Variable effects of large mammal herbivory on three non-native versus three native woody plants

Abstract

(1) The enemy release hypothesis posits that introduced species leave behind co-evolved pathogens and predators, thereby gaining an advantage over native competitors. On the other hand, introduced plants may encounter biotic resistance from local generalist herbivores such as large mammals. (2) We conducted a replicated, manipulative field experiment to compare the effects of large-mammal herbivory on growth and survival of three native and three invasive woody species over 2 years. Non-native Acer platanoides, Frangula alnus P. Mill. (= Rhamnus frangula L.) and Elaeagnus umbellata were each paired with a likely native competitor of similar life form and shade tolerance. Seedlings were planted with and without large-mammal exclosures, in open and understory environments. (3) In the open, E. umbellata grew taller than its paired native only when exposed to herbivory, but F. alnus grew taller than its paired native only within exclosures. The effects of exclosure on growth rate did not differ between A. platanoides and its native congener. In the understory, exposure to browsing reduced height growth rate overall in native species, but not in invasive species. (4) Browsing increased understory mortality only in the native shrub Viburnum dentatum, and did not affect mortality in the open. Within exclosures, there was a general trade-off between open growth and understory survival, but outside of exclosures, E. umbellata exhibited both greater open growth and greater understory survival than its native competitor. (5) Although large-mammal herbivory did not consistently favor non-natives, lack of browsing impact played an important facilitating role for E. umbellata in particular.