Jeremy M. Wojdak

Generality in ecology: testing North American grassland rules in South African savannas

Ecology has emerged as a global science, and there is a pressing need to identify ecological rules – general principles that will improve its predictive capability for scientists and its usefulness for managers and policy makers. Ideally, the generality and limits of these ecological rules should be assessed using extensive, coordinated experiments that ensure consistency in design and comparability of data. To improve the design of these large-scale efforts, existing data should be used to test prospective ecological rules and to identify their limits and contingencies. As an example of this approach, we describe prospective rules for grassland responses to fire and rainfall gradients, identified from long-term studies of North American grasslands and tested with existing data from long-term experiments in South African savanna grasslands. Analyses indicated consistent effects of fire on the abundance of the dominant (grasses) and subdominant (forbs) flora on both continents, but no common response of gr...

RELATIVE STRENGTH OF TOP-DOWN, BOTTOM-UP, AND CONSUMER SPECIES RICHNESS EFFECTS ON POND ECOSYSTEMS

Theory and experiments demonstrate that the number of species in a local area can determine rates of ecosystem processes, but we know little about how the strength of that control compares with other influences or how it might vary across ecological gradients. Here I report results of a pond mesocosm experiment where consumer (snail) species richness, predation intensity, and nutrient availability were crossed in a full-factorial design. This design allowed a direct comparison of the strength of the different factors on food web properties and ecosystem functioning (i.e., system-level fluxes of energy or matter), and an evaluation of how the factors may interact. Systems with higher snail species richness had greater secondary production, consumer biomass, ecosystem respiration, and macrophyte stem growth, and lower epiphyton and periphyton biomass. However, snail species richness effects on periphyton and epiphyton were context dependent; predators reduced the effect of increasing snail richness on the b...

Corticosterone Level Changes throughout Larval Development in the Amphibians Rana sylvatica and Ambystoma jeffersonianum Reared under Laboratory, Mesocosm, or Free-living Conditions

Abstract Studies of a few “model” amphibians continue to advance our mechanistic understanding of the endocrine control of larval amphibian development and metamorphosis, but there are few studies examining steroid profiles across species during larval amphibian development. We used censored regression analysis to address our primary objective, which was to examine baseline corticosterone level changes and responses to a standardized stressor throughout larval development in two amphibian species: one anuran (Wood Frogs, Rana sylvatica) and one caudate (Jefferson Salamanders, Ambystoma jeffersonianum). In addition, we looked at two additional factors that could influence the study of corticosterone during larval development, namely the rearing location of the animals (free-living, mesocosm-held, or laboratory-held) and for A. jeffersonianum, the method of induction of the stress response (ACTH injection or a confinement-agitation [CA] protocol). As has been documented for other anurans, baseline corticosterone content of R. sylvatica increased close to metamorphic climax in all rearing locations, although the absolute level varied with rearing location. Baseline corticosterone content of A. jeffersonianum increased gradually over development, and the increase in corticosterone content following CA mirrored the increase in baseline levels, although the absolute magnitude of the increase with CA varied based on rearing location. In larvae of A. jeffersonianum, both the CA method and ACTH injection significantly increased corticosterone content, with 30 min eliciting the maximum hormonal response level. Our results suggest that rearing location can influence corticosterone levels and the response to a standardized CA protocol, and that care should be taken in extrapolating results from laboratory studies to free-living amphibian populations.

The influence of temporally variable predation risk on indirect interactions in an aquatic food chain

We know little about how temporally variable predation risk influences prey behavior. The risk allocation hypothesis predicts that prey facing more frequent risk should show weak anti-predator responses, and should be particularly active foragers during rare periods of safety, compared to prey facing infrequent risk. Several studies offer support for the risk allocation hypothesis, but how these responses might propagate through the larger ecological community remains largely unknown. We experimentally investigated the relative strength of trait- and density-mediated indirect effects of a predator on its prey’s resource across predation treatments that varied the lethality (caged or free-swimming predators) and temporal variability (always, often, or sometimes present) of predation. We performed this experiment in pond mesocosms using a giant water bug predator (Belostoma lutarium), an herbivorous pond snail (Physa gyrina), and algae as the basal resource. Snails greatly reduced the abundance of their algal resource when in the absence of predation. Lethal predation at low and medium intensities had significant positive indirect effects on the abundance of algae, mostly by reducing snail density. Snails responded behaviorally to high levels of deadly predation by foraging more and hiding less than in other situations, as predicted by the risk allocation hypothesis, and thus ameliorated the density-mediated indirect effects of predators on algae. Behavioral responses to caged predators, and the subsequent trait-mediated indirect effects, were negligible regardless of predation intensity. Our previous work has demonstrated that trait-mediated indirect effects are weak when resources are abundant, as they were in this experiment. This work demonstrates that temporal variation in predation intensity plays a key role in determining the relative strength of TMIIs and DMIIs in an aquatic food chain.

Defensive Symbionts Mediate Host–Parasite Interactions at Multiple Scales

In protection mutualisms, defensive symbionts protect their hosts from natural enemies, including parasites. Protection mutualisms were historically considered rare ecological relationships, but recent examples demonstrate that defensive symbionts are both quite common and diverse. Defensive symbionts can have surprisingly large effects on host and parasite ecology at the individual, population, guild, and community scales. However, the highly context-dependent nature of protection mutualisms makes it difficult to identify and quantify the roles that defensive symbionts play in host-parasite systems. The mutualism-parasitism continuum framework can be used to understand and predict the outcomes of these interactions under variable environmental and ecological contexts. Embracing and expanding this theory will improve future research, and may better prepare us to use defensive symbionts as biocontrol agents.

Dispersal of a defensive symbiont depends on contact between hosts, host health, and host size

Symbiont dispersal is necessary for the maintenance of defense mutualisms in space and time, and the distribution of symbionts among hosts should be intricately tied to symbiont dispersal behaviors. However, we know surprisingly little about how most defensive symbionts find and choose advantageous hosts or what cues trigger symbionts to disperse from their current hosts. In a series of six experiments, we explored the dispersal ecology of an oligochaete worm (Chaetogasterlimnaei) that protects snail hosts from infection by larval trematode parasites. Specifically, we determined the factors that affected net symbiont dispersal from a current “donor” host to a new “receiver” host. Symbionts rarely dispersed unless hosts directly came in contact with one another. However, symbionts overcame their reluctance to disperse across the open environment if the donor host died. When hosts came in direct contact, net symbiont dispersal varied with both host size and trematode infection status, whereas symbiont density did not influence the probability of symbiont dispersal. Together, these experiments show that symbiont dispersal is not a constant, random process, as is often assumed in symbiont dispersal models, but rather the probability of dispersal varies with ecological conditions and among individual hosts. The observed heterogeneity in dispersal rates among hosts may help to explain symbiont aggregation among snail hosts in nature.

Host Density and Competency Determine the Effects of Host Diversity on Trematode Parasite Infection

Variation in host species composition can dramatically alter parasite transmission in natural communities. Whether diverse host communities dilute or amplify parasite transmission is thought to depend critically on species traits, particularly on how hosts affect each other’s densities, and their relative competency as hosts. Here we studied a community of potential hosts and/or decoys (i.e. non-competent hosts) for two trematode parasite species, Echinostoma trivolvis and Ribeiroia ondatrae, which commonly infect wildlife across North America. We manipulated the density of a focal host (green frog tadpoles, Rana clamitans), in concert with manipulating the diversity of alternative species, to simulate communities where alternative species either (1) replace the focal host species so that the total number of individuals remains constant (substitution) or (2) add to total host density (addition). For E. trivolvis, we found that total parasite transmission remained roughly equal (or perhaps decreased slightly) when alternative species replaced focal host individuals, but parasite transmission was higher when alternative species were added to a community without replacing focal host individuals. Given the alternative species were roughly equal in competency, these results are consistent with current theory. Remarkably, both total tadpole and per-capita tadpole infection intensity by E. trivolvis increased with increasing intraspecific host density. For R. ondatrae, alternative species did not function as effective decoys or hosts for parasite infective stages, and the diversity and density treatments did not produce clear changes in parasite transmission, although high tank to tank variation in R. ondatrae infection could have obscured patterns.

Pond acidification may explain differences in corticosterone among salamander populations.

Physiological tolerances play a key role in determining species distributions and abundance across a landscape, and understanding these tolerances can therefore be useful in predicting future changes in species distributions that might occur. Vertebrates possess several highly conserved physiological mechanisms for coping with environmental stressors, including the hormonal stress response that involves an endocrine cascade resulting in the increased production of glucocorticoids. We examined the function of this endocrine axis by assessing both baseline and acute stress–induced concentrations of corticosterone in larvae from eight natural breeding populations of Jefferson’s salamander Ambystoma jeffersonianum. We surveyed individuals from each pond and also examined a variety of environmental pond parameters. We found that baseline and stress-induced corticosterone concentrations differed significantly among ponds. Population-level baseline corticosterone concentrations were negatively related to pH and positively related to nitrate, and stress-induced concentrations were again negatively related to pH, positively related to nitrate, and positively related to temperature. We followed the field survey with an outdoor mesocosm experiment in which we manipulated pH and again examined baseline and acute stress–induced corticosterone in A. jeffersonianum larvae. As in the field survey, we observed an increase in the baseline corticosterone concentration of individuals exposed to the lowest pH treatment (pH 5–5.8). Examining physiological indices using a combined approach of field surveys and experiments can be a powerful tool for trying to unravel the complexities of environmental impacts on species distributions.

Echinostoma trivolvis (Digenea: Echinostomatidae) second intermediate host preference matches host suitability

Many trematodes infect a single mollusk species as their first intermediate host, and then infect a variety of second intermediate host species. Determining the factors that shape host specificity is an important step towards understanding trematode infection dynamics. Toward this end, we studied two pond snails (Physa gyrina and Helisoma trivolvis) that can be infected as second intermediate hosts by the trematode Echinostoma trivolvis lineage a (ETa). We performed laboratory preference trials with ETa cercariae in the presence of both snail species and also characterized host suitability by quantifying encystment and excystment success for each host species alone. We tested the prediction that trematodes might preferentially infect species other than their obligate first intermediate host (in this case, H. trivolvis) as second intermediate hosts to avoid potentially greater host mortality associated with residing in first intermediate hosts. In our experiments, ETa had roughly equivalent encystment success in Helisoma and Physa snails, but greater excystment success in Physa, when offered each species in isolation. Also, the presence of the symbiotic oligochaete Chaetogaster limnaei in a subset of Helisoma snails reduced encystment success in those individuals. When both hosts were present, we found dramatically reduced infection prevalence and intensity in Helisoma—ETa cercariae strongly preferred Physa. Thus, the presence of either an alternative host, or a predator of free-living parasites, offered protection for Helisoma snails from E. trivolvis lineage a infection.

Consequences of induced hatching plasticity depend on predator community

Many prey species face trade-offs in the timing of life history switch points like hatching and metamorphosis. Costs associated with transitioning early depend on the biotic and abiotic conditions found in the subsequent life stage. The red-eyed treefrog, Agalychnis callidryas, faces risks from predators in multiple, successive life stages, and can hatch early in response to mortality threats at the egg stage. Here we tested how the consequences of life history plasticity, specifically early hatching in response to terrestrial egg predators, depend on the assemblage of aquatic larval predators. We predicted that diverse predator assemblages would impose lower total predation pressure than the most effective single predator species and might thereby reduce the costs of hatching early. We then conducted a mesocosm experiment where we crossed hatchling phenotype (early vs. normal hatching) with five larval-predator environments (no predators, either waterbugs, dragonflies, or mosquitofish singly, or all three predator species together). The consequences of hatching early varied across predator treatments, and tended to disappear through time in some predation treatments, notably the waterbug and diverse predator assemblages. We demonstrate that the fitness costs of life history plasticity in an early life stage depend critically on the predator community composition in the next stage.