Experiment I: Genes and Environment

Objectives / Summary

J. Fridley and J.P. Grime, Winter 2005

Background

Processes underlying the maintenance of biodiversity are of great interest in modern biology, but little is known about whether the continuing erosion of genetic diversity (variation within species) influences the structure and composition of ecological communities. In 1997, J.P. Grime and colleagues used their extensive knowledge of an ancient, species-rich pasture ecosystem in northern England to create the first long-term experimental investigation of the effects of genetic impoverishment on plant species diversity and ecosystem processes. Results of the first five years of this study (Booth & Grime 2003) suggest that 1) species diversity declines as component populations lose genetic variation, and 2) communities of genetically diverse populations maintain a more consistent and predictable species composition than those more genetically impoverished. However, the processes responsible for these patterns remain unclear.

Rationale

A full understanding of how local genetic diversity influences community and ecosystem properties in the Cressbrookdale ecosystem requires knowledge of how such a large amount of genetically-controlled phenotypic variation is created and maintained in the first place. Is such polymorphism the result of neutral genetic processes or is it maintained by natural selection, in response to environmental heterogeneity in space or time? One hypothesis is that local genotypes are subject to the same adaptive tradeoffs that are observed at the species level—for example, selection pressures favoring tissue protection (from grazing or pathogens) necessarily require tradeoffs in traits related to tissue proliferation (such as response to nutrient-rich patches or apical growth rate). Such factors could be heterogeneous spatially at small scales at Cressbrookdale (e.g., nutrient patchiness from dung heaps, micro-scale variation in soil depth and thus water stress, grazing or pathogen spatial variance), or heterogeneous over time in response to different management regimes (particularly grazing intensity and timing) or climate fluctuation.

A second hypothesis is that selection is operating on establishing individuals to have phenotypes that depend in part on the phenotypes of neighbors; that is, interspecific interactions are an important component to local selection and are sensitive to genetic identity (Turkington and Harper 1979, Aarssen and Turkington 1985).

Together, these hypotheses focus on the role of local heterogeneity in environmental factors (Hypothesis 1) or biotic factors (Hypothesis 2) as adaptive explanations of the local genetically-controlled polymorphisms found in Booth & Grime (2003) and Whitlock (in preparation). A test of theses hypotheses should thus be in the form of common garden experiments with different genotypes of different species, assaying the response of genotypes to different environmental regimes and neighbor identities.

Questions / Objectives

1. [General] How is local (10 x 10 m) variation in genetically-controlled phenotype maintained in several key species of the Cressbrookdale ecosystem?

2. Do performance tradeoffs exist among genotypes in response to different environmental treatments, such that no one genotype performs the best in all environments?

TEST: Assay the response of genotypes of several species in different environments.

3. Is the outcome of interspecific interactions dependent upon the genetic identity of the competitors?

TEST: Assay the response of genotypes in the same environment to different genetic identities of competing species.

4. Does the persistence of a subdominant species depend on the genetic identity of its dominant neighbors?

TEST: Include subdominant species in species mixtures.