The role of mammals in maintaining plant species richness in a floating Typha marsh in southern Ontario

Animals are known to influence and sometimes help maintain plant species richness in terrestrial systems. This study investigated the effects of mammals in influencing plant species richness of a floating Typha marsh at Point Pelee National Park. The extent of use of the marsh by terrestrial mammals was documented by mapping mammal trails from air photographs. Trail densities ranging from 70 to 1550 m ha(-1) in various areas of the marsh, provided evidence for widespread, and in some places frequent, mammal activity. Plant surveys indicated a pattern of increased species richness and frequency associated with these trails, particularly in late summer when Typha biomass increased off trails. Three mechanisms by which mammals might influence species richness: disturbance of the dominant Typha, nutrient enrichment, and seed dispersal, were investigated using field and lab experiments. Two types of disturbance, trampling and herbivory, at two levels of intensity were simulated within 2 x 2m plots with and without fertilizer added. Disturbance significantly affected species richness; high levels of trampling decreased while high levels of clipping increased species richness. Nutrients had no effect; there were no significant disturbance x nutrients interactions. The importance of seed dispersal by raccoon was studied by collecting raccoon seat from within the marsh, incubating seat in a growth chamber and comparing the species that emerged to those growing in the marsh. Although substantial numbers of viable seeds of terrestrial species were found in seat, only one of these species was actually part of the Typha mat community. We concluded that mammals do influence plant species richness in this marsh primarily through disturbance of the dominant Typha.