of fish colonization and accumulation of invertebrate predators, drying and predation may be two mechanisms driving the peak of density and diversity towards intermediate values of pond size. We also found that not all intermediate sized ponds produced many larvae; in fact, some had low amphibian density, richness, and diversity. Further analyses of the subset of ponds represented in the peak of the area distribution showed that fish, hydroperiod, invertebrate density, and canopy are additional factors that drive density, richness and diversity of ponds up or down, when extremely small or large ponds are eliminated. Our results indicate that fishless ponds at intermediate sizes are more diverse, produce more larvae, and have greater potential to recruit juveniles into adult populations of most species sampled. Further, hylid and chorus frogs are found predictably more often in ephemeral ponds whereas bullfrogs, green frogs, and cricket frogs are found most often in permanent ponds with fish. Our data increase understanding of what factors structure and maintain amphibian diversity across large landscapes.

Life history differences influence the impacts of drought on two pond-breeding salamanders

Drought is a strong density-independent environmental filter that contributes to population regulation and other ecological processes. Not all species respond similarly to drought, and the overall impacts can vary depending on life histories. Such differences can necessitate management strategies that incorporate information on individual species to maximize conservation success. We report the effects of a short-term drought on occupancy and reproductive success of two pond-breeding salamanders that differ in breeding phenology (fall vs. spring breeder) across an active military base landscape in Missouri, USA: We surveyed ~200 ponds for the presence of eggs, larvae, and metamorphs from 2011 to 2013. This period coincided with before, during, and after a severe drought that occurred in 2012. The two species showed contrasting responses to drought, where high reproductive failure (34% of ponds) was observed for the spring breeder during a single drought year. Alternatively, the fall breeder only showed a cumulative 8% failure over two years. The number of breeding ponds available for use in the fall decreased during the drought due to pond drying and/or a lack of re-filling. Estimates of occupancy probability declined for the fall-breeding salamander between 2012 and 2013, whereas occupancy probability estimates of the spring breeder increased post-drought. The presence of fish, hydroperiod, the amount of forest cover surrounding ponds, and canopy cover were all found to affect estimates of occupancy probabilities of each species. Pond clustering (distance to nearest pond and the number of ponds within close proximity), hydroperiod, forest cover, and canopy cover influenced both estimates of colonization and extinction probabilities. Our results show life history variation can be important in determining the relative susceptibility of a species to drought conditions, and that sympatric species experiencing the same environmental conditions can respond differently. Consideration of the spatial network and configuration of habitat patches that act as refuges under extreme environmental conditions will improve conservation efforts, such as the placement of permanent ponds for aquatic organisms. A better awareness of species-specific tolerances to environmental filters such as drought can lead to improved management recommendations to conserve and promote habitat for a greater diversity of species across landscapes of spatially connected populations.

Structure and Dynamics of Ringed Salamander (Ambystoma annulatum) Populations in Missouri

Abstract:  We report a detailed account of the structure and dynamics of five populations of Ringed Salamanders (Ambystoma annulatum Cope, 1886) in Missouri, USA, studied over a 4-yr period. The average date of capture for breeding males varied from 30 August to 2 October, and average date of capture for breeding females varied from 9 September to 7 October. The sex ratio was consistently male biased and averaged 2.05 males to each female. The smallest breeding female was 77 mm snout–vent length ([SVL]; mean = 96.3 mm) and the smallest breeding male was 66 mm SVL (mean = 94.2 mm). Eggs and larvae were present in September and October each year, and the larval period varied from 7 to 9 mo. Juveniles metamorphosed in all years in every pond, albeit in low numbers (6–631). The mean SVL of metamorphosing juveniles among all years and ponds was 39.2 mm. Intra-annual body size of emerging juveniles declined over time; juveniles that metamorphosed early were larger in body size than those metamorphosing later in the summer. Production of juveniles per female averaged 0.76 (range, 0.056–2.929), and survival of juveniles from egg to metamorphosis averaged 0.2% (range, 0.01%–0.75%). Both females and males can reach reproductive maturity at 1 yr of age, but most return at 2–3 yr of age. Management of the Ringed Salamander must focus on all life-history stages; protecting the aquatic stage to maximize the number and fitness of metamorphosing juveniles, managing the terrestrial habitat to maximize survival to first reproduction, and monitoring connectivity to promote metapopulation dynamics.

Habitat traits and species interactions differentially affect abundance and body size in pond-breeding amphibians.

In recent studies, habitat traits have emerged as stronger predictors of species occupancy, abundance, richness and diversity than competition. However, in many cases, it remains unclear whether habitat also mediates processes more subtle than competitive exclusion, such as growth, or whether intra- and interspecific interactions among individuals of different species may be better predictors of size. To test whether habitat traits are a stronger predictor of abundance and body size than intra- and interspecific interactions, we measured the density and body size of three species of larval salamanders in 192 ponds across a landscape. We found that the density of larvae was best predicted by models that included habitat features, while models incorporating interactions among individuals of different species best explained the body size of larvae. Additionally, we found a positive relationship between focal species density and congener density, while focal species body size was negatively related to congener density. We posit that salamander larvae may not experience competitive exclusion and thus reduced densities, but instead compensate for increased competition behaviourally (e.g. reduced foraging), resulting in decreased growth. The discrepancy between larval density and body size, a strong predictor of fitness in this system, also highlights a potential shortcoming in using density or abundance as a metric of habitat quality or population health.

No evidence of natal habitat preference induction in juveniles with complex life histories

Individual variation in physiology and behaviour can have strong effects on ecological and evolutionary processes. Natal habitat, one source of individual variation, can influence individual phenotype, behaviour and fitness through effects on eventual habitat selection. Natal habitat preference induction occurs when individuals match stimuli in their selected habitat to those of their natal habitat. Natal habitat can also affect habitat selection through its influence on body condition (silver spoon effect). We tested for natal habitat preference induction and body-condition-dependent habitat selection in two species with complex life histories, the spotted salamander, Ambystoma maculatum, and the small-mouthed salamander, Ambystoma texanum. We reared salamanders from hatchlings in mesocosms with leaf, grass or control substrate, and tested juvenile habitat selection through two behavioural assays. We found weak evidence of larger salamanders having decreased latency and sampling more habitats, lending support to the body-condition-dependent habitat selection hypothesis in these species. Juveniles preferred grass litter cues regardless of the substrate in their natal mesocosm, suggesting natal habitat preference induction may not occur in species with complex life histories. We propose that species with complex life histories use simple movement rules, such as moving along habitat gradients, to select postnatal habitat when moving through a novel environment.

Non-additive response of larval ringed salamanders to intraspecific density

Conditions experienced in early developmental stages can have long-term consequences for individual fitness. High intraspecific density during the natal period can affect juvenile and eventually adult growth rates, metabolism, immune function, survival, and fecundity. Despite the important ecological and evolutionary effects of early developmental density, the form of the relationship between natal density and resulting juvenile phenotype is poorly understood. To test competing hypotheses explaining responses to intraspecific density, we experimentally manipulated the initial larval density of ringed salamanders (Ambystoma annulatum), a pond-breeding amphibian, over 11 densities. We modeled the functional form of the relationship between natal density and juvenile traits, and compared the relative support for the various hypotheses based on their goodness of fit. These functional form models were then used to parameterize a simple simulation model of population growth. Our data support non-additive density dependence and presents an alternate hypothesis to additive density dependence, self-thinning and Allee effects in larval amphibians. We posit that ringed salamander larvae may be under selective pressure for tolerance to high density and increased efficiency in resource utilization. Additionally, we demonstrate that models of population dynamics are sensitive to assumptions of the functional form of density dependence.

Differences in Larval Allometry among Three Ambystomatid Salamanders

Abstract Ecological and evolutionary processes commonly result in morphological variation among larval amphibians. Variation in head shape plays a critical role in both food capture and predation risk in gape-limited salamanders, yet in situ studies of head shape variability are rare outside of cannibal morph assessments. We examined allometry differences in larval head width (HW) and snout–vent length (SVL) among three sympatric species of ambystomatid salamanders from 166 ponds in Missouri, USA: Ringed Salamander (Ambystoma annulatum), Marbled Salamander (A. opacum), and Spotted Salamander (A. maculatum). We also tested whether several abiotic and biotic factors would predict HW after accounting for SVL. We found that larval HW and SVL were strongly correlated for all species but that the strength of this relationship varied among species. For early-stage larvae, Marbled Salamanders showed isometric scaling relationships, whereas both Spotted Salamanders and Ringed Salamanders were allometric. For late-stage larvae, all three species showed allometric patterns. At a small SVL, HW of small Ringed Salamanders was greater than the other species. As larvae increased in SVL, Marbled Salamander HW increased most rapidly and eventually exceeded both Ringed Salamanders and Spotted Salamanders of a similar size. We also found that both abiotic and biotic factors predicted significant differences in HW corrected for SVL among species, including predator density, competitor density, and hydroperiod. Overall, variability in scaling relationships may provide ecological advantages to each species at different points in ontogeny and different biotic and abiotic factors may induce such variation in asymmetric ways among species.

Agonistic Behavior and Resource Defense among Sympatric Juvenile Pond-Breeding Salamanders

Abstract Interference competition is frequently observed in sexually mature adults as they defend breeding territories. However, it remains unclear in many taxa whether juveniles respond aggressively to other juveniles or if they defend resources. To test whether postmetamorphic juveniles of a pond-breeding amphibian were aggressive towards other juveniles or were defending resources, we staged terrestrial encounters between three species of sympatric Ambystoma salamanders. We observed biting and other aggressive behaviors by juvenile Spotted (Ambystoma maculatum) and Marbled Salamanders (Ambystoma opacum). However, we did not observe aggressive behaviors by Ringed Salamanders (Ambystoma annulatum). In addition to species-level variations in aggression, these three species also differed in whether aggression was targeted primarily intra- or interspecifically. This study suggests that juveniles of pond-breeding amphibians of some species may defend essential habitat with agonistic behavior.

Pond-Breeding Amphibian Community Composition in Missouri

Abstract We examined pond-breeding amphibian community composition at 210 ponds in Missouri between 2002 and 2012 using drift fence, dipnet, and funnel trap data. We encountered a total of 20 pond-breeding amphibian species in the combined surveys. We also examined whether the presence of American Bullfrogs, Lithobates catesbeianus, and fish influenced these patterns of diversity. Our results indicate the presence of American Bullfrogs, fish, and their interaction influenced the community composition of amphibians at these sites but in opposite patterns. American Bullfrogs often had a positive relationship with the total number of species, total caudate species, and total anuran species, whereas fish presence was negatively associated overall with species diversity, and the presence of both American Bullfrogs and fish was negatively associated with anuran species diversity. It is important to have baseline community species composition data from wide geographical ranges so spatiotemporal changes in community structure can be noted and assessed.