Mark I. Cook

Sensitivity of Nesting Great Egrets (Ardea alba) and White Ibises (Eudocimus albus) to Reduced Prey Availability

ABSTRACT. Life-history theory suggests that long-lived bird species will adjust their nesting effort according to current conditions to balance the costs and benefits of current reproduction with their long-term needs for survival and future reproduction. However, responses to the same habitat conditions may differ between species, even within the same ecosystem, to produce different nesting and population patterns. We examined differences in the nesting ecology of two sympatric wading species, Great Egret (Ardea alba) and White Ibis (Eudocimus albus), between years with high (2006) and below-average (2007) prey availability in the Florida Everglades. Clutch size of White Ibises decreased by ∼19% from 2006 to 2007, whereas Great Egret clutch size remained constant. Model selection identified rain, water depth, Julian date, year, and prey biomass as parameters that most influenced daily survival rates (DSR) of White Ibis nests, whereas nest stage, region, Julian date, water depth, and the quadratic form of water recession rate most influenced Great Egret nest DSR. Daily survival for both Great Egret and Whites Ibis nests was higher in 2006 (DSR = 0.992 and 0.999, respectively) than in 2007 (DSR = 0.981 and 0.979). Our results support the hypothesis that prey availability and hydrological factors play crucial roles in regulating populations of wading birds in the Florida Everglades. Results also demonstrated that White Ibis reproduction was more sensitive to changes in hydrological conditions and prey availability than Great Egret reproduction.

The Effects of Water Depth and Emergent Vegetation on Foraging Success and Habitat Selection of Wading Birds in the Everglades

Abstract. Wading bird foraging success and habitat preference can be greatly affected by prey availability, which encompasses both prey density and the vulnerability of prey to capture. Two components of prey vulnerability, water depth and emergent vegetation, were manipulated within 10 m × 10 m enclosures to determine the relative effects on foraging habitat preference for eight species of wading birds and foraging success for a subset of four species that strike their prey. All species showed a strong preference for shallow water, and within this water depth showed a preference for the sparse vegetation density treatment. The preference for foraging habitat with a sparse or intermediate vegetation density has been documented in other studies, and may represent a tradeoff between selecting more heavily vegetated areas, which have a higher prey density, and more open areas, where prey are more vulnerable to capture. Almost all foraging occurred in the shallow water treatment, suggesting that preferred water depths constituted high quality habitat for wading birds. The weaker selection for sparse vegetation density and lack of an effect of vegetation density on capture rate and capture efficiency (p>0.05 for all tests, except Snowy Egret (Egretta thula) capture efficiency) suggested that emergent vegetation is of secondary importance to water depth as determinants of wading bird habitat quality.

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.

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.

Detrital floc and surface soil microbial biomarker responses to active management of the nutrient impacted Florida everglades.

Alterations in microbial community composition, biomass, and function in the Florida Everglades impacted by cultural eutrophication reflect a new physicochemical environment associated with monotypic stands of Typha domingensis. Phospholipid fatty acid (PLFA) biomarkers were used to quantify microbial responses in detritus and surface soils in an active management experiment in the eutrophic Everglades. Creation of open plots through removal of Typha altered the physical and chemical characteristics of the region. Mass of PLFA biomarkers increased in open plots, but magnitude of changes differed among microbial groups. Biomarkers indicative of Gram-negative bacteria and fungi were significantly greater in open plots, reflective of the improved oxic environment. Reduction in the proportion of cyclopropyl lipids and the ratio of Gram-positive to Gram-negative bacteria in open plots further suggested an altered oxygen environment and conditions for the rapid growth of Gram-negative bacteria. Changes in the PLFA composition were greater in floc relative to soils, reflective of rapid inputs of new organic matter and direct interaction with the new physicochemical environment. Created open plot microbial mass and composition were significantly different from the oligotrophic Everglades due to differences in phosphorus availability, plant community structure, and a shift to organic peat from marl-peat soils. PLFA analysis also captured the dynamic inter-annual hydrologic variability, notably in PLFA concentrations, but to a lesser degree content. Recently, use of concentration has been advocated over content in studies of soil biogeochemistry, and our results highlight the differential response of these two quantitative measures to similar pressures.