Search for: All records

ABSTRACT ObjectiveEnvironmental variability as a factor of climate change and water management can result in fluctuations in the abundance and distribution of fish populations from year to year, with either negative or positive effects depending on behavioral and physiological requirements and the ability to adapt to changing conditions. Variability in water levels can also influence prey availability, affecting predator abundance in seasonal foraging areas. In this study, our objective was to better understand how environmental variation has affected the relative abundance of Common Snook Centropomus undecimalis in the freshwater/estuarine habitats of Everglades National Park. MethodsElectrofishing data over 17 years (2004–2021) were analyzed in relation to a long-term time-series of environmental conditions, including water level, temperature, salinity, and precipitation. We used seasonal and trend decomposition via locally estimated scatterplot smoothing to isolate the effect of seasonality and identify nonlinear trends in the environmental time-series data and Common Snook abundance and Mann–Kendall trend tests to identify monotonic and directional trends over time. To identify the factors that best explain variation in Common Snook abundance, we used generalized linear models to relate relative abundance to the environmental covariates. ResultsWe found significant long-term trends of increasing water level and temperature and decreasing salinity in the study area. The generalized linear models indicated that Common Snook abundance had a negative relationship with water level and a positive relationship with temperature. Common Snook abundance over the 17 years of sampling was relatively stable; however, increases/decreases in Common Snook abundance corresponded to both seasonal changes in water level and the periodic occurrence of extreme conditions (e.g., cold spells, droughts, prolonged dry-season flooding). ConclusionsUnderstanding how past environmental change has affected fish populations can provide insight into how they may respond to future conditions. Our results suggest that water management decisions that maintain seasonal patterns of high/low water levels can potentially mitigate climate-driven shifts by providing conditions that promote prey production in the wet season and foraging opportunities in the dry season, increasing the relative abundance of ecologically and recreationally important species such as Common Snook. 

ABSTRACT ObjectiveThe objective of this study was to quantify the effects of temperature, hydrology, and body size on the diet and energy requirements of a generalist predator, Common Snook Centropomus undecimalis (hereafter, “snook”), to gain a better understanding of predator–prey dynamics in the wake of global change. We first ask how temperature, hydrology, and body size influence the occurrence of fish, invertebrates, and empty stomachs in the diet of snook. Next, we model the energetic requirements of snook as a function of body size and temperature. Last, we use predation simulations to test how changes in prey quality, together with snook energy requirements, interact to shape prey demand. MethodsThis study used long-term empirical diet information for snook that were collected from the Shark River, Everglades National Park, alongside models of consumer energetic needs and predation simulations. We used a set of generalized linear models to determine the relationships between snook diet and a suite of predictor variables representing hydrology, temperature, and body size. Models of consumer energetic requirements were used to better understand the total daily caloric needs of snook across a range of temperature and body sizes relative to the available energy in the fish and invertebrate prey that were collected from the system. Last, we conducted predation simulations to highlight the effects of variable diet scenarios on the foraging behaviors that are required to meet the total daily energetic requirements of snook at various temperatures and body sizes. ResultsSnook were observed consuming less fish, coupled with an increased likelihood of empty stomachs, at higher temperatures. Reliance on invertebrate taxa increased at high marsh stages. In addition to marsh stage, smaller-bodied individuals were more likely to consume invertebrates. The predation simulations revealed that snook that consumed invertebrate-dominated diets required greater prey biomass as well as an increased number of individual prey items to meet their daily energetic requirements relative to fish that consumed diets that contain fish. However, if snook maintained even a small proportion of fish in their diet, it substantially reduced the number and biomass of prey needed to meet their energetic requirements. ConclusionsOur predation simulations indicated that snook should select for high-quality fish prey as temperatures warm. However, the empirical data revealed a decrease in the probability of high-quality fish prey in the diets of snook. Furthermore, the empirical diet data showed that low-quality invertebrate prey were more likely to be seen in the diets of snook at high water levels. As temperatures increase and hydrology becomes increasingly variable because of global change, snook will likely need to consume larger quantities of lower quality prey (i.e., compensatory foraging) or disperse to forage in more optimal habitats. These results highlight the dynamic interplay between environmental conditions and consumer energetic needs for shaping the foraging ecology of a generalist predator. 

Abstract Embracing local knowledge is vital to conserve and manage biodiversity, yet frameworks to do so are lacking. We need to understand which, and how many knowledge holders are needed to ensure that management recommendations arising from local knowledge are not skewed towards the most vocal individuals. Here, we apply a Wisdom of Crowds framework to a data-poor recreational catch-and-release fishery, where individuals interact with natural resources in different ways. We aimed to test whether estimates of fishing quality from diverse groups (multiple ages and years of experience), were better than estimates provided by homogenous groups and whether thresholds exist for the number of individuals needed to capture estimates. We found that diversity matters; by using random subsampling combined with saturation principles, we determine that targeting 31% of the survey sample size captured 75% of unique responses. Estimates from small diverse subsets of this size outperformed most estimates from homogenous groups; sufficiently diverse small crowds are just as effective as large crowds in estimating ecological state. We advocate for more diverse knowledge holders in local knowledge research and application. 

Abstract Pulses of resource availability along environmental gradients can filter the local and regional distribution of macrophyte and microbial mat communities in wetlands. Wetlands that experience short hydroperiods (i.e., <6 months with standing water) may cause macrophyte and microbial mat competition for water. However, the stress gradient hypothesis predicts that abiotic stress should increase facilitative co‐regulation of producer dynamics. To determine if and how macrophyte and microbial mat biomass covary along a hydrologic gradient, we conducted two observational surveys and a biomass removal experiment in Everglades National Park, FL, USA. In the survey, macrophyte and microbial mat biomass were measured over a two‐year period across nine hydrologically regulated macrophyte community types to determine drivers of biomass and macrophyte–microbial mat interactions along a hydroperiod gradient (3–8 months) using a structural equation model. In the experiment, the effect of hydrology on the interaction between macrophytes and microbial mats was quantified by measuring the effect of bimonthly removal of macrophyte or microbial mat biomass on the biomass of both communities in plots in wetlands with contrasting hydroperiods (3–6 months). Hydrology and biological interactions influenced macrophyte and microbial mat biomass, with stronger interactions observed in the shortest hydroperiod transect sites dominated bySchoenus nigricansandCladium jamaicense. Along the hydrologic gradient, we found direct negative effects of macrophyte biomass on microbial biomass and vice versa, and a significant positive effect of microbial response to flooding duration on macrophyte biomass. Experimental macrophyte removal in shorter‐hydroperiod wetlands resulted in a significant increase in microbial biomass while microbial mat removal reduced biomass of the dominant macrophyteC. jamaicense. The facilitative effect of microbial mats on macrophyte biomass in shorter‐hydroperiod wetlands may be driven by mats prolonging soil moisture retention due to their desiccation‐resistant structure. Stress‐induced facilitation supported the stress gradient hypothesis across the short‐hydrologic gradient, while competitive interactions were also observed. As climate and human drivers continue altering hydrology in aquatic systems, the type and strength of community interactions will continue to shift and alter distributions across the landscape. 

Abstract Mangroves play a crucial role in mitigating hurricane impacts in coastal ecosystems, and their adaptive traits enable regeneration and forest recovery following these disturbances. Yet, how species‐specific regeneration varies across life stages and interacts with environmental conditions is poorly understood. We quantified regeneration rates of three dominant species of mangrove seedlings and saplings (Avicennia germinans,Laguncularia racemosa, andRhizophora mangle) recovering from a major hurricane. We selected forests with varying light availability and phosphorus (P) gradients in the Everglades (Florida, USA). From 2020 to 2022, we measured biannual stem elongation, height, and density of seedlings and saplings, and collected porewater variables (salinity, sulfide, and inorganic nutrients) and continuous light intensity to assess species‐specific drivers of regeneration. Species‐specific growth rates, total height, and density varied across sites, driven by differences in porewater P and light. Growth rates ofR. mangleseedlings and bothR. mangleandL. racemosasaplings were influenced by light, whileA. germinansgrowth rates were unaffected. OnlyR. mangleandL. racemosasaplings were influenced by porewater P, while growth of both seedlings and saplings was unaffected by porewater salinity and sulfide. Mangrove regeneration post‐disturbance is explained by spatial differences in subsidies and stressors and the composition of species and life stages, underscoring complex regeneration strategies in mixed‐species forests. 

Abstract Abundance-weighted averaging is a simple and common method for estimating taxon preferences (optima) for phosphorus (P) and other environmental drivers of freshwater-ecosystem health. These optima can then be used to develop transfer functions to infer current and/or past environmental conditions of aquatic ecosystems in water-quality assessments and/or paleolimnological studies. However, estimates of species’ environmental preferences are influenced by the sample distribution and length of environmental gradients, which can differ between datasets used to develop and apply a transfer function. Here, we introduce a subsampling method to ensure a uniform and comparable distribution of samples along a P gradient in two similar ecosystems: the Everglades Protection Areas (EPA) and Big Cypress National Preserve (BICY) in South Florida, USA. Diatom optima were estimated for both wetlands using weighted averaging of untransformed and log-transformed periphyton mat total phosphorus (mat TP) values from the original datasets. We compared these estimates to those derived from random subsets of the original datasets. These subsets, referred to as “SUD” datasets, were created to ensure a uniform distribution of mat TP values along the gradient (both untransformed and log-transformed). We found that diatom assemblages in BICY and EPA were similar, dominated by taxa indicating oligotrophic conditions, and strongly influenced by P gradients. However, the original BICY datasets contained more samples with elevated mat TP concentrations than the EPA datasets, introducing a mathematical bias and resulting in a higher abundance of taxa with high mat TP optima in BICY. The weighted averaged mat TP optima of BICY and EPA taxa were positively correlated across all four dataset types, with taxa optima of SUD datasets exhibiting higher correlations than in the original datasets. Equalizing the mat TP sample distribution in the two datasets confirmed consistent mat TP estimates for diatom taxa between the two wetland complexes and improved transfer-function performance. Our findings suggest that diatom environmental preferences may be more reliable across regional scales than previously suggested and support the application of models developed in one region to another nearby region if environmental gradient lengths are equalized and data distribution along gradients is uniform. 

Abstract Seagrasses are increasingly recognized for their ecosystem functions and services. However, both natural and anthropogenic stressors impact seagrass functional traits, for example by altering nutrient regimes. Here, we synthesize 27 yr of data from regional, long‐term seagrass and water quality monitoring programs of south Florida to investigate the impacts of relative nutrient availability on seagrass abundance (as expressed by percent cover) across an oligotrophic seascape. We employ linear mixed‐effect models and generalized additive models to show that seagrass abundance is driven by interannual variations in nutrient concentrations, which are ultimately controlled by climate oscillations (El Niño Southern Oscillation Atlantic Multidecadal Oscillation) via regional rainfall‐runoff relationships. Our study suggests that climate oscillations drive interannual variations in seagrass cover on a regional scale, with high‐rainfall years leading to increased nitrogen availability and higher seagrass abundance in typically nitrogen‐limited backreef meadows. Conversely, these periods are associated with reduced seagrass cover at the more P‐limited inshore sites and in Florida Bay, with yet unknown consequences for the provision of seagrass ecosystem services. We show that nutrient delivery from runoff can have diverging impacts on benthic communities, depending on spatial patterns of relative nutrient limitation, with some N‐limited seagrass meadows showing resilience to periodic nutrient enrichment. 

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 Mangrove forests are typically considered resilient to natural disturbances, likely caused by the evolutionary adaptation of species‐specific traits. These ecosystems play a vital role in the global carbon cycle and are responsible for an outsized contribution to carbon burial and enhanced sedimentation rates. Using eddy covariance data from two coastal mangrove forests in the Florida Coastal Everglades, we evaluated the impact hurricanes have on mangrove forest structure and function by measuring recovery to pre‐disturbance conditions following Hurricane Wilma in 2005 and Hurricane Irma in 2017. We determined the “recovery debt,” the deficit in ecosystem structure and function following a disturbance, using the leaf area index (LAI) and the net ecosystem exchange (NEE) of carbon dioxide (CO₂). Calculated as the cumulative deviation from pre‐disturbance conditions, the recovery debt incorporated the recapture of all the carbon lost due to the disturbance. In Everglades mangrove forests, LAI returned to pre‐disturbance levels within a year, and ecosystem respiration and maximum photosynthetic rates took much longer, resulting in an initial recovery debt of 178 g C m⁻²at the tall forest with limited impacts at the scrub forest. At the landscape scale, the initial recovery debt was 0.40 Mt C, and in most coastal mangrove forests, all lost carbon was recovered within just 4 years. While high‐intensity storms could have prolonged impacts on the structure of subtropical forests, fast canopy recovery suggests these ecosystems will remain strong carbon sinks. 

Abstract Ecosystem engineering is a facilitative interaction that generates bottom‐up extrinsic variability that may increase species coexistence, particularly along a stress/disturbance gradient. American alligators (Alligator mississippiensis) create and maintain ‘alligator ponds’ that serve as dry‐season refuges for other animals. During seasonal water recession, these ponds present an opportunity to examine predictions of the stress‐gradient (SGH) and intermediate disturbance hypotheses (IDH).To test the assumption that engineering would facilitate species coexistence in ponds along a stress gradient (seasonal drying), we modelled fish catch‐per‐unit‐effort (CPUE) in ponds and marshes using a long‐term dataset (1997–2022). Stomach contents (n = 1677 from 46 species) and stable isotopes of carbon and nitrogen (n = 3978 representing 91 taxa) from 2018 to 2019 were used to evaluate effects of engineering on trophic dynamics. We quantified diets, trophic niche areas, trophic positions and basal‐resource use among habitats and between seasons. As environmental stress increases, we used seasonal changes in trophic niche areas as a proxy for competition to examine SGH and IDH.Across long‐term data, fish CPUE increased by a factor of 12 in alligator ponds as the marsh dried. This validates the assumption that ponds are an important dry‐season refuge. We found that 73% of diet shifts occurred during the dry season but that diets differed among habitats in only 11% of comparisons. From wet season to dry season, both stomach contents and stable isotopes revealed changes in niche areas. Direction of change depended on trophic guild but was opposite between stable‐isotope and stomach‐content niches, except for detritivores.Stomach‐content niches generally increased suggesting decreased competition in the dry season consistent with existing theory, but stable‐isotope niches yielded the opposite. This may result from a temporal mismatch with stomach contents reflecting diets over hours, while stable isotopes integrate diet over weeks. Consumptive effects may have a stronger effect than competition on niche areas over longer time intervals.Overall, our results demonstrated that alligators ameliorated dry‐season stress by engineering deep‐water habitats and altering food‐web dynamics. We propose that ecosystem engineers facilitate coexistence at intermediate values of stress/disturbance consistent with predictions of both the SGH and IDH.