PARENT SESSION
Oral Session # 9: Invasive Species I: Theory and Modeling.
Presiding: E Rykiel
Monday, August 4. 8:00 AM to 11:30 AM, SITCC Meeting Room 201.

Scale matters: How spatial scale alters the diversity-invasibility relationship.

Davies, Kendi^*,1, Chesson, Peter², Harrison, Susan¹, Inouye, Brian^{1, 3}, ¹ Department of Environmental Science and Policy, Davis, CA² Section of Evolution and Ecology, Davis, CA³ Department of Biological Science, Tallahassee, FL

ABSTRACT- Elton suggested that diverse communities should be less susceptible to invasion. In agreement, models and experiments at small spatial scales have tended to show a negative relationship between native diversity and the number of invaders. In contrast, studies at large spatial scales have tended to find positive relationships between native and exotic diversity. This paradox is likely the product of different processes operating at different spatial scales: interactions between species at small scales and environmental constraints at large scales. Shea and Chesson (2002) formalized this with a conceptual model in which the negative slopes of the relationships between mean native and mean exotic richness for multiple sites at local scales, form a cloud of points that make up the relationship between mean native and exotic richness at a larger spatial scale. We point out that most empirical studies at larger scales contrast the total rather than the mean native and exotic richness. Thus there is a mismatch between an elegant concept and empirical practice. We show that if beta diversity is not constant, the slope of the relationship between mean native and exotic richness is not the same as the slope of the relationship between total native and exotic richness. As a case study, we examined the relationship of native and exotic diversity for four nested spatial scales in a California grassland community. For means, the relationship between native and exotic richness was negative for all four scales. However, for totals, the relationships at the two smaller scales were negative but the relationships at the larger scales were positive. In the literature, nested spatial data have been treated in many different ways, often with the wrong approach used to address the intended hypothesis. To test hypotheses that match processes to spatial scale, we need to specify statistical models that properly partition processes to the relevant scales.

Key words: invasibility, California grasslands, diversity, spatial scale