Seedling Herbivory in Grassland: Relative Impact of Vertebrate and Invertebrate Herbivores

Abstract

1 Exclosure techniques were used to assess the relative roles of arthropods, molluscs and rodent herbivores in the mortality of herbaceous seedlings in temperate grassland. The fate of over 13 000 seedlings comprising seventeen plant species was studied in a grassland and meadow. Analysis examined two components of seedling herbivory: seedling encounter (the probability of at least one seedling being damaged) and seedling exploitation (the proportion of seedlings damaged once encountered). 2 The study examined the effect of season (spring and autumn), plant species as well as betweenand within-habitat variation in the frequency and severity of herbivory attributable to the different guilds of herbivores. Detailed examination of plant species variation attempted to identify whether different guilds of herbivores share similar seedling 'preferences' and if certain seedling characteristics (such as size) make them more prone to herbivory. 3 Only molluscs and rodents encountered groups of seedlings to any significant extent, with arthropods playing only a minor role in seedling mortality. In spring, seedling herbivory was infrequent, with neither molluscs or rodents encountering more than 10% of seedlings in either habitat. In autumn there was an increased abundance of molluscs and rodents in both habitats. Seedling encounter by molluscs rose in both habitats but the clumped spatial distribution of rodent activity in the meadow led to seedling encounter by rodents increasing only in the grassland. Both molluscs and rodents appeared to encounter large seedlings more frequently than small seedlings leading to significant correlation in plant species encounter between these two guilds of herbivores. 4 In general, once a group of seedlings was encountered, molluscs and rodents exploited a similar number of seedlings, on average about 30%. With increasing seedling size, mollusc seedling exploitation decreased whereas in contrast, rodent seedling exploitation increased. Thus rodents exploited a greater proportion of large seedlings than molluscs and molluscs exploited a greater proportion of small seedlings. 5 Molluscs and rodents differed in the way they handled seedlings. Molluscs most frequently grazed seedlings, removing less than 75 % of tissue, while rodents consumed seedlings completely. Forb seedlings (mostly legumes) tended to suffer less damage from herbivores than grasses. The possible influence of seedling defences in these feeding patterns is discussed and the relative roles of molluscs and rodents in seedling mortality in temperate habitats is assessed.