Matthew R. Pintar

Out with the Old, in with the New: Oviposition Preference Matches Larval Success in Cope's Gray Treefrog, Hyla chrysoscelis

Abstract Oviposition site selection by breeding amphibians determines larval habitat for their offspring. Many amphibians breed in temporary ponds, which vary in hydroperiod and levels of competition, predation, and resource abundance. Newly filled ponds have fewer competitors and predators and more periphyton and phytoplankton, which are more palatable food sources for anuran larvae. We tested for oviposition site preference between old and new water pools in Hyla chrysoscelis, a species known to have better larval performance in newly filled pools. Consistent with larval performance and the observation that anurans often breed in ponds immediately after they fill, H. chrysoscelis selectively oviposited in new pools. Conductivity was significantly lower and dissolved oxygen significantly higher in new versus old pools, and these may be cues of pond age and productivity, respectively. We demonstrate that adult oviposition site selection preferences in our study match larval performance differences seen in previous work and that this preference is not simply for newly filled ponds but for ponds with a recent influx of water. These results further suggest novel ways for land managers to increase amphibian populations. Draining ponds will increase their attractiveness to breeding females who simultaneously avoid fish and choose new water.

Tree leaf litter composition drives temporal variation in aquatic beetle colonization and assemblage structure in lentic systems

Tree leaf litter inputs to freshwater systems are a major resource and primary drivers of ecosystem processes and structure. Spatial variation in tree species distributions and forest composition control litter inputs across landscapes, but inputs to individual lentic habitat patches are determined by adjacent plant communities. In small, ephemeral, fishless ponds, resource quality and abundance can be the most important factor affecting habitat selection preferences of colonizing animals. We used a landscape of experimental mesocosms to assess how natural populations of aquatic beetles respond over time to variation in tree leaf litter composition (pine or hardwood). Patches with faster-decomposing hardwood leaf litter were initially colonized at higher rates than slower-decomposing pine pools by most species of Hydrophilidae, but this pattern reversed later in the experiment with higher colonization of pine pools by hydrophilids. Colonization did not differ between pine and hardwood for dytiscids and the small hydrophilid Paracymus, but there were distinct beetle assemblages between pine and hardwood patches both early and late in the experiment. Our data support the importance of patch quality and habitat selection as determinants of species abundances, richness, and community structure in freshwater aquatic systems, not only when new habitat patches are formed and initial conditions set, but as patches change due to interactions of processes such as decomposition with time.

Variation in Pond Hydroperiod Affects Larval Growth in Southern Leopard Frogs, Lithobates sphenocephalus

Size at metamorphosis is an important correlate of amphibian fitness. Several, often interrelated factors, such as hydroperiod, competition, and food availability, determine metamorph size. Hydroperiod is a defining characteristic of ponds, but pond drying is not always a deterministic process, and refilling can occur prior to complete drying. We conducted an experiment to test if a recurring four-week partial drying/filling cycle affected amphibian growth and size using a population of overwintering larval Lithobates sphenocephalus (Southern Leopard Frogs). Effects of the drying/filling cycle varied over time on larval mass, but this cycle had no overall effect on other measures of larval or metamorph size. Metamorph size covaried with both survival and larval period length: individuals were larger at metamorphosis when survival was lower and among individuals with longer larval periods. We observed the first reported instance of prolonged larval periods across multiple winters in L. sphenocephalus, wherein some of our larvae exposed to the drying/filling cycle required 18 months to reach metamorphosis. Individuals that overwintered as larvae for a second winter were larger at metamorphosis than those that metamorphosed at the end of the previous summer. Lithobates sphenocephalus maximize metamorphic size rather than optimize metamorphic timing, and metamorph size can also be affected by larval survival.

Prey‐driven control of predator assemblages: zooplankton abundance drives aquatic beetle colonization

Trophic interactions are critical determinants of community structure and ecosystem function. In freshwater habitats, top predators are traditionally viewed as drivers of ecosystem structure, shaping populations of consumers and primary producers. The temporary nature of small water bodies makes them dependent on colonization by many organisms, particularly insects that form highly diverse predator assemblages. We conducted mesocosm experiments with naturally colonizing populations of aquatic beetles to assess how prey (zooplankton) abundances influenced colonization and assemblages of natural populations of aquatic beetles. We experimentally demonstrate that zooplankton populations can be proximate regulators of predator populations and assemblages via prey-density-dependent predator recruitment. Our results provide support for the importance of prey populations in structuring predator populations and the role of habitat selection in structuring communities. We indicate that traditional views of predators as drivers of ecosystem structure in many systems may not provide a comprehensive picture, particularly in the context of highly disturbed or ephemeral habitats.

Colonization across gradients of risk and reward: Nutrients and predators generate species‐specific responses among aquatic insects

Handling Editor: Colleen Seymour Abstract 1. Predation risk and resource abundance are two primary characteristics that determine species abundances and community composition. Colonizing organisms should attempt to minimize the risk of mortality and maximize growth through selection of patches with the highest expected fitness. However, maximizing fitness across multiple gradients of patch quality involves accurate cue assessment, integration and behavioural responses that consider multiple factors that affect fitness simultaneously. 2. Our goal was to simultaneously and factorially assess the effects of predation risk and resource abundance among an assemblage of aquatic insects to determine the relative importance of each factor, and whether the two factors interact to affect colonization, oviposition and community assembly. 3. We conducted a field mesocosm experiment in which we crossed predator density (0, 1, 2 fish, Fundulus chrysotus) with supplemental nutrient abundance (0, 4, 8 g rabbit chow) in a 3 × 3 factorial design. We then assayed colonization by natural populations of aquatic beetles and oviposition by natural populations of Culex mosquitoes. 4. We observed species-specific responses, with many species avoiding fish and some selecting habitats with more nutrients. Nutrients and predator presence only interactively affected oviposition by Culex mosquitoes, and the effect of fish presence exceeded that of nutrients in all but one analysis. 5. Our results illustrate the primacy of predation risk in generating colonization patterns and structuring communities in aquatic habitats, but that colonization responses to variation in multiple components of patch quality are often species-specific. Simultaneous assessments of multiple aspects of patch quality allow for the determination of potential interactions among cue sources and the relative importance of various patch characteristics to colonizers.

Relative predation risk and risk of desiccation co-determine oviposition preferences in Cope’s gray treefrog, Hyla chrysoscelis

Habitat permanence and threat of predation are primary drivers of community assembly and composition in lentic freshwater systems. Pond-breeding amphibians select oviposition sites to maximize fitness and minimize risks of predation and desiccation of their offspring, typically facing a trade-off between the two as predation risk often increases as desiccation risk decreases. To experimentally determine if Hyla chrysoscelis partition oviposition along gradients of relative desiccation risk and predation risk, we tested oviposition site preference in a natural population of treefrogs colonizing experimental ponds that varied in water depth and contained predatory larvae of two Ambystoma salamander species. Hyla chrysoscelis selected habitats with both lower predation risk, avoiding A. talpoideum over A. maculatum, and lower desiccation risk, selecting ponds with three times greater depth. We demonstrate that adult oviposition site choices simultaneously minimize relative predation risk and desiccation risk and that closely related salamander species produce functionally different responses among colonizing animals.

Larval Development Varies Across Pond Age and Larval Density in Cope's Gray Treefrogs, Hyla chrysoscelis

Abstract Ovipositing amphibians select breeding sites that determine their offsprings larval habitat. Preference–performance theory predicts that adults will select habitat patches that match the expected performance of their offspring in those patches. Many amphibians breed in temporary ponds immediately after they fill, and one species, Hyla chrysoscelis (Copes Gray Treefrogs), selects between breeding sites based on patch age differences of as little as 7 d. Prior work established that H. chrysoscelis have better larval performance in more recently filled ponds, but not in ponds less than 15 d after filling or among ponds that differed in age by less than 65 d. Our objective was to determine experimentally whether larval H. chrysoscelis performance varies across short time scales after filling (3- and 23-d-old ponds) and across two larval densities (15 and 30 larvae per mesocosm). Higher larval densities reduced metamorph mass and body condition, but did not otherwise affect larval performance. There were lower survival, shorter larval periods, and slightly lower mass and body condition among larvae from 3-d-old ponds than those in 23-d-old ponds. These mixed performance differences across our two levels of habitat age might not necessarily be representative of what happens in natural systems, however, where older ponds have more established communities of competitors and predators.