Ecology Letters, (2009) 12: 229–238 doi: 10.1111/j.1461-0248.2008.01279.x LETTER Do differences in food web structure between organic and conventional farms affect the ecosystem service of pest control?
Abstract Sarina Macfadyen,1 Rachel While many studies have demonstrated that organic farms support greater levels of Gibson,1 Andrew Polaszek,2 biodiversity, it is not known whether this translates into better provision of ecosystem Rebecca J. Morris,1,3 Paul services. Here we use a food-web approach to analyse the community structure and 4 5 G. Craze, Robert Planque´ , function at the whole-farm scale. Quantitative food webs from 10 replicate pairs of William O.C. Symondson6 and 1 organic and conventional farms showed that organic farms have significantly more Jane Memmott * species at three trophic levels (plant, herbivore and parasitoid) and significantly different 1School of Biological Sciences, network structure. Herbivores on organic farms were attacked by more parasitoid species University of Bristol, Woodland on organic farms than on conventional farms. However, differences in network structure Road, Bristol BS6 7EU, UK 2Department of Entomology, did not translate into differences in robustness to simulated species loss and we found no Natural History Museum, difference in percentage parasitism (natural pest control) across a variety of host species. London SW7 5BD, UK Furthermore, a manipulative field experiment demonstrated that the higher species 3Department of Zoology, richness of parasitoids on the organic farms did not increase mortality of a novel University of Oxford, South herbivore used to bioassay ecosystem service. The explanation for these differences is Parks Road, Oxford OX1 3PS, UK likely to include inherent differences in management strategies and landscape structure 4Biology and Environmental between the two farming systems. Science, University of Sussex, Brighton BN1 9QG, UK Keywords 5 Department of Mathematics, Agro-ecology, biodiversity, networks, parasitoid diversity, species interactions. VU University Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands 6Cardiff School of Biosciences, Cardiff University, Biomedical Sciences Building, Museum Avenue, Cardiff CF10 3US, UK *Correspondence: E-mail: [email protected] Current address: CSIRO Entomology, Black Mountain, GPO Box 1700, Canberra, ACT 2601, Australia Ecology Letters (2009) 12: 229–238
a large pool of species is necessary to maintain the INTRODUCTION functioning of ecosystems, especially in intensive agricul- The extent to which biodiversity can be lost before it affects tural landscapes (Loreau et al. 2001; Tscharntke et al. 2005). the provision of ecosystem services is unclear. Services such For example, for insect pollinators, maintaining a diversity as natural pest control are the result of the complex network of species is essential for sustaining plant pollination of interactions among species in a community. These services because of yearly variations in community compo- services provide substantial, if not invaluable economic sition (Kremen et al. 2002). For the service of natural pest benefits to humans particularly in food production (Losey & control the situation is more complex. Snyder et al. (2006) Vaughan 2006; Klein et al. 2007). Many studies suggest that found that a greater number of predator species increased