Nathan Kraft

The central goal of my research is to understand the processes that generate and maintain diversity in the identity, characteristics, and abundance of organisms that occur together in nature. I am interested in two of the most pressing questions in community ecology: first, quantifying the relative contributions of equalizing and stabilizing mechanisms to species coexistence, particularly in diverse communities and second, predicting how species and communities will respond to climate change. While many avenues exist to address these questions, I am primarily motivated to pursue more general answers that emerge from a consideration of the functional ecology and phylogenetic identity of species. Within a clade such as angiosperms, functional traits provide a powerful way to predict key physiological features of a species in the absence of other information, while phylogenetic patterns can provide an additional means to estimate the overall similarity of species. Functional traits and phylogenetic distances can therefore be used as a common currency among species to predict responses and draw generalizations.

In my work I synthesize approaches from community ecology, ecophysiology and phylogenetics. I find this to be one of the most exciting areas in ecology, where the ongoing unification of previously unrelated research frameworks continues to yield novel insights. I primarily focus on land plants, for which our understanding of the ecological ramifications of morphological and physiological variation among species is relatively well developed, though the questions and approaches are broadly applicable. Details of several of my research projects can be found below.

While many species coexistence theories have been put forward based on research in high diversity systems such as tropical forests, little consensus has been reached. One productive way forward is to classify coexistence mechanisms into processes that have a stabilizing effect on diversity, such as niche differentiation and enemy-mediated density dependence, and mechanisms that equalize the fitness differences between co-occurring taxa, such as life history tradeoffs. Neutral theory, in which equalizing mechanisms dominate, is of particular relevance to high-diversity systems where it seems implausible that traditional ecological stabilizing mechanisms such as niche differentiation could apply to all species.

Past Research: As part of my dissertation research, I used an observational approach based on the ecological strategies (functional traits) of co-occurring trees to test predictions of different diversity maintenance mechanisms. Working in Ecuador in one of the most diverse tropical forests census plots on the planet (Yasuni FDP), I combined field-collected trait measurements from the 1,100 tree species co-occurring in the 25 hectare plot with a null model analysis to test if co-occurring species were more or less ecological similar than expected by a niche-free model of the forest. Strikingly, I found evidence for stabilizing mechanisms within one of the most diverse forests in the world (Kraft et al. 2008) at a range of spatial scales (Kraft and Ackerly 2010). Taken together, my results support a niche-based view of tropical forest dynamics in which subtle but pervasive habitat specialization and strategy differentiation contribute to species coexistence.

Ongoing Research: Ongoing tropical forest projects include a quantification of the role of intraspecific trait variation to the assembly dynamics of tropical forests, an extension of earlier analyses to include seedling traits and seedling dynamics, and a analysis of the influence of the functional traits of neighbors on growth and mortality of focal trees.

For more details see:

Nathan Kraft, Renato Valencia, and David Ackerly, 2008. Functional traits and niche-based tree community assembly in an Amazonian forest. Science, Vol. 322, pp. 580-582.

Nathan Kraft and David Ackerly, 2010. Functional trait and phylogenetic tests of community assembly across spatial scales in an Amazonian forest. Ecological Monographs Vol. 80, pp 401-422. pdf

Patterns of phylogenetic relatedness can often contain valuable ecological and conservation information. In a community context, relatedness can often be used as proxy for the degree of ecological similarity of two species, and can therefore be a powerful tool for ecological analysis in the absence of other species-specific information. I have a long-standing interest in the use and development of these approaches (Kraft et al. 2007, Kraft and Ackerly 2010), as they can be used to gain insight into logistically challenging communities where the key functional traits of the organisms are unknown or cannot be sampled.

As part of my dissertation, I developed a novel simulation modeling framework to test the connections between trait evolution, community assembly process and phylogenetic structure (Kraft et al. 2007). These modeling efforts have provided support for the heuristic arguments originally put forth as justification for the methods, and have also revealed a number of subtleties essential for the proper interpretation of phylogenetic community analyses. Application of these methods to the tropical forests of Ecuador revealed patterns largely consistent with trait-based approaches (Kraft and Ackerly 2010). Currently I am using phylogenetic approaches with collaborators at NCEAS to improve our understanding of the phenological responses of plant species to climate change.

For more details see:

Nathan Kraft, William Cornwell, Campbell Webb and David Ackerly, 2007. Trait evolution, community assembly, and the phylogenetic structure of ecological communities. The American Naturalist, Vol. 170(2), pp. 271-283.

Nathan Kraft and David Ackerly, 2010. Functional trait and phylogenetic tests of community assembly across spatial scales in an Amazonian forest. Ecological Monographs Vol. 80, pp 401-422. pdf

Functional trait-based analyses provide a powerful way to infer ecological processes and to predict species responses to environmental change, but they hinge on successfully identifying functional traits that are important for establishment and persistence of a species in a local community. Recent projects have found strong trait-demographic rate correlations for four key traits in a tropical forest in Panama (Wright et al. 2010) and wood density- mortality correlations across 10 tropical forest plots using a Bayesian Hierarchical model (Kraft et al. 2010).

As part of my postdoctoral research I am involved in two related projects focused on understanding the consequences of trait variation for species coexistence within communities. The first project experimentally manipulates trait diversity among pairs of California vernal pool annual plants and measures the intensity of competitive interactions as a function of the functional and phylogenetic distance between the focal plants. The second project expands on the first by adding intraspecific variation in traits, a key unknown in many trait-based studies (Kraft and Ackerly 2009), into the experimental design, working instead with multiple genotypes of two species of Populus (poplar). The goal of these analyses is to strengthen our understanding of the ecological ramifications of functional trait variation in communities.

Related publications:

Nathan Kraft, Margaret Metz, Richard Condit and Jerome Chave, 2010. The relationship between wood density and mortality rates in a global tropical forest dataset. New Phytologist, Vol. 188, 1124-1136. pdf.

S. Joseph Wright, Kaoru Kitajima, Nathan Kraft, Peter Reich, Ian Wright, Daniel Bunker, Richard Condit, James Dalling, Stuart Davies, Sandra Díaz, Bettina Engelbrecht, Kyle Harms, Stephen Hubbell, Christian Marks, Maria Ruiz-Jaen, Cristina Salvador, Renato Valencia, and Amy Zanne, 2010. Functional traits and the growth- mortality tradeoff in tropical trees. Ecology Vol. 91, 3664-3674. pdf

I am part of several ongoing projects aimed at predicting species responses to climate change. One set of projects focuses on predicting the impacts of climate change on California native plants using a combination of distribution data, niche models, and climate projections. This work is in collaboration with David Ackerly and other members of his lab (project link). A related set of projects has also emerged from a NCEAS distributed graduate seminar on using functional traits to predict species responses to climate change.

Related publications:

Brody Sandel, Leah Goldstein, Nathan Kraft, Jordan Okie, Michal Shuldman, David Ackerly, Elsa Cleland and Katharine Suding, 2010. Contrasting trait responses in plant communities to experimental and geographic variation in precipitation. New Phytologist Vol. 188, 565-575. pdf.

David Ackerly, Scott Loarie, Will Cornwell, Stu Weiss, Healy Hamilton, Ryan Branciforte and Nathan Kraft, 2010. The geography of climate change: implications for conservation biogeography. Diversity and Distributions Vol. 16, pp. 476-487. pdf

Evolutionary perspectives in the assessment of biodiversity have become increasingly important to conservation efforts in recent years. This shift has been driven in part by the realization that sustaining biological diversity requires the protection of the ecological and evolutionary processes that generate it, along with the associated features of the environmental landscape. To this end, we have recently identified hotspots of evolutionary diversification within the California flora by combining species distributions, phylogenetic information, and molecular-based estimates of divergence time.

I am also part of a NCEAS working group examining the role of phylogenetic niche conservatism in shaping diversity gradients.

Related publications:

Nathan Kraft, Bruce Baldwin, David Ackerly, in press. Range size, taxon age, and hotspots of neoendemism in the California flora. Diversity and Distributions Vol. 16, pp. 403-413. Abstract,

Lauren Buckley, Jonathan Davies, David Ackerly, Nathan Kraft, Susan Harrison, Brian Anacker, Howard Cornell, Ellen Damschen, John-Avid Grytnes, Bradford Hawkins, Christy McCain, Patrick Stephens and John Weins, in press. Phylogeny, niche conservatism, and the latitudinal diversity gradient in mammals. Proceedings of the Royal Society B Vol. 277, pp. 2131-2138. pdf