William J. Resetarits

CHOICE OF OVIPOSITION SITE BY HYLA CHRYSOSCELIS: ROLE OF PREDATORS AND COMPETITORS'

The role of predators and competitors in the choice of oviposition site by the treefrog Hyla chrysoscelis was examined in a randomized complete block experiment using 90 replicated experimental ponds. Control ponds containing neither predators nor competitors were contrasted with treatment ponds into which one of four species of predators (Ambystoma maculatum larvae, Enneacanthus chaetodon adults, Notophthalmus viridescens adults, Tramea carolina larvae) or one of two species of competitors (Rana catesbeiana, Hyla chrysoscelis) was added. Treatments had significant effects on the mean number of eggs deposited in ponds. Fewer eggs were laid in ponds with Ambystoma, Enneacanthus, or Hyla, as a result of fewer females laying eggs and fewer eggs laid per visit, compared with control ponds. Notophthalmus, Rana, and Tramea had no effect on the number of eggs laid. Ovipositing Hyla discriminated among potential oviposition sites based on the species present. Choice of oviposition site can determine the success of a females reproductive investment, and it can be a mechanism affecting the structure of ecological communities as well. Our results emphasize the importance of oviposition site choice in the evolution of reproductive patterns and implicate species avoidance by ovipositing females as a mechanism important in generating variability in ecological communities.

PREDATOR IDENTITY AND ECOLOGICAL IMPACTS: FUNCTIONAL REDUNDANCY OR FUNCTIONAL DIVERSITY?

Different species occupy similar trophic positions in natural communities. However, ecologists have often overlooked the consequences of this variation for local communities by assuming that species occupying similar trophic positions are functionally similar. There have been few experimental tests of this important assumption. We tested the assumption of functional similarity by comparing the effect of six different predators (three fish and three salamander species) on an ensemble of larval anuran prey. Our experiment identified substantial variation in the impact of different predators on a variety of responses. Differences among predators in their selection of prey caused the structure of the larval anuran ensemble to vary continuously as opposed to producing discrete alternative states. Predators also differed in their ability to suppress either the total number or biomass of anurans. Thus, performance of larval anurans was dependent upon the identity of the predator. Moreover, the identity of predators judged similar and their degree of similarity depends on the specific response variable. In our study we found no predominance of weak or strong interactors, as predators appear to be evenly distributed along a gradient of interaction strengths. Knowledge of the identity of the species occupying a particular trophic position is crucial to predicting the impact of that trophic position on a community. Ecologists should not simply assume that species are functionally similar. The differing effects of different predators suggest that variation in their distribution across complex landscapes influences prey number, biomass, population dynamics, distribution, and ultimately local and regional species diversity.

LARVAL PERFORMANCE AND OVIPOSITION SITE PREFERENCE ALONG A PREDATION GRADIENT

The ability of females to assess habitat quality for offspring can strongly influence individual reproductive success as well as population dynamics and community assembly. However, the specific relationship between oviposition site selection and larval performance is often unclear. Ovipositing females of several species of treefrogs (Hyla) avoid predatory fish. The fish density required to elicit such avoidance and its relationship to larval performance are central to our understanding of how behavior can drive population dynamics and community assembly in complex landscapes. We conducted experiments investigating both oviposition preference and larval performance in pinewoods treefrogs (Hyla femoralis) along a density gradient of predatory fish (0-6 Umbra pygmaea). Female H. femoralis detected and strongly avoided the nonlethal presence of even a single 2-g fish, ovipositing almost exclusively in predator-free controls. In a separate experiment, larval performance largely matched adult preferences; larvae were totally eliminated in all but the lowest fish density. Given a landscape of breeding sites of varying risks, ovipositing H. femoralis behaviorally partition available habitats into those with and without fish, largely matching the associated fitness consequences.

Habitat selection behaviour links local and regional scales in aquatic systems

The role of habitat selection behaviour in the assembly of natural communities is an increasingly important theme in ecology. At the same time, ecologists and conservation biologists are keenly interested in scale and how processes at scales from local to regional interact to determine species distributions and patterns of biodiversity. How important is habitat selection in generating observed patterns of distribution and diversity at multiple spatial scales? In theory, habitat selection in response to interacting species can generate both positive and negative covariances among species distributions and create the potential to link processes of community assembly across multiple scales. Here I demonstrate that habitat selection by treefrogs in response to the distribution of fish predators functions at both the regional scale among localities and the local scale among patches within localities, implicating habitat selection as a critical link between local communities and the regional dynamics of metacommunities in complex landscapes.

ECOLOGICAL INTERACTIONS AMONG PREDATORS IN EXPERIMENTAL STREAM COMMUNITIES

Field observations on the effect of brook trout on an assemblage (sometimes described as a guild) of predatory salamanders suggest that the interactions among predators in small, headwater streams may be important in determining the structure of this assem- blage of predators and thereby, the predation pressure felt by species at lower trophic levels. The interactions among predators and the potential role of those interactions in headwater stream communities was examined in 16 replicated experimental streams. I examined the ecological interactions among four species of co-occurring predators, adult Salvelinusfon- tinalis (98-122 mm standard length (SL)), larval Gyrinophilus porphyriticus (36-58 mm snout-vent length (SVL)), larval Eurycea bislineata (13-21 mm SVL), and Cambarus bartondi (6-17 mm carapace length (CL)), in an experiment that began in mid-June and ran for 3 mo. Brook trout affected both survival and growth of the larger salamander species, Gyrinophilus, and Salvelinus and Gyrinophilus affected the growth of the smaller salamander Eurycea and the crayfish Cambarus. The presence of Gyrinophilus had no effect on relative condition or fecundity of Salvelinus. Larger predators caused Cambarus and Eurycea to alter their activity levels and habitat, but Gyrinophilus made no adjustments to the presence of Salvelinus. Results demonstrate that Gyrinophilus is vulnerable to pre- dation/aggression from brook trout, but that Eur cea and Cambarus are able to behaviorally avoid predation by both large predators at a significant cost to growth. The effect of brook trout on the two species of salamander in the experimental streams parallels the observed differences between salamanders in trout and trout-free waters in the field. Thus, inter- actions among predators in headwater streams directly and indirectly influence population dynamics of the interacting species by affecting survival, growth, behavior, and habitat use. The interactions among these predators can determine, in part, the predation envi- ronment experienced by individuals at lower trophic levels.

CALLING SITE CHOICE BY HYLA CHRYSOSCELIS: EFFECT OF PREDATORS, COMPETITORS, AND OVIPOSITION SITES'

We examined the effect of predators, competitors, and conspecifics on the choice of calling sites by male gray treefrogs, Hyla chrysoscelis, and the correlations between choice of calling sites by males and choice of oviposition sites by females. The experiment was conducted in an array of wading pools and utilized a replicated block design for variance analysis. Pools were open to natural colonization by male and female treefrogs. The treat- ments consisted of controls containing no predators or competitors, four treatments con- taining one species of predator (either Ambystoma maculatum larvae, Enneacanthus chae- todon adults, Notophthalmus viridescens adults, or Tramea carolina larvae), and two treatments containing one species of competitor (either Rana catesbeiana or Hyla chryso- scelis). Male and female treefrogs avoided pools containing nonspecific tadpoles and pools containing adult black-banded sunfish, Enneacanthus chaetodon. Females also avoided pools containing larval spotted salamanders, Ambystoma maculatum. Correspondence be- tween calling sites and oviposition sites was examined in response to treatment, block (consisting of a location and a time), and location. Correspondence between male choices and female choices was weak. Number and location of calling males showed no significant correlation with the number and location of eggs along any of the axes of choice. Both males and females chose sites based on the species present, and both showed preferences in regard to location and time. The differences between male and female choices indicate that the potential choices have different values to each sex, or that different criteria are used to rank potential choices. Anuran reproductive behavior is not a simple conse- quence of the direct interactions between males and females, but is a result of complex interactions between the behavior of the two sexes and the environment in which their behavior is embedded.

Interactions Between the Salamander Siren Intermedia and the Keystone Predator Notophthalmus Viridescens

Is the ability to function as a keystone predator a property of a species or an emergent property of the community? We addressed this question in a temporary-pond community where the broken-striped newt Notophthalmus viridescens dorsalis is known to act as a keystone predator on larval anurans. We independently manipulated the initial density of adult Notophthalmus (two or four) and the presence or absence of one adult Siren intermedia in a set of 20 artificial ponds to determine if this additional predator affected the ability of Notophthalmus to function as a keystone predator. Each pond received a diverse assemblage of larval anuran prey: a spring-breeding assemblage of 275 Rana utricularia, 100 Pseudacris crucifer and 25 Bufo americanus tadpoles, and a summer assemblage of 200 Hyla chrysoscelis and 150 Gastro- phryne carolinensis tadpoles. Two additional ponds received neither predator, to assay the outcome of competition among the tadpoles. The effect of the salamander Siren on Notophthalmus survival and fecundity depended on the density of Notophthalmus. At low newt density Siren had no effect on survival, but at high newt density competition reduced the survival and growth rates of Notophthalmus. The density of Notophthalmus and the presence of Siren interacted to determine the fe- cundity of Notophthalmus. At low newt density Siren reduced Notophthalmus reproductive success by preying on larvae. At high newt density Siren indirectly enhanced reproductive success by reducing survival of adults, thus releasing larvae from intraspecific competition and cannibalism. Notophthalmus density had no effect on adult Siren survival or growth rate in this experiment, but other evidence indicates that competition with Notophthalmus reduces the growth rates of Siren. Notophthalmus acted as a keystone predator on the assemblage of spring-breeding anurans; it increased the number of metamorphs of the weak competitor Pseudacris crucifer by releasing them from interspecific competition. Siren preyed on tadpoles in a non- selective manner. An additive model was sufficient to describe the effects of Siren and initial density of Notophthalmus on the structure of the tadpole assemblage. In our system of experimental ponds, the strong interaction between Siren and Notoph- thalmus density did not extend its effects to lower trophic levels, and the direct and indirect effects of Siren did not alter the role of Notophthalmus as a keystone predator.

Mapping Functional Similarity of Predators on the Basis of Trait Similarities

Theoretical and empirical studies in community ecology often simplify their study system by lumping together species on the basis of trait similarities (e.g., their taxonomy, resource or microhabitat usage) and then assume species sharing similar traits are functionally similar. To date, no study has directly tested whether species more similar with respect to any of these traits are more similar in their functional effects on population or ecosystem processes. In this study, we examined the association between traits and functional effects of six different aquatic predatory vertebrates. We demonstrated that functional similarity across multiple response variables was not correlated with trait similarity, but differences in trait values were associated with significantly different effects on individual response variables. The exact relationship between species traits and functional effect of predators, however, was different for each response variable. Using traits to predict functional similarity among species may be useful when considering individual response variables, but only if it is known which traits have the greatest influence on the response variable of interest. It is doubtful that any one scheme will predict the functional similarity of species across a diverse array of response variables because each response will likely be strongly influenced by different traits.

Competitive asymmetry and coexistence in size-structured populations of brook trout and spring salamanders

The question of coexistence of competing species has generated much of the basic theory of community ecology. The question becomes more complex when competition occurs between size-structured populations of predators in which the nature of the interaction extends beyond exploitative competition. I conducted a field experiment examining the interactions between fingerling brook trout, Salvelinus fontinalis (total length 78-101 mm), and larval spring salamanders, Gyrinophilus porphyriticus (snoutvent length 35.7-62.4 mm), in an array of replicate experimental streams. Brook trout affected both the growth and survival of larval spring salamanders, reducing survival by 50 % and growth by > 90 % for mass and 44 % for SVL. Brook trout also altered the habitat use of larval G. porphyriticus. G. porphyriticus had no effect on any of the responses of fingerling S. fontinalis. These results parallel those found in an earlier experiment on interactions between small adult brook trout and larval spring salamanders. Interactions between size-structured populations of G. porphyriticus and S. fontinalis are characterized by strong asymmetry in favor of S. fontinalis across a wide range of relative body sizes. This calls into question the mechanisms allowing continued coexistence of G. porphyriticus and S. fontinalis, and in general, the persistence of stream salamanders with predatory fish. Persistence is likely a complex function of the interactions between the life history and local demography of both G. porphyriticus and S. fontinalis, coupled with characteristics of the local environment, rather than the result of classic mechanisms of species coexistence such as niche divergence or alpha selection. Such complex mechanisms of species coexistence are likely more common in situations involving competition between size-structured populations than single factor explanations based on models of exploitative competition between competitors of fixed body sizes and life stages

Effects of forest canopy on habitat selection in treefrogs and aquatic insects: implications for communities and metacommunities

The specific dispersal/colonization strategies used by species to locate and colonize habitat patches can strongly influence both community and metacommunity structure. Habitat selection theory predicts nonrandom dispersal to and colonization of habitat patches based on their quality. We tested whether habitat selection was capable of generating patterns of diversity and abundance across a transition of canopy coverage (open and closed canopy) and nutrient addition by investigating oviposition site choice in two treefrog species (Hyla) and an aquatic beetle (Tropisternus lateralis), and the colonization dynamics of a diverse assemblage of aquatic insects (primarily beetles). Canopy cover produced dramatic patterns of presence/absence, abundance, and species richness, as open canopy ponds received 99.5% of propagules and 94.6% of adult insect colonists. Nutrient addition affected only Tropisternus oviposition, as females oviposited more egg cases at higher nutrient levels, but only in open canopy ponds. The behavioral partitioning of aquatic landscapes into suitable and unsuitable habitats via habitat selection behavior fundamentally alters how communities within larger ecological landscapes (metacommunities) are linked by dispersal and colonization.