Chapter 27: Biotic Interactions
Interactions between individual organisms
Organisms interact with each other and these interactions can have significant consequences to the participants. Most students are familiar with the classification scheme below which organizes interactions into types based on the consequences of the interaction on the two participants:
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effect on the ‘larger’ organism |
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(-) negative | (0) neutral | (+) positive | |
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effect on the ‘smaller’ organism |
(-) negative | competition | amensalism | predation, herbivory |
| (0) neutral | amensalism | ‘neutralism’ | commensalism | |
| (+) positive | parasitism | commensalism | mutualism |
There are multiple problems with this scheme and the definitions that stem from it. Larger vs. smaller is sometimes an arbitrary distinction. It is not clear what level, organism or population, it is focused on or how effects might be measured. At the level of individuals ‘ positive’ might be monitored by organism size, growth rate, longevity or reproductive success. But at the level of populations one might monitor population density or population growth rate. Sometimes what is ‘positive’ and what is ‘negative’ may not be obvious. A bird eating a poisonous butterfly is negative to both the individual butterfly and the bird, but at the population level one could argue that it is good for both the bird and butterfly populations. Some fungi living in plants (endophytes) substantially increase the growth of plants that they infect (positive effect), yet at the same time reduce or eliminate the likelihood of producing offspring (negative effect). Nitrogen-fixing bacteria infecting roots may benefit plants (faster growth, bigger plants) if soil nitrogen is low, but harm plants (slower growth, smaller plants) if nitrogen levels are high. Pollinator visits may benefit plants if they transport pollen to other members of the same species but not if their next visits are to different flower species.
Perhaps the most significant biological context for the terms would be evolutionary (positive = enhanced reproductive success) but this may depend on circumstances that are difficult to evaluate. Predators (Fig. 1) would generally be thought to have a negative on prey populations, but in a number of situations, predators are thought to ‘benefit’ prey populations by preventing overpopulation. Seed predation (e.g., Clark’s nutcracker eating pine seeds, Fig. 2)) is clearly harmful to the individual pine organisms (i.e., the embryonic pine individual present in a seed) but apparently benefits the pine populations by allowing for dispersal. In short, the terms defined in Table 1 are not always useful.
Another way to organize biotic interactions is not based upon arbitrary considerations of what ‘benefits’ or ‘harms’ the organisms/populations involved, instead, it is based upon the medium through which the interaction occurs:
- trophic interactions—one organism eats another or part of another, obtaining mat