PARENT SESSION
Symposium 10: The invasion paradox: Reconciling biodiversity and invasion patterns across spatial scales
Organized by: DF Sax, GD Tilman, and TJ Stohlgren
Wednesday, August 10, 8:00 AM - 11:30 AM, Meeting Room 517 A, Level 5, Palais des congrès de Montréal

Biodiversity as homeland defense: Resisting invasion once borders have been breached.

Naeem, Shahid^*,1, ¹ Ecology, Evolution, and Environmental Biology, New York, NY, USA

ABSTRACT- The homeland of a plant species assemblage consists of the collection of its plant neighborhoods bordered by a region across which transport of propagules of non-resident or invader plant species is negligible. Changes that result in significant increases in invader propagule transport constitute breaches of these borders, at which point only intrinsic factors can regulate the success of invader propagules. A three year study of the success of 1178 invading plants in 6 - 8 old field/tall grass prairie plant assemblages at Cedar Creek Natural History Area, Minnesota, USA, suggest that the strongest of these intrinsic factors is resident plant neighborhood species richness. Invaders, however, often exhibit threshold behavior, transitioning from low density and low rates of spread, or seemingly naturalized invaders, to invasive species that have high densities and spread rapidly within a homeland. A simple threshold model demonstrates that resident plant neighborhood diversity regulates this threshold. Thus, resident plant neighborhood richness lies at the fulcrum of two opposing forces that govern the phase transition between a species being naturalized and invasive; global scale forces that determine site fertility, disturbance, and propagule pressure, and local scale forces that resist the spread of individual propagules. These finding do not support the existence of a cross-scale paradox in plant invasion research, merely ecological process operating in opposition at different scales.

Key words: Invasion, scale, biodiversity, grassland