Dorn, Nathan

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.

In this thesis I examined factors limiting population success of apple snails (Pomacea spp.) in Florida wetlands. First, I examined effects of summertime hydropattern in replicated wetlands on reproduction and juvenile growth. Annual reproductive effort of Florida apple snails (Pomacea paludosa) and the invasive apple snail P. maculata under was not affected by deeper water in the summer. When juvenile P. paludosa were forced to grow in reportedly favorable and relatively deeper summertime depths survival was high between treatments and growth was unaffected. In the other chapter I examined interactions between the two snails with a series of observations and experiments. I examined historical data on the assemblages and found that P. paludosa were locally extinguished in one of the wetlands within six years of invasion by P. maculata. Two field experiments gave conflicting results about the importance of interspecific inhibition of P. paludosa by the invasive snail, but in either case resource reduction could not be demonstrated. Lab exposure to P. maculata chemical cues strongly inhibited P. paludosa growth, although P. paludosa did not behaviorally avoid mucus of P. maculata. In mesocosms Pomacea paludosa growth was reduced with increasing exposure to adult P. maculata waterborne cues, and the results suggest that contact with mucus at a low exposure may even have a greater inhibiting effect. This interspecific direct chemical growth inhibition was novel for gastropods, but could be more generally important, and its potential impacts to P. paludosa populations should be explored further.

Avian reproduction is generally resource dependent, and resource availability can vary through time and space. Wading birds breeding activity in southern Florida wetlands is limited by availability of aquatic prey, which is largely influenced by seasonal hydrodynamics. Restoration of natural hydrologic patterns is expected to increase populations and return breeding activity in the southern Everglades, but which prey support successful breeding at coastal colonies is unknown. To address this, I examined prey use of nesting White Ibis (Eudocimus albus) at coastal colonies in Everglades National Park (ENP) in a two-year observational study. I also examined ibis diets at nesting colonies located in three regions of the Everglades to better understand regional variation in trophic support of ibis reproduction. I found that crayfish and fish were used extensively in ENP in a year with exceptional ibis breeding activity, and I provide evidence that lengthened hydroperiods in higher elevation marl prairies may enhance crayfish production and availability in the southern Everglades. I found that crayfish were the most important prey type in all regions of the Everglades when reproductive effort was highest. These results add to the growing body of evidence that crayfish are important prey for ibis reproductive success in all parts of the Everglades, and suggest that increasing water flowing into the southern Everglades may incite greater nesting at historic colony locations. With this knowledge we can forecast how wading birds, and more generally wetlands of southern Florida, will respond to a changing environment and potential restoration scenarios.

The hydrological and topographical variation of wetlands can affect the behavior,
population growth, and local densities of aquatic species, which in turn can drive the
behavior and density dynamics of gleaning predators. Prey availability, primarily
determined by prey density and water depth in wetlands, is an important limiting factor
for nesting wading bird populations, top predators in the south Florida Everglades. The
Everglades is able to support large colonies of nesting wading birds because of the
microtopographic variation in the landscape. Some types of prey concentrate in flat,
shallow sloughs or become trapped in isolated pools as they move down from higher
elevation ridges with receding water levels. Manipulations to the hydrology and
landscape of the Everglades has negatively impacted nesting wading bird populations in
the past, and may continue to be detrimental by allowing flat, shallow sloughs to be
intersected by deep canals, a potential refuge for wading bird prey. In addition, the subtle
elevation differences between the ridge and slough landscape may be an important mechanism for increasing slough crayfish (Procambarus fallax) prey availability for the
most abundant and seemingly depth-sensitive Everglades wading bird, the White Ibis
(Eudocimus albus). I implemented a 2-year experimental study in four replicated manmade
wetlands with controlled water recession rates in order to determine the effects of
proximate deep water (akin to canals) on fish prey concentrations in the sloughs, as water
levels receded similarly to a natural Everglades dry season. I also calculated average
daily wading bird densities with game cameras (Reconyx PC800 Hyperfire) using timelapse
imagery over 60 days to determine when and where wading birds responded to
changing prey concentrations. I completed an additional observational study on White
Ibis and slough crayfish prey from the first year of data (2017). Crayfish make up the
majority of the diet for nesting White Ibis, and literature has suggested crayfish are most
abundant at slough depths much deeper than previously proposed foraging depth
limitations for White Ibis. This study specifically compared recent determinations about
crayfish movement dynamics in the ridge and slough system with White Ibis foraging
behavior and depth limits. Results from the first experimental study suggest that canals
might be an attractive refuge for relatively large prey fishes (> 3 cm SL) in sloughs, but it
is uncertain if the fencing blocked all prey fish movement. The second observational
study determined White Ibis foraging activity was primarily driven by a down-gradient
crayfish flux from ridge to slough, with the majority of foraging activity occurring at
much deeper slough depths than previously suggested water depth limitations for White
Ibis. Results from both of these studies support the importance of preserving the ridgeslough
landscape of the Everglades to sustain high prey availability for wading birds.

Purpose: To quantify Reptile and Amphibian community assemblages in freshwater
wetlands and determine biotic and abiotic factors e.g., wetland permanence and presence of large
fish that may influence assemblage structure. Methods: Wetland Reptile and Amphibians were
monitored via passive trapping at 20 wetland sites at Jonathan Dickinson State Park JDSP in
Hobe Sound, FL. Trapping at JDSP occurred over a single wet season June-November period
and community assemblages were analyzed statistically using multivariate techniques. Results:
Hierarchical clustering revealed two primary assemblages, which included 70 of wetlands. One
assemblage, typical in wetlands containing large-bodied fish, consisted of two or three snake
species and low abundances of anurans in the family Ranidae. The second assemblage, typical of
fishless wetlands, generally had abundant anurans in the families Hylidae and Ranidae.
Conclusion: Though distance from other wetlands as well as wetland permanence also influenced
assemblage structure, fish presence/absence was a larger determinant of herpetofaunal
communities.

The purpose of this research was to quantify wading
bird hourly and daily foraging activity in a drying
wetland. We sought to determine the hours of peak
activity and identify water depths that facilitate high
foraging activity. Our results indicated that 56% of
daily activity occurred between 0600 – 0800 h and
>95% of the daily activity was found in images taken
between 0600 and 1400 h. The wading bird activity
showed little variation and no obvious correlation with
water depth fluctuations (highest recorded density =
2.1 birds/ha at 14.9 cm depth; lowest recorded density
= 0.05 birds/ha at 35.1 cm depth). The wading bird
activity occurring between 0600 – 1400 h accounts
for >95% of daily activity. We conclude that a study of
water depth and average wading bird density earlier
in the nesting season is needed for a clearer effect of
water depth on wading bird foraging activity.

Theory predicts that when prey can reach a size refuge from predation, prey
vulnerability to predation is a function of hatchling size, growth rate, and the handling
limitations of its predator, which collectively influence the amount of time prey spend
vulnerable. I examined the mechanistic role of prey size for the predator-prey interaction
between predatory crayfish (Procambarus fallax) and apple snail prey (Pomacea
paludosa and P. maculata) and found that crayfish feeding rates decreased with snail
size, such that smaller hatchling P. maculata were more than twenty times more
vulnerable than hatchling P. paludosa. Experimental manipulations of productivity
increased apple snail growth rates, reducing the effects of predatory crayfish on P.
maculata survivorship, but not P. paludosa survivorship. My results indicate that when
prey can reach a size refuge from predation, increased system productivity decreases
predator limitation of that prey.

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.