INTRODUCTION
Herbivory serves as a critical top-down mechanism within plant
communities that regulates biodiversity, productivity, and ecosystem
function (Picala and Crawley, 1992; Mortensen et al., 2018; Moorhead et
al., 2017). Herbivory often influences local-scale spatial ecology of
plant communities via direct and indirect impacts (Picala and Crawley,
1992). Herbivores regulate plant communities directly via consumption of
plant biomass. Plant communities are impacted indirectly, however,
through changes in resource availability by the removal of dominant,
highly productive plant species by herbivores that may facilitate the
establishment of rare or subordinate species, potentially increasing
richness and community evenness (Pacala and Crawley, 1992; Holt, Grover
and Tilman, 1994; Becerra, Noge, and Venable, 2009; Mortensen et al.,
2018).
Herbivory by vertebrates in temperate ecosystems can reduce plant
productivity, increase plant diversity, and alter lower trophic levels
and soil chemistry (Sala et. al, 1986; Ritchie, Tilman, and Knops, 1998;
Schowalter, 2012; Wilson et. al., 2012; Moorhead et al. 2017; Ramirez et
al., 2020). In North American forests, the density of white-tailed deer
(Odocoileus virginianus ), a ruminant herbivore, is often strongly
related to reductions in understory plant productivity, diversity, and
tree recruitment (Ritchie et al., 1998; Shelton et al., 2014; Habeck and
Schultz, 2015). Herbivory by mammals can also have cascading impacts on
ecosystem function (Van Der Wal et al, 2004; Moorhead et al. 2017;
Ramirez et al., 2020). For instance, even small mammal consumption and
redistributing of plant biomass have been shown to alter woody biomass
and plant community composition leading to indirect effects on plant
tissue chemistry and soil enzyme activities (Moorhead et al. 2017).
While the ecological impacts of mammalian and insect herbivores have
garnered much attention in the literature (Ritchie et al., 1998; Goheen
et. al, 2010; Wilson et. al, 2012; Hodkinson, 2012), fewer studies have
investigated the role of herbivorous reptiles in regulating plant
communities (King, 1996; Olesen and Valido, 2004; Fourqurean et al.,
2010).
Herbivorous reptiles are relatively uncommon because of the
physiological constraints imposed by ectothermy on thermoregulation and
the digestion of nutrient-poor plant materials. However, herbivorous
reptiles can serve as the dominant herbivore in a variety of
environments, such as arid deserts, forest canopies, tropical
rainforests, and both marine and freshwater aquatic systems,
particularly on island systems (Nagy and Shoemaker, 1975; Moll and
Jansen, 1995; King, 1996; Olesen and Valido, 2003; Fourqurean et al.,
2010). In the southeastern US, herbivorous chelonians can be observed in
both aquatic (e.g. , Pseudemys floridanus andChelonia mydas ) and terrestrial environments (e.g. ,Gopherus polyphemus and Terrapene carolina ) (Bonin et al.,
2006; Fourqurean et al., 2010). However, we have a minimal understanding
on the role of chelonian herbivores as regulators of plant community
diversity and composition (Estill and Cruzan, 2001; Tuberville et al.,
2005).
The gopher tortoise (Gopherus polyphemus ) is an herbivore found
throughout the upland pine forests of the southeastern Coastal Plain in
North America. Gopher tortoise herbivory and burrow excavation increase
habitat heterogeneity by altering light exposure, temperature, soil-pH,
soil-moisture content, and the distribution of nutrients in sandhill
substrates (Diemer, 1986; Kaczor and Hartnett, 1990; Pike and Mitchell,
2013). Browsing gopher tortoises may remove dominant, palatable plant
species and increase the relative abundance of rarer species (MacDonald
and Mushinsky, 1988). Thus, gopher tortoises may impact plant community
composition and primary production, however, there have only been a few
attempts to document these impacts (Diemer, 1986; Breininger et al.,
1991; McCoy et al., 2006).
Gopher tortoise herbivory and burrowing provide frequent, low-intensity
bouts of disturbance due to low metabolic costs and intimate home range.
(McRae et al., 1981, King, 1996). Studies that quantify the impacts of
herbivory commonly deploy experiments that selectively exclude
herbivores and/or control grazing intensity (Ritchie et al., 1998;
Hester et. al., 2000; Habeck and Schultz, 2015). Exclusion experiments
allow researchers to understand thresholds of grazing intensity, effects
on primary productivity, herbivore-herbivore interactions, and impacts
on biodiversity and whole-system dynamics (Noy-Meir, 1975; Mitchell and
Kirby 1990; Milchunas and Lauenroth, 1993; Ritchie et al., 1998; Hester
et. al., 2000; Habeck and Schultz, 2015; Fourqurean et al., 2010;
Richardson and Stiling, 2019). In the case of the gopher tortoise,
Richardson and Stiling, 2019, completed a two-year tortoise-exclusion on
coastal Florida shrubland islands devoid of mammalian herbivores finding
a reduction in both plant richness and species diversity. An exclusion
experiment in upland longleaf pine forest that is significantly more
rich in understory plant species alongside mammalian herbivores offers
an ideal system to test how the removal of a low-intensity, frequent
disturbance of a reptilian herbivore effects: 1) univariate measures of
richness, evenness, and diversity, 2) aboveground plant community
production, and 3) the overall effects on plant community composition.
We hypothesize that gopher tortoise exclusion will increase aboveground
plant production but will reduce plant richness, evenness, and diversity
as fast-growing, dominant species quickly respond to the removal of
herbivory. This will result in a significant shift in overall plant
community composition between access and exclosure plots.