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A Comparison Of Dung Beetle Diversity In Pasture Versus Forest

Back To the Classroom      Rubric        Table 1       References    Chelsea Crawford  Granada Hills High School, Granada Hills, CA Tracy Gleghorn  Austwell-Tivoli High School, Tivoli, TX Nichia Huxtable  Sunnyside High School, Fresno, CA Introduction: One of the most diverse and abundant groups of animals is the Coleoptera, otherwise known as beetles. Within Coleoptera, the largest and most studied family is Scarabaeidae, the scarab beetles. Several subfamilies of scarab beetles are referred to as "dung beetles" because their life cycle revolves around the excrement of mammals (Borror & White, 1970). Dung beetles are found in all areas of the world, except the polar regions (Janzen, 1983). Dung beetles can be divided into four groups based on their dung interaction behaviors. In the first group, females dig tunnels under dung piles and create nests at the bottom. Both the male and female then carry dung into the nest, and the female deposits a single egg. This egg will develop into a larva that will eat the dung as it grows into an adult. In the second group, the female, helped by the male, again digs a tunnel and fills it with dung. She then cuts smaller sections from this dung and deposits an egg into each section. During the development of the larvae, the female tends the nest and, in many cases, dies there. In the third group, the female burrows into the dung pile and makes a small cavity in which she deposits an egg. The larva that develops will eat the excrement around it as it grows into an adult. The fourth and final group of dung beetles is the most commonly observed because they are diurnal and because their behaviors are the most conspicuous. Males of this group form a ball of dung and roll it away from the original pile. The awaiting female then deposits an egg inside the ball, after which the ball is buried. The developing larva eats the dung ball and emerges as an adult from the ground (Blanco, 1988). Due to these feeding and reproductive behaviors, dung beetles help disperse piles of dung that would otherwise accumulate. Dung beetles are also important in the dispersal of seeds found in that dung. For example, Estrada et al. (1999) suggest that dung beetles help maintain a diverse seed bank, helping preserve the regenerating capacity of the forest. Dung beetles are also known to fertilize and aerate soil, increase the rate of nutrient cycling, as well as reduce transmission of some vertebrate parasites (Andresen, 2001). There are 150 species of dung beetles found in Costa Rica. While many of these species are generalists, some are adapted to a specific habitat, such as forest or open areas (Blanco, 1988). Expected changes in land use could affect the biodiversity of dung beetle populations. In order to access the impact that land use change may have on dung beetle biodiversity, we designed an investigation to compare the diversity of dung beetles in pasture and forest habitats. The investigation was designed to test both diversity of dung beetle morphotype as well as extent of morphotype overlap between the two habitats. It was expected that the forest habitat would support a greater diversity of dung beetle morphotypes both in richness and evenness. This hypothesis was based on the assumption that the higher floral and faunal diversity of forests would provide a greater diversity of dung resources, which, in turn, would support a greater diversity of dung beetles. Methods: Four 100g samples of fresh, undisturbed cow dung were collected and stored in plastic resealable bags. Within the same day, the samples were taken to four study sites within two different habitats and used to attract dung beetles. Both habitats were located within the Palo Verde Biological Research Station in the Guanacaste region of Costa Rica. The first two sites were within a former lowland dry forest that was converted to pastureland. The second two sites were located within secondary lowland dry forest approximately 26 years old. At each site, the dung was placed onto the ground and slightly flattened to increase surface area and, therefore, diffusion of odor. The dung was observed for a minimum of 90 minutes. During the observation period, arrival of each dung beetle was recorded, as well as descriptions of all behaviors and they time at which they occurred. Results: A total of thirteen beetles representing two morphotypes were observed in the pasture. In the forest, 25 beetles representing six morphotypes were found. Of the behaviors observed, five were restricted to the forest habitat. All five represented interactions between different morphotypes of dung beetles. In one case, a morphotype A inexplicably abandoned the dung ball it had just completed. In another, a morphotype C stole a newly formed dung ball from a morphotype E, who tried unsuccessfully to reclaim the ball. The morphotype C thief quickly rolled the ball away from the dung pile, even though it was smaller than the average ball normally formed by a morphotype C. A few minutes later, the morphotype C thief was killed by a green rove beetle. The original morphotype E found the victim’s abandoned ball, reclaimed it, and rolled it away (Justice was served). The green rove beetle is a dung beetle predator that lies in wait under the dung pile to ambush unsuspecting dung beetles. During our observations, three green rove beetles attacked a total of five dung beetles. Two attempts were considered successful resulting in the severe dismemberment of the victimized beetle. In the other three attacks, the dung beetles were able to escape after much struggle. However, these victims lost appendages and appeared traumatized by the event. After escaping, the traumatized beetles would move away from the dung pile and rest for an indefinite period of time. Click on this link  to see table 1: Morphotypes and associated behaviors of dung beetles at four study sites. Analysis: Richness: There was not enough repetition to conduct a statistical analysis of morphotype richness. Figure 1 illustrates the difference in the number of morphotypes found in each habitat.   Evenness: A Shannon-Weiner Diversity Index was calculated to determine the evenness of morphotypes within each habitat. The evenness for the forest was 0.89 and the evenness for the pasture was 0.85. However, because the number of morphotypes found is most likely an underestimation due to limited sampling, the measure of evenness is probably overestimated (Zar, 1999).. Discussion: This investigation indicates that there is a greater richness of dung beetle morphotypes in the forest habitat than the pasture. However, it should be noted that the limited scope of the data collected may not be representative of these habitats in general. There does not appear to be a significant difference in the evenness of dung beetle morphotypes in each habitat. In other words, there is no overwhelming dominance of any one type of beetle in either habitat. It is important to also look at the extent of species overlap between these two habitats. Both types of beetles found in the pasture were also present in the forest, whereas the other four types of beetles in the forest were not found in the pasture. Beetles of morphotypes A & B are most likely generalists that can thrive under varying environmental conditions. Beetles such as these would be favored as forest is converted into pasture. Morphotypes C-F may represent specialists who are only adapted to forest environments. If indeed this is the case, our data supports the argument that deforestation is leading to loss of biodiversity of dung beetle species in Costa Rica, which may lead to instability in nutrient cycling. This is just one more example of how land use change continues to have an adverse effect on ecosystem health.