Experiment II: Genes vs. Species

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. These results suggest that genetic diversity has feedbacks to ecosystem functioning by means of controlling the abundance of local populations, but it remains unclear how such effects compare or interact with local diversity at the species level.

Rationale

The last 10 years have seen increasing popularity—some now say paradigm—of the idea of biodiversity as a driver of ecosystem processes. Most studies have been conducted locally (communities the size of a 100 m² or smaller) and have focused exclusively on diversity at the species level or higher (taxonomic or functional diversity). There is now perhaps general, albeit contentious, recognition that local species diversity can enhance ecosystem processes such as annual production, rates of nutrient cycling, and decomposition. It is also clear that the effects of individual species are typically much more important than interactions based on species synergisms (such as complementarity), although complementary has been found to be significant in some circumstances.

Genetically-controlled phenotypic differences within populations have been found to be considerable within surprisingly small areas (Linhart and Grant 1996), and they have recently been found to influence local species abundance (Booth and Grime 2003). This opens up the possibility that local genetic diversity can influence ecosystem properties by the same mechanisms found at the species level—through phenotypic complementarity in genotypes (Aarssen and Turkington 1985), either within or among species, or, more likely, by influencing the abundance and thus ecosystem effect of particularly important species. Indeed, “biodiversity” includes all relevant phenotypic variation among individuals in a community, and is nested both within and among species. So how does such variance both within and among species contribute to ecosystem processes? That is, if both types of diversity contribute to ecosystem functioning, what is the relative importance of each type? Do they interact? These questions have, to our knowledge, never been investigated.

Furthermore, it is probable that local environmental heterogeneity in both space and time is a key factor that contributes to local diversity effects. Realistic environmental heterogeneity has rarely been included in synthetic studies of biodiversity and ecosystem functioning.

Questions / Objectives

1. What are the relative effects of genetic and species diversity on ecosystem functioning in the Cressbrookdale ecosystem?

2. Do genetic and species diversity interact to influence ecosystem functioning?

3. Do species and genotypes within species tradeoff in their performance on different substrates and in response to different management / climate regimes?

A hypothesis

If genetic diversity enhances the performance of particularly important species in this system (such as the drought tolerant Festuca ovina), and the contribution of species synergisms to yield is minor, then the “dilution” of F. ovina abundance by increasing species richness may lead to a negative relationship between genetic diversity and species diversity at this small scale. That is, the best long-term performing community may be a genetically diverse monoculture of F. ovina, and the poorest may be a genetically uniform species mixture.