Mercury Exposure and Habitat Fragmentation Affect the Movement, Foraging Behavior, and Search Efficiency of the Marsh Periwinkle (Littorina irrorata)

The interactions between habitat fragmentation and other stressors are considered a key knowledge gap. The present study tested the hypotheses that mercury enhances the effects of fragmentation by (1) reducing the cumulative daily movement of organisms, (2) shifting their foraging behavior, and (3) altering the vertical movement of the marsh periwinkle (Littoraria irrorata) in a field experiment. Random walk simulations were used to access how changes in movement affect the search efficiency of organisms in the long term. Eighteen 1.5 m2 plots were constructed in a salt marsh where landscapes characteristics were manipulated to reach three different levels of habitat cover. Daily movement of 12 marked control and mercury-exposed snails were measured in each plot. Bayesian models were used to analyze the data and evidence ratios were used to test the hypotheses. The results showed that the effects of fragmentation were consistent in both control and exposed treatments, with an increase in the cumulative displacement of organisms. However, mercury significantly reduced the movement of organisms in all levels of fragmentation, shifting their foraging behavior (evidence ratio > 1000). Exposed snails were more likely to be found inactive in comparison with the control treatment (evidence ratio > 1000). Fragmentation also reduced the vertical movement of organisms in both treatments. In contrast, mercury increased the vertical movement of organisms (evidence ratio > 1000). The search efficiency of organisms also increased in a highly fragmentated landscape, suggesting that changes in foraging behavior are likely due to reduced resources and consequently increase in foraging effort. The present study shows that mercury exposure can enhance the effects of habitat fragmentation by changing organisms' movement, foraging behavior, and search efficiency. Environ Toxicol Chem 2023;42:1971-1981. © 2022 SETAC.

Genetic and Epigenetic Differentiation Across Intertidal Gradients in the Foundation Plant Spartina alterniflora

Ecological genomics approaches have informed us about the structure of genetic diversity in natural populations that might underlie patterns in trait variation. However, we still know surprisingly little about the mechanisms that permit organisms to adapt to variable environmental conditions. The salt marsh foundation plant Spartina alterniflora exhibits a dramatic range in phenotype that is associated with a pronounced intertidal environmental gradient across a narrow spatial scale. Both genetic and non-genetic molecular mechanisms might underlie this phenotypic variation. To investigate both, we used epigenotyping-by-sequencing (epiGBS) to evaluate the make-up of natural populations across the intertidal environmental gradient. Based on recent findings, we expected that both DNA sequence and DNA methylation diversity would be explained by source population and habitat within populations. However, we predicted that epigenetic variation might be more strongly associated with habitat since similar epigenetic modifications could be rapidly elicited across different genetic backgrounds by similar environmental conditions. Overall, with PERMANOVA we found that population of origin explained a significant amount of the genetic (8.6%) and epigenetic (3.2%) variance. In addition, we found that a small but significant amount of genetic and epigenetic variance (<1%) was explained by habitat within populations. The interaction of population and habitat explained an additional 2.9% of the genetic variance and 1.4% of the epigenetic variance. By examining genetic and epigenetic variation within the same fragments (variation in close-cis), we found that population explained epigenetic variation in 9.2% of 8,960 tested loci, even after accounting for differences in the DNA sequence of the fragment. Habitat alone explained very little (<0.1%) of the variation in these close-cis comparisons, but the interaction of population and habitat explained 2.1% of the epigenetic variation in these loci. Using multiple matrix regression with randomization (MMRR) we found that phenotypic differences in natural populations were correlated with epigenetic and environmental differences even when accounting for genetic differences. Our results support the contention that sequence variation explains most of the variation in DNA methylation, but we have provided evidence that DNA methylation distinctly contributes to plant responses in natural populations.

Pesticide tolerance induced by a generalized stress response in wood frogs (Rana sylvatica)

Increasing evidence suggests that phenotypic plasticity can play a critical role in ecotoxicology. More specifically, induced pesticide tolerance, in which populations exposed to a contaminant show increased tolerance to the contaminants later, has been documented in multiple taxa. However, the physiological mechanisms of induced tolerance remain unclear. We hypothesized that induced pesticide tolerance is the result of a generalized stress response based on previous studies showing that both natural stressors and anthropogenic stressors can induce tolerance to pesticides. We tested this hypothesis by first exposing larval wood frogs (Rana sylvatica) to either an anthropogenic stressor (sublethal carbaryl concentration), a natural stressor (cues from a caged predator), or a simulated stressor via exogenous exposure to the stress hormone corticosterone (125 nM). We also included treatments that inhibited corticosterone synthesis with the compound metyrapone (MTP). We then exposed the larvae to a lethal carbaryl treatment to assess time to death. We found that prior exposure to 125 nM of exogenous CORT and predator cues induced tolerance to a lethal concentration of carbaryl through a slight delay in time to death. Pre-exposure to sublethal carbaryl, as well as MTP alone or in combination with predator cues, did not induce tolerance to the lethal carbaryl concentration relative to the ethanol vehicle control treatment. Our study provides evidence that pesticide tolerance can be induced by a generalized stress response both in the presence and absence (exogenous CORT) of specific cues and highlights the importance of considering physiological ecology and environmental context in ecotoxicology.

Sea-level rise and the emergence of a keystone grazer alter the geomorphic evolution and ecology of southeast US salt marshes

Human disturbances, climate change, and their combined effects on species distributions and environmental conditions are increasingly modifying the organization of our world’s oceans, forests, grasslands, wetlands, tundras, and reefs. Here, we reveal that these contemporary conditions can trigger the emergence of novel keystone species. Across the southeastern US coastal plain, sea-level rise is outpacing salt marsh vertical accretion, causing these grasslands to be tidally inundated for longer and softening marsh substrates to levels optimal for crab burrowing. Using field experiments, measurements, surveys, and models, we show that these conditions amplify the burrowing and grazing effects of a previously inconspicuous crab, enabling it to redefine predator–prey interactions, eco-geomorphic feedbacks, and the mechanisms by which salt marshes are responding to climate change.

Keystone species have large ecological effects relative to their abundance and have been identified in many ecosystems. However, global change is pervasively altering environmental conditions, potentially elevating new species to keystone roles. Here, we reveal that a historically innocuous grazer—the marsh crab Sesarma reticulatum—is rapidly reshaping the geomorphic evolution and ecological organization of southeastern US salt marshes now burdened by rising sea levels. Our analyses indicate that sea-level rise in recent decades has widely outpaced marsh vertical accretion, increasing tidal submergence of marsh surfaces, particularly where creeks exhibit morphologies that are unable to efficiently drain adjacent marsh platforms. In these increasingly submerged areas, cordgrass decreases belowground root:rhizome ratios, causing substrate hardness to decrease to within the optimal range for Sesarma burrowing. Together, these bio-physical changes provoke Sesarma to aggregate in high-density grazing and burrowing fronts at the heads of tidal creeks (hereafter, creekheads). Aerial-image analyses reveal that resulting “Sesarma-grazed” creekheads increased in prevalence from 10 ± 2% to 29 ± 5% over the past <25 y and, by tripling creek-incision rates relative to nongrazed creekheads, have increased marsh-landscape drainage density by 8 to 35% across the region. Field experiments further demonstrate that Sesarma-grazed creekheads, through their removal of vegetation that otherwise obstructs predator access, enhance the vulnerability of macrobenthic invertebrates to predation and strongly reduce secondary production across adjacent marsh platforms. Thus, sea-level rise is creating conditions within which Sesarma functions as a keystone species that is driving dynamic, landscape-scale changes in salt-marsh geomorphic evolution, spatial organization, and species interactions.

Periwinkle climbing response to water- and airbone predator chemical cues may depend on home-marsh geography

The salt marsh periwinkle, Littorina irrorata, exhibits a spatial refuge from predation by climbing the stems of Spartina alterniflora in order to avoid benthic predators. Salt marsh periwinkles have a broad geographic distribution, and for many species, responses to predators also varies with biogeography. This study sought to determine if the geographical location of the home marsh influenced the response of periwinkles (climbing height) to blue crab predator cues both via air and water. Snails from Louisiana (LA) climbed higher in general than those from North Carolina (NC), regardless of chemical cue. However, LA snails climbed 11 cm higher in the presence of waterborne predators than control snails with no cue, while NC snails only climbed five cm higher in the same comparisons. Airborne chemical cue tended to have snails climbing at intermediate heights. These responses were significantly enhanced when both populations of snails were housed together. Periwinkle response to predator cues was stronger in LA than NC, and so it is possible that the behavioral response of these snails to predators varies with biogeography of the home marsh. Also interestingly, the results of this study also suggest that cue delivery is probably occurring via mechanisms other than water, and potentially via airborne cues. Therefore, salt marsh periwinkles likely respond to numerous cues that initiate behavioral responses, including airborne cues, and these responses may vary by home-marsh geography.

Nowhere to hide: Effects of linear features on predator-prey dynamics in a large mammal system

Rapid landscape alteration associated with human activity is currently challenging the evolved dynamical stability of many predator-prey systems by forcing species to behaviourally respond to novel environmental stimuli. In many forested systems, linear features (LFs) such as roads, pipelines and resource exploration lines (i.e. seismic lines) are a ubiquitous form of landscape alteration that have been implicated in altering predator-prey dynamics. One hypothesized effect is that LFs facilitate predator movement into and within prey refugia, thereby increasing predator-prey spatial overlap. We evaluated this hypothesis in a large mammal system, focusing on the interactions between boreal woodland caribou (Rangifer tarandus caribou) and their two main predators, wolves (Canis lupus) and black bears (Ursus americanus), during the calving season of caribou. In this system, LFs extend into and occur within peatlands (i.e. bogs and nutrient-poor fens), a habitat type highly used by caribou due to its refugia effects. Using resource selection analyses, we found that LFs increased predator selection of peatlands. Female caribou appeared to respond by avoiding LFs and areas with high LF density. However, in our study area, most caribou cannot completely avoid exposure to LFs and variation in female response had demographic effects. In particular, increasing proportional use of LFs by females negatively impacted survival of their neonate calves. Collectively, these results demonstrate how LFs can reduce the efficacy of prey refugia. Mitigating such effects will require limiting or restoring LFs within prey refugia, although the effectiveness of mitigation efforts will depend upon spatial scale, which in turn will be influenced by the life-history traits of predator and prey.

Long-distance interactions regulate the structure and resilience of coastal ecosystems

Mounting evidence indicates that spatial interactions are important in structuring coastal ecosystems. Until recently, however, most of this work has been focused on seemingly exceptional systems that are characterized by regular, self-organized patterns. In this review, we document that interactions that operate at long distances, beyond the direct neighborhood of individual organisms, are more common and have much more far-reaching implications for coastal ecosystems than was previously realized. We review studies from a variety of ecosystem types-including cobble beaches, mussel beds, coral reefs, seagrass meadows, and mangrove forests-that reveal a startling interplay of positive and negative interactions between habitats across distances of up to a kilometer. In addition to classical feeding relations, alterations of physical conditions constitute an important part of these long-distance interactions. This entanglement of habitats has crucial implications for how humans manage coastal ecosystems, and evaluations of anthropogenic impact should explicitly address long-distance and system-wide effects before we deem these human activities to be causing little harm.

Environmental complexity reduces tadpole predation by water bugs

We assessed the role of habitat structure in the outcome of predation by measuring how aquatic vegetation influences predation rates of water bugs (Belostoma oxyurum (Dufour, 1863), Hemiptera, Belostomatidae) on tadpoles of Dendropsophus minutus (Peters, 1872) and Scinax curicica Pugliese, Pombal, and Sazima, 2004 (Anura, Hylidae). Considering that previous studies have shown that some tadpole species preferentially use microhabitats with aquatic vegetation at sites in southeastern Brazil, we hypothesized that these tadpoles may select such complex microhabitats because they can offer some protection against co-occurring predatory aquatic insects. We used field enclosures containing tadpoles of D. minutus and S. curicica and one predator (B. oxyurum), placed on natural substrata in sites both with and without aquatic vegetation, according to treatment. We measured the combined effects of predation and habitat structure on the survivorship of tadpoles, monitoring each enclosure daily during 10 days to survey surviving tadpoles. Treatments with predators reduced tadpole survivorship significantly in relation to controls for both tadpole species. The interaction between predator and vegetation was also significant, predation rates being lower when vegetation was present.

Cricket frogs maintain body hydration and temperature near levels allowing maximum jump performance

One goal of this study was to determine the combination of hydration and temperature in the northern cricket frog Acris crepitans that allowed maximum jump distance in the laboratory. Second, environmental variables in the field were measured to determine the best predictor(s) of mean body temperature and hydration and to determine whether frogs maintain levels of temperature and hydration yielding maximum jump distance. Laboratory data revealed that hydration and the hydration-temperature interaction significantly affected jump performance. Frogs at 95% and 85% hydration jumped significantly better than frogs at 75% hydration, but frogs at 95% hydration at 15 degrees C jumped significantly poorer than those at 95% hydration at 30 degrees C. Animals at 85% hydration at 30 degrees C and 85% hydration at 15 degrees C jumped just as well as those at 95% hydration at 30 degrees C. Mean body temperature of 55 frogs in the field was 28.0 degrees C, and hydration was 97.4%. Sky condition (sunny, cloudy, or partly cloudy) was the best predictor of frog hydration, and air temperature was the best predictor of frog body temperature. Cricket frogs in the field maintain a hydration and temperature near those found to yield maximum jump distances in laboratory trials. This may be a behavioral adaptation to allow maximum jump distance during predator avoidance.