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A narrative in ecology is that prey modify traits to reduce predation risk, and the trait modification has costs large enough to cause ensuing demographic, trophic and ecosystem consequences, with implications for conservation, management and agriculture. But ecology has a long history of emphasising that quantifying the importance of an ecological process ultimately requires evidence linking a process to unmanipulated field patterns. We suspected that such process-linked-to-pattern (PLP) studies were poorly represented in the predation risk literature, which conflicts with the confidence often given to the importance of risk effects. We reviewed 29 years of the ecological literature which revealed that there are well over 4000 articles on risk effects. Of those, 349 studies examined risk effects on prey fitness measures or abundance (i.e., non-consumptive effects) of which only 26 were PLP studies, while 275 studies examined effects on other interacting species (i.e., trait-mediated indirect effects) of which only 35 were PLP studies. PLP studies were narrowly focused taxonomically and included only three that examined unmanipulated patterns of prey abundance. Before concluding a widespread and influential role of predation-risk effects, more attention must be given to linking the process of risk effects to unmanipulated patterns observed across diverse ecosystems.

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 by95 %, two fishes declined by80 %, 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.

The marbled crayfish (Procambarus virginalis) is a triploid and parthenogenetic freshwater crayfish species that has colonized diverse habitats around the world. Previous studies suggested that the clonal marbled crayfish population descended as recently as 25 years ago from a single specimen of P. fallax, the sexually reproducing parent species. However, the genetic, phylogeographic, and mechanistic origins of the species have remained enigmatic. We have now constructed a new genome assembly for P. virginalis to support a detailed phylogeographic analysis of the diploid parent species, Procambarus fallax. Our results strongly suggest that both parental haplotypes of P. virginalis were inherited from the Everglades subpopulation of P. fallax. Comprehensive whole-genome sequencing also detected triploid specimens in the same subpopulation, which either represent evolutionarily important intermediate genotypes or independent parthenogenetic lineages arising among the sexual parent population. Our findings thus clarify the geographic origin of the marbled crayfish and identify potential mechanisms of parthenogenetic speciation., Gutekunst et al. explore the phylogeographic origins of the marbled crayfish, Procambarus virginalis, a parthenogenetic freshwater species. Based on genomic data of the parent species, they demonstrate that both parental haplotypes of P. virginalis were inherited from the Everglades subpopulation of P. fallax.

The biological changes caused by autotriploidy are poorly studied in animals. To investigate this issue in depth, we compared genetics, morphology, life history, ecology and behaviour of the triploid marbled crayfish and its diploid parent, slough crayfish Procambarus fallax. We performed a meta-analysis of our data and literature data. Our COI based molecular tree, consisting of 27 species of Cambaridae, confirmed the close taxonomic relationship between marbled crayfish and P. fallax. Comparison of both crayfish revealed similarities in mitochondrial gene sequences, morphological characters, colouration, body proportions and behaviours. Considerable differences were recorded with respect to chromosome number in somatic cells, haploid genome size, DNA methylation level, body size, fecundity, longevity, population size structure, invasiveness, and the range of inhabited biomes. These differences have dimensions that are otherwise only observed between species supporting earlier proposed raising of marbled crayfish from P. fallax forma virginalis to a new species named Procambarus virginalis. Particularly noteworthy is the enhancement of the fitness traits that probably resulted from evolutionary changes in gene expression. These alterations and the transition from sexual reproduction to parthenogenesis are likely responsible for the increased invasiveness of marbled crayfish in tropical to cold-temperate biomes.

The mechanisms causing native displacement by invasive species are challenging to identify. We used survey data to demonstrate a local extinction of a native gastropod ( Pomacea paludosa ) from a Florida wetland coincident with invasion of a non-native gastropod ( P. maculata ). We then examined the potential for P. maculata to displace P. paludosa . We used two field experiments to examine effects of densities and heterospecific interactions on juvenile growth in situ and we used lab and mesocosm experiments to explore chemically-mediated interactions between the species. Field experiments produced variable results; evidence for reduced P. paludosa growth in the presence of P. maculata was found under higher nutrient conditions with fast snail growth rates, but growth inhibition disappeared under oligotrophic conditions. Resource reduction could not be demonstrated in the field. Juvenile P. paludosa did not avoid mucus of P. maculata and did not reduce feeding in response to cues. Nevertheless, exposure to mucus and waterborne cues of P. maculata reduced P. paludosa growth by 53% relative to conspecific cue exposure. In mesocosms juvenile P. paludosa growth was significantly negatively correlated with adult P. maculata waterborne cues and was particularly affected at exposures ≥60% (~0.6 · m -2 ). Combined mucus and waterborne cues appeared to further reduce growth at low exposure (20%). We documented the first apparent local extinction of P. paludosa by P. maculata. We found no evidence for competition, but the results collectively pointed to direct chemical inhibition of growth that could be important at field scales, even at low densities.

As avian reproductive success is generally prey limited, identifying important prey types or sizes and understanding mechanisms governing prey availability are important objectives for avian conservation ecology. Irruptive White Ibis (Eudocimus albus) nesting at coastal colonies in the southern Everglades numbered over 100,000 nests in the 1930s. A century of drainage and altered hydrologic patterns reduced aquatic prey availability and eliminated large nesting events; nesting activity in recent decades has been typically less than 5% of historical peaks. Hydrologic restoration is expected to increase ibis nesting activity, but which prey types will support high nesting effort is less clear. In 2017 and 2018, we collected food boluses from White Ibis chicks at coastal colonies in Everglades National Park. We also monitored regional nesting activity from 1999 to 2018. In 2017, the region had 1,075 nests, typical of the past several decades; but in 2018, there were 30,420 nests, representing the highest recorded nesting activity in 87 yr. Prey composition varied between years; estuarine crabs dominated nestling boluses in 2017, while crayfish and fish were dominant prey in 2018. Crayfish, especially Procambarus alleni, were heavily exploited by ibis early in the 2018 breeding season, while fish were used more at the end. Crayfish abundances in wetlands near the colonies were higher prior to 2018, and more crayfish-producing short-hydroperiod wetlands remained available for ibis foraging in 2018. Our results support previous studies indicating that crayfish are important prey for breeding ibises and suggest that unprecedented, extensive flooding of seasonal wetlands promoted crayfish production and initiated the irruptive breeding in 2018. Our observations indicate that rehydration of the southern Everglades could restore ibis nesting activity at coastal colonies, but further investigations of hydrologic variation, crayfish production, and ibis foraging and nesting activity will be helpful to understand these dynamics and the importance of short-hydroperiod wetlands.LAY SUMMARYIncreasing White Ibis breeding in the southern Everglades and increasing the frequency of large nesting events (tens of thousands of nest attempts) are stated ecological goals of Everglades restoration.Little is known about which prey support White Ibis breeding and large nesting events in the southern Everglades.We found that White Ibis nesting in the southern Everglades used freshwater crayfish heavily in a year with extraordinarily high breeding activity and used estuarine crabs heavily in a year with poor breeding activity.Extensive flooding of seasonal wetlands, following hurricane Irma, was correlated with higher crayfish abundances an foraging habitat availability in the year with higher White Ibis breeding activity.Our observations indicate that ecological processes supporting large nesting events of White Ibis could be restored with additional freshwater flooding of the southwestern Everglades.

The vulnerability of gastropods to their predators varies with life history traits such as morphology, body size, behavior, and growth rates as well as predator size. A recent study suggested that the invasive apple snail, Pomacea maculata, was considerably more vulnerable to crayfish predators than the native Florida apple snail, P. paludosa. The difference was hypothesized to be caused by the relatively small hatchling size of P. maculata. To test this hypothesis, we conducted a series of feeding assays designed to quantify maximum feeding rates and selective foraging of crayfish on apple snails. The rate at which crayfish killed individual P. maculata (i.e., kill rates) decreased with snail size, and kill rates on both species increased with crayfish size. Kill rates on juvenile P. maculata were higher than kill rates on size-matched hatchling P. paludosa, and crayfish fed selectively on P. maculata when offered mixed groups of size-matched snails. Further analyses revealed that hatchling P. paludosa possess shells 1.8× heavier than size-matched P. maculata suggesting differences in vulnerability to crayfish were consistent with interspecific differences in shell defenses. Differences in hatchling size and defensive traits in combination make crayfish kill rates on hatchling P. maculata approximately 15.4× faster than on hatchling P. paludosa, but the relative contribution of hatchling size to differences in apple snail vulnerability was >3× greater than the contribution of defensive traits.

Aquatic macrophyte patches are natural features of wetland ecosystems that serve as habitat for aquatic animals. Previous studies suggest animal densities in Everglades, USA, wetlands are generally less numerous in sawgrass ridges than in deeper lily sloughs. We studied the density distribution of a population of Procambarus fallax in ridge and slough habitat types over a 2-year period, spanning two wet–dry cycles and estimated growth and survival rates under flooded conditions to understand comparative value of each to the crayfish population. Procambarus fallax individuals inhabited and recruited in both marsh habitats. During periods of high water, crayfish densities were similar in both habitats; however, densities in both habitats varied seasonally, leading us to postulate some degree of population redistribution in response to fluctuating water depths. Analysis of size distributions over time revealed juveniles in both habitats and two major recruitment periods each year; distinct juvenile cohorts were present in early winter (Nov–Dec) and mid-summer (July–Aug). An in situ experiment of juvenile growth demonstrated that slough habitat type supported faster growth over ridge habitat. To understand habitat-specific mortality risk, a tethering study during flooded conditions indicated that relative predation risk by aquatic predators was greater in sloughs for all sizes and higher for smaller individuals in both habitats. The comparative importance of ridge and slough balances growth potential and survival probability during flooded conditions. This is the first study through time and across both habitat types analyzing the distribution and size structure of P. fallax population in the Everglades.

The obligately parthenogenetic marbled crayfish, Procambarus virginalis, is the first formally described asexual species of the Crustacea Decapoda. It is a triploid descendant of the sexually reproducing slough crayfish, Procambarus fallax. Here we describe the morphology of cultured and wild marbled crayfish of wide size ranges in detail and photodocument all taxonomically relevant characters. Some morphological traits and coloration showed considerable variation within populations despite the monoclonal nature of marbled crayfish. There were also significant differences between wild and laboratory populations with respect to body proportions, coloration and spination. Comparison with Procambarus fallax revealed no qualitative morphological characters that unambiguously identify the marbled crayfish. Analysis of the mitochondrial cytochrome c oxidase subunit I gene (COI) and nuclear microsatellites of marbled crayfish and Procambarus fallax from different sources indicated that the tri-allelic microsatellite PclG-02 is better suitable than COI to identify the marbled crayfish. A respective identification key is provided. The COI and microsatellites of Procambarus fallax from different areas of Florida and southern Georgia suggest that the parents of the first marbled crayfish may have come from northern Union County, northern Florida.

Both site-selective oviposition and interactions following colonization can play a role in structuring communities, but the relative importance of each has not been well studied for many animals. We manipulated the presence of a small-bodied fish predator (Eastern Mosquitofish, Gambusia holbrooki) and submerged aquatic vegetation (SAV; Utricularia spp.) in 24 mesocosms (n = 6 replicates, 4 treatments) to determine the effects of predators and habitat structure on dragonfly oviposition and naiad success. Adults did not avoid ovipositing in mesocosms with mosquitofish predators, but some species did select for or against SAV. No dragonfly naiads emerged from mesocosms with mosquitofish that lacked SAV. In treatments with SAV, total emergence was almost 3× higher in mesocosms without mosquitofish than mesocosms with mosquitofish. Oviposition patterns generally could not account for emergence patterns in the mesocosms, suggesting that libellulid dragonfly production can be severely limited by postcolo...

The marbled crayfish is the only obligately parthenogenetic decapod crustacean and a novel research model and invasive animal on three continents. It is regarded either as a parthenogenetic form of slough crayfishProcambarus fallaxor as a separate species namedProcambarus virginalis.In order to investigate the species question of this unusual crayfish in detail we have identified the similarities and differences in morphology, life history, genetics, behaviour, ecology and biogeography between marbled crayfish and its most likely parent speciesP. fallax.We have investigated specimens from natural habitats, laboratory colonies and museum collections and performed a meta-analysis of our data and published data. OurCOIbased molecular tree with 27 Cambaridae confirms closest relationship of marbled crayfish withP. fallax.Marbled crayfish andP. fallaxare similar with respect to morphological characters, coloration and body proportions, but differ considerably with respect to body size, fertility and longevity. The mitochondrial genes of both crayfish are similar, but ploidy level and haploid genome size are markedly different. Both crayfish are eurytopic and have two major annual recruitment periods, but marbled crayfish show different population structure and higher invasiveness. Marbled crayfish occur in tropical to cold temperate habitats of the old world, butP. fallaxis confined to subtropical and warm-temperate habitats of the southeastern USA. Cross-breeding experiments with both crayfish revealed reproductive isolation. The application of the Evolutionary Genetic Species Concept for asexuals to all available data supports raising marbled crayfish from “forma” to species rank. A determination key is provided to discriminateProcambarus virginalis, the first asexual decapod species, from its parent speciesP. fallax.

There was an error in the abstract of the original publication. The 9th sentence of abstract should be:"Our results are inconsistent with predictions about the sorting effects of predators across productivity gradients because the more resistant prey dominated at low productivity."

Recent studies suggest native aquatic predators can favor native over invasive species and provide biotic resistance to invasions. The invasive apple snail, Pomacea maculata continues to spread in freshwaters of Florida and appears to be a strong competitor of the native Florida apple snail (P. paludosa). Little is known about effects of predators on either species and we compared the effects of a common native crayfish predator (Procambarus fallax) on hatchlings and juveniles. Because crayfish feed size-selectively, we predicted that the smaller P. maculata hatchlings would be more vulnerable than the hatchlings of native P. paludosa. In experimental wetland mesocosms, crayfish reduced survival of both species, but none of the P. maculata survived the 44 days exposure, such that predatory effects of crayfish were >8× stronger on the non-native P. maculata than on native P. paludosa. Crayfish in the lab selectively consumed the smaller P. maculata when offered both hatchlings simultaneously. We combined the observed survival rates with published life history data and the results (i.e., projected survivors clutch−1) suggested that wetlands with crayfish could have greater limiting effects on non-native P. maculata than the native P. paludosa. Wetland conditions favoring populations of native crayfish may also favor the relatively predator-resistant native P. paludosa.

Predator–prey interactions are often size-structured and focused on smaller vulnerable size classes. Predators are also predicted to sort prey communities according to relative vulnerabilities. Increased system productivity and juvenile growth may benefit some species more than others, making relative vulnerability non-static and growth-mediated. We hypothesized that increased system productivity would weaken juvenile-stage predation generally, and potentially shift the community sorting effects of a predator. Using replicated wetland mesocosms we quantified the effects of a generalist size-specific crayfish predator (Procambarus fallax) on juveniles of two species of apple snails (Pomacea spp.) under two levels of system productivity (low vs. high). After 6 weeks of exposure, we quantified predator and productivity effects on snail survival, biomass, and composition of the assemblage. Crayfish depressed the final density and biomass of snails, and sorted the assemblage, selectively favoring survival of the native P. paludosa over the intrinsically more vulnerable invasive P. maculata. Both snails grew faster at higher productivity, but growth differentially increased survival of the invasive snail in the presence of crayfish and weakened the sorting effect. The native P. paludosa hatches at a larger less vulnerable size than the invasive P. maculata, but higher productivity reduced the relative advantage of P. paludosa. Our results are inconsistent with predictions about the sorting effects of predators across productivity gradients, because the more vulnerable prey dominated at low productivity. Our findings highlight that the relative vulnerabilities of prey to a common predator are not always fixed, but can be growth-mediated.

Summary Disturbances caused by drying can eliminate fully aquatic organisms and alter wetland communities. However, the net effects of pulsed drying followed by re-wetting on populations of benthic macroinvertebrates with greater tolerance to drying have been poorly studied. We quantified the population responses of two large macroinvertebrates, dragonflies (a number of species; Family: Libellulidae) and a crayfish (Procambarus fallax), to variable drying history and predatory sunfish (Family: Centrarchidae) in experimental subtropical wetlands. To simulate naturally occurring combinations of drying and sunfish presence, the experimental treatments included a drying and re-wetting sequence that either eliminated sunfish or did not, with a third treatment of continuously flooded conditions with sunfish present. The activity of adult dragonflies was similar over all experimental wetlands, but larval density after 6 months was highest in continuously flooded wetlands (with sunfish) and some species (Erythemis simplicicollis, Celithemis eponina) were absent from wetlands that experienced drying. There was no evidence that sunfish limited larval density. After 6 months, overall crayfish biomass was greatest in wetlands that had dried and lacked sunfish. Crayfish density was similar in all wetlands, but individual crayfish body mass (g) was greatest in wetlands that lacked sunfish. Because of sunfish predation, few crayfish survived to full juvenile size. In the presence of sunfish, drying did not affect crayfish biomass or individual body mass. The abundance of libellulid dragonfly larvae was indirectly reduced by the drying. In wetlands where submerged vegetation was reduced by drying, we suggest that libellulid populations were limited by a small-bodied fish (Gambusia holbrooki). In contrast, survival of juvenile crayfish was directly limited by sunfish, and therefore, crayfish (but not libellulid) populations could be temporarily enhanced by drying events when and where sunfish populations are reduced.

One of the best studied freshwater predator–prey interactions is arguably that between crayfish and gastropods. In many ecosystems, crayfish limit populations of thin-shelled snails and induce changes in snail traits. However, the connection between these 2 phenomena and, therefore, the mechanism by which crayfish have population-dynamic effects on snails, has been little studied. I examined the population-level effects of 2 procambarid crayfish (Procambarus alleni and Procambarus fallax) on a 2-species assemblage of snails (physids: Hatia cubensis, planorbids: Planorbella duryi) in experimental mesocosms. Observations early in the experiment suggested that both snail species were responding behaviorally to the crayfish, especially to P. alleni. At the end of the study (8 wk), both species of crayfish had reduced filamentous periphyton by 50% and snail density by >95%. The 2 crayfish species had quantitatively similar effects on both trophic levels. Physids laid eggs early in the study, but they h...

The intensity of competitive interactions between fishes is partly determined by prey use and ontogenetic niche shifts. In a wetland where distinct habitat shifts are missing we compared prey use of three generalist benthivorous sunfishes to look for evidence of ontogenetic, interspecific, and “seasonal” variation in prey composition. Diet analysis revealed evidence of diet ontogeny in warmouth (Lepomis gulosus, 30–152 mm standard length, SL), but not in bluespotted sunfish (Enneacanthus gloriosus, 30–47 mm SL) or dollar sunfish (Lepomis marginatus, 30–60 mm SL). Bluespotted and dollar sunfishes consumed small dipteran and amphipod prey and had similar diets in both seasons suggesting a potential for strong interspecific competition. In the dry season, warmouth shifted from using smaller insect prey to larger decapod and fish prey with increasing size. This shift to prey types that were little used by the other species reduced dietary niche overlap with the other sunfishes. After drought and re-flooding (in the wet season), decapods and small fish were less abundant in the wetland and the warmouth ontogenetic shift was less distinct. When matched for gape width, prey composition differed between warmouth and both dollar and bluespotted sunfishes in the wet season, suggesting differences in sunfish foraging modes, but prey use differences were less clear in the dry season when prey were abundant. Both warmouth ontogenetic diet shifts and seasonal variation in prey use (probably mediated by prey abundance) had strong influences on diet overlap and therefore the potential for intra- and interspecific competition between sunfishes in this wetland ecosystem.

Effects of predators on prey populations can be especially strong in aquatic ecosystems, but disturbances may mediate the strength of predator limitation and even allow outbreaks of some prey populations. In a two-year study we investigated the numerical responses of crayfish (Procambarus fallax) and small fishes (Poeciliidae and Fundulidae) to a brief hydrological disturbance in replicated freshwater wetlands with an experimental drying and large predatory fish reduction. The experiment and an in situ predation assay tested the component of the consumer stress model positing that disturbances release prey from predator limitation. In the disturbed wetlands, abundances of large predatory fish were seasonally reduced, similar to dynamics in the Everglades (southern Florida). Densities of small fish were unaffected by the disturbance, but crayfish densities, which were similar across all wetlands before drying, increased almost threefold in the year after the disturbance. Upon re-flooding, juvenile crayfish survival was inversely related to the abundance of large fish across wetlands, but we found no evidence for enhanced algal food quality. At a larger landscape scale (500 km2 of the Everglades), crayfish densities over eight years were positively correlated with the severity of local dry disturbances (up to 99 days dry) during the preceding dry season. In contrast, densities of small-bodied fishes in the same wetlands were seasonally depressed by dry disturbances. The results from our experimental wetland drought and the observations of crayfish densities in the Everglades represent a large-scale example of prey population release following a hydrological disturbance in a freshwater ecosystem. The conditions producing crayfish pulses in the Everglades appear consistent with the mechanics of the consumer stress model, and we suggest crayfish pulses may influence the number of nesting wading birds in the Everglades.

Wading bird (Ciconiiformes) nesting success is influenced by the availability of aquatic prey, but principle prey may differ among species. During an excellent nesting year (2009) 118 boluses were collected from nestlings of three species, White Ibis (Eudocimus albus), Tricolored Heron (Egretta tricolor) and Snowy Egret (Egretta thula) in a mixed colony in the northern Everglades. Although these species have similar foraging depths and foraging flight distances from nesting colonies, crayfish dominated the ibis boluses while small-bodied fishes dominated egret boluses. Fish prey species composition in Snowy Egret and Tricolored Heron boluses did not differ. Compared to available fish species from nearby wetlands, the Egretta spp. did not exhibit taxonomic selectivity but did feed selectively on larger (2–4 cm standard length) fish. Whether restoration activities in the Everglades, including hydroperiod lengthening, will simultaneously enhance prey for both invertivores like White Ibis and pisciv...

Predators and dry-disturbances have pronounced effects on invertebrate communities and can deterministically affect compositional turnover between discrete aquatic habitats. We examined the effect of sunfish (Lepomis spp.) predators on two native crayfish, Procambarus alleni and P. fallax, that regionally coexist in an expansive connected wetland in order to test the hypotheses that sunfish predation limits crayfish recruitment (both species) and that it disproportionately affects P. alleni, the species inhabiting temporary wetlands. In replicate vegetated wetlands (18.6 m(2)), we quantified summertime crayfish recruitment and species composition across an experimental gradient of sunfish density. Separately, we quantified effects of sunfish on crayfish growth, conducted a complimentary predation assay in mesocosms, and compared behavior of the two crayfish. Sunfish reduced P. alleni summertime recruitment by99% over the full sunfish gradient, and most of the effect was caused by low densities of sunfish (0.22-0.43 m(-2)). P. alleni dominated wetlands with few or no sunfish, but the composition shifted towards P. fallax dominance in wetlands with abundant sunfish. P. fallax survived better than P. alleni over 40 h in smaller mesocosms stocked with warmouth. Sunfish reduced P. fallax recruitment 62% in a second wetland experiment, but growth rates of caged crayfish (both species) were unaffected by sunfish presence, suggesting predatory effects were primarily consumptive. Consistent with life histories of relatively fish-sensitive invertebrates, P. alleni engaged in more risky behaviors in the laboratory. Our results indicate that sunfish predators limit recruitment of both species, but disproportionately remove the more active and competitively dominant P. alleni. Spatially and temporally variable dry-disturbances negatively co-varying with sunfish populations allow for regional coexistence of these two crayfish and may release populations of either species from control by predatory fishes.

Prey availability is known to limit reproduction of some species of nesting birds, but identifying the primary prey types of a species with a flexible diet can be challenging. For the White Ibis Eudocimus albus, a tactile feeding, medium-sized wading bird, nestling prey composition is suggested to depend on landscape water depths ⁄availability of foraging habitat at the time of nesting and on historical drying events affecting prey production. We collected and compared inter- and intra-annual diet variation of White Ibis chicks reared in the Everglades over two years that were independently identified as being relatively good (2006) and poor (2007) nesting seasons. We collected 127 nestling boluses and analysed the temporal variation in biomass of eight functional prey groups using multivariate techniques. The boluses from 2006 in the central Everglades were dominated by fish, but in 2007, after fish had been reduced by the previous year of drying, the boluses from the same region were more variable and dominated by garbage (i.e. scavenging). Analysis of five different collections taken from a different colony in the northern Everglades indicated that boluses were characterized by crayfish and had fewer fish or less garbage when landscape water depths were relatively higher and more preferred habitat was available. At lower landscape water depths in 2007 the bolus composition shifted away from crayfish towards small fish and urban food (terrestrial insects and garbage). Our results support the suggestion of depth-dependent diets; prey composition depends on the current landscape water levels around the colonies, and also suggests that previous drying events can lead to increased reliance on alternative food sources. White Ibis partially compensated for unavailable aquatic prey with alternative urban foods, but their nesting success appears to have suffered.

A mysterious parthenogenetic cambarid crayfish (the Marmorkrebs) has been spreading across the globe for the past decade. We compare this crayfish directly to two other cambarids, Procambarus fallax and P. alleni, that have been suggested to be related or even identical to the Marmorkrebs. Using external morphology and sequences of two mitochondrial genes we show clear correspondences between Marmorkrebs and P. fallax, a species found natively throughout peninsular Florida, USA. Based on these congruent results we suggest that the Marmorkrebs is the parthenogenetic form of P. fallax. This finding has potential evolutionary and ecological implications at several levels. The Marmorkrebs might be a type of geographical parthenogenesis, but a natural population in the wild is so far unknown. Furthermore, challenges arise in regard to the respective species status of the Marmorkrebs. Taxonomically we suggest that the Marmorkrebs is treated as ‘parthenogenetic form’ of P. fallax. Last but not least, the identity of this animal and its ecology has an impact for considering potential spread and effects of this species across the globe.

Wetland or stream drying can be a severe disturbance for aquatic animals, but the severity of a drying disturbance is context dependent. We used a controlled drying experiment to investigate the resistance of populations of 2 crayfish species (Procambarus fallax and Procambarus alleni) found in south Florida wetlands in 3 sediment types (sand, marl, and peat). The species had different putative burrowing abilities, and we tested the hypothesis that the severity of drying disturbance for populations of burrowing crayfish would be species and substrate dependent. Observations of initial burrowing success indicated that the species could burrow equally well in peat, whereas P. alleni burrowed more effectively than P. fallax in marl and sand. No P. fallax burrowed in the dense sand substrate. The number of crayfish surviving 2 mo underground was a function of particular combinations of species and substrate. Survival of both species was high in peat (∼66%) and similarly high for P. alleni in marl. Sur...

We investigated the use of crayfish gastroliths and exoskeleton pieces for quantifying crayfish abundance and size-structure in diets of White Ibis (Eudocimus albus) chicks in the Everglades. We then quantified crayfish and fish abundance from various small hard parts and intact fish heads in 23 boluses, taken from two nesting colonies in Water Conservation Area 3 (WCA 3) and Arthur R. Marshall Loxahatchee National Wildlife Refuge (Lox) in the Florida Everglades during May 2006. We determined that using gastroliths to estimate crayfish numbers in boluses had significant drawbacks; only a small fraction of field-caught crayfish bear gastroliths and the two crayfish species in the Everglades differ in percentage bearing gastroliths. In contrast, counts of crayfish rostrums and chelae pairs gave simple and similar estimates of crayfish in the boluses. The two colonies had strikingly different diets in May 2006; New Colony 3 (Lox) boluses were dominated by crayfish while birds from the Alley North (W...

Aquatic predators and habitat permanence can jointly affect benthic invertebrate biomass and community composition. In 2006 I sampled fish and invertebrates in ten ponds embedded in a seasonal wetland before and after a natural drought. Drought reduced fish biomass and density leaving some ponds in a fishless condition when rains returned in July. In July, large aquatic insects and crayfish colonized and reproduced in the ponds, but did not colonize all of the ponds equally. Using measurements of fish abundance and other environmental parameters of the ponds, I conducted linear regression analyses to explore potential drivers of variable invertebrate biomass in July. Fish biomass had a negative effect on invertebrate biomass and it explained more of the variation in total invertebrate biomass and total non-shrimp biomass than fish abundance (number of fish caught). Dissolved oxygen and pond depth were both correlated with fish biomass, but were poorer predictors of invertebrate biomass. Ponds with few or no fish had 20× greater total biomass and 200× more non-shrimp biomass than ponds with high fish biomass. Shrimp dominated the invertebrate composition, and were only found in the two deepest ponds with the highest fish biomass; predatory insects and crayfish dominated the other eight ponds. When taxa were analyzed separately, fish biomass explained a large portion of the variation for predatory insects (Coleoptera, Hemiptera, and Odonata) and crayfish (Procambarus alleni), but dissolved oxygen was the best predictor of larval stratiomyid (order Diptera) biomass. These results are generally consistent with studies demonstrating negative effects of fish on large predatory invertebrates, but also suggest that more severe local droughts can seasonally enhance insect and crayfish populations by generating fishless or nearly fishless conditions.

Summary 1. Faster growing, larger and/or more aggressive crayfish species are predicted to dominate permanent waterbodies. We tested this prediction using a 9 year dataset for two species of crayfish (Procambarus alleni and Procambarus fallax) co-existing in a sub-tropical flowing slough in southern Florida. Using a series of laboratory and mesocosm experiments we also compared life history traits and performance of the respective species to test mechanisms that could explain dominance shifts in the local crayfish assemblages. 2. Over the 9-year period, P. alleni densities were the greatest in shallower, shorter-hydroperiod areas bordering the slough, while P. fallax densities were higher in deeper, longer-hydroperiod central areas. These areas were separated by 0.8–2 km of continuous wetland with no apparent barriers to movement between them. 3. Density of P. fallax was not strongly affected by any measures of hydrological variation, while P. alleni density increased with more severe drought conditions. Following the strongest droughts, P. alleni colonized areas in the centre of the slough where they had been absent or scarce in wetter years. 4. We conducted experiments to compare growth rates, drought tolerance, and competitive dominance of these species. P. alleni survived drought conditions better, had higher growth rates, and was the dominant competitor for space and food. While drought probably limits P. fallax in the drier slough habitats, neither drought sensitivity nor interspecific competition with P. fallax can explain decreases of P. alleni with wetter conditions. 5. Our results indicate that a competition-colonization tradeoff cannot explain the crayfish compositional dynamics in this wetland because P. alleni is both the best competitor and the best at surviving in and colonizing areas with the strongest droughts. Future attention should focus on the potential for selective effects of predators that co-vary with hydrology. 6. The traits (large size, fast growth, competitive dominance) exhibited by P. alleni, which is absent in long-hydroperiod wetlands, are those exhibited by dominant crayfish in permanent lakes and streams containing fish. Although these traits make crayfish less vulnerable to fish in some lakes and streams, life-history models of community structure across permanence gradients suggest the opposite traits should be favoured for co-existence with fish.

The relationship between species diversity and the stability and production of trophic levels continues to receive intense scientific interest. Though facilitation is commonly cited as an essential underlying mechanism, few studies have provided evidence of the impact that indirect facilitation may have on diversity/ecosystem functioning relationships. In this laboratory study, we examined the effect of zooplankton species diversity on trophic structure (total algal and zooplankton biomass) and temporal stability of total zooplankton biomass. We utilized four species of pond zooplankton grown in either monoculture or in polyculture. When comparing responses in polycultures with responses averaged across monocultures, a positive effect of diversity on total zooplankton biomass was observed. This occurred as a result of positive facilitative effects among competing zooplankton. Daphnia pulex , a biomass dominant in monoculture, was negatively affected by the presence of interspecific competitors. In contrast, Diaphanosoma brachyurum , a species that performed poorly in monoculture, was strongly and positively affected by the presence of interspecific competitors, driving positive diversity effects on total zooplankton biomass. Positive temporal covariances among zooplankton were detected in several polyculture replicates, increasing temporal variability of total zooplankton biomass. However, this destabilizing effect was weak relative to effects of high biomass yields in polyculture which caused temporal biomass variability (as measured by the coefficient of variation) to be lower in polyculture relative to monocultures. Zooplankton diversity effects on total algal biomass were not detected. However, increased zooplankton diversity significantly altered the size structure of algae, increasing the relative abundance of large, grazer-resistant algae.

We evaluated the sampling efficacy of 1-m2 throw traps (active sampler) and baited minnow traps (passive sampler) across an experimental density gradient (1, 2, 4, 6, 8, 10, and 15/m2) of the slough crayfish (Procambarus fallax) in 2 trials with different crayfish populations. In both trials, throw-trap density estimates were highly correlated with actual crayfish density (r2 = 0.96). The form of the relationships between density estimates and stocked densities was consistent between trials, and indicated that throw traps captured a similar proportion of the stocked crayfish regardless of the stocked density. When we adjusted the relationships to account for clearing efficiency (proportion of captured animals actually recovered from the trap), the slopes of the regressions were not significantly different from 1 in either trial. Size distributions and sex ratios of crayfish collected by the throw traps accurately reflected those of the stocked populations. Baited minnow traps performed inconsisten...

While crayfish are traditionally considered fish prey, they are capable of feeding on substrate-bound fish eggs and their introductions have been blamed for the decline in fish populations in Europe and North America. To investigate their potential effects on fish reproductive success we measured the effects of a native crayfish (Orconectes virilis) on the reproductive success of two substrate-nesting sunfish, pumpkinseed (Lepomis gibbosus) and bluegill (Lepomis macrochirus), in replicated pond experiments. Crayfish were observed feeding on eggs in both experiments. Crayfish presence delayed successful reproduction by pumpkinseeds in densely vegetated ponds, resulting in lower young-of-the-year biomass in ponds with crayfish. In the second experiment, with bluegills in less-vegetated ponds, crayfish prevented successful reproduction entirely. However, when we added crayfish-proof exclosures to the crayfish ponds late in the summer, bluegills located the crayfish-free habitat and successfully reproduced inside the exclosures (1 month after first successful reproduction in control ponds). The results of these experiments demonstrate the potential strong negative effects of crayfish on sunfish reproduction and suggest that the spatial distribution of crayfish and other egg predators may influence fish nesting behaviors and habitat choices. Further studies are needed to determine the magnitude of crayfish effects in natural lakes and ponds where sunfish and crayfish co-occur.

Mortality (e.g. predation, disturbance) is often thought to lower the intensity of interspecific competition and thereby promote the coexistence of competing species. However, surprisingly few tests of this idea exist, especially for metazoans feeding on a self-renewing resource. Here we examined the effect of density-independent mortality on the coexistence of four species of pond zooplankton (microcrustacean grazers) in a series of laboratory microcosms. Across the experimental mortality gradient, consumer biomass decreased and resource abundance increased with an increase in mortality. Thus, the treatments resulted in an increase in resource availability per consumer (one measure of reduced competitive intensity). There was no significant effect of mortality treatment on species relative abundances or species evenness, and the zooplankter Diaphanosoma dominated community biomass at all mortality levels. Mortality rate did have a marginally significant effect on species richness (p < 0.07), but richness did not increase monotonically with mortality level. Instead, richness tended to be highest in the low- and no-mortality treatments.

Stable isotope enrichment experiments offer a potentially powerful way to examine the base of aquatic food webs, but interpretation of the data from these experiments may be confounded by problems such as selective ingestion/assimilation of bulk food sources by consumers, variable tracer enrichment over time, and the failure of consumers to approach isotopic equilibrium with the tracer in their diets over the course of the experiment. Our study examined data from a stable isotope addition experiment in which 15N-labeled NH4+ was added to a midwestern US stream. A compartment model was used to provide insights into the kinetics of 15N uptake and release from algae, heterotrophic microbes colonizing detritus, and invertebrate consumers. The model accounted for temporal variation in the degree of isotopic enrichment and did not require the assumption of isotopic equilibrium between consumers and their diets. The importance of instream production (i.e., growth of algae and microbes within the study re...

Large omnivorous predators may play particularly important roles determining the structure of communities because of their broad diets and simultaneous effects on multiple trophic levels. From June 2001 to June 2002 we quantified community structure and ecosystem attributes of six newly establishing freshwater ponds (660 m2 each) after populations of omnivorous crayfish (Orconectes virilis) were introduced to three of the ponds. Crayfish preyed heavily on fish eggs in this experiment, which reduced recruitment of young-of-year fish. This effect indirectly enhanced zooplankton biomass in crayfish ponds. Phytoplankton abundance exhibited a more complex pattern and was probably influenced by non-trophic (e.g., bioturbation) effects of crayfish. Peak dissolved oxygen levels were lower in the crayfish ponds indicating that they had lower primary production: respiration ratios. Metaphytic algae were strongly affected by crayfish presence; filamentous greens quickly disappeared and the blue-green Gleotrichia (a less preferred food item) eventually dominated the composition in crayfish ponds. Chara vulgaris and vascular macrophytes established 34% cover in control ponds by June 2002, but were not able to establish in crayfish ponds. Two important periphyton herbivores (tadpoles and gastropods) were absent or significantly reduced in the crayfish ponds, but periphyton differences were temporally variable and not easily explained by a simple trophic cascade (i.e., crayfish-snails and tadpoles-periphyton). Our results indicate that crayfish can have dramatic direct and indirect impacts on littoral pond communities via feeding links with multiple trophic levels (i.e., fish, invertebrates, and plants) and non-trophic activities (bioturbation). Although the effects of omnivorous crayfish on littoral communities can be large, their complex effects do not fit neatly into current theories about trophic interactions or freshwater community structure.

Although host preferences in phytophagous insects may be generated by several factors, few studies have simultaneously examined several potential host choice determinants. In this study we tested the impact of the following potential host choice determinants on host preference of the semi-aquatic lepidopteran Munroessa gyralis (Pyralidae): growth on different host plants; protein content, polyphenolic content, toughness, and chemical extracts of different host plants; prior feeding experience; and predation pressure on the caterpillar by fishes. Two water lilies, Brasenia schreberi and Nymphaea odorata, were preferred in cafeteria-style feeding experiments over 14 other species of vascular plants. The most preferred water lily (Brasenia) also afforded the fastest growth relative to three other species on which growth was measured. Feeding preferences across species were unrelated to protein content, polyphenolic content, or toughness. Domiciles constructed by caterpillars from leaf fragments were protective from field assemblages of fishes, but domiciles made from preferred or unpreferred host species conferred no significant protection from fish in the laboratory. Caterpillars responded positively to chemical cues of water lilies, and prior feeding experience increased preference for an otherwise unpreferred water lily (Nuphar advena) within the life-span of individual caterpillars. M. gyralis is a generalist herbivore exhibiting modest preference induction and preferences for and among members of the family Nymphaeaceae. Our results suggest that relative growth rates, chemical cues, and previous feeding experience are important factors determining feeding preference. Protein content, polyphenolic content, and toughness appear less important, and the importance of fish predators remains in question. As pupation seems to occur exclusively on Nymphaea, we suggest that host use may be restricted due to life-stage-specific developmental constraints that are not apparent from the results of growth or preference assays. It is currently unknown how often specific life-stages may restrict host use, but our work suggests this as a potentially important area of inquiry.