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.