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Research Update 2 Rees, M. et al. (2002) Snow tussocks, chaos and the evolution of mast seeding. Am. Nat. 160, 44–59 3 Kelly, D. (1994) The evolutionary ecology of mast seeding. Trends Ecol. Evol. 9, 465–470 4 Silvertown, J. (1980) The evolutionary ecology of mast seeding in trees. Biol. J. Linn. Soc. 14, 235–250 5 Busgen, M. and Munch, E. (1929) The Structure and Life of Forest Trees, Chapman & Hall 6 Norton, D.A. and Kelly, D. (1988) Mast seeding over 33 years by Dacrydium cupressinum Lamb. (rimu) TRENDS in Ecology & Evolution Vol.17 No.11 November 2002 (Podocarpaceae) in New Zealand: the importance of economies of scale. Funct. Ecol. 2, 399–408 7 Kelly, D. et al. (2001) Evaluating the wind pollination benefits of mast seeding. Ecology 82, 117–126 8 Satake, A. and Iwasa, Y. (2000) Pollen coupling of forest trees: forming synchronized and periodic reproduction out of chaos. J. Theor. Biol. 203, 63–84 9 Kelly, D. and Sullivan, J.J. (1997) Quantifying the benefits of mast seeding on predator satiation and wind pollination in Chionochloa pallens (Poaceae). Oikos 78, 143–150 495 10 Isagi, Y. et al. (1997) How does masting happen and synchronize? J. Theor. Biol. 187, 231–239 11 Wallinga, J. and van Oijen, M. (1997) Level of threshold weed density does not affect the long-term frequency of weed control Crop Protection 16, 273-278 Rob P. Freckleton Dept of Zoology, University of Oxford, South Parks Road, Oxford, UK OX1 3PS. e-mail: [email protected] 2002: the year of the ‘diversity–ecosystem function’ debate Tom Cameron The main conclusion of a detailed review on biodiversity and ecosystem functioning in 2001 was to recommend continuing research into general patterns between species loss and ecosystem processes, in relation to habitat management. Four key papers have emerged from an explosion of recent experimental tests addressing this question. Studies by Pfisterer and Schmid, and by Schaffers provide empirical evidence for the lack of a unique relationship between increased ecosystem diversity and ecosystem function or stability. Paine and, in a separate report, Willms et al. describe the problems associated with ignoring trophic links in the experimental testing of the ‘diversity – production’ debate. These findings highlight the importance of rigorous testing of general ecological theory before recommending it for use by habitat or population managers. Published online: 03 September 2002 There has never been so much attention focused on the implications of biodiversity loss for ecosystem functioning or primary productivity. Two pairs of reports published this year highlight the importance of careful testing of ecological paradigms and the thorough evaluation of the focus of biodiversity research [1–4]. In a two-part report, Schaffers [1] investigates the roles of soil, biomass and management as factors that control plant species diversity. The author identified interesting hump-shaped species-richness relationships with biomass and productivity; a normal distribution, as identified in Mediterranean-type ecosystems [5]. However, he points out that these relationships fail if soil or management http://tree.trends.com variables are considered. What Schaffers is saying is that the biomass or productivity – diversity relationships that are found in plant systems can arise from covariation of biomass and /or productivity with other abiotic or management factors, which are typically ignored in such studies. More importantly he found that management practices, such as mowing and hay removal (i.e. grazing) promoted a positive relationship with species evenness and the number of rare or endangered species present. This suggests that vulnerable species might be threatened by competitive exclusion in increased biomass environments, and that a tradeoff between predation and competition exists that promotes the coexistence of rare and common species and not simply a productivity relationship. Complementing these findings, Pfisterer and Schmid [2] report on an experimental grassland system where increased biodiversity increases biomass production. However, they stress the importance of a link between such systems and an inverse relationship between the diversity and the stability of ecosystem functioning [6]. The authors found that biomass production in species-poor ecosystems was reduced less following perturbation, and returned to pre-perturbation levels faster, than did species-rich systems. Artificially increasing spatial heterogeneity to accommodate increased diversity and production might therefore prove catastrophic if these habitats contain rare or endangered populations. This is in direct contrast to the conclusions of a recent review by Loreau et al. [7]. Instead, the indirect interactions discovered by Pfisterer and Schmid could promote increased extinctions in increasingly variable environments. These two reports [1,2] have questioned the use of previous experimental results to look for generalities in the ‘diversity begets productivity’ debate because confounding variables, such as soil microbe diversity, soil quality and ecosystem resistance, have been ignored [7,8]. Such problems could be overcome with more complete experimental null hypotheses. Where have all the rare species gone? Part of the burning interest in biodiversity and ecosystem function relationships is born from the concern over declining species numbers in recent years and the consequences of this decline to ecosystem services and life on Earth [9]. This interest has generated a large portfolio of studies that describe the importance of ecosystem function for diversity, and vice versa [7,10,11]. But is it ecosystem function that is important? Has this generalization been experimentally tested and, if so, how rigorously? Recently, Paine [3] tested the hypothesis that trophic links cloud the relationship between species richness and ecosystem output as measured by production (see also [10,11]). Collecting data over seven years in the low intertidal zone at Tatoosh Island, WA, USA, he found that experimental plots with restricted grazer access had higher species richness and primary production. However, this increase in production was accounted for by the enemy release of only one or two highly productive algal species. Moreover, the increased richness (presence of rare species) was accounted for by the lack of grazers rather than the increased production. These key results clearly demonstrate a weakness in the ‘diversity begets production’ debate. Paine’s report is 0169-5347/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S0169-5347(02)02618-6 496 Research Update further evidence, and perhaps the most convincing to date, that species-specific properties in real systems (i.e. with trophic links included) can prevent general patterns emerging across ecosystems. Complementing this is the finding by a team from Canada [4] who, using a 70-year grazer exclusion experiment in a mixed prairie plant community, found a neutral or positive effect on primary production of disturbance exclusion (i.e. fire, grazing, etc.). However, this was due to increased cover of the most highly productive species, which brought about a reduction in species diversity. Willms et al. [4] conclude that the potential effect that reduced diversity might have on reducing production stability is more than compensated for by increased litter mass. This is similar to the findings of Schaffers [1]. Both Paine [3] and Willms et al. [4] have demonstrated the difficulty in finding a general ecological relationship between ecosystem function and diversity because of species-specific effects and important trophic links. Similar conclusions have been drawn from laboratory experiments [12]. …but how do I get it? I know what I want… It might be reasonable to assume that increased production will lead to increased coexistence of species because competition is relaxed (in the absence of adverse indirect trophic links, which is unlikely). It is also reasonable to accept that an increased number of species could increase ecosystem functioning through a more complete utilization of resources [7] and facilitation. However, it is not reasonable for habitat managers to assume that increasing the productivity of their habitats would secure persistence or increase diversity. The same caveat applies to the advice that managing for increased species richness, or preventing their loss, will help increase or maintain productivity. TRENDS in Ecology & Evolution Vol.17 No.11 November 2002 For the most part, these hypotheses have not been accurately tested (but see [13]). These four papers highlight fundamental questions, particularly regarding the conservation of biodiversity and the methods recommended to habitat managers and politicians. Can increased production and its relationship with ecosystem stability be exploited to promote species conservation, more appropriate habitat management strategies, and sustainable biodiversity? I would suggest not, because of the species-specific effects and the complexities of indirect interactions between biomass, decomposition, competition and predation illustrated by these studies. This is especially the case in disturbed ecosystems, which contain introduced species that can monopolize ecosystems when released from natural enemies and competition for resources [14,15]. So, we know we want conservation of vulnerable species in ecosystems, but how do we achieve this? Does increasing diversity or biomass promote population viability or longevity for rare species in ecosystems? Given the universally accepted importance of the ‘diversity–ecosystem function’ debate, I believe two questions now need to be addressed. First, is there an unequivocal link between increased ecosystem function and the protection of rare or declining populations? Second, does stable ecosystem function encourage persistence of stable populations, rather than the dominance of single highly productive or invasive species? These are both urgent issues in conservation ecology. The four reports I have discussed [1–4] stress that we need to change our thinking towards such experimentation, because too many applied projects and conservation management plans are based on untested theory. Acknowledgements I thank Steve Sait, Helen Wearing, Pejman Rohani and two referees for assistance with this article. References 1 Schaffers, A.P. (2002) Soil, biomass, and management of semi-natural vegetation. Part II. Factors controlling species diversity. Plant Ecol. 158, 247–268 2 Pfisterer, A.B. and Schmid, B. (2002) Diversitydependent production can decrease the stability of ecosystem functioning. Nature 416, 84–86 3 Paine, R.T. (2002) Trophic control of production in a rocky intertidal community. Science 296, 736–739 4 Willms, W.D. et al. (2002) Response of the mixed prairie to protection from grazing. J. Range Manage. 55, 210–216 5 Bond, W. (1983) On Alpha diversity and the richness of the cape flora: A study in southern Caper Fynbos. In Mediterranean-type Ecosystems (Kruger, F.J. et al., eds), pp. 337–356, Springer-Verlag 6 Sankaran, M. and McNaughton, S.J. (1999) Determinants of biodiversity regulate compositional stability of communities. Nature 401, 691–693 7 Loreau, M. et al. (2001) Biodiversity and ecosystem functioning: current knowledge and future challenges. Science 294, 804–808 8 Cavigelli, M.A. and Robertson, G.P. (2000) The functional significance of denitrifier community composition in a terrestrial ecosystem. Ecology 81, 1402–1414 9 Woodruff, D.S. (2001) Declines of biomes and biotas and the future of evolution. Proc. Natl. Acad. Sci. U. S. A. 98, 5471–5476 10 Tilman, D. (1999) Diversity and production in European grasslands. Science 286, 1099–1100 11 Hector, A. et al. (1999) Plant diversity and productivity experiments in European grasslands. Science 286, 1123–1127 12 Bjornstad, O.N. et al. (2001) The impact of specialized enemies on the dimensionality of host dynamics. Nature 409, 1001–1006 13 Schmid, B. (2002) The species richness–productivity controversy. Trends Ecol. Evol. 17, 113–114 14 Crawley, M.J. et al. (1999) Invasion-resistance in experimental grassland communities: species richness or species identity? Ecol. Lett. 2, 140–148 15 Byers, J.E. et al. (2002) Directing research to reduce the impacts of nonindigenous species Conserv. Biol. 16, 630–640 Tom Cameron Centre for Biodiversity & Conservation, School of Biology, University of Leeds, Leeds, UK LS2 9JT. e-mail: [email protected] Meeting Report Metersticks and microarrays Jessica Ruvinsky The annual meeting of The American Society of Naturalists was held in Banff, Canada, from 11 to 14 July 2002. Published online: 11 September 2002 http://tree.trends.com The American Society of Naturalists (ASN) met independently of the Society for the Study of Evolution for the first time since 1987 this summer in Banff, Canada. Among the mountains, the posters and the few concurrent sessions, members discussed the role and direction of the Society. Is the future in genomics or in natural history? 0169-5347/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S0169-5347(02)02623-X