Search for: All records

Abstract The Trophic Disruption Hypothesis (TDH) predicts that invasive species may cause native species to undergo trophic dispersion (change in trophic‐niche area) and trophic displacement (diet switching), predictably altering food‐web structure and biodiversity. In Everglades National Park, Florida, USA, African Jewelfish (Rubricatochromis letourneuxi) density has recently (2012–2017) undergone a boom‐bust cycle, linked to declines of native taxa and altered aquatic‐community composition that persist after the bust. Everglades restoration efforts seek to restore historic hydrologic conditions that may contribute to food‐web changes unfolding coincidentally with the jewelfish boom. We used complementary datasets of stomach contents and stable isotopes (δ¹⁵N and δ¹³C) to quantify pre‐ and post‐invasion consumer diets, trophic positions, trophic niches, basal energy use (autotrophic vs. heterotrophic), and energy fluxes to test assumptions of the TDH. The direction of change for these metrics from dry season to wet‐season post‐invasion (i.e., effect of adding water) was used as a proxy for the direction of effects from restored water delivery. For trophic shifts attributable to jewelfish invasion, we tested assumptions of the TDH. Comparing pre‐ versus post‐invasion for native consumers, we observed trophic displacement in 42% of species size classes (based on stomach contents), trophic dispersion for 57% of species (based on stable isotopes) and 54% of species size classes (based on stomach contents), and overall greater reliance on autotrophic energy. Altered trophic dynamics were more frequent pre‐ versus post‐invasion than among habitats or between seasons, and the direction of those responses was in the opposite direction of dry‐season to wet‐season differences and/or occurred at a higher frequency. Post‐invasion food‐web structure and function revealed increased relative abundance of mesopredators (including African Jewelfish) and reduced biomass and energy fluxes into and out of small fishes (e.g., Cyprinodontiformes). Our results show that African Jewelfish invasion is linked to altered spatiotemporal trophic dynamics and energy fluxes through declines in native fishes and invertebrates, which indirectly affected trophic relationships at the regional scale in the Everglades. As a result, we suggest extending the TDH to explicitly include the potential for invasive species to alter basal energy use, spatiotemporal trophic dynamics, and energy fluxes. 

Abstract Disturbances can produce a spectrum of short‐ and long‐term ecological consequences that depend on complex interactions of the characteristics of the event, antecedent environmental conditions, and the intrinsic properties of resistance and resilience of the affected biological system.We used Hurricane Harvey's impact on coastal rivers of Texas to examine the roles of storm‐related changes in hydrology and long‐term precipitation regime on the response of stream invertebrate communities to hurricane disturbance.We detected declines in richness, diversity and total abundance following the storm, but responses were strongly tied to direct and indirect effects of long‐term aridity and short‐term changes in stream hydrology. The amount of rainfall a site received drove both flood duration and flood magnitude across sites, but lower annual rainfall amounts (i.e. aridity) increased flood magnitude and decreased flood duration. Across all sites, flood duration was positively related to the time it took for invertebrate communities to return to a long‐term baseline and flood magnitude drove larger invertebrate community responses (i.e. changes in diversity and total abundance). However, invertebrate response per unit flood magnitude was lower in sub‐humid sites, potentially because of differences in refuge availability or ecological‐evolutionary interactions. Interestingly, sub‐humid streams had temporary large peaks in invertebrate total abundance and diversity following recovery period that may be indicative of the larger organic matter pulses expected in these systems because of their comparatively well‐developed riparian vegetation.Our findings show that hydrology and long‐term precipitation regime predictably affected invertebrate community responses and, thus, our work underscores the important influence of local climate to ecosystem sensitivity to disturbances. 

Wetlands are dynamic environments where aquatic organisms are affected by both predictable and unpredictable changes in hydrology. Understanding how abundant large-bodied predators respond to these changes is especially important in context of wetland restoration. We used satellite telemetry to investigate how individual (e.g., sex, size, body condition) and environmental factors influenced movement behaviors of American Alligators [Alligator mississippiensis (Daudin, 1801)] in a managed freshwater marsh ecosystem of the Florida Everglades. We quantified space use, movement activity, and habitat selection of animals (n = 18) across hydrological seasons and the breeding period and performed stable isotope analyses to infer seasonal dietary changes. Though individual animals did not change space use across seasons, movement activity was lower for some individuals and δ15 Nitrogen isotopic values were higher in the dry season possibly reflecting greater foraging opportunities when marsh dry down concentrates prey. Alligators may be using canals as foraging sites which have abundant prey year-round and shallow sawgrass habitats as spots for basking. Based on our findings, ongoing restoration of water inflow will likely change the distribution and movement behavior of alligators. 

Abstract The potential for animals to modify spatial patterns of nutrient limitation for autotrophs and habitat availability for other members of their communities is increasingly recognized. However, net trophic effects of consumers acting as ecosystem engineers remain poorly known. The American AlligatorAlligator mississippiensisis an abundant predator capable of dramatic modifications of physical habitat through the creation and maintenance of pond‐like basins, but its role in influencing community structure and nutrient dynamics is less appreciated.We investigated if alligators engineer differences in nutrient availability and changes to community structure by their creation of ‘alligator ponds’ compared to the surrounding phosphorus (P)‐limited oligotrophic marsh.We used a halo sampling design of three distinct habitats extending outward from 10 active alligator ponds across a hydrological gradient in the Everglades, USA. We performed nutrient analysis on basal food‐web resources and quantitative community analyses, and stoichiometric analyses on plants and animals.Our findings demonstrate that alligators act as ecosystem engineers and enhance food‐web heterogeneity by increasing nutrient availability, manipulating physical structure and altering algal, plant and animal communities. Flocculent detritus, an unconsolidated layer of particulate organic matter and soil, showed strong patterns of P enrichment in ponds. Higher P availability in alligator ponds also resulted in bottom‐up trophic transfer of nutrients as evidenced by higher growth rates (lower N:P) for plants and aquatic consumers. Edge habitats surrounding alligator ponds contained the most diverse communities of invertebrates and plants, but low total abundance of fishes, likely driven by high densities of emergent macrophytes. Pond communities exhibited higher abundance of fish compared to edge habitat and were dominated by compositions of small invertebrates that track high nutrient availability in the water column. Marshes contained high numbers of animals that are closely tied to periphyton mats, which were absent from other habitats.Alligator‐engineered habitats are ecologically important by providing nutrient‐enriched ‘hotspots’ in an oligotrophic system, habitat heterogeneity to marshes, and refuges for other fauna during seasonal disturbances. This work adds to growing evidence that efforts to model community dynamics should routinely consider animal‐mediated bottom‐up processes like ecosystem engineering. 

Extreme climate events such as hurricanes can influence the movement and distribution of fish and other aquatic vertebrates. However, our understanding of the scale of movement responses and how they vary across taxa and ecosystems remains incomplete. In this study, we used acoustic telemetry data to investigate the movement patterns of common snook (Centropomus undecimalis) in the Florida Coastal Everglades during Hurricane Irma, which made landfall on the southwest Florida coast as a Category 3 storm on 10 September 2017 after passing in close proximity to our study site. We hypothesized that the hurricane resulted in shifts in distribution and that these movements may have been driven by environmental cues stemming from changes in barometric pressure associated with hurricane conditions, fluctuations in water levels (stage) characterizing altered riverine conditions, or a combination of both hurricane and riverine drivers. The data revealed large-scale movements of common snook in the time period surrounding hurricane passage, with 73% of fish detected moving from the upper river into downriver habitats, and some individuals potentially exiting the river. Furthermore, regression model selection indicated that these movements were correlated to both hurricane and riverine conditions, showing increased common snook movement at higher river stage and lower barometric pressure, and stage explaining a larger proportion of model deviance. Animal movement has widespread and diverse ecological implications, and by better understanding the factors that drive movement, we may anticipate how future extreme climate events could affect fish populations in impact-prone regions. 

Predicting the responses of animals to environmental changes is a fundamental goal of ecology and is necessary for conservation and management of species. While most studies focus on relatively gradual changes, extreme events may have lasting impacts on populations. Animals respond to major disturbances such as hurricanes by seeking shelter, migrating, or they may fail to respond appropriately. We assessed the effects of Hurricane Irma in 2017 on the behavior and survival of juvenile bull sharks (Carcharhinus leucas) within a nursery of the Florida coastal Everglades using long-term acoustic telemetry monitoring. Most of our tagged sharks (n = 14) attempted to leave the shallow waters of the Shark River Estuary before the hurricane strike, but individuals varied in the timing and success of their movements. Eight bull sharks left within hours or days before the hurricane, but three left more than a week in advance. Nine of 11 bull sharks (~ 82%) eventually returned to the array within weeks or months of the storm. Six of these returning individuals were detected in a different coastal array in nearshore waters ca. 80 km away from the mouth of the estuary during their absence. The remaining three bull sharks moved downstream relatively late (after the hurricane) and may have died. We used binomial generalized linear mixed models to estimate the probability of presence within the array as a function of several environmental variables. Departure from the array was predicted by declining barometric pressure, increasing rate of change in pressure, and potentially fluctuations in river stage. Juvenile bull sharks may weigh multiple environmental cues, perceived predation risk, their own physical size, and shifting prey resources when making decisions during and after hurricanes. 

Coastal ecosystems display consistent patterns of trade-offs between resistance and resilience to tropical cyclones.