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Tools for ecological investigations
Lecture
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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:
- Control of independent variables
- Matching of sites
- Maximum temporal scale
- Maximum spatial scale
- Range of possible manipulations
- Realism
- Generality
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:
- Spp. are restricted to different microhabitats,
and so do not interact
- 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
- Agents such as predators independently control
each species and restrict each to a different set
of conditions
- 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:
- Spp. distributional limits are regularly spaced
- Spp. distributional limits are randomly arranged
- 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)
- Spp. may have similar distributional limits
because of similar physiological tolerance limits
- Clusters of distributional limits may be
attributed to the way the observer divided the
gradient
- Herbivores may stop at a certain point along the
gradient, and therefore create discontinuities
- 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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