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.

Physiological performance measures and tolerance limits of estuarine indicator species in south Florida

Current water management practices in South Florida have negatively impacted many species inhabiting Florida Bay. Variable and high salinity has been identified as a key stressor in these estuaries. The Comprehensive Everglades Restoration Plan (CERP) includes water redistribution projects that will restore natural freshwater flows to northeastern Florida Bay. My studies focused on the following central theme and hypotheses: Biological performance measures (i.e., growth, reproduction, survival), behavior (i.e., habitat preference and locomotor behavior) and diversity of estuarine fish will be controlled by changes in salinity and water quality that will occur as a result of the restoration of freshwater flow to the bay. A series of acute and subchronic physiological toxicity studies were conducted to determine the effects of salinity changes on the life stages (embryo/larval, juvenile, adult) and fecundity of four native estuarine fish (Cyprinodon variegatus, Floridichthys carpio, Poecilia latipinna, and Gambusia holbrooki). Fish were exposed to a range of salinity concentrations (freshwater to hypersaline) based on salinity profiles in the study areas. Growth (length, weight) and survival were measured. Salinity trials included both rapid and gradual change events. Results show negative effects of acute, abrupt salinity changes on fish survival, development and reproductive success as a result of salinity stress. Other studies targeted reproduction and critical embryo-larval/neonate development as key areas for detecting long-term population effects of salinity change in Florida Bay. Adults of C. variegates and P. latipinna were also examined for behavioral responses to pulsed salinity changes. These responses include changes in swimming performance, locomotor behavior and zone preference. Finally, an ecological risk assessment was conducted for adverse salinity conditions in northeastern Florida Bay. Using the U.S. EPA's framework, the risk to estuarine fish species diversity was assessed against regional salinity profiles from a 17-year database. Based on the risk assessment, target salinity profiles for these areas are recommended for managers.^

Antipredator Responses by Native Mosquitofish to Non-Native Cichlids: An Examination of the Role of Prey Naiveté

The strong impact of non-native predators in aquatic systems is thought to relate to the evolutionary naiveté of prey. Due to isolation and limited dispersal, this naiveté may be relatively high in freshwater systems. In this study, we tested this notion by examining the antipredator response of native mosquitofish, Gambusia holbrooki, to two non-native predators found in the Everglades, the African jewelfish, Hemichromis letourneuxi, and the Mayan cichlid, Cichlasoma urophthalmus. We manipulated prey naiveté by using two mosquitofish populations that varied in their experience with the recent invader, the African jewelfish, but had similar levels of experience with the longer-established Mayan cichlid. Specifically, we tested these predictions: (1) predator hunting modes differed between the two predators, (2) predation rates would be higher by the novel jewelfish predator, (3) particularly on the naive population living where jewelfish have not invaded yet, (4) antipredator responses would be stronger to Mayan cichlids due to greater experience and weaker and/or ineffective to jewelfish, and (5) especially weakest by the naive population. We assayed prey and predator behavior, and prey mortality in lab aquaria where both predators and prey were free-ranging. Predator hunting modes and habitat domains differed, with jewelfish being more active search predators that used slightly higher parts of the water column and less of the habitat structure relative to Mayan cichlids. In disagreement with our predictions, predation rates were similar between the two predators, antipredator responses were stronger to African jewelfish (except for predator inspections), and there was no difference in response between jewelfish-savvy and jewelfish-naive populations. These results suggest that despite the novelty of introduced predators, prey may be able to respond appropriately if non-native predator archetypes are similar enough to those of native predators, if prey rely on general antipredator responses or predation cues, and/or show neophobic responses.

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.

Using data from an encounter sampler to model fish dispersal

A method to estimate speed of free-ranging fishes using a passive sampling device is described and illustrated with data from the Everglades, U.S.A. Catch per unit effort (CPUE) from minnow traps embedded in drift fences was treated as an encounter rate and used to estimate speed, when combined with an independent estimate of density obtained by use of throw traps that enclose 1 m2 of marsh habitat. Underwater video was used to evaluate capture efficiency and species-specific bias of minnow traps and two sampling studies were used to estimate trap saturation and diel-movement patterns; these results were used to optimize sampling and derive correction factors to adjust species-specific encounter rates for bias and capture efficiency. Sailfin mollies Poecilia latipinna displayed a high frequency of escape from traps, whereas eastern mosquitofish Gambusia holbrooki were most likely to avoid a trap once they encountered it; dollar sunfish Lepomis marginatus were least likely to avoid the trap once they encountered it or to escape once they were captured. Length of sampling and time of day affected CPUE; fishes generally had a very low retention rate over a 24 h sample time and only the Everglades pygmy sunfish Elassoma evergladei were commonly captured at night. Dispersal speed of fishes in the Florida Everglades, U.S.A., was shown to vary seasonally and among species, ranging from 0· 05 to 0· 15 m s−1 for small poeciliids and fundulids to 0· 1 to 1· 8 m s−1 for L. marginatus. Speed was generally highest late in the wet season and lowest in the dry season, possibly tied to dispersal behaviours linked to finding and remaining in dry-season refuges. These speed estimates can be used to estimate the diffusive movement rate, which is commonly employed in spatial ecological models.

Hydroperiod and seasonal effects on fish decomposition in an oligotrophic Everglades marsh

In the Everglades, the majority of fish detrital inputs occur during the dry scason, when waterlevel drawdown reduces aquatic habitat. While these mortality events are highly seasonal, the remineralization and recycling of fish detrital nutrients may represent an important stimulus to the ecosystem in the following wet season. The goal of this study was to quantify the rate of detrital fish decomposition during three periods of the year to determine seasonal variations in decomposition patterns in this ecosystem. A multiple regression analysis showed that hydroperiod and water depth both played a role in determining fish decomposition rates within this ecosystem. Decomposition rates ranged from a low of 13% day−1 in December 2000 to a high of 50% day−1 in April 2001, the height of the dry season. Phosphorus analysis showed that Gambusia holbrooki, the dominant small fish species in the Everglades, contains 7.169±1.46 mg P g−1 wet fish weight. Based on the observed decomposition rates and the average biomass added, the estimafed daily flux of phosphorus from the experimental detrital loading ranged from a low of 27.04 mg P day−1 to a high of 108.14 mg P day−1 during the decomposition period. We estimated that these inputs could represent an input of 43 μg P m−2 day−1 to the total temporal Everglades phosphorus budget. Although much of this phosphorus is likely incorporated into the macroinvertebrate pool, detrital inputs peak during the dry season when nutrients are most likely to be incorporated into the soil and occur when decomposition of vegetative material is moisture-limited. These inputs may therefore play an important role in stimulating vegetative production during the early wet season.

A preliminary analysis of the correlation of food-web characteristics with hydrology and nutrient gradients in the southern Everglades

We estimated trophic position and carbon source for three consumers (Florida gar, Lepisosteus platyrhincus; eastern mosquitofish, Gambusia holbrooki; and riverine grass shrimp, Palaemonetes paludosus) from 20 sites representing gradients of productivity and hydrological disturbance in the southern Florida Everglades, U.S.A. We characterized gross primary productivity at each site using light/dark bottle incubation and stem density of emergent vascular plants. We also documented nutrient availability as total phosphorus (TP) in floc and periphyton, and the density of small fishes. Hydrological disturbance was characterized as the time since a site was last dried and the average number of days per year the sites were inundated for the previous 10 years. Food-web attributes were estimated in both the wet and dry seasons by analysis of δ15N (trophic position) and δ13C (food-web carbon source) from 702 samples of aquatic consumers. An index of carbon source was derived from a two-member mixing model with Seminole ramshorn snails (Planorbella duryi) as a basal grazing consumer and scuds (amphipods Hyallela azteca) as a basal detritivore. Snails yielded carbon isotopic values similar to green algae and diatoms, while carbon values of scuds were similar to bulk periphyton and floc; carbon isotopic values of cyanobacteria were enriched in C13compared to all consumers examined. A carbon source similar to scuds dominated at all but one study site, and though the relative contribution of scud-like and snail-like carbon sources was variable, there was no evidence that these contributions were a function of abiotic factors or season. Gar consistently displayed the highest estimated trophic position of the consumers studied, with mosquitofish feeding at a slightly lower level, and grass shrimp feeding at the lowest level. Trophic position was not correlated with any nutrient or productivity parameter, but did increase for grass shrimp and mosquitofish as the time following droughts increased. Trophic position of Florida gar was positively correlated with emergent plant stem density.

Relative roles of dispersal dynamics and competition in determining the isotopic niche breadth of a wetland fish

1. The niche variation hypothesis predicts that among-individual variation in niche use will increase in the presence of intraspecific competition and decrease in the presence of interspecific competition. We sought to determine whether the local isotopic niche breadth of fish inhabiting a wetland was best explained by competition for resources and the niche variation hypothesis, by dispersal of individuals from locations with different prey resources or by a combination of the two. We analysed stable isotopes of carbon and nitrogen as indices of feeding niche and compared metrics of within-site spread to characterise site-level isotopic niche breadth. We then evaluated the explanatory power of competing models of the direct and indirect effects of several environmental variables spanning gradients of disturbance, competition strength and food availability on among-individual variation of the eastern mosquitofish (Gambusia holbrooki). 2. The Dispersal model posits that only the direct effect of disturbance (i.e. changes in water level known to induce fish movement) influences among-individual variation in isotopic niche. The Partitioning model allows for only direct effects of local food availability on among-individual variation. The Combined model allows for both hypotheses by including the direct effects of disturbance and food availability. 3. A linear regression of the Combined model described more variance than models limited to the variables of either the Dispersal or Partitioning models. Of the independent variables considered, the food availability variable (per cent edible periphyton) explained the most variation in isotopic niche breadth, followed closely by the disturbance variable (days since last drying event). 4. Structural equation modelling provided further evidence that the Combined model was best supported by the data, with the Partitioning and the Dispersal models only modestly less informative. Again, the per cent edible periphyton was the variable with the largest direct effect on niche variability, with other food availability variables and the disturbance variable only slightly less important. Indirect effects of heterospecific and conspecific competitor densities were also important, through their effects on prey density. 5. Our results support the Combined hypotheses, although partitioning mechanisms appear to explain the most diet variation among individuals in the eastern mosquitofish. The results also support some predictions of the niche variation hypothesis, although both conspecific and interspecific competition appeared to increase isotopic niche breadth in contrast to predictions that interspecific competition would decrease it. We think this resulted from high diet overlap of co-occurring species, most of which consume similar macroinvertebrates.

Fish Community Responses to the Combined Effects of Decreased Hydroperiod and Nonnative Fish Invasions in a Karst Wetland: Are Everglades Solution Holes Sinks for Native Fishes?

Short-hydroperiod Everglades wetlands have been disproportionately affected by reductions in freshwater inflows, land conversion and biotic invasions. Severe hydroperiod reductions in these habitats, including the Rocky Glades, coupled with proximity to canals that act as sources of invasions, may limit their ability to support high levels of aquatic production. We examined whether karst solution holes function as dry-down refuges for fishes, providing a source of marsh colonists upon reflooding, by tracking fish abundance, nonnative composition, and survival in solution holes throughout the dry season. We paired field surveys with an in situ nonnative predation experiment that tested the effects of predation by the recent invader, African jewelfish (Hemichromis letourneuxi) on native fishes. Over the 3 years surveyed, a large number of the solution holes dried before the onset of the wet season, while those retaining water had low survivorship and were dominated by nonnatives. In the experiment, mortality of eastern mosquitofish (Gambusia holbrooki) in the presence of African jewelfish was greater than that associated with deteriorating water quality. Under current water management, findings suggest that solution holes are largely sinks for native fishes, given the high frequency of drydown, extensive period of fish residence, and predation by nonnative fishes.

Seasonal Fish Dispersal in Ephemeral Wetlands of the Florida Everglades

We hypothesized that fishes in short-hydroperiod wetlands display pulses in activity tied to seasonal flooding and drying, with relatively low activity during intervening periods. To evaluate this hypothesis, sampling devices that funnel fish into traps (drift fences) were used to investigate fish movement across the Everglades, U.S.A. Samples were collected at six sites in the Rocky Glades, a seasonally flooded karstic habitat located on the southeastern edge of the Everglades. Four species that display distinct recovery patterns following drought in long-hydroperiod wetlands were studied: eastern mosquitofish (Gambusia holbrooki) and flagfish (Jordanella floridae) (rapid recovery); and bluefin killifish (Lucania goodei) and least killifish (Heterandria formosa) (slow recovery). Consistent with our hypothesized conceptual model, fishes increased movement soon after flooding (immigration period) and just before drying (emigration period), but decreased activity in the intervening foraging period. We also found that eastern mosquitofish and flagfish arrived earlier and showed stronger responses to hydrological variation than either least killifish or bluefin killifish. We concluded that these fishes actively colonize and escape ephemeral wetlands in response to flooding and drying, and display species-specific differences related to flooding and drying that reflect differences in dispersal ability. These results have important implications for Everglades fish metacommunity dynamics.

Physiological performance measures and tolerance limits of estuarine indicator species in South Florida

Current water management practices in South Florida have negatively impacted many species inhabiting Florida Bay. Variable and high salinity has been identified as a key stressor in these estuaries. The comprehensive Everglades Restoration Plan (CERP) includes water redistribution projects that will restore natural freshwater flows to northeastern Florida Bay. My studies focused on the following central theme and hypotheses: Biological performance measures (i.e., growth, reproduction, survival), behavior (i.e., habitat preference and locomotor behavior) and diversity of estuarine fish will be controlled by changes in salinity and water quality that will occur as a result of the restoration of freshwater flow to the bay. A series of acute and subchronic physiological toxicity studies were conducted to determine the effects of salinity changes on the life stages (embryo/larval, juvenile, adult) and fecundity of four native estuarine fish (Cyprinodon variegatus, Floridichthys carpio, Poecilia latipinna, and Gambusia holbrooki). Fishe were exposed to a range of salinity concentrations (freshwater to hypersaline) based on salinity profiles in the study areas. Growth (length, weight) and survival were measured. Salinity trials included both rapid and gradual change events. Results show negative effects of acute, abrupt salinity changes on fish survival, development and reproductive success as a result of salinity stress. Other studies targeted reproduction and critical embryo-larval/neonate development as key areas for detecting long-term population effects of salinity change in Florida Bay. Adults of C. variegatus and P. latipinna were also examined for behavioral responses to pulsed salinity changes. These responses include changes in swimming performance, locomotor behavior and zone preference. Finally, an ecological risk assessment was conducted for adverse salinity conditions in northeastern Florida Bay. Using the U.S. EPA's framework, the risk to estuarine fish species diversity was assessed against regional salinity profiles from a 17-year database. Based on the risk assessment, target salinity profiles for these areas are recommended for managers.