By John Liu, Wildlife, Fish, and Conservation Biology ‘21

Author’s Note: In this article, I will explore the overwhelming impact that the teeny tiny dung beetles have on American grasslands. Dung beetles, along with reintroduced bison and prescribed fires, are stomping, rolling, and burning through the landscape; all in efforts to revive destroyed grassland habitats. Barber et. al. looks at how the beetles are reacting to the bison herds and prescribed fires. Seemingly unrelated factors interact with each other closely, producing results that bring hope to one of the most threatened habitats.

Watching grass grow: its lit

Grasslands are quiet from afar, often characterized by windblown tallgrasses and peaking prairie dogs. But in fact, they are dynamic. Historically, grasslands were constantly changing: fires ripping through the landscape, bison stampedes kicking up dust, and grasses changing colors by the season [2]. However, climate change, increasing human populations, and agricultural conversions all contribute to an increasing loss of critical habitats; grasslands being amongst the most affected [7]. A loss of grasslands not only results in the extermination of previously residing fauna, but also a reduction of ecosystem services that they once provided. Thus, it is of increasing concern to restore grassland habitats. With the help of bison, dung beetles, and prescribed fires, recovery of grasslands is promising and likely swift.

Eat, poop, burn, repeat

As previously mentioned, grasslands thrive when continuously disturbed. The constant disturbance keeps woody vegetation from encroaching, nonnative plants from invading, and biodiversity from declining as a result of competitive exclusion between species [12]. To accomplish this, grasslands rely on large herbivore grazers such as American bison (Bison bison) to rip through the vegetation and fires to clear large areas of dry debris [9].

American bison are herbivore grazers- animals that feed on plant matter near the ground. The presence of these grazers alter available plant biomass, vegetation community structures, and soil conditions. This is the result of constant trampling, consuming, and digesting of the plant matter [9, 11]. Due to their valuable impact on the landscape, bison are considered keystone species- species that have an overwhelming, essential role in the success of an ecosystem [8]. Grasslands would look vastly different without bison walking, eating, and defecating on them [9].

But bison do not aimlessly roam the grasslands, eating anything they come across. They specifically target areas that have been recently burned. These scorched areas present themselves with new growth, higher in nutritional content [3, 5]. Historically, lightning strikes or intense summer heats caused these fires, driving the movement of grazers, but human intervention inhibits these natural occurrences. Instead, prescribed fires- planned, controlled burnings performed by humans- now mitigate the loss of natural fires, encouraging the bison’s selective foraging behaviors [4, 12]. Inciting bison to follow burned patches benefits the grasslands in more ways than one. First, this prevents overgrazing of any one particular area. By moving throughout the landscape, particular areas will reestablish while others are cleared by the bison. Second, the simple act of traversing large distances physically changes the landscape. Bison are large animals that travel in herds. When moving about the grasslands, they trample vegetation and compact the soil beneath their hoofs. Finally, grazing bison interrupt the process of competitive exclusion- limiting success as a result of competition for resources- amongst native plants. They indiscriminately consume vegetation in these areas, leaving little room for any one species of plant to out compete another [9].

The world is your toilet…. with dung beetles!

What goes in must come out, and bison are no exception to that rule. After digestion of the grasses they eat, bison leave behind a trail of dung and urine. The nitrogen rich waste feeds back into the ecosystem, offering valuable nutrients to the plants and soil-dwelling organisms alike [1]. But a recent study by Barber et. al. highlights a small, but critical component that ensures nutrient distribution is maximized in grasslands: the dung beetles (Scarabaeidae: Scarabaeinae and Aphodiinae, and Geotrupidae).

Dung beetles rely on the solid waste from their mammalian partners. The beetles eat, distribute, and even bury the dung; which helps with carbon sequestration [10]. They are found around the world- from the rainforests of Borneo to the grasslands of North America- and interact with each environment differently. In Borneo, dung beetles distribute seeds found in the waste of fruit loving Howler monkeys (Alouatta spps) [6]. While in North America, they spread nutrients found in the waste of grazing bison. They provide unique ecosystem functions- shattering of nutrient rich dung throughout vast landscapes. These attributes led to their increasing popularity in science research as a study taxon in recent years.

Figure 1: Grassland health is largely dependent on the interplay of multiple living and non-living elements. In 1.1, the area is dominated by woody vegetation and few grasses due to a lack of disturbance. In 1.2, the introduction of prescribed fires clears some woody vegetation, allowing grasses to compete. In 1.3, bison introduce nutrients into the landscape, increasing productivity. However, the distribution of dung is limited. In 1.4, the addition of dung beetles lead to better distribution of nutrients thus more productivity and species diversity. 

Barber et. al. took a closer look to see how exactly dung beetles were reacting to bison grazing and prescribed fires blazing through their grassy fields. They found significant contributions from each; both noticeably directing the movement and influencing the abundance of these beetles. As the bison followed the flames, so did the beetles. The beetles’ dependence on the bison’s dung showed when researchers looked at beetle abundance in two key areas: those with bison and those without. There were significantly more beetles in areas with bison, likely feeding on their dung, scattering it, and burying it; all while simultaneously feeding the landscape. Prescribed fires also lead to increases in beetle abundance. Whether it be 1.5 years post-restoration or 30 years post-restoration, researchers consistently saw increases in beetle abundance when prescribed fires were performed. This further amplifies the importance of disturbances in grassland habitats, for ecosystem health but also for species richness.

And the grass keeps growing

The reintroduction of bison in the grasslands of America proved successful in rebuilding a lost habitat, with the help of dung beetles and prescribed fires. However, bison and dung beetles are just one of many examples of unlikely pairings rebuilding lost habitats. Although the large-scale ecological processes have been widely studied, species-to-species interactions are often overlooked. Continued surveys of the grasslands will reveal more about the interactions of contributing factors and their effects on each other and the habitat around them.

Citations

1. Barber, Nicholas A., et al. “Initial Responses of Dung Beetle Communities to Bison Reintroduction in Restored and Remnant Tallgrass Prairie.” Natural Areas Journal, vol. 39, no. 4, 2019, p. 420., doi:10.3375/043.039.0405.
2. Collins, Scott L., and Linda L. Wallace. Fire in North American Tallgrass Prairies. University of Oklahoma Press, 1990.
3. Coppedge, B.R., and J.H. Shaw. 1998. Bison grazing patterns on seasonally burned tallgrass prairie. Journal of Range Management 51:258-264.
4. Fuhlendorf, S.D., and D.M. Engle. 2004. Application of the fire–grazing interaction to restore a shifting mosaic on tallgrass prairie. Journal of Applied Ecology 41:604-614.
5. Fuhlendorf, S.D., D.M. Engle, J.A.Y. Kerby, and R. Hamilton. 2009. Pyric herbivory: Rewilding landscapes through the recoupling of fire and grazing. Conservation Biology 23:588-598.
6. Genes, L. , Fernandez, F. A., Vaz‐de‐Mello, F. Z., da Rosa, P. , Fernandez, E. and Pires, A. S. (2018), Effects of howler monkey reintroduction on ecological interactions and processes. Conservation Biology. . doi:10.1111/cobi.13188
7. Gibson, D.J. 2009. Grasses and Grassland Ecology. Oxford University Press, Oxford, UK.
8. Khanina, Larisa. “Determining Keystone Species.” Ecology and Society, The Resilience Alliance, 15 Dec. 1998, www.ecologyandsociety.org/vol2/iss2/resp2/.
9. Knapp, Alan K., et al. “The Keystone Role of Bison in North American Tallgrass Prairie: Bison Increase Habitat Heterogeneity and Alter a Broad Array of Plant, Community, and Ecosystem Processes.” BioScience, vol. 49, no. 1, 1999,
10. Menendez, R., P. Webb, and K.H. Orwin. 2016. Complementarity of ´ dung beetle species with different functional behaviours influence dung–soil carbon cycling. Soil Biology and Biochemistry 92:142-148
11. Mcmillan, Brock R., et al. “Vegetation Responses to an Animal-Generated Disturbance (Bison Wallows) in Tallgrass Prairie.” The American Midland Naturalist, vol. 165, no. 1, 2011, pp. 60–73., doi:10.1674/0003-0031-165.1.60.
12. Packard, S., and C.F. Mutel. 2005. The Tallgrass Restoration Handbook: For Prairies, Savannas, and Woodlands. Island Press, Washington, DC.
13. Raine, Elizabeth H., and Eleanor M. Slade. “Dung Beetle–Mammal Associations: Methods, Research Trends and Future Directions.” Proceedings of the Royal Society B: Biological Sciences, vol. 286, no. 1897, 2019, p. 20182002., doi:10.1098/rspb.2018.2002.