Facilitative and competitive effects of a large species with defensive traits on a grazing-adapted, small species in a long-term deer grazing habitat

Abstract

Plants can adapt to grazing environments by developing defensive traits, such as spines and toxins, or having a small phenotype, such as short and prostrate growth forms. This study examined facilitative and competitive interactions between species with different types of grazing adaptation. We predicted that large species with defensive traits sometimes protect grazing-adapted species without defensive traits from herbivores, but competitively suppress them overall. We conducted an experiment using fences and removals of an unpalatable plant in the long-term deer grazing habitat of Nara Park in Nara, Japan. We evaluated the seasonal variations in the facilitative and competitive effects of a defensive perennial, Urtica thunbergiana, on the growth, survival, reproduction, and final fitness of a small palatable annual species, Persicaria longiseta, during a growing season. The populations of the two species in the park have adapted to the grazed habitat by increasing the density of stinging hairs (Urtica) and developing inherently short shoots and small leaves (Persicaria). We found that Urtica individuals had facilitative effects on the growth of Persicaria individuals under grazing during a few periods of the growing season, but had neutral effects on survival and plant fitness throughout the season. In the fenced plots, Urtica had negative effects on the growth, survival, and reproduction of Persicaria. These results suggest that the relative importance of the facilitative and competitive effects of Urtica on Persicaria fluctuated due to seasonal variations in grazing pressure and vegetative productivity. Although well-defended plants often facilitate less-defended species, we conclude that the facilitative effects of Urtica on Persicaria are limited in a plant community with a long history of intensive grazing.