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Strong effects of weak interactions in ecological communities


The loss or removal of individual species can cause dramatic changes in communities1,2,3,4,5. Experiments indicate that in many communities only a few species will have such strong effects, whereas most will have weak effects owing to small per capita effects and/or low abundance3,6,7,8,9,10,11,12,13,14,15,16. But extinction of these ‘weak’ interactors could significantly alter natural communities because they play important stabilizing or ‘noise-dampening’ roles14,15,17,18,19,20,21,22,23. I demonstrate here that some ‘weak’ interactors may also be important by magnifying spatiotemporal variation in community structure. An analysis of published interaction strength data shows that the greatest variation in species effect occurred for the weakest interactions. A field experiment corroborates this and shows how indirect interactions can generate an inverse relationship between the mean and variance of a consumer's impact on its prey. When a species' effects are highly variable in sign and magnitude, they may average to seem weak over broad scales but be strong in local contexts. Thus, what is frequently considered to be ‘noise’ in species interaction data may be a critical part of the signal.

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Figure 1: Estimated range of variation in interaction strength versus the mean strength for three studies that quantified per capita interaction strengths among a suite of consumer–prey interactions.
Figure 2: Mean (±s.e.) direct, pairwise effect of whelks on mussels that were transplanted to achieve constant initial cover, for each of the three densities of whelks enclosed.
Figure 3: Mean (±s.e., n = 4) effect of low- and high-density predator treatments on mussel colonization (relative to predator exclusions) for each of the three experimental starting dates.
Figure 4: Variation in mussel colonization rates (per cent cover per month) between individual replicates (n = 4 disturbance patches) within each separate experimental starting date and between means of each starting date.


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I thank K. Buzzard, B. Daley, J. and T. Panek, B. Profit and S. Suskie for assistance in the field, B. and A. King for the use of their front yard, and G. Allison, C. Blanchette, F. S. Chapin, J.Corbin, C. M. D'Antonio, J. Dunne, S. Hobbie, D. Hooper, B. Langford, H. Lefcort, J. Levine, J.Lubchenco, N.D.Martinez, B. McCune, B. Menge, S. Navarrete, R. Paine, I. Parker, M. Power, W. Riley, A. Smyth, P.van Tamelen and J. T. Wootton for invaluable advice during the study. This work was supported in part by the Oregon State University Zoology Research Fund, the Holt Marine Education Fund, Sigma Xi, NSF and the Andrew W. Mellon Foundation. It was also greatly facilitated by the Sierra Nevada Aquatic Research Laboratory and the 1997–98 El Niño.

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Berlow, E. Strong effects of weak interactions in ecological communities. Nature 398, 330–334 (1999).

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