Kimberly J. Babbitt

Influence of Habitat Complexity on Predator-Prey Interactions between the Fish (Gambusia holbrooki) and Tadpoles of Hyla squirella and Gastrophryne carolinensis

Abstract We examined the role of habitat complexity in influencing predator–prey interactions between fish and tadpoles. Tadpoles of the Squirrel Treefrog (Hyla squirella) and the Eastern Narrow-Mouth Toad (Gastrophryne carolinensis) were exposed to Eastern Mosquitofish (Gambusia holbrooki) under different degrees of habitat complexity (no cover, low cover, and high cover) in a randomized block, replicated, controlled experiment using wading pools. This study indicates that Gambusia can quickly and dramatically impact tadpole populations even at low predator densities and can forage effectively in vegetated areas that might normally serve as prey refugia from larger predatory fish (e.g., Lepomis spp.). Moreover, the influence of habitat complexity on predator-prey interactions may be species-specific. The number of H. squirella tadpoles consumed was not affected significantly by the degree of habitat complexity; however, consumption of G. carolinensis by Gambusia decreased with increasing habitat complexity. We attribute this finding to the observation that G. carolinensis are less active than H. squirella and, therefore, more difficult for Gambusia to detect with increasing habitat complexity.

The relative impacts of native and introduced predatory fish on a temporary wetland tadpole assemblage

Understanding the relative impacts of predators on prey may improve the ability to predict the effects of predator composition changes on prey assemblages. We experimentally examined the relative impact of native and introduced predatory fish on a temporary wetland amphibian assemblage to determine whether these predators exert distinct (unique or non-substitutable) or equivalent (similar) impacts on prey. Predatory fish included the eastern mosquitofish (Gambusia holbrooki), golden topminnow (Fundulus chrysotus), flagfish (Jordanella floridae), and the introduced walking catfish (Clarias batrachus). The tadpole assemblage included four common species known to co-occur in temporary wetlands in south-central Florida, USA: the oak toad (Bufo quercicus), pinewoods treefrog (Hyla femoralis), squirrel treefrog (Hyla squirella), and eastern narrowmouth toad (Gastrophryne carolinensis). Tadpoles were exposed to different predators in wading pools under conditions similar to those found in surrounding temporary wetlands (particularly in terms of substrate type, the degree of habitat complexity, and temperature). Native predators were similar with respect to predation rate and prey selectivity, suggesting similar energy requirements and foraging behavior. Conversely, native fish predators, especially G. holbrooki, were distinct from the introduced C. batrachus. In contrast to expectations, C. batrachus were less voracious predators than native fish, particularly G. holbrooki. Moreover, survival of G. carolinensis and H. femoralis were higher in the presence of C. batrachus than G. holbrooki. We suggest that C. batrachus was a less efficient predator than native fish because the foraging behavior of this species resulted in low predator-prey encounter rates, and thus predation rate. In combination with a related field study, our results suggest that native predatory fish play a stronger role than C. batrachus in influencing the spatial distribution and abundance of temporary wetland amphibians in the landscape.

Predator induced phenotypic plasticity in the pinewoods tree frog, Hyla femoralis: necessary cues and the cost of development

Predator-induced defenses can result from non-contact cues associated with the presence of a feeding predator; however, the nature of the predator cue has not been determined. We tested the role of two non-contact cues, metabolites of digestion of conspecific prey released by the predator and alarm pheromones released by attacked conspecific prey, in the development of inducible defenses by exposing pinewoods tree frog (Hyla femoralis) tadpoles to non-lethal dragonfly (Anax junius) larvae fed either inside experimental bins or removed from the bins for feeding to eliminate alarm pheromones. The costs associated with the development of the induced morphology were also investigated by providing the tadpoles with two food levels intended to provide adequate or growth limiting resources. The generalized morphological response of H. femoralis tadpoles to predators included the development of bodies and tails that were both deeper and shorter, smaller overall body size, and increased orange tail fin coloration and black tail outline. Metabolites of digestion were sufficient to initiate development of inducible defenses; however, the combination of metabolites and alarm cue resulted in a greater response. Furthermore, growth and development were slowed in tadpoles that expressed the induced morphology; however, this growth cost was insufficient to preclude the development of the induced morphology when food resources were low. These results indicate that two aspects of the indirect predator cue work together to trigger a morphological anti-predator response.

Effects of biotic and abiotic factors on the distribution and abundance of larval two-lined salamanders (Eurycea bislineata) across spatial scales

Abstract. We sampled eight streams in the White Mountain National Forest, New Hampshire, throughout their elevational reach for larval salamanders and predatory fish to examine the effects of abiotic factors and predation on the distribution and abundance of larval salamanders. Eurycea bislineata (two-lined salamander) and Salvelinus fontinalis (brook trout) abundance varied among and within streams. Eurycea bislineata showed a negative association with S. fontinalis across spatial scales (micro-scale, among quadrats; meso-scale, among pool/riffle pairs; macro-scale, among streams). At the smallest scale, the average density of larval E. bislineata was greatest in microhabitats with relatively high boulder cover and low sand and bare rock cover only in the presence of S. fontinalis; no such relationship was observed in the absence of S. fontinalis. In a mesocosm experiment, larval salamander survival was higher in enclosures containing cobbles than enclosures containing a gravel mix, illustrating the advantage of coarse substrates with interstitial spaces that are inaccessible to predatory fish. At the meso-scale, E. bislineata larvae were less abundant in stream sections with S. fontinalis than those without. Among streams, those with many S. fontinalis had fewer E. bislineata. Of the abiotic parameters measured, water temperature and pH were positively related to E. bislineata presence, and elevation, water temperature, pH, canopy cover, and gradient were positively related to E. bislineata abundance. Larval Plethodontid salamanders can reach high densities and appear to have strong interactions with stream biota, thus their functional role in stream communities deserves further attention.

Benefits of Eating Conspecifics: Effects of Background Diet on Survival and Metamorphosis in the Cuban Treefrog (Osteopilus septentrionalis)

Abstract Cannibalism provides a potential mechanism for enhanced growth and survival for species with complex lifecycles developing in habitats of unpredictable duration. We asked whether background resource quality affected survival and metamorphosis of Cuban Treefrog (Osteopilus septentrionalis) tadpoles fed conspecifics. In a factorial experiment, we crossed two levels of background resources, a high-quality (commercial rabbit chow) or low-quality (algae and periphyton) diet with no conspecifics, the presence of a dead conspecific (in a bag), or ad libitum availability of dead conspecifics. Survival of tadpoles did not differ significantly among treatments. Availability of conspecifics resulted in larger mass and decreased larval periods for tadpoles on low-quality background treatments; however, for tadpoles on high-quality background treatments, availability of conspecifics resulted in prolonged larval periods. These results suggest that profitability of feeding on conspecifics is dependent on background resource level and may partially explain why cannibalism is facultative and associated with low availability of alternative food or crowded conditions in most larval anurans.

Influence of agricultural upland habitat type on larval anuran assemblages in seasonally inundated wetlands.

Conservation initiatives in agricultural landscapes play an increasingly important role in ensuring the long-term persistence of amphibian biodiversity because native habitats continue to be lost to urban and commercial development. We examined larval anuran structure within seasonally inundated wetlands in four upland habitat types of southcentral Florida differing in degree of upland habitat modification for cattle ranching: native prairie at the Kissimmee Prairie Sanctuary (KPS); and improved pasture, rangeland (semi-native prairie), and woodland habitats at the MacArthur Agro-Ecology Research Center (MAERC). We sampled 24 wetlands monthly for tadpoles using throw-traps and dipnets during the 1999 wet season (June–October), recording 10,126 tadpoles of 10 species. Species richness, mean total abundance, and species abundances differed among upland habitat types. The most heavily modified habitat (pasture) had both species-poor assemblages and low tadpole abundances. Species richness in woodland, rangeland, and native prairie wetlands were similar, but woodland wetlands had higher tadpole abundances. Wetlands in the four habitats differed in the amount of nearby woodland and wetland habitats, length of hydroperiod, and percent cover of aquatic vegetation, which likely contributed to the observed amphibian richness and abundance patterns. Cattle ranches in Florida that retain a large proportion of woodland, rangeland, and temporary wetlands in the landscape are likely to contribute significantly to amphibian conservation initiatives on protected native lands.

Fine-Scale Population Structure and Asymmetrical Dispersal in an Obligate Salt-Marsh Passerine, the Saltmarsh Sparrow (Ammodramus caudacutus)

ABSTRACT. Understanding the spatial scale of gene flow can yield valuable insight into the ecology of an organism and guide conservation strategies. Fine-scale genetic structure is uncommon in migratory passerines because of their high vagility and presumed high dispersal abilities. Aspects of the behavior and ecology of some migratory species, however, may promote structure on a finer scale in comparison to their mobility. We investigated population genetic structure in the Saltmarsh Sparrow (Ammodramus caudacutus), a migratory passerine that breeds along the northeastern coast of the United States, where it is restricted exclusively to a narrow strip of patchily distributed tidal marsh habitat. Using genotyping with 10 microsatellite loci, we detected weak but significant population structure among Saltmarsh Sparrows from nine marshes on the breeding grounds between Scarborough, Maine, and Oceanside, New York. Genetic variation among marshes was largely consistent with a pattern of isolation by distance, with some exceptions. One inland marsh was genetically divergent despite its proximity to other sampled marshes, which suggests that mechanisms besides geographic distance influence population genetic structure. Bayesian clustering, multivariate analyses, and assignment tests supported a population structure consisting of five groups. Estimates of migration rates indicated variation in gene flow among marshes, which suggests asymmetrical dispersal and possible source-sink population dynamics. The genetic structure that we found in Saltmarsh Sparrows may result from natal philopatry and breeding-site fidelity, combined with restricted dispersal due to obligate dependence on a patchy habitat. Our findings suggest that fine-scale population structure may be important in some migratory passerines.

Effects of Red-Backed Salamanders on Ecosystem Functions

Ecosystems provide a vast array of services for human societies, but understanding how various organisms contribute to the functions that maintain these services remains an important ecological challenge. Predators can affect ecosystem functions through a combination of top-down trophic cascades and bottom-up effects on nutrient dynamics. As the most abundant vertebrate predator in many eastern US forests, woodland salamanders (Plethodon spp.) likely affect ecosystems functions. We examined the effects of red-backed salamanders (Plethodon cinereus) on a variety of forest ecosystem functions using a combined approach of large-scale salamander removals (314-m2 plots) and small-scale enclosures (2 m2) where we explicitly manipulated salamander density (0, 0.5, 1, 2, 4 m−2). In these experiments, we measured the rates of litter and wood decomposition, potential nitrogen mineralization and nitrification rates, acorn germination, and foliar insect damage on red oak seedlings. Across both experimental venues, we found no significant effect of red-backed salamanders on any of the ecosystem functions. We also found no effect of salamanders on intraguild predator abundance (carabid beetles, centipedes, spiders). Our study adds to the already conflicting evidence on effects of red-backed salamander and other amphibians on terrestrial ecosystem functions. It appears likely that the impact of terrestrial amphibians on ecosystem functions is context dependent. Future research would benefit from explicitly examining terrestrial amphibian effects on ecosystem functions under a variety of environmental conditions and in different forest types.

Genetic Barcode RFLP Analysis of the Nelson's and Saltmarsh Sparrow Hybrid Zone

Abstract Hybridization between Saltmarsh Sparrow (Ammodramus caudacutus) and Nelsons Sparrow (A. nelsoni) has been documented in areas where the two species occur sympatrically, increasing the difficulty of accurate species identification. We developed a DNA barcoding restriction fragment length polymorphism (RFLP) test to discriminate between Nelsons Sparrows and Saltmarsh Sparrows and applied it to 426 putative Saltmarsh Sparrows sampled from Maine to New York, USA. All individuals were identified in the field as Saltmarsh Sparrows based on morphology, but 34 (8%) had Nelsons specific mitochondrial DNA, indicating they were of hybrid origin. This discrepancy in morphological and genetic data highlights the difficulties associated with accurate field identification and may hinder conservation efforts by confounding attempts to identify and monitor “pure” populations. Mitochondrial DNA of Nelsons Sparrow was prevalent at the most southern point of the previously documented overlap zone and was also found in one individual 150 km south of the overlap zone. Our findings offer new insights into the extent of hybridization between the two species and underscore the need for further investigation into the consequences of hybridization on conservation of Saltmarsh Sparrows.

Contrasting Patterns of Habitat Use by Prawns and Crayfish in a Headwater Marsh of the St. Johns River, Florida

Abstract We compared seasonal patterns of habitat use by the prawn Palaemonetes paludosus and the crayfish Procambarus alleni in Blue Cypress Marsh Conservation Area, Florida. Prawn densities were similar to those found in other oligotrophic wetlands of southern Florida, whereas crayfish densities were much greater than reported previously for other wetlands in the area. Prawns and crayfish had strikingly different patterns of habitat use. Prawn density and biomass were similar in wet prairies and sloughs, whereas crayfish density and biomass were significantly higher in wet prairies. Within habitats, the abundance of prawns and crayfish generally increased with increasing structural complexity and the abundance of crayfish generally decreased with increasing water depth. Differences in risk of predation, frequency of agonistic encounters, food availability, and other factors likely contributed to observed patterns of habitat use. Because of differences in their ability to burrow and avoid concentration into dry-season refugia, prawns and crayfish responded very differently to seasonal variation in hydrologic conditions. Prawn densities were initially low (following a severe drought) and then increased during much of the study period, whereas crayfish densities were relatively stable throughout the study period. Overall, it appears that prawns are more responsive to antecedent hydrologic conditions and crayfish are more responsive to the availability of suitable habitats such as wet prairies.