Nathan J. Dorn

Colonization and reproduction of large macroinvertebrates are enhanced by drought-related fish reductions

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.

Hydrological disturbance diminishes predator control in wetlands

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.

Estimation of Crayfish Abundance and Size-structure in Diets of White Ibis Chicks

Abstract 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 (WCA 3) boluses were fish-dominated and had few crayfish. Using measurements of the crayfish rostrums we determined the size-structure of crayfish found in the diets of the New Colony 3 birds, and determined that the crayfish in the diet were relatively large (mean = 19 mm carapace length) when compared to the available crayfish in the marsh. These crayfish were also large relative to previous reports of crayfish found in White Ibis diets in the Everglades.

Nestling Diet of Three Sympatrically Nesting Wading Bird Species in the Florida Everglades

Abstract. 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 piscivores, such as the egrets, remains an open question.

Importance of Crayfish Prey to Nesting White Ibis (Eudocimus albus)

Abstract. Wading bird prey populations, prey availability and wading bird nesting success are all thought to be associated with hydrologic conditions in wetlands, but the relationship between successful nesting and essential prey types fed to chicks is poorly understood in many cases. Prey fed to White Ibis (Eudocimus albus) chicks were quantified and compared for three colonies in the northern Everglades within and between the 2008 and 2009 nesting seasons. Crayfish were the dominant prey type in both years, being present in 58–88% of chick boluses across all collections. Prey composition was not temporally variable within 2008, but chick diet shifted slightly toward more fish at the end of 2009 when the wetland wras at its driest. Crayfish were the dominant (55–65%) energetic component of White Ibis chick diets in the northern Everglades during both a good nesting year (6,000 nests) with higher water levels (2008) and during an excellent nesting year (9,300 nests) when water levels were lower (2009). The results from this study and an earlier study suggest fish, along with terrestrial insects and urban refuse are only secondary prey for nesting White Ibis and that wetlands with abundant crayfish populations should promote high nesting effort.

Mosquitofish predation and aquatic vegetation determine emergence patterns of dragonfly assemblages

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 postcolonization interactions with mosquitofish. The dominant species emerging from the 3 treatments with naiad success varied consistently, a result suggesting that emerging assemblage composition was altered primarily by tolerances to mosquitofish/interspecific interactions. In mesocosms with SAV, the emerging assemblages were more species rich and more similar in the absence than in the presence of mosquitofish. We suggest that stochastic postcolonization egg or early-naiad survival may account for some assemblage variation in the presence of an efficient stage-specific predator like mosquitofish. This assemblage of libellulids appears to be filtered primarily according to vulnerability to fish predators, with SAV serving to reduce intensity of postcolonization interactions.

Benefits of adjacent habitat patches to the distribution of a crayfish population in a hydro-dynamic wetland landscape

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.

Life history traits determine the differential vulnerability of native and invasive apple snails ( Pomacea spp.) to a shared juvenile-stage predator

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.

In-depth investigation of the species problem and taxonomic status of marbled crayfish, the first asexual decapod crustacean

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 crayfish Procambarus fallax or as a separate species named Procambarus 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 species P. fallax. We have investigated specimens from natural habitats, laboratory colonies and museum collections and performed a meta-analysis of our data and published data. Our COI based molecular tree with 27 Cambaridae confirms closest relationship of marbled crayfish with P. fallax. Marbled crayfish and P. fallax are 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, but P. fallax is 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 discriminate Procambarus virginalis, the first asexual decapod species, from its parent species P. fallax.

System productivity alters predator sorting of a size-structured mixed prey community

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.”