Community assembly theory follows a continuous spectrum from neutral species assemblage to niche species assemblage. Disturbance, such as drought, and other potential environmental filters can provide a strong structuring force, such that the communities follow patterns of niche assemblage. Anthropogenic changes to landscapes cause shifts in community structure and composition. Cattle grazing has many ecological impacts, including increased soil erosion, nutrient additions, and shifts in plant communities. Small ponds, modified to be water sources for cattle, are also directly impacted through reduced vegetation, increased turbidity, and increased nutrient inputs. Researchers are increasingly recognizing the important role parasites play in community interactions. Parasites are often missing from community level studies. This study explored two community assembly questions, (1) does cattle grazing intensity affect community composition, and (2) are ponds that experience high intensity grazing more structured than ponds that experience no or low intensity grazing, with a particular interest in host and parasite communities. To evaluate these questions, we conducted a field survey of 75 ponds, determining the species richness at two component communities and their infracommunities; aquatic snails and their trematode parasites, amphibians and their macroparasites, and grouping them into three categories, no, low, or high grazing intensity.
Results/Conclusions
There is a significant difference between the compositions of aquatic communities across grazing categories. There is no difference in species richness across grazing categories, but the community composition shifted in that common species are found more often and rarer species are found less often with increasing grazing intensity. The variation within each treatment is not significantly different, showing that there is not increased structuring with increased grazing intensity. These results indicate that community composition shifts with increased grazing, but that unlike other disturbances, are not providing an environmental filter by which communities assemble. A possible mechanism for this shift in community structure is the productivity-biodiversity relationship, which shows that common species become more dominant with increasing nutrient levels. Overall, these results support that cattle grazing is not an environmental filter that promotes niche community assembly, but that it does shift community composition. These results illustrate another way in which anthropogenic land use change is altering communities, with the potential for shifts in the overall functioning of these ecosystems.
