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Boom-bust population dynamics are long-recognized phenomena during species invasions, but few studies documented impacts of these dynamic changes. The Florida Everglades is the largest wetland in the United States, is undergoing a multi-decade hydro-restoration effort, and has been invaded by several tropical freshwater fishes. We used a 26-year dataset of small native marsh fishes and decapods to assess potential effects of African Jewelfish (Hemichromis letourneuxi) invasion and compared their effects to those of a more recently invading species, Asian Swamp Eels (Monopterus albus/javanensis), and a long-established non-native species, Mayan Cichlids (Mayaheros urophthalmus). Unlike boom-bust dynamics of jewelfish, swamp eel abundance increased and stabilized over the course of this study. After accounting for effects of hydrologic variation, the densities of several native species were more reduced by either jewelfish or swamp eels than by native fish predators, while effects of Mayan Cichlids were similar to those of native fish predators. Impacts of the jewelfish boom in Shark River Slough were smaller (density reductions ≤ 50%) and more temporally limited than those of swamp eels, which produced near-complete loss of four species in Taylor Slough. Following the jewelfish bust, the density of affected species approximated pre-invasion predictions based on hydrology, but their recovery is now threatened by the subsequent invasion of swamp eels in Shark River Slough. Long-term monitoring data provide opportunities to probe for population-level effects at field scales, and indicate that impacts of non-native species can be context-dependent and vary across ecosystems and temporal scales. 

The predator-permanence hypothesis predicts that as hydroperiod increases in lentic ecosystems, biotic interactions—mainly predation—replace physical factors like drying as the main determinant of community structure and population dynamics. We propose that the same transition occurs over time in seasonally flooded ecosystems that are connected to permanent water bodies. To test for evidence of successional changes that are similar to spatial changes in the relative importance of drying and predation, we used a 12-y time series of snail density, predator density, and water depth at 4 sites arranged along a nutrient gradient in a subtropical, seasonally flooded wetland, the Florida Everglades, USA. The rate of change in snail population size was negatively correlated with their density at all 4 sites, suggesting that density-dependent factors such as resource limitation regulate snail dynamics. The strength of the relationship varied among sites such that when water depth changes were less important, snail population size was more important in predicting changes in snail population size. At the site that consistently had the greatest snail density, crayfish density negatively affected the rate of snail population change, suggesting that crayfish predation may limit snail population growth in areas with more or higher-quality resources that support larger snail populations. Tethering studies were also conducted, which revealed higher snail mortality in the wet season, primarily because crushing predators (e.g., molluscivorous fishes) were more common at that time and added to the chronic mortality by entry-based predators (e.g., crayfish, which access snails through their aperture). In summary, 3 of the sites resembled temporary or permanent fishless ponds where snail populations were primarily structured by abiotic factors, intraspecific competition, and invertebrate predators (e.g., crayfish) during the wet season, whereas 1 site showed evidence that snail populations were also influenced by molluscivorous fish. This temporal change in importance of water permanence factors to fish that affected population dynamics supports the spatial pattern proposed by the predator-permanence hypothesis. 

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.

 

Recruitment has been linked to decreases in the ratio of age-specific mortality (M′) to mass-specific growth (G′), and year-class strength may be predicted by the age when M′/G′ = 1. Hydrological stress adversely affects these parameters for species inhabiting floodplains; however, the relationship between M′ and G′ in hydrologically variable environments is poorly understood. We evaluated age-specific mortality for six species from a 20-year time series and growth curves from otolith length-at-age data. We assessed the effect of hydrology on the transitional age (age M′/G′ = 1) at 21 sites representing a hydrological gradient. Disturbance intensity influenced age-specific mortality but had no effect on mass-specific growth. The transitional age was inversely correlated with annual density, but weakly associated with population biomass. Hydrological disturbance shifted the transitional age to older ages, reducing recruitment overall. We demonstrated that the M′/G′ transition was affected adversely by hydrological stress and can be applied to a diverse group of taxa. Growth, survivorship, and the transitional age should be evaluated to improve population modelling efforts used to predict the influence of future restoration actions.