Ecosystem engineering by bison (Bison bison) wallowing increases arthropod community heterogeneity in space and time

Abstract

Ecosystem engineers have important effects on abundance and diversity of organisms and are vital for conservation efforts. Some large mammalian grazers are engineers because their grazing activity radically changes plant community structure, an effect which then cascades to other consumers. Many large grazers also behaviorally modify the physical environment. American bison (Bison bison) are known for their wallowing, a behavior that creates distinct areas of high disturbance with modified biological and physical characteristics, but how this behavior affects other consumers is poorly understood. In this study, we investigated arthropod abundance and diversity patterns in active and abandoned wallows compared to those in surrounding tallgrass prairie. We found that active wallows contained lower arthropod abundance and diversity compared to surrounding prairie. Herbivorous arthropods were particularly affected and only about 50% as abundant in wallows, while carnivores and detritivores were affected similarly, but to a lesser degree. In contrast, abandoned wallows had higher arthropod abundance seasonally and higher species richness in several feeding groups. Because of arthropod differences in active and abandoned wallows compared to adjacent prairie, it appears that the impact of wallowing is dependent on time since occurrence, with long-term effects creating patches of higher arthropod abundance and richness. These patches are likely important, at least seasonally, for other consumers higher on the food web. Together with publications documenting positive effects of bison grazing on arthropod abundance and diversity, our results indicate that wallowing effects are potentially additive. These results suggest that physical changes caused by bison behavior are important for maintaining arthropod biodiversity of tallgrass prairies, and bison may therefore be valuable conservation tools. Bison have been proposed as important candidates for rewilding portions of North America, and our results suggest that they could indeed be valuable toward this end.