Do community changes persist after irruptive population dynamics? A case study from an invasive species boom and bust.

Irruptive or boom-and-bust population dynamics, also known as 'outbreaks', are an important phenomenon that has been noted in biological invasions at least since Charles Elton's classic book was published in 1958. Community-level consequences of irruptive dynamics are poorly documented and invasive species provide excellent systems for their study. African Jewelfish (Rubricatochromis letourneuxi, "jewelfish") are omnivores that demonstrate opportunistic carnivory, first reported in Florida in the 1960s and in Everglades National Park (ENP) in 2000. Twelve years after invasion in ENP, jewelfish underwent a 25-fold increase in density in one year. By 2016, jewelfish represented 25-50% of fish biomass. Using a 43-year fish community dataset at two sites (1978-2021), and a 25-year dataset of fish and invertebrate communities from the same drainage (1996-2021), with additional spatial coverage, we quantified differences in fish and invertebrate communities during different phases of invasion. During jewelfish boom, abundant, native cyprinodontiform fishes decreased in density and drove changes in community structure as measured by similarity of relativized abundance. Density of two species declined by > 70%, while four declined by 50-62%. Following the jewelfish bust, some species recovered to pre-boom densities while others did not. Diversity of recovery times produced altered community structure that lagged for at least four years after the jewelfish population declined. Community structure is an index of ecological functions such as resilience, productivity, and species interaction webs; therefore, these results demonstrate that irruptive population dynamics can alter ecological functions of ecosystems mediated by community structure for years following that population's decline.

Phenotypic plasticity in the sailfin molly III: Geographic variation in reaction norms of growth and maturation to temperature and salinity.

Phenotypic plasticity, the ability of a single genotype to produce different phenotypes under different environmental conditions, plays a profound role in several areas of evolutionary biology. One important role is as an adaptation to a variable environment. While plasticity is extremely well documented in response to many environmental factors, there is controversy over how much of that plasticity is adaptive. Evidence is also mixed over how often conspecific populations display qualitative differences in the nature of plasticity. We present data on the reaction norms of growth and maturation to variation in temperature and salinity in male and female sailfin mollies (Poecilia latipinna) from three locally adjacent populations from South Carolina (SC). We compare these reaction norms to those previously reported in locally adjacent populations from north Florida (NF). In general, patterns of plasticity in fish from SC were similar to those in fish from NF. The magnitude of plasticity differed; fish from SC displayed less plasticity than fish from NF. This was because SC fish grew faster and matured earlier at the lower temperatures and salinities compared to NF fish. This is a countergradient pattern of variation, in which SC fish grew faster and matured earlier in conditions that would otherwise slow growth and delay maturity. Among fish from both regions, males were much less plastic than females, especially for length at maturity. While there was no detectable heterogeneity among populations from NF, males from one of the SC populations, which is furthest from the other two, displayed a qualitatively different response in age at maturity to temperature variation than did males from the other two SC populations. The pattern of population variation in plasticity within and among regions suggests that gene flow, which diminishes with distance in sailfin mollies, plays a critical role in constraining divergence in norms of reaction.

An apex predator engineers wetland food-web heterogeneity through nutrient enrichment and habitat modification.

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 Alligator Alligator mississippiensis is 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.

Hydrology-mediated ecological function of a large wetland threatened by an invasive predator.

Invasive species are one of the greatest threats to ecosystems, disrupting ecosystem function and leading to the collapse and extinction of native species. While populations of native fishes in the Everglades are tied to the system's natural hydrological dynamics, Asian Swamp Eels (Monopterus albus/javanensis) are drought-resistant fish first reported from Florida in 1997 and the Everglades in 2007. Using a 26-year dataset that included a 13-year baseline period prior to swamp eel arrival in Taylor Slough, we assessed population changes of common small fishes and decapods that are important prey for larger vertebrate predators. After invasion, populations of two crayfishes collapsed by >95 %, two fishes declined by >80 %, two fishes had intermediate declines of 44-66 %, and three species remained unchanged. Species most strongly reduced were those dependent on predator-free habitats at the onset of the wet season, indicating drought-resistant swamp eels have introduced novel predator effects and disrupted the hydrology-mediated production of aquatic animals that are prey for many larger predators. Ongoing Everglades restoration is designed to restore hydrological conditions that support production of crayfishes and fishes, and nesting wading birds reliant on them. Water management may have facilitated the invasion of swamp eels. Our results suggest that the continued spread of swamp eels may result in adverse consequences for Everglades trophic dynamics and potentially diminish benefits expected from the $20B+ restoration.

Timing and magnitude of net methylmercury effects on waterbird reproductive output are dependent on food availability.

Mercury (Hg) is a globally distributed pollutant. Its sub-lethal effects on reproduction of birds have been used as indicators of contamination and of potential demographic effects. However, studies typically used single endpoints that might not be representative of entire reproductive cycle. To estimate timing and net cumulative effects of Hg exposure under field conditions, we used observational data over 11 years from >1200 nests of great egrets breeding under temporally and spatially varying food availability and Hg exposures in the Florida Everglades. We collected measures of fish biomass and availability (>100 locations annually) and used four avian reproductive endpoints that represented the entire breeding cycle. We calculated net reproductive loss by adding estimated Hg effects on failures prior to egg laying, clutch size, hatching success and nestling survival in response to food availability and Hg exposure. To validate and assess results of the observational egret study, we ran the same analyses with data of captive breeding white ibises experimentally exposed to Hg with ad libitum food over 3 years. We found large (>50 %) reductions in great egret offspring with high Hg exposure (18 µg/g dw THg nestling feather, ~0.7 µg/g ww whole egg THg) and high food availability, and even larger reductions (up to 100 %) with high Hg exposure and low food. Timing and the relative contribution of different endpoints to overall reproductive failure varied with food availability. Failures prior to egg laying were relevant at all food availabilities and proportionally most important during high food availability (~70 % of total losses). Under high food, post-hatching failures increased moderately with increasing exposure (~10 % of total losses), and under low food, hatching failures became dominant (~50 % of total losses). Patterns of failure of captive white ibis fed ad libitum resembled those of great egrets under high food availability but differed in total magnitude. We suggest that, a) net reproductive effects of Hg in free-ranging animals are probably much higher than generally reported in studies using single endpoints, b) Hg effect sizes vary considerably among different endpoints and c) food availability is a strong driver of timing and net effects of Hg exposure.

Accounting for food availability reveals contaminant-induced breeding impairment, food-modulated contaminant effects, and endpoint-specificity of exposure indicators in free ranging avian populations.

It remains unclear how sub-lethal effects of contaminants play out in relation to other stressors encountered by free-ranging populations. Effects may be masked or influenced by interactions with field stressors such as food availability. We predicted that (1) including food availability, and particularly its interaction with Hg, would reveal or enhance associations between Hg and breeding endpoints. We further predicted that (2) breeding impairment associated with Hg would be higher under food stress conditions. We monitored Hg and nest success of great egrets (Ardea alba) in eight breeding colonies in the Florida Everglades over 11 years. We characterized variation in local food availability among colonies and years using fish biomass and recession range -a proxy to fish vulnerability. We used two Hg exposure indicators (egg albumen Hg and nestling feather Hg) and six breeding endpoints (clutch-size, brood-size, fledged-size, hatching success, post-hatching success and fledglings per egg) to assess whether variation in food availability influenced associations between Hg and these endpoints. Accounting for interactions between Hg and food availability, we identified statistically significant associations in all 12 indicator-endpoint combinations, while only three were detectable without food. Further, 10 combinations showed interactions between Hg and components of food availability. Our results also indicated an endpoint-specific affinity, with albumen [Hg] explaining more variation in hatching success while nestling feather [Hg] explained more variation in post-hatching survival. Both Hg indicators accounted for relevant (6-10%) amounts of variation in fledglings produced per egg laid, an integrative endpoint. Increased Hg exposure resulted in overall reduced reproductive success when food availability was low, but our models predicted low or no effects of increasing Hg exposure when food availability was high. Our results indicate that Hg induced impairment is strongly driven by food availability, providing a framework that accommodates previously contradictory results in the literature.

Early Breeding Failure in Birds Due to Environmental Toxins: A Potentially Powerful but Hidden Effect of Contamination.

Toxin emissions and legacies are major global issues affecting many species through, among other effects, endocrine disruption and reproductive impairment. Assessment of toxin risk to wildlife focuses mostly on offspring-related metrics, while the lack of breeding initiation or early breeding failure has received less attention. We tested whether exposure to methyl mercury (MeHg) results in early breeding failure and reduced number of breeding birds using observational and experimental data. We used 21 years of numbers of breeding pairs of colonially breeding wild Great Egrets (Ardea alba) in response to annual and geographical variation upon exposure to environmental MeHg. After controlling for food availability, we found a strong negative association between MeHg exposure and the number of breeding Great Egrets. We report reductions of >50% in breeding numbers under exposure levels otherwise associated with <20% reduction in post-egg-laying breeding success. Experimental exposure of White Ibises (Eudocimus albus) to MeHg also caused early breeding failure and a ∼20% reduction in breeding numbers at environmentally relevant exposures. The demographic consequences of reductions in breeding pairs are additive to known and typically studied impairments in postlaying reproductive success. Net demographic effects of exposure to endocrine disruptors may often be strongly underestimated if early breeding failure is not measured.

Correction to: Speed and directedness predict colonization sequence post-disturbance.

Authors would like to update the incorrect version of Fig. 4 which was incorrectly published in original publication.

Speed and directedness predict colonization sequence post-disturbance.

Foundational ecological models characterize dispersal with two behavioral traits, speed and directional bias. We hypothesized that these two traits can predict the order of colonization by fishes in a heterogenous landscape. Colonization patterns following hydrological disturbance were documented from a 20-year multi-site time series of marsh fish, and we evaluated the ability of a two-parameter model to predict these patterns. The maximum aerobic swimming speed (UCRIT) for six coexisting fish species were estimated using endurance tests; field estimates of directedness and swimming speed were previously documented using encounter samplers. We incorporated interspecific variation in speed, direction, and density into several Agent Based Models to simulate dispersal following disturbance. Six virtual "species" with varying levels of directedness, "swam" in an artificial environment to reach a refuge habitat. The time of first arrival for each species was saved at the end of each run and used to calculate the probability of arrival order. Our simulated results generated predictions on order of arrival consistent with observed colonization patterns in our long-term dataset. Swim tunnel results revealed that fast (high UCRIT) estimates were characteristic of early colonizing species; whereas, slow (low UCRIT) estimates were characteristic of late colonizing species. Directional bias better predicted order of arrival than speed and was robust to inter-specific variation in density. This study demonstrated that two parameters were adequate to predict the order of species colonization in a complex landscape. These results support the use of relatively simple trait-based models to generate realistic community assembly dynamics.

When is an herbivore not an herbivore? Detritivory facilitates herbivory in a freshwater system.

Herbivory is thought to be an inefficient diet, but it independently evolved from carnivorous ancestors in many metazoan groups, suggesting that plant-eating is adaptive in some circumstances. In this study, we tested two hypotheses to explain the adaptive evolution of herbivory: (i) the Heterotroph Facilitation hypothesis (herbivory is adaptive because herbivores supplement their diets with heterotrophic microbes); and (ii) the Lipid Allocation hypothesis (herbivory is adaptive because algae, which have high lipid concentrations, are nutritionally similar to carnivory). We tested these hypotheses using enclosure cages placed in the Everglades and stocked with Sailfin Mollies (Poecilia latipinna), a native herbivore. Using shading and phosphorus addition (P), we manipulated the heterotrophic microbe and lipid composition of colonizing epiphyton and examined the effects of varying food quality on Sailfin Molly life history. Epiphyton grown in "shade only" conditions had a 55% increase in bacterial fatty acids and 34% lower ratios of saturated + monounsaturated to polyunsaturated fatty acids relative to the other treatments. Ratio of autotroph to heterotroph biovolume varied throughout the experiment, with a 697% increase at 3 weeks and 98% decrease at 6 weeks compared to the other treatments. Gut contents revealed that fish fed selectively on epiphyton to compensate for apparent deficiencies in the available food. Fish raised in "shade only" cages experienced the highest survival, which was best explained by autotrophic biovolume and algal- and bacterial-derived fatty acids at 3 weeks (2-6× more likely than alternative models with ∆AICc > 2.00), and by percentage of bacterial fatty acids in the diet at 6 weeks (3-8× more likely than alternative models with ∆AICc > 2.00). There were no differences in fish growth among treatments. Autotrophic lipids play a role in early fish life history, but we did not find these to be the best predictors of life history later in the juvenile period. Instead, heterotrophic lipids facilitated the herbivorous diet and enhanced survival of juvenile fish in our experiment. Bacterial fatty acid content of the diet promoted herbivore survival, consistent with the Heterotroph Facilitation hypothesis. This is the first study to explicitly contrast Heterotrophic Facilitation and Lipid Allocation hypotheses for the adaptive evolution of herbivory in an aquatic system.

Mechanisms That Generate Resource Pulses in a Fluctuating Wetland.

Animals living in patchy environments may depend on resource pulses to meet the high energetic demands of breeding. We developed two primary a priori hypotheses to examine relationships between three categories of wading bird prey biomass and covariates hypothesized to affect the concentration of aquatic fauna, a pulsed resource for breeding wading bird populations during the dry season. The fish concentration hypothesis proposed that local-scale processes concentrate wet-season fish biomass into patches in the dry season, whereas the fish production hypothesis states that the amount of dry-season fish biomass reflects fish biomass production during the preceding wet season. We sampled prey in drying pools at 405 sites throughout the Florida Everglades between December and May from 2006-2010 to test these hypotheses. The models that explained variation in dry-season fish biomass included water-level recession rate, wet-season biomass, microtopography, submerged vegetation, and the interaction between wet-season biomass and recession rate. Crayfish (Procambarus spp.) biomass was positively associated with wet-season crayfish biomass, moderate water depth, dense submerged aquatic vegetation, thin flocculent layer and a short interval of time since the last dry-down. Grass shrimp (Palaemonetes paludosus) biomass increased with increasing rates of water level recession, supporting our impression that shrimp, like fish, form seasonal concentrations. Strong support for wet-season fish and crayfish biomass in the top models confirmed the importance of wet-season standing stock to concentrations of fish and crayfish the following dry season. Additionally, the importance of recession rate and microtopography showed that local scale abiotic factors transformed fish production into the high quality foraging patches on which apex predators depended.

Reciprocal transplant reveals trade-off of resource quality and predation risk in the field.

Balancing trade-offs between avoiding predators and acquiring food enables animals to maximize fitness. Quantifying their relative contribution to vital rates in nature is challenging because predator abundance and nutrient enrichment are often confounded. We employed a reciprocal transplant study design to separate these confounded effects on growth and reproduction of snails at wetland sites along a gradient of predator threats and phosphorus (P) enrichment associated with a canal. We held snails in mesh bags that allowed the passage of waterborne predator cues and fed them local or transplanted periphyton. Molluscivores were more abundant near the canal, and snails tethered near the canal suffered 33% greater mortality than those tethered far from it (far sites). The greatest difference in snail growth rates was at the far sites where growth on far periphyton was 48% slower than on P-enriched (near canal) periphyton. Close proximity to the canal reduced growth on near periphyton by 21% compared to growth on the same periphyton far from the canal; there was no difference in growth rate on either periphyton type when snails were raised near the canal. Snails laid 81% more egg masses at far sites than at near sites, regardless of periphyton origin. Top-down and bottom-up processes were elevated near the canal, and their effects canceled on growth, but not reproduction. Phenotypic trade-offs such as these may explain why some taxa show little response to nutrient enrichment, compared to others, or that the effects of nutrient enrichment may be context dependent.

Using scenario planning to evaluate the impacts of climate change on wildlife populations and communities in the Florida Everglades.

It is uncertain how climate change will impact hydrologic drivers of wildlife population dynamics in freshwater wetlands of the Florida Everglades, or how to accommodate this uncertainty in restoration decisions. Using projections of climate scenarios for the year 2060, we evaluated how several possible futures could affect wildlife populations (wading birds, fish, alligators, native apple snails, amphibians, threatened and invasive species) across the Everglades landscape and inform planning already underway. We used data collected from prior research and monitoring to parameterize our wildlife population models. Hydrologic data were simulated using a spatially explicit, regional-scale model. Our scenario evaluations show that expected changes in temperature, precipitation, and sea level could significantly alter important ecological functions. All of our wildlife indicators were negatively affected by scenarios with less rainfall and more evapotranspiration. Under such scenarios, habitat suitability was substantially reduced for iconic animals such as wading birds and alligators. Conversely, the increased rainfall scenario benefited aquatic prey productivity and apex predators. Cascading impacts on non-native species is speculative, but increasing temperatures could increase the time between cold events that currently limit expansion and abundance of non-native fishes, amphibians, and reptiles with natural ranges in the tropics. This scenario planning framework underscored the benefits of proceeding with Everglades restoration plans that capture and clean more freshwater with the potential to mitigate rainfall loss and postpone impacts of sea level rise.

Genetic evidence for multiple sources of the non-native fish Cichlasoma urophthalmus (Günther; Mayan Cichlids) in southern Florida.

The number and diversity of source populations may influence the genetic diversity of newly introduced populations and affect the likelihood of their establishment and spread. We used the cytochrome b mitochondrial gene and nuclear microsatellite loci to identify the sources of a successful invader in southern Florida, USA, Cichlasoma urophthalmus (Mayan cichlid). Our cytochrome b data supported an introduction from Guatemala, while our microsatellite data suggested movement of Mayan Cichlids from the upper Yucatán Peninsula to Guatemala and introductions from Guatemala and Belize to Florida. The mismatch between mitochondrial and nuclear genomes suggests admixture of a female lineage from Guatemala, where all individuals were fixed for the mitochondrial haplotype found in the introduced population, and a more diverse but also relatively small number of individuals from Belize. The Florida cytochrome b haplotype appears to be absent from Belize (0 out of 136 fish screened from Belize had this haplotype). Genetic structure within the Florida population was minimal, indicating a panmictic population, while Mexican and Central American samples displayed more genetic subdivision. Individuals from the Upper Yucatán Peninsula and the Petén region of Guatemala were more genetically similar to each other than to fish from nearby sites and movement of Mayan Cichlids between these regions occurred thousands of generations ago, suggestive of pre-Columbian human transportation of Mayan Cichlids through this region. Mayan Cichlids present a rare example of cytonuclear disequilibrium and reduced genetic diversity in the introduced population that persists more than 30 years (at least 7-8 generations) after introduction. We suggest that hybridization occurred in ornamental fish farms in Florida and may contribute their establishment in the novel habitat. Hybridization prior to release may contribute to other successful invasions.

Perceived risk of predation affects reproductive life-history traits in Gambusia holbrooki, but not in Heterandria formosa.

Key to predicting impacts of predation is understanding the mechanisms through which predators impact prey populations. While consumptive effects are well-known, non-consumptive predator effects (risk effects) are increasingly being recognized as important. Studies of risk effects, however, have focused largely on how trade-offs between food and safety affect fitness. Less documented, and appreciated, is the potential for predator presence to directly suppress prey reproduction and affect life-history characteristics. For the first time, we tested the effects of visual predator cues on reproduction of two prey species with different reproductive modes, lecithotrophy (i.e. embryonic development primarily fueled by yolk) and matrotrophy (i.e. energy for embryonic development directly supplied by the mother to the embryo through a vascular connection). Predation risk suppressed reproduction in the lecithotrophic prey (Gambusia holbrokii) but not the matrotroph (Heterandria formosa). Predator stress caused G. holbrooki to reduce clutch size by 43%, and to produce larger and heavier offspring compared to control females. H. formosa, however, did not show any such difference. In G. holbrooki we also found a significantly high percentage (14%) of stillbirths in predator-exposed treatments compared to controls (2%). To the best of our knowledge, this is the first direct empirical evidence of predation stress affecting stillbirths in prey. Our results suggest that matrotrophy, superfetation (clutch overlap), or both decrease the sensitivity of mothers to environmental fluctuation in resource (food) and stress (predation risk) levels compared to lecithotrophy. These mechanisms should be considered both when modeling consequences of perceived risk of predation on prey-predator population dynamics and when seeking to understand the evolution of reproductive modes.

Space-for-time substitution works in everglades ecological forecasting models.

Space-for-time substitution is often used in predictive models because long-term time-series data are not available. Critics of this method suggest factors other than the target driver may affect ecosystem response and could vary spatially, producing misleading results. Monitoring data from the Florida Everglades were used to test whether spatial data can be substituted for temporal data in forecasting models. Spatial models that predicted bluefin killifish (Lucania goodei) population response to a drying event performed comparably and sometimes better than temporal models. Models worked best when results were not extrapolated beyond the range of variation encompassed by the original dataset. These results were compared to other studies to determine whether ecosystem features influence whether space-for-time substitution is feasible. Taken in the context of other studies, these results suggest space-for-time substitution may work best in ecosystems with low beta-diversity, high connectivity between sites, and small lag in organismal response to the driver variable.

Large-scale natural disturbance alters genetic population structure of the sailfin molly, Poecilia latipinna.

Many inferences about contemporary rates of gene flow are based on the assumption that the observed genetic structure among populations is stable. Recent studies have uncovered several cases in which this assumption is tenuous. Most of those studies have focused on the effects that regular environmental fluctuations can have on genetic structure and gene flow patterns. Occasional catastrophic disturbances could also alter either the distribution of habitat or the spatial distribution of organisms in a way that affects population structure. However, evidence of such effects is sparse in the literature because it is difficult to obtain. Hurricanes, in particular, have the potential to exert dramatic effects on population structure of organisms found on islands or coral reefs or in near shore and coastal habitats. Here we draw on a historic genetic data set and new data to suggest that the genetic structure of sailfin molly (Poecilia latipinna) populations in north Florida was altered dramatically by an unusually large and uncommon type of storm surge associated with Hurricane Dennis in 2005. We compare the spatial pattern of genetic variation in these populations after Hurricane Dennis to the patterns described in an earlier study in this same area. We use comparable genetic data from another region of Florida, collected in the same two periods, to estimate the amount of change expected from typical temporal variation in population structure. The comparative natural history of sailfin mollies in these two regions indicates that the change in population structure produced by the storm surge is not the result of many local extinctions with recolonization from a few refugia but emerged from a pattern of mixing and redistribution.

Temporal population genetic structure of eastern mosquitofish in a dynamic aquatic landscape.

We analyzed the effect of periodic drying in the Florida Everglades on spatiotemporal population genetic structure of eastern mosquitofish (Gambusia holbrooki). Severe periodic drying events force individuals from disparate sources to mix in dry season relatively deep-water refuges. In 1996 (a wet year) and 1999 (a dry year), we sampled mosquitofish at 20 dry-season refuges distributed in 3 water management regions and characterized genetic variation for 10 allozyme and 3 microsatellite loci. In 1996, most of the ecosystem did not dry, whereas in 1999, many of our sampling locations were isolated by expanses of dried marsh surface. In 1996, most spatial genetic variation was attributed to heterogeneity within regions. In 1999, spatial genetic variation within regions was not significant. In both years, a small but significant amount of variation (less than 1% of the total variation) was partitioned among regions. Variance was consistently greater than zero among long-hydroperiod sites within a region, but not among short-hydroperiod sites within a region, where hydroperiod was measured as time since last marsh surface dry-down forcing fishes into local refuges. In 1996, all sites were in Hardy-Weinberg equilibrium. In 1999, we observed fewer heterozygotes than expected for most loci and sites suggesting a Wahlund effect arising from fish leaving areas that dried and mixing in deep-water refuges.

Food web dynamics in a seasonally varying wetland.

A spatially explicit model is developed to simulate the small fish community and its underlying food web, in the freshwater marshes of the Everglades. The community is simplified to a few small fish species feeding on periphyton and invertebrates. Other compartments are detritus, crayfish, and a piscivorous fish species. This unit food web model is applied to each of the 10,000 spatial cells on a 100 x 100 pixel landscape. Seasonal variation in water level is assumed and rules are assigned for fish movement in response to rising and falling water levels, which can cause many spatial cells to alternate between flooded and dry conditions. It is shown that temporal variations of water level on a spatially heterogeneous landscape can maintain at least three competing fish species. In addition, these environmental factors can strongly affect the temporal variation of the food web caused by top-down control from the piscivorous fish.