PARENT SESSION
Wednesday, August 9, 8:00-11:30 am
Symposium 10 - Niche versus neutral: a look at an iconic idea in community ecology, its challenger, and the middle ground, Part I
Ballroom B, Ballroom Level, Cook Convention Center
Organized by: AM Ostling (aostling@princeton.edu), N Sanders, and J Lake

In this symposium and accompanying organized oral session, we will explore the latest advances in our understanding of how communities assemble, focusing on the ongoing debate over niches versus neutrality and keeping an eye towards finding the middle ground.

A trait-based approach to community assembly: linking niche breadth, plasticity, and functional diversity.

Ackerly, David^*,1, Cornwell, W², ¹ University of California, Berkeley, Berkeley, CA² Stanford University, Stanford, CA

ABSTRACT- The study of species distributions along environmental gradients provides important insights into the role of niche differentiation and functional diversification in the assembly of communities. We present a trait-based approach to community assembly using community-level mean trait values to define environmental gradients. The analysis of individual species trait values vs. community-level trait means provides a shared framework to quantify niche breadth (b), the slope of intraspecific plasticity and/or ecotypic variation (s), and a novel separation of trait values into _T and _T components: _T refers to a species’ position along the trait gradient, based on the community-level mean trait values of occupied sites; _T describes whether species tend to have higher or lower trait values relative to the mean of the communities in which they occur. In idealized communities, the range of trait values within individual communities (a measure of functional diversity) is then shown to be a simple additive function of the range of species _T values plus niche breadth multiplied by one minus the intraspecific slope [range(T) = range(_T) + b(1-s)]. This framework is exemplified by analyses of specific leaf area (SLA) in a local and a global data set. In woody plant communities of coastal California, _T contributes twice as much to local trait diversity as niche breadth, and intraspecific variation reduces trait diversity by about 5%. In the global GLOPnet data set (where niche breadth and intraspecific data are only sparsely available), most of the interspecific variance in SLA is associated with differences in community mean values (_T), with the remainder associated with _T values within communities. Phylogenetic analysis reveals that the evolution of _T is more deeply conserved than _T values. Some clades, such as coniferales, occupy intermediate positions on the trait gradient, but have characteristically low _T values within communities. Others, such as Proteaceae, tend to occupy low-SLA communities, and have average _T values over all species close to 0. The development of trait-based approaches, illustrated here, provides a powerful framework to integrate organismal function, evolutionary history, and community assembly along environmental gradients.

Key words: niche breadth, functional diversity, community assembly