COS 72-1 - Why the explanation for diversity lies at the individual scale, and why it is relevant for global change

Wednesday, August 10, 2011: 1:30 PM
6A, Austin Convention Center
James S. Clark, Duke University, Durham, NC
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Individuals compete, species do not.  The mechanisms of coexistence are hidden at the species (aggregate) level, because the relevant processes operate at the individual level.  Species differences required for coexistence need not be apparent in the species-level parameter values that are estimated in empirical studies and implemented in theoretical models.  A recent analysis showed that disaggregated (individual-level) dynamics are consistent with high-dimensional coexistence--individuals respond to spatio-temporal variation more like others of the same species, thus concentrating competition within the species (Clark 2010, Science).  However, no analysis has yet shown how the disaggregated (individuals within species) relationships differ from the species aggregates.  If disaggregation is critical to understanding coexistence in a high-dimensional environment, there should be evidence that individual level data change the way we interpret species differences. 

Using a 20-yr experiment to determine how individuals of different species respond to recruitment opportunities following canopy gap formation and spatio-temporal climate variation we show that disaggregation fundamentally changes the interpretation of species differences, revealing relationships much more likely to contribute to coexistence.  A hierarchical Bayes model yields inference on the joint distribution of individuals within species, allowing us to compare responses inferred at both levels.  To evaluate the effects of many input combinations on multiple demographic rates of 40 species, we quantify direct effects and interactions at the individual scale.  Sensitivity analysis is implemented in a way that permits direct comparisons of the effects of all input variables on each demographic rate of all species.  


We show that the correlations between species aggregate responses to resources and climate often reverse at the disaggregated scale.  For example, species having individuals that respond most when already doing well are often the ones that respond least when previously under stress.  Species aggregates show the broad overlap that has been observed in many previous studies of trait and demographic differences.  Individuals partition climate and resource space in different ways, depending on the species identity.   The disaggregated result lends support to a role for environmental variation contributing to coexistence, when the aggregated result does not.

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