In a guest post, Timothy Crews quotes two representative statements from my book in which I argue that (in contrast to natural selection's improvement of individual adaptations) neither natural selection nor other natural processes have improved the overall organization of natural ecosystems over millennia. My doubts about "other natural processes" were based on actual data comparing the productivity and stability of natural and managed ecosystems. Dr. Crews argues that shorter-term ecological processes, specifically succession, may improve "ecosystem function with respect to agricultural goals." He doesn't seem to be claiming longer-term improvements, such as succession improving ecosystems more today than it did millions of years ago, which would be analogous to the longer-term improvement of individual adaptations by natural selection.
I don't doubt that some successional changes in some ecosystems qualify as improvements by criteria relevant to agriculture. So I agree that agriculture might "benefit from studies of niche complementarity" etc. As I wrote on page 1:
"Once we drop the assumption of perfection, however, we can learn much from studying natural communities.""The assumption of perfection" may be an exaggeration of the viewpoint I intended to criticize. I could rephrase as "Once we drop the assumption that there are ecological processes that consistently improve the overall organization of natural ecosystems in ways that would make it safe for agriculture to copy their organization without first testing all the effects of that organization", but that seemed a bit lengthy for page 1.
But how consistently do successional processes improve productivity, stability, nutrient retention, etc.? If there's a good recent review of this question, I'd appreciate hearing about it. The papers I've found do not support the hypothesis that these measures of ecosystem performance consistently improve with succession.
Gower et al.(Gower et al. 1996) analyzed 13 datasets for forests around the world and reported that "Aboveground net primary production (ANPP) commonly reaches a maximum in young forest stands and decreases by O-76% as stands mature." In agriculture, productivity is arguably our most-important criterion, even for environmentalists, as it reduces the amount of land needed to grow a given amount of food. But other measures of performance are also important.
Crews' guest post emphasizes nutrient retention. Vitousek and Reiners (Vitousek and Reiners 1975) presented data compared nitrate loss in streams from younger versus older forests. The older forests lost more nitrate. They also discussed similar data from other forests. Similarly, Lamb (Lamb 1980) found higher nitrate levels in soils of older rather than younger tropical rainforests.
Comparisons among ecosystems are complicated by the possibility of differences unrelated to succession. The closest thing I know to a controlled comparison is that of Wardle et al.(Wardle et al. 1997) They compared islands whose differences in successional stage depended on how recently they had been struck by lightning and burned. This depended mostly on their size. Species diversity was greatest on islands in later successional stages, but "ecosystem process rates were lowest on those islands," perhaps because microbial biomass was greatest at early successional stages. The effects of succession on overall productivity and stability were apparently not measured, unfortunately.
Until I see some data to the contrary -- more than an isolated example or two -- I conclude that succession does not improve ecosystem function consistently enough that we can safely copy the overall organization of even late-successional ecosystems without extensive testing.
Gower, S. T., R. E. McMurtrie, and D. Murty. 1996. Aboveground net primary production decline with stand age: potential causes. Trends in Ecology & Evolution 11:378-382.
Lamb, D. 1980. Soil nitrogen mineralisation in a secondary rainforest succession. Oecologia 47:257-263.
Vitousek, P. M., and W. A. Reiners. 1975. Ecosystem succession and nutrient retention: a hypothesis. Bioscience 25:376-381.
Wardle, D. A., O. Zackrisson, G. Hornberg, and C. Gallet. 1997. The influence of island area on ecosystem properties. Science 277:1296-1299.