Grass type, vegetation cover, and predation affect abundance of Microtus californicus and Thomomys bottae in a coastal Mediterranean ecosystem

Abstract

Invasive grasses negatively impact ecosystems and have become dominant over native grasses in central California. This study measured vegetation cover, the extent of ungulate grazing, vegetation height and predation in invasive and native grasses and examined the response of Pocket gophers (Thomomys bottae) and California meadow voles ( Microtus californicus) to invasive and native grasses. It was predicted that both T. bottae and M. californicus are more abundant in native grasses compared to invasive grasses. This study also predicted that M. californicus would prefer less grazed grasses, but T. bottae would prefer grazed grass. Finally, it was predicted that there would be a negative relationship between predatory bird activity and small mammal abundance. Results suggested that while T. bottae preferred mixed grasses, M. californicus preferred Harding grass ( Phlalaris aquatica ), which is an invasive grass type. Furthermore, vegetation cover, but not tallest grass height, affected small mammal abundance. Results indicated a negative relationship between the presence of T. bottae and M. californicus . However, there was a positive relationship between predatory bird activity and small mammal abundance. It was proposed that other factors such as location of the native grass affected small mammal abundance. The negative relationship between T. bottae and M. californicus was thought to result from T. bottae reduction of vegetation cover, which caused a reduction in food supply and shelter from predatory birds. Studying the negative impact of invasive grasses on ecological systems in central California is a critical step towards developing invasive species management techniques.