Wetland ecology

Wetlandscape topography creates spatial variation in hydroperiod, which in turn creates spatial variation in biotic communities. Such spatiotemporal variation occurs on a large scale in some of the most productive wetlands in the world, including those of south Florida, U.S.A. Wading birds (Pelicaniformes and Ciconiiformes) are iconic, top-level consumers of such ecosystems. Infrequent drying is necessary to maintain the primary nest substrate (Salix caroliniana), and prey availability is regulated by production of prey biomass in the wet season and the subsequent concentration of prey biomass into shallow pools during the dry season. The goal of this dissertation was to explicitly model wading bird nest abundance and survival as functions of water-level fluctuations, which were spatially nested (i.e., the effects of water-level fluctuations that occurred over a large scale were measured separately from the effects of water-level fluctuations that occurred over a small scale). In Chapter 2, I modeled colony-specific effects of wetlandscape water level fluctuations on wading bird nest abundance. Modeling the response at the colony level allowed the inclusion of important parameters that cannot be measured at the wetlandscape scale. For instance, each colony had its own optimal range of lake stage, which depended on local topography. I used the models to predict cumulative nest abundance under six hydrologic scenarios that were based on potential water management actions at Lake Okeechobee. I found that increasing water levels at the lake would marginally benefit the Great Egret but would substantially reduce long-term Snowy Egret and White Ibis populations. In Chapter 3, I modelled spatiotemporal distributions of fish biomass density in Lake Okeechobee’s littoral zone as a function of hierarchically nested hydrological variables. These models were consistent with the dynamic landscape connectivity model previously described in the literature. I modified the models to predict a binomial response which could then be linked to wading bird foraging threshold. The model predictions were used to estimate the number of available patch days during the breeding season, which was highly correlated with the number of nests for the great egret (Ardea alba), the snowy egret (Egretta thula), and the white ibis (Eudocimus albus). In Chapter 4, I used spatial statistics to better understand how interannual variability in resource wave patterns in the littoral zone influenced wading bird nest abundance. I found that more birds nested in years when the drying edge of the marsh moved further across the landscape. Great egret nest survival increased also, but small heron nest survival decreased. This decrease was likely because small herons continued to nest late into the season in years with longer waves, and, as with most bird species, nests that are initiated later in the season. In Chapter 5, I compiled conventional nestling diet data from 5 wading bird species sampled in 4 wetland types from 2010 to 2020 (not every wetland type was sampled in every year). This chapter provides a comprehensive, broad description of wading bird diets in south Florida, and quantifies interspecies, spatial, and interannual variation in nestling diets. By using a model-based approach to quantify the relative biomass of prey species and prey traits in nestling diets, I provide the first diet analysis that is fully reproducible across the large sympatric range of the wading bird species in the study (great egret, snowy egret, tricolored heron [Egretta tricolor], little blue heron [Egretta caerulea], and wood stork [Mycteria americana]).

Macroinvertebrates make up a large fraction of secondary production in wetlands
and are strongly influenced by hydrologic alterations. However, little is known of the
effect flow has on macroinvertebrate production and community composition in wetlands.
Reintroducing measurable water velocities (1-5 cm/s) to the oligotrophic (phosphoruslimited)
Everglades has the potential to affect macroinvertebrate production and
community structure by increasing the supply of phosphorus (P) to periphyton, changing
basal food quality, and physically altering the habitat. In Chapter 2, I investigated the
potential effects of flow-mediated nutrient-loading on the growth rate of herbivorous
grazers, apple snails (Pomacea maculata), by growing periphyton on standard substrates,
in four sloughs within a landscape-scale flow addition experiment; two sloughs received
elevated flow velocities (2.9-5.2 cm/s) and two control sloughs (0.3-0.4 cm/s). Snails fed
periphyton from the flowing sloughs gained more than 3.7-fold greater total mass than
snails fed periphyton from the control sloughs. The highest velocity slough produced the
greatest snail growth. Snail growth was correlated with P accumulation and C:P ratios in
the periphyton. Water column nutrients were low and the water column TP differences among sloughs could not fully explain differences in periphyton nutrients or snail growth.
Increasing flow above background conditions by as little as 2.4 cm/s in this oligotrophic
wetland altered periphyton food quality by flow loading, which subsequently increased
growth rates of primary consumers. In Chapter 3, I investigated potential changes in
macroinvertebrate standing stock biomass and community composition caused by flow by
conducting two sweep net surveys. Surveys were conducted within two groups of sloughs,
flow sloughs (6 sloughs) and control sloughs (12 sloughs), within the same landscape-scale
flow addition experiment as the growth experiment. Surveys were conducted from January
to March in 2018 and 2021. Biomass was compared between the flow sloughs and control
sloughs using generalized linear models. Community composition was analyzed at the
slough level using Bray-Curtis similarity. Biomass of macroinvertebrates was similar
among transects in 2018 but higher in the flow sloughs in 2021 with the highest biomasses
further from the L67A (velocities <2cm/s). The two highest flowing sloughs (typically
>3cm/s) had a different community composition from the other 16 sloughs in both 2018
and 2021 with a predator resistant non-native snail appearing in 2021. The results of the
surveys show an increase in macroinvertebrate production in the flow sloughs possibly
related to increased nutritional value of food, however, with higher biomass further from
the L67A and the invasion of a predator resistant snail at the high flow sloughs, it also
appears that there is increased top-down pressure on the macroinvertebrates at the sloughs
closer to the L67A (were cover from predation has been reduced). The overall results of
these studies indicate flow produces more nutritional food for herbivorous
macroinvertebrates and increases standing stock biomass but can change the community
composition when periphyton cover is reduced.

Distribution and abundance of water impacts population dynamics and habitat structure within a wetland ecosystem. It is known that drought can interfere with trophic dynamics and temporarily free crayfish from the threat of predation by population limiting fish predators in seasonal freshwater wetlands. Less is known about wet season water depths, and their effect on predator prey interactions. In order to address this, I conducted a two-and-a-half-year study in which I experimentally manipulated the wet season water depth in four 8 ha replicate wetlands of the central Everglades, allowing two wetlands to be flooded as natural rainfall patterns would allow (unconstrained hydro-pattern) and two wetlands to experience a constrained hydro-pattern in which the maximum depths to which they were flooded was limited. I discovered that crayfish, small marsh fish, and large bodied predatory fish populations were unaffected by the water depth difference between the hydro-pattern treatments. Unlike fauna, flora did respond to the hydro-pattern treatments with stem densities increasing in the wetlands under the constrained hydro-pattern treatment.

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.

Peatlands act as carbon sinks while representing major sources of biogenic gases
such as methane (CH4) and carbon dioxide (CO2), two potent greenhouse gases. Gas
production and release in these peats soils are also influenced by overall warm
temperatures and water table fluctuations due to the naturally shallow water table in the
Florida Everglades. Releases of biogenic gases from Florida Everglades peat soils are not
well understood and the temporal distribution and dynamics are uncertain. The general
objective of this work was geared towards a methodological approach which aimed to
examine the feasibility of capacitance moisture probes to investigate biogenic gas
dynamics in various Florida Everglades peat soils at high temporal resolution. This work
has implications for establishing capacitance moisture probes as a method to monitor gas
dynamics in peat soils at high temporal resolution and better understanding patterns of
gas build-up and release from peat soils in the Everglades.

Shortened periods of inundation due to water management have led to the encroachment and expansion of Carolina willow (Salix caroliniana) in sawgrass (Cladium jamaicense) marsh communities. Morphologic and physiologic differences between sawgrass and willow have potential consequences for microhabitat conditions and ecosystem function such as a reduction in temperatures and light availability and changes in primary productivity. Since it is a woody shrub, willow is often assumed to exhibit higher rates of transpiration than non woody plants, which in turn can affect photosynthesis and carbon exchange and ultimately wetland water management. In this study willow was found to have higher rates of stomatal conductance (gs) and photosynthesis (Anet) than sawgrass. However, sawgrass had greater intrinsic water use efficiency (WUE) than willow. This suggests that willow is capable of greater gas exchange and carbon assimilation than sawgrass but requires more water. Understanding the implications of willow expansion will improve landscape models of wetland water and carbon exchange and inform water management decisions.

Many herpetofaunal species are imperiled, and the causes of this are often a
synergy of multiple factors. In wetlands specifically, two of the possible determinants of
species occurrence and faunal community assemblage are fish presence and wetland
permanence, which are not always correlated. Twenty wetlands were sampled in
Jonathan Dickinson State Park, Florida, USA to observe how wetland herpetofaunal
communities vary with fish, wetland permanence and other environmental factors.
Herpetofaunal communities with and without fish were significantly different from one
another and differences between herpetofaunal communities were primarily due to the
contribution of four species of frogs, two generalist ranids and two specialist hylids.
Wetland permanence had no observable effect on community structure. Fishless wetlands were significantly more species-rich and possessed higher numbers of individuals even for species that occurred in both fishless and fish wetlands, regardless of their permanence. These findings have implications for wetlands restoration and herpetofaunal conservation.

Saltwater intrusion is threatening coastal wetlands through alteration of the natural communities leading to native species loss. In southern Florida, the Pond Apple Slough is experiencing tidally driven saltwater intrusion, which is thought to be responsible for the invasion of a marine isopod, Sphaeroma terebrans, into the native, dominant pond apple trees. The isopod appears to be responsible for the death of pond apple trees in the site. The isopod's distribution on trees within the slough appears to be controlled by the period of inundation and distance from the source of saltwater. Restriction of tidal water entering the site may limit the isopod's distribution on the pond apple trees.

Life history strategy suggests long lived bird species will adjust their nesting effort according to current conditions, balancing the costs of reproduction with their long-term needs for survival and future reproduction. The habitat conditions that produce these responses may differ between species, even within the same ecosystem, producing different nesting and population trends. I traced the pathway by which food availability influences the physiological condition of pre-breeding great egrets and white ibises through to reproductive measures, and the physiological condition of chicks. I focused on these two species with contrasting foraging strategies, in relation to foraging and habitat conditions to maximize the likelihood of application of these results to other wading bird species. Experimental food supplementation and physiology research on white ibis chicks demonstrated that in years with low prey availability white ibis were food limited, with increased levels of stress protein 60 and fecal corticosterone. This is the first study to demonstrate experimentally the response of stress protein 60 to changing levels of food availability. During a year with low prey availability (2007) white ibis adults and chick physiological condition was lower than that of great egrets. During the same year, fledging success was lower for both species (20% for white ibis versus 27% for great egret) but the magnitude of the decrease was particularly severe for the white ibis (76% decline versus 66% decline for the great egret). Results suggest white ibises modify their clutch size during years with poor habitat in accordance with life history traits of a long-lived species, whereas great egrets maintained their clutch size during years with poor habitat.

It can be difficult to disentangle the factors that determine population success in freshwater systems, particularly for organisms with disturbance-resistant life stages like aquatic invertebrates. Nevertheless, the effects of environmental variation and habitat structure on animal population success in wetlands are important for understanding both trophic interactions and biodiversity. I performed two experiments to determine the factors limiting crayfish (Procambarus fallax) and dragonfly (Family: Libellulidae) populations in wetland environments. A simulation of a dry-disturbance and subsequent sunfish (Family: Centrarchidae) re-colonization revealed that crayfish populations are sensitive to sunfish, while dragonfly naiads seemed to be limited by other drying-related factors. A second manipulation revealed that small-bodied fishes and habitat structure (submerged vegetation) shaped dragonfly communities primarily through postcolonization processes.