Gurevitch et al 2000... or... The Sequel: Bigger, Better, and More Amazing?

"Ecologists working with a range of organisms and environments have carried out manipulative field experiments that enable us to ask questions about the interaction between competition and predation (including herbivory) and about the relative strength of competition and predation in the field. Evaluated together, such a collection of studies can offer insight into the importance and function of these factors in nature."

Thus are the opening lines of the abstract of Jessica Gurevitch, Janet A. Morrison, and Larry V. Hedges' April 2000 Paper in The American Naturalist (Vol 155 No 4 pp 435-453). These two sentences seem to state some simple realities about the science of ecology, and ways to look at and think about data collected to come to a few simple (if not somewhat broadly overgeneralized) understandings of the interactions between organisms and between populations. A seemingly easy enough premise to make some sense out of, yes?

No. Nineteen pages later, I'm not sure my understandings of the importance of competition and predation are any more refined or concrete.

I want to respect the integrity of any paper published in a journal like The American Naturalist, but I find it quite easy to get a bit cynical about the meta analysis work of Dr Gurevitch and her collaborators (a feeling shared by at least a few other individuals in this class). The abstract of this paper goes on to say that a "new factorial meta analysis technique" will be employed to address the interactions between predation and competition, but many of the stumbling blocks encountered in Gurevitch's 1992 paper in the same journal, this time co authored by L. Morrow, A. Wallace, and J. Walsh, are encountered in this 2000 paper.

In analyzing the studies of other scientists, the playing field must be leveled so as to compare apples to apples and oranges to oranges. Published summaries of studies may not have included all of the parameters needed by Gurevitch et al to normalize the data for their studies, and we all know that with the fear of arm chair ecologists stealing our lives' works, raw data can be hard to come by. Gurevitch et al are only able to find 20 useable articles on 39 field experiments, a considerably smaller sample size than the 1992 paper, which points out the differences in meta analysis results when looking at different trophic levels and different communities (marine, freshwater, terrestrial, etc). The smaller sample size in the 2000 paper makes conclusions reached and arguments made that much less convincing.

While the sample size and breadth is an issue, some of my problems with the paper are even more basic in their nature. Competition and predation. When we think of one something murdering and eating one something else, the difference would seem obvious. On a population level, however, both are interactions between organisms where one is negatively effected by the other. Interactions where one organism or group just lowers the fitness of another can be quite the sticky wicket to parse out; herbivory and allelopathy in plants would seem to be two of many particularly slippery slopes in these respects. We have toyed with the idea in class that competition might be thought of as mostly between trophic levels, while competition is within a trophic level, but where on that spectrum would cannibalism lie? My point here is that in order to address a question of competition versus predation, one must first define these two classes to a tee. In my humble opinion, Dr Gurevitch and her collaborators do not succeed at this task.

Mention of equilibrium and non equilibrium theories and states of communities and their structuring are also referenced multiple times, but concise definitions of these terms are also lacking. Are all ecological processes not in a constant state of change on some level? At what point do Gurevitch et al define a community or even a species pair as being at equilibrium?

All of this nitpicking over words aside, does the paper come to any amazing conclusions or revelations? The opening line of the Discussion section states "The patterns that emerge from this factorial meta-analysis are strikingly clear, make sense, and are relevant to larger issues of community structure." Bold statement; but less than three paragraphs later, there is a line that is colored a bit differently in my opinion. "Our meta-analysis cannot address these types of questions, but it shows how consistently important predation can be as a factor that diminishes competitive interactions in the field." I hate to parse things a la Fox News to get at my point, but words like "can be", "a factor" (of many?) and "diminishes" are quite relative and wishy washy for something that is "strikingly clear". The ambiguity continues two pages later when Gurevitch et al states "At a very basic level, our conclusions agreed with some of those of Sih et al (1985); they reported that competition and predation were both generally important in field experiments." Independent of the need for an adverb like "generally", it is quite confusing to say in one's discussion that a pair of factors were both important when one of the main points of the paper was to parse out the importance of each factor.

Are Jessica Gurevitch, Janet A. Morrison, and Larry V. Hedges amazing scientists? Undoubtedly. Does this particular paper show all of their amazingnesses in a good (comprehensible) light to a budding ecologist like myself? I unfortunately have to say perhaps not so much. Are there more convincing uses of these meta analysis techniques? Probably. Let's look them up when we find some free time... in July maybe?