Gawlik, Dale E.

Avian reproductive success varies depending on environmental conditions,
degree of predation, location, and food availability. I examined food availability and
nesting habitat of wading birds in South Florida. Evaluating prey availability and prey
used by two small heron species demonstrated the ability for small herons to maintain
consistent diets through fluctuating environmental conditions, within and across different
wetland types. Small herons may be able to cope with environmental changes by altering
foraging locations and strategies. Regarding nesting habitat, man-made sites may
provide habitat comparable to natural sites, at least in the short-term. The nest’s position
can influence its susceptibility to increased temperatures and predators, particularly for.
Climatic conditions such as rainfall can also impact nesting success by altering foraging
conditions and fish behavior. Understanding the effects of hydrologic regimes on biota
can have practical applications since ecosystems around the world share similar problems with competing demands for resources, and there is a concern for how use can affect the
quality of the resource.

The Florida apple snail (Pomacea paludosa) is found
in tropical/subtropical freshwater ecosystems climate
(Cordeiro & Perez, 2011). The Florida apple snail
is of special management interest because survival
of the endangered Snail Kite (Rostrhamus sociabilis
plumbeus) depends on a healthy populations of apple
snails. Results of this study show Florida apple
snails occur independently of habitat type (slough,
prairie etc.), which should indicate abundant foraging
habitats available to Snail Kites. However, Snail
Kites show preferences for specific foraging habitats,
which suggests that they are more restricted in available
habitat than what snail habitat use would suggest.

Understanding the link between indicator species and their environment is imperative to managing and conserving anthropogenically-altered ecosystems. Seasonally-pulsed wetlands are uniquely complex ecosystem where water-level fluctuations shape trophic interactions. Anthropogenic manipulation of water-level fluctuation threatens the integrity of these systems worldwide. Wading birds, a group of species sensitive to landuse changes and fluctuating habitat conditions, serve as important indicators for wetland health. I used wading birds in the Everglades, as a model system to address the challenges of environmental restoration within an ecosystem heavily impacted by anthropogenic activities. Specifically, I 1) identified the nesting response of Great Egret (Ardea alba), White Ibis (Eudocimus albus), and Wood Stork (Mycteria americana) to hydrologically-mediated changes in food availability and 2) quantified spatiotemporal foraging-habitat selection of Great Egrets, White Ibis, and Wood Storks to fluctuating hydrologic conditions. Collectively, model selection results suggest food availability, generated through dynamic hydrological conditions, is a strong predictor of the abundance of nesting birds in a given year. Great egret and white ibis produce the highest nests numbers in years when the frequency of days of rising water is low. Wood stork nest numbers are the highest in years with high prey production coupled with continuous prey availability. My study of resource selection indicated wading birds select foraging sites based on similar hydrologic parameters, but the response varies by species. Wood storks are more likely to forage in shallow cells (< 10 cm) drying with high recession rates (0.5-1.5 cm/day), and long time since last drydown (600 days). White ibises selected foraging cells with relatively shallow water depths (0-15 cm), intermediate recession rates (0.5-1.0 cm/day), and long time since drydown (600 days). Great egrets selected foraging cells with a wider range of water depths (0-20 cm) where recession rates were lower (0.5 cm/day). All species are more likely to forage in cells where water has not increased by more than 3 cm in the previous two weeks. These differences in resource selections correspond to morphological and behavioral differences in the species, whereby wood storks were more constrained hydrologically and would be more affected by water-level manipulation.

One factor that potentially controls the distribution and density of wading bird
prey within open-water marsh habitats during seasonal drying events is the amount of
available aquatic habitat, which is partly a function of the amount of microtopographic
relief at a given location. To determine how microtopographic relief affects prey
concentrations during dry-downs a simulation model was developed and run using
empirical microtopographic data collected from the Everglades. The simulation suggests
that those locations within the marsh with higher levels of microtopographic relief
concentrate prey earlier during the dry-down period and potentially to greater densities
overall. In addition, a model selection analysis was performed on field data to determine
which set of factors displayed the greatest effects upon prey concentrations during drydown
events. When examining the best selected a priori model it appears that the
amount of available aquatic habitat, water depth, and macrophyte density have the
strongest affects upon concentrations of prey during a seasonal drying event.

The vulnerability of prey to capture plays a fundamental role in determining
overall prey availability for wading birds. Structural complexity can act to decrease prey
vulnerability and influence foraging habitat selection. To determine how structural
complexity can affect habitat selection I conducted a use vs. availability study throughout
the Florida Everglades in 2005 and 2006. Results indicated that wading birds chose
foraging sites that had less emergent vegetation and a thicker flocculent layer relative to
random sites. Submerged vegetation, and the height of emergent vegetation did not
affect wading bird site selection. A difference in habitat selection between years was
evident due to hydrological conditions. Ideal hydrological conditions are probably the
most important parameter to wading bird success. Other factors affecting prey
vulnerability became increasingly important in years of poor hydrology, probably
because the penalty for choosing low quality foraging habitat would be greater than in
years of more optimal conditions.

Understanding the foraging and nesting ecology of Wood Storks will play an
important role in Everglades restoration because this species has specific resource
requirements during the breeding season which ultimately affect nest success. I
conducted a foraging habitat use and selection study, which indicated that Wood Storks
from coastal colonies, Paurotis Pond and Rodgers River Bay, require a narrow range of
water depths in the mangrove-saltwater marsh ecotone near their colonies as well in the
freshwater marsh habitats of the inland Everglades. Wood Storks nesting at the inland
colony, Tamiarni West, relied heavily upon nearby freshwater marsh habitat and selected
foraging sites associated with shrub swamp habitat as well as optimal water depths. The
observational nesting ecology study of Wood Storks showed marked differences in
parental nest attendance and food delivery rates between the two years of study, 2005 and
2006, which had different hydrological patterns.

Wetlands are some of the most diverse and productive ecosystems on earth. Water-level fluctuations determine the ecological function of shallow lakes and wetlands. Currently, anthropogenic modification to water-level fluctuations is the leading source of ecological degradation in lake and wetland ecosystems worldwide. I used wading birds nesting in Lake Okeechobee, as a model system to address the challenges of environmental restoration within an ecosystem greatly impacted by anthropogenic activities. Specifically, I 1) identified environmental factors most important for predicting the number of wading bird nests, 2) tested the assumptions of both the match-mismatch and the threshold hypothesis by modeling the relationship between nesting success and prey density with foraging habitat availability, and 3) measured the stress response of Great (Ardea alba) and Snowy Egrets (Egretta thula) to hydrologically-mediated changes in food availability. Collectively, the results suggest that the number of nests was greatest when area of nesting substrate was high and water-levels were moderate (3.9 - 4.4 m). Nest numbers dropped when either nesting substrate or foraging habitat was limited. My investigation into the predictions of the match-mismatch and threshold hypotheses found that indeed, prey density can reduce or intensify the effects of a mismatch event. The interaction of prey density and foraging habitat availability was significant and positive in both models. Saturation thresholds existed for both fledging success (147 prey (m^2)^-1) and total productivity (189 prey (m^2)^-1), above which high concentrations of prey could sustain nesting when foraging habitat availability was low. Finally, my studies of the stress response support the hypothesis that hydrologic factors associated with prey availability play an important role in regulating nesting patterns, although the level of food limitation the birds experience at the lake was not as severe as expected. Model selection identified foraging habitat availability as most influential to the nestling Great Egret stress response, whereas foraging habitat availability and prey density both influenced nestling Snowy Egret stress response. Moreover, the Snowy Egret stress response was more sensitive to changes in prey availability than was the Great Egret stress response. Temperature and foraging conditions influenced yolk corticosterone concentrations for both egret species.

Anthropogenic impacts, such as habitat destruction and spread of exotic species,
are contributing to the sixth major extinction event in Earth’s history. To develop
effective management and conservation plans, it is important to understand the ecological
drivers of at-risk populations, assess the ability of a population to adapt to environmental
change, and develop research methods for long-term ecosystem monitoring. I used
wading birds nesting in the Florida Everglades, USA as a model system to address the
challenges of managing and monitoring populations within an ecosystem greatly
impacted by anthropogenic activities. Specifically, my project investigated 1) the prey
selection of wading bird species, and the role of prey and foraging habitat availability on
annual nesting numbers, 2) the ability of using diet change to predict species adaptability
to a rapidly changing environment, and 3) the use of sensory data to provide low-cost,
long-term monitoring of dynamic wetlands. I found that tricolored herons, snowy egrets, and little blue herons consumed marsh fish larger than those generally available across
the landscape. Additionally, number of nests initiated by tricolored herons, snowy egrets,
and little blue herons was strongly correlated with the annual densities of large fish
available within the Everglades landscape. Conversely, number of nests initiated by
wood storks, great egrets, and white ibises was more correlated with the amount of
foraging habitat availability across the nesting season. Wood stork diets changed
considerably since the 1960’s, consisting of mainly sunfish and exotic fish as opposed to
marsh fishes dominant in historical diet studies. Storks also consumed more exotic fish
species than they did historically. This diet plasticity and the species’ ability to exploit
anthropogenic habitats may be conducive to maintaining population viability as storks
experience widespread human-induced changes to their habitat. Sensory-only data
models generated complementary results to models that used site-specific field data.
Additionally, sensory-only models were able to detect different responses between size
classes of fish to the processes that increase their concentrations in drying pools.
However, the degree to which sensory variables were able to fit species data was
dependent upon the ability of sensors to measure species-specific population drivers and
the scale at which sensors can measure environmental change.

The Roatan Spiny-tailed Iguana (Ctenosaura oedirhina) is endemic to the 146-km2 island of Roatn, Honduras. Harvesting for consumption, fragmentation of habitat, and predation by domestic animals threaten this lizard. It is currently listed as Endangered by the International Union for Conservation of Nature (IUCN), as threatened by the Honduran government, and is on Appendix II of the Convention on International Trade in Endangered Species (CITES). This species has been geographically fragmented and genetically isolated into small subpopulations that are declining in density. With data gathered from use/availability surveys, resource selection functions were used to identify habitats and environmental variables associated with their presence. Results indicate that protection from harvesting is the most important factor in determining their distribution. These high-density populations are currently restricted to ~0.6 km2. Organisms living in small, isolated populations with very restricted ranges are at higher risk of extirpation due to various direct and indirect forces. Mark-recapture-resight surveys and distance sampling have been used to monitor the populations since 2010 and 2012 respectively. The data show that the high-density populations are declining. The current population size is estimated to be 4130-4860 individuals in 2015. A population viability analysis (PVA) was conducted to identify the most pressing threats and specific life history traits that are affecting this decline. The analysis estimates that if current trends persist, the species will be extinct in the wild in less than ten years. Adult mortality is a main factor and female mortality specifically characterizes this decline. In order for this species to persist over the next fifty years, adult mortality needs to be reduced by more than 50%. A lack of enforcement of the current laws results in the persistence of the main threat, poaching for consumption, thus altering the species distribution and causing high adult mortality. This is complicated by social customs and a lack of post primary education. Management changes could mitigate this threat and slow the population decline. Recommendations include an education campaign on the island, increased enforcement of the current laws, and breeding of C. oedirhina in situ and ex situ for release into the wild.

The spread of nonnative invasive species has become the second greatest threat to global biodiversity, making management of invasive species a critical component of the conservation of biodiversity worldwide. Managers and conservation biologists often lack basic life history data, as well as quantitative and theoretical models to predict risk of invasion or other negative effects. I contribute information to both categories by providing life history information (diet and morphology) of the Purple Swamphen (Porphyrio porphyrio) and by characterizing the invasion pathways that nonnative avian species in Florida follow. I found Purple Swamphens are predominantly eating and selecting for Eleocharis cellulosa. Additionally, there is a large amount of variation in nonnative avian species’ propensity to colonize natural habitat and the time it takes to do so. Nine out of 15 species investigated colonized natural habitat and the time it took them to do so ranged from 8 to 41 years. It is through a combination of various techniques that ecologists will begin to fully understand the importance of studying nonnative species as well as reducing the impact that nonnatives have on native ecosystems.