Papers in the Biological Sciences


Date of this Version



Published in Ecology, 85(10), 2004, pp. 2901–2910. Copyright (c) 2004 by the Ecological Society of America. Used by permission.


An exchange regarding the article: Bradley, K. L., E. I. Damschen, L. M. Young, D. Kuefler, S. Went, G. Wray, N. M. Haddad, J. M. H. Knops, and S. M. Louda. 2003. Spatial heterogeneity, not visitation bias, dominates variation in herbivory. Ecology 84:2214–2221.

Cahill et al. write:
Here we show that the experimental design and statistical analysis used by Bradley et al. (2003) do not adequately control for Type II experimental error (accepting the null hypothesis when it is in fact false). This is particularly important when responses have low effect sizes such as those previously demonstrated for visitation effects (Hik et al. 2003). Type II errors are of utmost concern with respect to confounding effects of experimental methodology, as their undetected effects greatly hinder understanding the ecology of the system under investigation. We use the study of Bradley et al. (2003) to illustrate the impact that alternative interpretations of experimental design and statistical analysis have on the resulting conclusions. We describe specific shortcomings of the methodological choices made by Bradley et al. and show that a more appropriate statistical model indicates their data are consistent with our prior findings.

Louda et al. respond:
Cahill et al. (2004) criticize our recent study (Bradley et al. 2003) that contradicted the cause that they have adopted to explain an interesting phenomenon— high levels of variation in the intensity of herbivory among plants in nature. Their hypothesis is that visiting plants generally and significantly alters the intensity of herbivory (Cahill et al. 2001). If this visitation hypothesis were supported, then the interpretation of existing studies of the intensity and impact of herbivory on plant performance would need to be reexamined. However, few data support their hypothesis (Schnitzer et al. 2002, Bradley et al. 2003), including the evidence in their own studies (Cahill et al. 2001, 2002) and their reanalysis of our data (Cahill et al. 2004). ... In the interests of progress in the study of herbivory, we suggest that further defense of an ‘‘herbivory uncertainty principle,’’ or more precisely the visitation hypothesis that has not been well substantiated by field data, should be abandoned. The evidence does not support the putative ‘‘principle.’’ Instead, more effort should be placed on increasing our mechanistic, predictive understanding of the influence of environmental context on the intensity of herbivory, the biological mechanisms driving the spatial and temporal variation in herbivory, and the consequences of herbivory on plant abundance, distribution, and dynamics.

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