Proffitt, C. Edward

The main objective of this research was to analyze how well my proposed Foundation Species Interaction model explained the benthic communities found on red mangrove (Rhizophora mangle) prop roots. This research investigated the connections between the primary foundation species (mangroves), the dominant secondary foundations species (oysters, sponges, and barnacles), and the resulting biodiversity in order to understand the temporal and spatial variability of the ecosystems at different community levels. Chapter 1 was dedicated to explaining my change in ecological theory, the Foundation Species Interaction model. The interactions included in the model between the foundations species that were examined were the mutualistic (+, +), commensal (+, 0), and parasitic (+, -) interactions. Chapter 2 focused on exploration of the mangrove ecosystem in Southeast Florida and establishing where the secondary foundation species and prop root epibionts where found along the latitudinal gradient. The survey investigated the connections between mangroves, the dominant secondary foundations species (e.g. oysters, sponges, and barnacles), and the resulting biodiversity to understand the temporal and spatial variability of the ecosystems at different community levels. Chapter 3 was dedicated to testing the Foundation Species Interaction model’s ability to predict the biodiversity along the latitudinal gradient of the survey. The difference in mangrove prop root communities were largely explained by where the communities laid along the latitudinal gradient and by secondary foundation species presence. The shift from one foundation species to another had sizable effects on biodiversity. Chapter 4 combined the Foundation Species Interaction model with the established predation hypothesis and tested their abilities to explain biodiversity along the latitudinal gradient. This allowed for shifts in community structure to be examined for top-down and bottom-up influences. Predation effects changed along the latitudinal gradient, as the predation effects changed so did the effects of each of the individual foundation species.

Sphaeroma terebrans, is an ecological engineer that can significantly modify the habitat of free-hanging aerial prop roots of Rhizophora mangle. The wood-boring isopod extensively burrows into red mangrove aerial prop roots for habitat and protection from desiccation and access to phytoplankton. However, the burrows created have major consequences on the mangrove habitat and aerial root inhabitants. It has been suggested that sessile species residing in aerial root communities can either encourage or discourage colonization by S. terebrans. Abiotic factors can affect the distribution and abundance of mangrove forest and are the same factors which determine the composition and abundance of organisms living on the roots. Surveys indicated that burrowing damage was found predominately in the first 20 cm of the root tip. Exploratory Structural Equation Modeling (SEM) was used to test multivariate hypothesized models looking at habitat relationships with S. terebrans in aerial root communities. Temperature and dissolved oxygen were shown to be important drivers in affecting submerged root length of aerial roots. Ultimately, the indirect effects between these parameters proved to be stronger in influencing the barnacle – isopod association, which causes direct negative effects on submerged root length. Colonial tunicates showed weak effects in masking aerial roots from the damaging barnacle – isopod association. Chlorophyll a was used as a proxy for phytoplankton biomass and proved to be less influential than habitat protection for S. terebrans. Results highlight the need for experimentation in addition to modeling in order to determine the mechanisms influencing aerial root community inhabitants and further effects on the habitat.

Climate change is causing shifts in species geographic distributions. This trend is seen
throughout the globe but the impact is especially noticeable in marine environments, which are
highly sensitive to phenological and ecological alterations. Here, systemic shifts have cascading
effects on the food web, productivity, and event timing. Throughout the tropics and the
subtropics, mangrove trees act as the primary foundation species that dominate the intertidal
zone. In particular, red mangroves Rhizophora mangle play a crucial role by acting as substrate
for sessile species within their ecosystems. In these ecosystems, secondary foundation species
that can colonize the prop roots of the red mangroves thereby further affecting the structure of
the community. The original habitat architecture limits species variety and the effectiveness of
species to utilize the space. Habitat architecture is strongly influenced by the foundation species
that form the base for community structure. Investigating the connections between a primary
foundation species, secondary foundation species, and the resulting biodiversity of sessile
species is critical to understanding the variability of the ecosystem. Association with certain
foundation species may provide a more positive environment for certain taxa than others and
thus ease stressors that otherwise could functionally eliminate a species from the ecosystem. In
addition, these associations can have cascading effects on neighboring species and neighboring
ecosystems. Here, we conducted a presence/absence survey from Key West to the Kennedy
Space Center to identify the species that utilized mangrove prop roots as habitat, their
associations, and distributions.

In Florida, mangroves have responded to climate change by slowly migrating
northward into traditional salt marsh habitat. However, little is understood about the
relationships among mangrove growth form plasticity and environmental conditions. In
addition, the effects of the mangrove northward expansion on pre-existing salt marsh
communities are unknown, especially any influences of differences in tree morphology.
The size, canopy structure, and root structure of the three mangrove species Rhizophora
mangle, Avicennia germinans, and Laguncularia racemosa were measured at six sites
along the east coast of Florida. Structural equation modeling was used to evaluate the
multivariate relationships between environmental and biotic variables. Mangrove growth
form varied widely with environmental variables. The results of this study suggest that R.
mangle expansion into salt marsh may rely on interactions with salt marsh and shading as
well as on climatic variables, which has implications for future mangrove expansion
northward in Florida.

Seagrasses are important foundation species in coastal ecosystems. Genetic
diversity of seagrasses can influence a number of ecological factors including, but not
limited to, disturbance resistance and resilience. Seagrasses in the Indian River Lagoon
(IRL), Florida are considered to be highly disturbed due to frequent events, like algal
blooms, that impair water quality, reducing available light for seagrass growth. Halodule
wrightii is a dominant seagrass throughout the IRL, but its genetic diversity has only been
quantified in a few Gulf of Mexico and Florida Bay populations and little is known about
its potential ecological consequences. I quantified the genetic variation of H. wrightii
using microsatellite markers in the southern IRL to determine: (i) how disturbance history
influenced genetic diversity, (ii) if morphology of clones was, in part, genetically
controlled and related to disturbance history, and (iii) if genotypes showed phenotypic
plasticity in response to disturbances. In the IRL, H. wrightii populations exhibited moderate genetic diversity that varied with disturbance history. The disturbance history
of a population was classified by the variance in the percent occurrence of H. wrightii
over a 16-year period. Genotypic richness and clonal diversity of H. wrightii increased
with increasing disturbance histories. Other genetic diversity measures (e.g., allelic
richness, observed heterozygosity) did not change with disturbance history. These
findings suggest that impacts to seagrass coverage over time can change the genotypic
composition of populations. When different genotypes of H. wrightii were grown in a
common garden, differences in leaf characteristics among genotypes provided evidence
that morphological trait variation is, in part, explained by genetic variance. The
disturbance history of genotypes did not directly affect morphological traits. However,
significant genotype x site (within disturbance history) interactions found greater
variation in shoot density and below ground traits of H. wrightii genotypes from sites of
intermediate disturbance history. Traits of H. wrightii were shown to be phenotypically
plastic. Significant genotype x environment interactions for shoot density and height
demonstrated that genotypes responded differently by increasing, decreasing, and not
changing sizes in response to light reduction. Genetic diversity of H. wrightii has strong
implications for ecological function in coastal communities.

Plant interactions (e.g., competition, facilitation) are critical drivers in
community development and structure. The Stress Gradient Hypothesis (SGH)
provides a predictive framework for how plant species interactions vary inversely
across an environmental stress gradient, predicting that facilitation is stronger with
increasing levels of stress. The SGH has been supported in numerous ecosystems
and across a variety of stress gradients, but recent research has demonstrated
contradictory results. These discrepancies have led to SGH revisions that expand its
conceptual framework by incorporating additional factors, such as other stressor
types and variations in species life history strategies. In this dissertation, I examine
a further modification of the SGH by proposing and testing a Multiple Stress
Gradient Hypothesis (MSGH) that considers how plant interactions vary along a continuous gradient of two co-occurring stressors using mangrove and salt marsh
communities as a case study. In Chapter 1, I outline the predictive framework of a
MSGH, by creating a series of predictions of species interactions. The components
of the MSGH predict that stressors of similar types (e.g., resource and nonresource)
will have similar effects and be additive. On the other hand, varying
species life history strategies and life stages will lead to extremes of plant
interactions. In Chapter 2, I performed a series of experiments to test the various
components of the MSGH. In Chapter 3, I performed a large-scale observational
study to test whether multiple co-occurring stressors altered the cumulative effects
on plant interactions, and if these stressors should be grouped (e.g., resource and
non-resource, abiotic and biotic, etc.) to enhance predictability. From a series of
studies conducted herein, I concluded that co-occurring stressors are important
factors that control complex species interactions as shown in my MSGH modeling
approach. Further, future theories need to incorporate species-specific and stressor specific
grouping when modeling how species interactions shape communities.