Monument Valley

Tools for ecological investigations

Lecture graphics

Choosing the tools for studying nature

Diamond (1983, Nature 304:586-587) gave an overview of approaches for studying ecological concepts

classified research into 2 types (while recognizing that these represent regions along a continuum of possibilities):

comparative and experimental studies [right-hand-side of Fig. 4.1 Keddy 1989, p. 83]

in comparative studies ("natural experiments") the researcher exploits naturally occurring perturbations



Diamond divided comparative studies into 2 types:

"natural trajectory experiments" are comparisons of the same community before, during, and after a perturbation

"natural snapshot experiments" are comparisons of communities assumed to be different principally w/ respect to one IV

Diamond also divides experimental studies into 2 types, depending on the location (and, by inference, the level of control exerted by the researcher): laboratory and field experiments

Keddy (1989) adds an additional category: descriptive studies

data sets are collected from a community and are then statistically manipulated to look for patterns which the researcher believes can be attributed to competition

this approach was popularized by animal ecologists in the 1980s

Diamond evaluates the strengths and weaknesses of 4 types of studies relative to 7 criteria:



DESCRIPTIVE STUDIES

Keddy (1989): "generations of plant ecologists have been occupied w/ tallying the contents of quadrats in the summer, and then trying to draw inferences about these observations in the winter."

Many statistical techniques have been developed just to look for patterns in these data sets

The biggest problem w/ this approach can be illustrated w/ a simple example of association analysis using the 2 x 2 contingency table

Data are collected from sample units (usu. quadrats) and the association between any pair of species is calculated using the chi-square test

The null hypothesis is that the species are independently distributed; the alternative hypothesis is that the 2 spp. are either positively or negatively associated

Negative associations are often interpreted as being evidence of competition; actually, at least 4 interpretations are possible:

  1. Spp. are restricted to different microhabitats, and so do not interact

  2. Spp. are positively associated but the sample unit was so small that only a few indivs. fit in it, thereby obscuring the pattern which occurred at a larger scale

  3. Agents such as predators independently control each species and restrict each to a different set of conditions

  4. The spp. compete, and competition leads to habitat segregation

It is not possible to distinguish between these causes w/ descriptive data alone

A variation on using association analysis is to choose natural environmental gradients and examine distributional limits of spp. along these gradients in order to infer the existence of competition

Assumption: systems that are structured by competition have different kinds of patterns than those not structured by competition

Problem: it is not clear what type of patterns competition would produce

Nonetheless, statistical tests have been developed to determine distributions. Three alternatives are recognized:

  1. Spp. distributional limits are regularly spaced

  2. Spp. distributional limits are randomly arranged

  3. Spp. distributional limits are clustered along the gradient, producing apparent communities

Again, departures from random patterns do not tell us anything about competition (or any other process)

  1. Spp. may have similar distributional limits because of similar physiological tolerance limits

  2. Clusters of distributional limits may be attributed to the way the observer divided the gradient

  3. Herbivores may stop at a certain point along the gradient, and therefore create discontinuities

  4. One or more competitive dominants may set the distributional limits of an entire group of spp.

Only the last hypothesis is consistent w/ competition, and hypotheses 1-3 are very difficult to reject



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