DISCUSSION
Following a single growing season of gopher tortoise exclusion, we found
a 35% increase in total plant cover, 45% increase in plant height,
15% reduction in plant species richness and 12% reduction in plant
diversity. Even short-term gopher tortoise exclusion can have
significant measurable impacts on the understory plant communities of
upland pine forests. Our results also suggest that gopher tortoise
exclusion led to an increase in small stature woody species
(Geobalanus oblongifolius and Toxicodendron radicans ),
grasses (Sorghastrum secundum ), and ferns (Pteridium
pseudocaudatum ), many of which are known dietary components of
the gopher tortoise (MacDonald and Mushinsky, 1988; Mushinsky, Stilson
and McCoy, 2003). Our results suggest that the persistence of gopher
tortoises may be useful for maintaining the heterogeneous and biodiverse
understory plant assemblages of coastal upland pine forests alongside
fire (Kaczor and Hartnett, 1990; Breininger et al., 1991; McCoy et al.,
2006).
The effects of tortoise exclusion on plant richness and diversity over
six months are comparable to longer-term studies in scrub-shrub habitats
(Richardson and Stiling, 2019). After two years of gopher tortoise
exclusion in a coastal Florida shrubland, Richardson and Stilling (2019)
found a 37% reduction in plant richness (9 plant species total) and a
38% reduction in Gini-Simpson diversity. While our 15% reduction in
richness and 12% in plant diversity were about half the effect size
found by Richardson & Stilling, we suspect that future studies will
reveal that the effects of tortoise exclusion will increase with time.
Significant impacts of herbivore exclusion experiments after one year of
exclusion are rare in the literature, and significant effects build up
after multiple years (Moorhead et al. 2017, Shelton et al. 2014, Frank,
Rathfon and Saunders, 2018; Schäfer et al., 2019; Bloodworth, Ritchie
and Komatsu, 2020). In longleaf pine forests in southern Alabama,
white-tailed deer exclusion had no effect on plant diversity and cover
after four years of exclusion (Brockway and Lewis, 2003). As a potential
working hypothesis to explain the differences between gopher tortoise
and mammal exclusion, most mammalian herbivores have significantly
larger home ranges relative to gopher tortoises. For example,
white-tailed deer have home ranges up to 1000 ha and eastern cottontail
rabbit have home ranges of 4–8 ha, while gopher tortoise home ranges
are typically limited to 0.004–3.2 ha (Trent and Rongstad, 1974;
Gálvez-Bravo et al., 2011; Bowers, 1993; Guyer at al., 2012). The strong
effects of gopher tortoise exclusion may be driven by the more
concentrated impacts of gopher tortoise herbivory relative to the
diffuse impacts of grazing mammals (McRae et al., 1981, Bowers, 1993).
We found that plot spatial location explained 48% of the variation in
plant community composition, which was substantially more when compared
tortoise exclusion, which was ~5%. Plant species
richness ranged between 7-24 individual species and percent cover varied
between 0.5% - 55% coverage yielding high amounts of heterogeneity
among our plots. We found that 24% of observed plant species were found
in one or two plots and only 13 species were found in more than half of
our plots. High heterogeneity is commonly reported within understory
communities of upland longleaf pine forests (Drew, Kirkman and Gholson,
1998; Kirkman et al., 2001; Mitchell et al., 2006; Blaustein, 2008;
Agrawal, Lau and Hambäck, 2006).
While gopher tortoises directly impact plant assemblages by defoliation,
they also influence plants by burrow excavation, urine and feces
deposition, and routine trampling of vegetation during foraging and
frequent burrow-to-burrow migrations (Diemer, 1986; Kaczor and Hartnett,
1990; Pike and Mitchell, 2013). The small size of our exclosures (1
m2) likely fail to account for the full impacts of
gopher tortoises on understory plant communities. To assess for impacts
other than direct ingestion of plant material, larger exclosures might
be better equipped coupled with experimental manipulation to isolate
trampling effects, from nutrient deposition and burrow construction to
quantify impacts by these alternative disturbances. In other herbivore
species that live in social networks and burrow systems, large areas are
used in quantifying impacts to the surrounding plant communities and not
necessarily herbivore-exclusion. For example, European rabbits and their
warren (burrow) systems were used to show that rabbit activity promoted
vegetation productivity and diversity by increase in soil disturbance
and resources provided by latrines in (15.5 m2) open
influence areas (Gálvez-Bravo et al., 2011). Similarly, gopher tortoises
have a life history that relies on complex interacting social networks
and frequent movements amount multiple burrows which would need a larger
area to assess to understand the full impacts of how these reptiles
regulate the forest floor understory (McRae et al., 1981; Boglioli,
Guyer, and Michener, 2003; Guyer, Johnson and Hermann, 2012).
We found that exclusion of gopher tortoise herbivory had effects on
plant diversity and productivity that are similar to large and small
mammalian herbivores. While reptilian herbivores are relatively rare in
most ecosystems and have been traditionally under-represented in the
herbivory literature, some studies have shown reptilian herbivory to
impact arid deserts (Nagy and Shoemaker, 1975), tropical rainforests
(Moll and Jansen, 1995), and coastal upland forest (King, 1996; Olesen
and Valido, 2003). Future studies should investigate the role of gopher
tortoises in maintaining biodiversity of longleaf pine understories in
comparison to other commonly used management strategies, like prescribed
burning. (Pacala and Crawley, 1992). Gopher tortoises may work in
conjunction with fire to promote and maintain biodiversity in the
understory of longleaf pine ecosystems. While arguments for gopher
tortoises as a keystone species have largely focused on the animal
assemblages that access gopher tortoise burrows (Guyer and Bailey 1993,
Catano and Stout 2015), our results suggest that gopher tortoises have a
role as regulators of plant diversity and productivity through herbivory
as well. Gopher tortoises may play a key role in maintaining the
productivity and diversity of longleaf pine understory communities;
however, human development and habitat loss are having negative effects
on gopher tortoise populations throughout the species range. Thus, our
results highlight the importance of continued protections and
conservation efforts for gopher tortoises in the southeastern United
States, because tortoises play an important role in regulating diversity
of plant communities.