How much is definitional? What is "good"? What is "health"?

"3 Studies Indicate That Biodiversity Is Not Inherently Good for Ecosystems"

• By KIM A. McDONALD (Chronicle of Higher Ed.)
•  

Three studies of biological communities show that increasing the diversity of species doesn't necessarily improve the health of ecosystems.

More important than biodiversity in driving ecosystems to change for the better or the worse, according to the studies, are the biological traits of the ecosystem's dominant species.

Those traits include such factors as how the most ubiquitous plants capture their nutrition and how that process affects the rest of the ecosystem

Published in today's issue of Science, the results are certain to spark debate among scientists and policy makers, most of whom have long assumed that the health of ecosystems was intimately tied to maintaining biodiversity.

In the first study, David A. Wardle and colleagues at the Swedish University of Agricultural Studies, in Ume=E5, examined 50 Swedish islands, many of which had been ravaged by forest fires. On some islands, they discovered that species-rich ecosystems had slower ecological processes, because the dominant plant species produced decaying organic matter of poor quality.

In the second study, David U. Hooper, of the University of California at Berkeley, and Peter M. Vitousek, of Stanford University, found that variations in nutrient cycling in a California grassland were related more to the biological characteristics of the dominant organisms than to species diversity.

Similar results were obtained in the third study, in which David Tilman, of the University of Minnesota, and colleagues studied a reserve in Minnesota planted specifically to test ecosystem models.

While evidence from those studies suggests that maintaining a high diversity of species is not as important as the biological function of dominant species, efforts to maintain high biodiversity shouldn't be abandoned, wrote J.P. Grime, of Britain's University of Sheffield, in an accompanying article. "It is obvious that for all ecosystems a point could be reached at which further loss of key species could impair functioning and usefulness to humans," he wrote. "The most immediate problem is to identify irreplaceable species and functional types and to discover whether there are situations in which ecosystem viability depends on unusually high biodiversity."

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"Credulity is the man's weakness, but the child's strength." Charles Lamb

• Robert J. Swenson

Andy's Reply:

Bob and other recipients,

A few comments on your email. Other thoughts? Andy

The context of this piece is that ecologists have recently been asking how "biodiversity" (usually expressed as species richness which is the number of species in an ecosystem) influences ecosystem functions like

energy production and flow, and nutrient cycling. Four or five recent studies have found that ecosystems with highest species richness had either higher rates of energy production or a quicker recovery in energy

production following disturbance (e.g., drought) than did ecosystems with lower species richness. This is important because it suggests that polyculture may sometimes have enhanced agricultural productivity over monoculture. It also says that human elevation of extinction rates in ecosystems may reduce their productivity. So, in this context, "good" and "healthy" are inprecise terms for stable, productive ecosystems.

The piece below is an important step because it argues that who the species are that comprise the species richness can matter to ecosystem function. Ecologists have known this for a very long time and have referred to species that have a disproportional influence on ecosystems as "keystone" species. The piece below reminds us of this. However, this piece in no way suggests that biodiversity has been overrated. First, we usually do not know who the keystone species in a system are, so keeping as many species as

possible in the system increases the chances of having some species that enhance productivity and stability. Secondly, no one species can be good at everything. So having several plant species, for example, with different architectures results in more solar energy and nutrient capture and higher productivity than having one or a few species. An analogy is whether you would rather have a football team where all players have the body build and skills of the quarterback or the range of bodies that typically makes up a team.

So I would conclusion that species richness is often a correlate with the community characteristics that confer productivity and stability. We need to know more about the causal factors of these relationships. In the meantime, maintaining biodiversity is a good way to hedge our bets against uncertainty, as all financial investors well know.

Andy Hansen

Montana State University

Ecologist

Response to Jack Decker's question: "So is weed species diversity good?"

Your comments on weed diversity are well placed. In terms of the ecological literature the "diversity -stability" hypothesis is being debated. note J.P. Grime 1997. Biodiversity and ecosystem function: the debate deepens. Science vol. 277, August 1997, page 1260. Intuitively, one tends to consider that the greater the diversity the more stable is the ecosystem. The alternative hypothesis considers functional groups ie. it is not how many different species you have but what function within the ecosystem do they perform.

This debate is relevant to weed science in-terms of our management practices. We have published two papers on weed diversity issues: Clements et. al. 1994. Integrated weed management and weed species diversity. Phytoprotection 75 (1) 1-18. This paper reviews the terminologies and issues related to diversity in natural systems. We then attempted to bridge this information across to weed communities and basically show how various components of IWM influence weed diversity. Diversity may very well mean stability in weed populations. Herbicide resistance is an example of where diversity is lost and a single species dominates the weed population. This is not desirable. Several practices notably, reduced till systems tend to increase diversity. If an increase in diversity does not affect weed community structure, and the same species are dominant as before then the management implications are few. Our second paper Derksen et al. 1995. Impact of post-emergence herbicides on weed community diversity within conservation-tillage systems . Weed Res. 35: 311-320. This paper dealt with the effect of non-residual post herbicides on weed community diversity. We found that non-residual post herbicides acted much in the same way as a natural disturbance. Weed diversity was not reduced by the use of these herbicides but actually increased. Dominant species were controlled by the herbicide thereby allowing a more diverse weed population to establish.

Clear answers do not exist with this issue. From our past and current research , diversity is not an issue to be ignored by agriculturalist. Our data would indicate that there are many ways via management that we can increase biodiversity within agriculture and not threaten economic viability.

Dr. Clarence Swanton,

Professor,

Department of Crop Science,

Univ. of Guelph

Ah, what is the meaning of diversity?

We recently completed an analysis of seed bank diversity over 5 years in the Maine Potato Ecosystem Project, an on-going cropping systems study. Potato is grown in a two year rotation and among other treatments are two contrasting soil management systems, unamended (barley/red clover rotation crop; 1x blended fertilizer for potato), and amended (pea/oat/hairy vetch green manure rotation crop; manure, compost, and 0.5x blended fertilizer for potato), that contrast practices typical for the region to those designed to achieve rapid improvements in soil quality.

In 1991, the first year of the study, Simpson's diversity was similar in all treatments. By 1995, in plots managed without herbicides (cultivation used for weed control) diversity was greater in the amended than the unamended soil management system. In contrast, where full or reduced herbicide rates were used for weed control diversity was similar in both soil management systems.

Relative abundance of the top six species was unaffected by soil management system in 1991 but by 1995 the relative abundance of common lambsquarters had increased in the unamended compared to the amended soil management system (84% vs. 54%, respectively) whereas low cudweed had decreased (6% vs. 24%, respectively).

Greater diversity in the amended system could reflect inadvertent additions of weed seed with manure. In 1995 use of manure resulted in the application of 670 seeds m-2 comprising 12 species; use of compost resulted in the addition of 5 species at a density of 4 seeds m-2. Alternatively, lower diversity in the unamended may simply reflect poor control of lambsquarters.

In this example I would say that diversity is a good thing, or at least lack of diversity is undesirable because it means that lambsquarters is increasing. Greater diversity in the amended system could result in beneficial interspecific interactions but it would take some clever field studies to know for certain. On the other hand, diversity maintains species that could become problematic if some critical management stress were unknowingly removed.

Still searching for truth in diversity,

Eric Gallandt

Washington State University

Kareiva, P. 1996. Diversity and sustainability on the prairie. Nature 379:673-674.

Moffat, A.S. 1996. Biodiverstiy is a boon to ecosystems, not species. Science (Research News) 271:1497

Tilamn, D. 1999. Diversity by default. Science 283:495-496.

Tilman, D. 1996. Biodiversity: population versus ecosystem stability. Ecology 77:350-363.

Tilman, D., D. Wadin and J. Knops. 1996. Productivity and sustainability influenced by biodiversity in grassland ecosystems. Nature 379:718-720.