E-Letter responses to:

reports: Oswald J. Schmitz Effects of Predator Hunting Mode on Grassland Ecosystem Function Science 2008; 319: 952-954 [Abstract] [Full text] [PDF]

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Published E-Letter responses:

Response to J. A. Newman's E-Letter

Oswald J. Schmitz   (24 April 2008)

A Failure to Test the Hypothesis of Interest

Jonathan A. Newman   (24 April 2008)

Response to J. A. Newman's E-Letter 24 April 2008
Oswald J. Schmitz Department of Forestry and Environmental Studies, Yale University, New Haven, CT 06511, USA Respond to this E-Letter: Re: Response to J. A. Newman's E-Letter	As elaborated in Schmitz (1), the study follows from the general insight derived from an empirical synthesis of a broad range of predator taxa (2) that the nature of predators' direct effects on their prey and indirect effects on the abundance and composition of the prey’s resources is related to predator hunting mode. That synthesis was motivated by natural history observations in my own particular system (3) that hunting mode differences of Pisaurina mira and Phidippus rimator might explain differences in patterns at the plant community level. The current study takes the next step by evaluating whether earlier insights (2) can be extended to explain ecosystem level functions. I agree with Newman that theoretically it would be highly desirable to replicate hunting mode in a single study. However, the practical reality is that my study system, and perhaps many others, is constrained by the number of predator species within the community that have different hunting modes. In my system, the sit-and-wait mode is represented by just a single species. The implication of Newman’s argument that we can only conduct such studies within species-rich assemblages, is itself a kind of bias that will be counterproductive to the general ecological scientific goal of having experiments on the role of predator hunting mode carried out in many kinds of systems. After all, it is replication among many kinds of systems that is likely to lead to the robust conclusions we seek in trying to understand how community composition influences the functioning of ecosystems (4). Oswald J. Schmitz Department of Forestry and Environmental Studies, Yale University, 370 Prospect Street, New Haven, CT 06511. 1. O. J. Schmitz, Science 319, 952 (2008). 2. O. J. Schmitz, in Ecology of Predator-Prey Interactions, P. Barbosa, I. Castellanos, Eds. (Oxford Univ. Press, Oxon, 2005), pp. 256–278. 3. O. J. Schmitz, K. B. Suttle, Ecology 82, 2072 (2001). 4. S. Naeem, Science 319, 913 (2008).
A Failure to Test the Hypothesis of Interest 24 April 2008
Jonathan A. Newman, Professor University of Guelph, Guelph, ON , Canada Respond to this E-Letter: Re: A Failure to Test the Hypothesis of Interest	O. J. Schmitz (Reports, Effects of predator hunting mode on grassland ecosystem function, 15 February 2008, p. 952) attempts to test the hypothesis that "differences in foraging modes alter key ecosystem functions." While his experimental results are very interesting, I suggest that his experiment does not test the hypothesis of interest. His experiment only tests the hypothesis that the different predators, Pisaurina mira and Phidippus rimator, cause differences in key ecosystem functions. And I accept that Schmitz has admirably demonstrated that result. To properly test the hypothesis about hunting mode, one would have to replicate "hunting mode" with a (preferably random) sample of predators that differ by hunting mode. As it is, Schmitz is cherry-picking one particular difference between the two predators (hunting mode) and attributing the results to that particular difference. Yet these two species undoubtedly differ in many more subtle characteristics besides their obvious difference in hunting mode. Any of these more subtle differences could potentially be responsible for the results. Suppose, for the sake of arguement, that the grasshoppers react not to the differences in foraging mode, but to differences between the two predators in their odor. Should the title of the paper then be: "Effects of Predator Odor on Grassland Ecosystem Function"? We must remind ourselves that evidence consistent with a hypothesis does not constitute a test of that hypothesis. Schmitz's results are certainly consistent with the hypothesis that "differences in foraging modes alter key ecosystem functions" but they are not a test of that hypothesis. Jonathan A Newman University of Guelph, Guelph, ON N1G 2W1, Canada.