Tools for ecological investigations (continued)

Lecture graphics

COMPARATIVE STUDIES

Follow directly from descriptive studies, in that observational data are used to describe patterns, and resulting patterns are compared to infer differences in process

Value in comparative studies lies in the spatial and temporal scales which they can consider

Caution:

Since there is not an experiment, researcher is always forced to compare patterns and then invoke mechanisms

As w/ descriptive studies, it is tempting to collect data w/o first asking a question

EXPERIMENTAL STUDIES

An experiment requires you to specify a question and a means of answering the question in advance, so experimental studies tend to be better-designed than descriptive or comparative studies

e.g., a common hypothesis proposed for many studies of competition is that competition is structuring a community in a certain manner

a manipulation is then performed to test this hypothesis

dependent and independent variables must be specified

dependent: characteristic that measure the performance of indivs. or pop'ns

independent: abundance of neighbors; a negative relationship is predicted between abundance of neighbors and performance measures

Laboratory experiments

tell what could potentially happen in nature under specific sets of conditions which can not be produced in the field

advantage: researcher can manipulate a wide range of biotic and abiotic variables

disadvantage: limited scope, extreme unrealism; however convincingly phenomena are demonstrated in lab, can not extrapolate from controlled environment to the actual existence of a process in nature

Field experiments

unlike lab studies, there is a reference point: current performance of indivs. or pop'ns of interest [Fig. 4.6 Keddy 1989 p. 94]

Descriptive and comparative studies do not test for an ecological process--they test for a pattern and infer a mechanism

Experiments do not necessarily overcome this problem--they may eliminate the largest number of alternative hypotheses, but manipulations of, for example, 'interference', really test for density dependence

'Apparent' competition (resulting from indirect effects) has been proposed to explain density dependence; examples:

the removed species may have been a host for a pathogen which also damaged the remaining species

the removed species may have attracted a herbivore which also fed upon the remaining species

Parker and Root (1981 Ecology 62:1390-1392) showed that a herbaceous plant species was excluded by some habitats by a grasshopper assoc. w/ a common shrub; removal of the shrub --> incr. in herb, but w/o interaction between the 2 spp.

these examples indicate:

1. mechanisms of ecological interactions may be very complex

2. natural history can not be divorced from experiments

Because ecologists usually enjoy field work, it is tempting to rush into data collection w/o first taking the time to think

An appropriate strategy for conducting ecological research is presented by Keddy [Table 4.4 p. 97]

In the absence of generality, ecology is nothing more than natural history. How can we increase the generality of field experiments? Keddy (1989) proposes:

1. Demonstrate that the pattern is a general one

2. Use increased no. of spp.

   tradeoff may be whether to pose complex questions w/ a few spp. or simple ones w/ a lot of spp.

3. Provide a comparative context

   select spp. because they are representative (in some way) of a large group of spp. of interest

   groups can be defined after the experiment is conducted, too (e.g., gap-colonizers vs. shade-tolerant spp.)

   then, present results not based on species responses, but on responses of groups [Table 5.2 Keddy 1989 p. 108]

4. Use general experimental factors

5. Arrange experiments along gradients

   incorporates variation --> expanded generality

   variation is incorporated systematically, so you can tell how trt effects vary along the gradient

   e.g., Gurevitch (1986)