Our research focuses on the population dynamics of plants and how they are influenced by impacts of natural disturbances and global environmental change. We are particularly interested in the interactive effects of fire, grazing and drought in grasslands and woodlands in southern Australia, and how climate change, fragmentation and shrub encroachment affect ecosystems.

Thursday, 14 January 2016

Cracking open the relationship between biological diversity and ecosystem productivity solves a long-standing riddle

Biodiversity has been hypothesized to be of critical importance for the stability of natural ecosystems and their abilities to provide positive benefits such as oxygen production, soil genesis, and water detoxification to plant and animal communities, as well as to human society. Many of the efforts of conservation agencies around the world are driven by the assumption that this hypothesis is true. Elucidating this connection, and the processes that underpin it, is important on many levels such as anticipating how ecological communities may change (a) in response to anthropogenic perturbations (i.e. changes to the abiotic environment), (b) the introduction of new species or (c) the loss of established ones. While theoretical studies have supported this claim, scientists have struggled for the last half-century to clearly isolate such an effect in the real world. Indeed, a comprehensive, mechanistic understanding of the relationship between biodiversity and ecosystem productivity remains elusive. 

The fundamental mechanisms underlying global productivity-diversity patterns
have been debated by ecologists for decades. Methodological advances are now permitting a glimpse at the processes that lie behind surface patterns.
I was lucky enough to be part of a paper just published in Nature by Jim Grace et al. where we argue that the interplay between these factors is more fully understood when both are placed in a rich network of cause-and-effect pathways, as opposed to being regarded as entities engaged in an isolated back-and-forth.

We considered data generated by the Nutrient Network, a global scientific cooperative that examined 40+ grass-dominated plant communities from across the planet. Indeed, we used data from over a thousand grassland plots spanning five continents.
Location of the NutNet study sites across the world

Several interesting results emerged:
1.  the rate of biomass production increases with the number of species found at a site (its species richness). This effect holds steady across the observed variation in species richness, instead of saturating in communities with greater richness, as a generation of experimental and theoretical work has suggested that it may.
2. with increasing accumulated above-ground biomass (live plant tissue and dead litter)  species richness declines.  We think this provides evidence that competition between species — primarily for light — is an important force in determining why communities contain as many species as they do. Competition seemed to be influential regardless of how productive a community was, contrary to the historical suggestion that competition is a stronger structuring force in more-productive communities.

These results show that you cannot have sustainable, productive ecosystems without maintaining biodiversity in the landscape. Indeed, as Debra Willard, Coordinator for the USGS Climate Research & Development Program, put it: “These results suggest that if climate change leads to reduced species or genetic diversity, which is a real possibility, that then could lead to a reduced capacity for ecosystems to respond to additional stresses.”

One of the Australian NutNet sites, from Western Australia

Further information: Grace et al. (2016) Integrative modelling reveals mechanisms linking productivity and plant species richness. Nature doi:10.1038/nature16524

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