Discussion
Our survey shows an abundant and diverse arthropod community in pastures
of the Southeastern U.S. In total, 126,25 specimens were identified to
morphospecies for this survey, making it one of the largest assessments
of arthropod diversity in pasture systems in the region. Previous
surveys of arthropods in grazing lands of the Southeastern U.S. focused
on specific groups of arthropods (e.g., pests, dung beetles, biocontrol
agents, and pollinators), or on specific habitats within pastures (e.g.,
plant foliage or dung pats) (Wilson 1963, Kramer et al. 1985, Hu 1995,
Pompo et al. 2016, Leppla et al. 2017). To our knowledge, our
bioinventory is one of the most comprehensive surveys of arthropod
communities in pastures of the Southeastern U.S., including foliage,
soil and dung microhabitats. Furthermore, this bioinventory identified
the majority of collected arthropods to the family level. The magnitude
and comprehensive identification of specimens from our bioinventory will
serve as a valuable reference for future biodiversity studies.
Our bioinventory shows both similarities and differences in community
composition compared to previous studies conducted in the region. For
example, a survey of dung-dwelling arthropods in north-central Florida
found the orders with the highest species richness to be Coleoptera (109
species), Diptera (35 species), and Hymenoptera (24 species) (Hu 1995),
while our study found a similar pattern of species richness in the dung
community with Coleoptera (105 species), Diptera (53 species), and
Hymenoptera (40 species). The higher number of Diptera and Hymenoptera
species reported by our study relative to Hu (1995) may be the result of
the larger geographic range conducted by our survey. Another example of
arthropod bioinventories in grasslands of the Texas panhandle found the
canopy-dwelling to have the highest specimen abundance in the orders of
1) Hemiptera, 2) Araneae, 3) Orthoptera, 4) Coleoptera, and
5) Hymenoptera (Bhandari et al. 2018). These results differ from our
survey, which documented the five most abundant orders to be 1)
Hemiptera, 2) Diptera, 3) Hymenoptera, 4) Orthoptera, and 5) Araneae.
While Hemiptera tops the list of both bioinventories, the remainder of
the orders differ between the two inventories. The differences in
arthropod abundance between the inventories could be for a multitude of
reasons, e.g., time of sampling, grassland management, surrounding
landscape, etc. One likely mechanism that contributed to the differences
between inventories was variances in grassland habitat between the two
distant geographic locations, i.e., the Southern Great Plains versus the
Southeastern U.S. The results of previous arthropod bioinventories
relative to our bioinventory highlights the differences that exist in
arthropod communities from similar habitats but different geographic
locations. This information underpins the need for future arthropod
inventories from various habitats and different regions as a means to
accurately assess biodiversity declines during the Anthropocene.
The assemblage of functional guilds was distinct amongst the three
arthropod communities sampled for this study (Figure 2). To begin,
herbivores were the most abundant guild in the foliar community, while
predators were the most abundant guild in the soil and dung communities.
The high abundance of herbivores in the foliar community is not
surprising, as herbivores are often the most abundant group found in
plant canopies of grasslands (Cagnolo et al. 2002, Hironaka and Koike
2013). However, the relatively low abundance of pollinators in the
foliar community was unexpected to us. But this result does align with a
recent rangeland bioinventory, which documented pollinators to be only a
minor portion of the community (Bhandari et al. 2018). The abundance of
pollinators in pasture systems is likely influenced by forb abundance.
Consequently, the small number of pollinators may be a result of pasture
management that prioritizes production of grasses over forbs for cattle
production. Although grass production is the primary concern of ranchers
raising cattle, a balanced cattle diet includes forbs (Grant et al.
1985). Thus, increasing the abundance of forbs in pastures could improve
pollinator abundance, while also fulfilling cattle dietary requirements.
Unlike the foliar community, the soil and dung communities were
dominated by predatory arthropods. Due to the lack of experimental
manipulation in the design of this study, it is difficult to ascertain
why predators were the most abundant group in the soil and dung.
However, it should be noted that a significant portion of the collected
specimens were classified as unknown in terms of functional guild. We
hypothesize that many of the unknown arthropods in our survey
constituted food sources for the predator population, helping to support
the large predator portion of the community. We hypothesize many of the
unknown specimens are detritovores, but lacking creditable sources to
classify these specimens. It is also worth noting that pest species
comprised a very small portion of the arthropod community in all three
habitats, comprising only 1.01%, 0.34%, and 0.46% of arthropod
abundance in the foliar, soil, and dung habitats, respectively. The
predators combined with competition from other specimens of other
functional guilds seem to be capable of holding pest populations in
check in these pasture systems. Further studies are needed to better
understand life histories of many of the organisms and further our
understanding of arthropod food webs in pasture ecosystems.
Describing arthropod communities of various habitats and agriculture
systems requires further attention in order to provide meaningful data
for future studies (Goldstein 2004, Apfelbaum and Haney 2012). For
instance, bioinventories of pastures can inform land managers and
government agencies about livestock and land management practices that
promote conservation efforts of endangered species or guilds of
conservation interest, e.g., rusty-patch bumblebee or pollinators.
Furthermore, bioinventories serve as reference points to compare shifts
in community composition as novel pasture management methods are tested.
Perhaps most importantly, bioinventories serve as a reference during
biodiversity declines of the Anthropocene. Without bioinventories, we
are essentially flying blind during this time of biodiversity loss.
Leaving us unable to make informed decisions to conserve at risk
species. While bioinventories demand considerable time, energy, and
resources to accomplish, they are vital for the future of biodiversity
on our planet, and their value cannot be overstated.