1. Predicting the Native-Exotic Richness Relationship Using Native Beta Diversity
Gabriela Nunez-Mir and Songlin Fei
Department of Forestry and Natural Resources Purdue University
Princess Tree (Paulownia tomentosa)
Bill Johnson Nature Stock Photography, Inc -
Abstract
Expected Native-Exotic Diversity Patterns
Preliminary Results
Invasion paradox, a change of direction in native-exotic richness relationships (NERRs) within and across scales
We hypothesize that the directional changes in NERRs can be explained by the degree of beta, which describes heterogeneity
We used data from the Forest Inventory and Analysis (FIA) dataset to evaluate the relationship between regional beta diversity and the slope of the NERR for different units at multiple scales
Strong, yet inconsistent, relationships were observed at the largest scale
High heterogeneity areas:
Small increases in extent result in increases in beta diversity that generate positive NERRs
Adj R2= 0.006
P-value < 0.001
Adj R2= 0.027
P-value= 0.02
Adj R2= 0.17
P-value = 0.006
Research Problem
Low heterogeneity areas:
Larger increases in extent are required to increase beta diversity and generate positive NERRs.
Contradicting relationships between native and exotic diversity complicate our understanding of the role of native diversity in invasiibility and hinder the development of generalizations. Contradicting relationships are observed:
Across scale: fine scale versus broader scales
Within scale:
E.g., two studies on North American grasslands at the ≤1m2 scale
The inconsistencies observed across and within scale suggest that other variables or perspectives to the study of these relationships remain underexplored
Adj R2= 0.005
P-value= 0.03
Methods
Only positive or nonsignificant native- exotic richness relationships found in Cleland et al. 2004
Only negative native-exotic richness relationships found in Davies et al
Adj R2= 0.008
P-value= 0.52
Adj R2= 0.08
P-value= 0.08
Native and exotic species richness data for FIA plots in the eastern US
For each ecoregion at increasing levels of spatial scale:
Obtained the slope of NERRs
for plots within region
Measured beta diversity:
βt (Wilson and Shmida 1984)
Regressed NERR slopes as a function of beta diversity
Adj R2= 0.008
P-value= 0.21
Adj R2= 0.02
P-value= 0.7
Adj R2= 0.2
P-value= 0.05
Beta Diversity
Many community processes are driven by heterogeneity
Beta diversity describes biotic heterogeneity by measuring the amount of variation in species assemblages in an area
Are the inconsistencies observed across spatial scales in NERRs resolved along a biotic heterogeneity (beta diversity) gradient?
Biotic heterogeneity generates positive NERRs by enhancing coexistence mechanisms and masking patterns generated by local processes, such as competitive exclusion.
Conclusions
Strong relationships between beta diversity and the slope of native-exotic richness relationship were only observed at the largest scale
Inconsistent relationships  Possible evidence of aggregation bias
Unclear whether beta diversity is able to explain inconsistencies. Aggregation bias might.
Central Question
Funded through NSF-Macrosystems Biology
Acknowledgements
Hypothesis
References
Cleland, E. E., Smith, M. D., Andelman, S. J., Bowles, C., Carney, K. M., Claire Horner‐Devine, M., ... & Vandermast, D. B. (2004). Invasion in space and time: non‐native species richness and relative abundance respond to interannual variation in productivity and diversity. Ecology Letters, 7(10),
Davies, K. F., Chesson, P., Harrison, S., Inouye, B. D., Melbourne, B. A., & Rice, K. J. (2005). Spatial heterogeneity explains the scale dependence of the native-exotic diversity relationship. Ecology, 86(6),