So far, we have considered only survival and reproductive rates of single species. Before we can answer the questions we have posed about populations, we also need to look at the ways in which populations of different species interact with one another (Table 54.2) These interactions fall into five general categories:
► A mutualism is an interaction in which both participants benefit (+/+ interaction).
► A commensalism is an interaction in which one participant benefits but the other is unaffected (+/0 interaction).
► An amensalism is an interaction in which one participant is harmed but the other is unaffected (0/- interaction).
► A predator-prey or parasite-host interaction is one in which one participant is harmed, but the other benefits (+/- interaction).
► If two organisms use the same resources and those resources are insufficient to supply their combined needs, the organisms are called competitors, and their interactions constitute competition (-/- interactions).
Mutualistic interactions exist between plants and microorganisms, protists and fungi, plants and insects, among animals, and among plants. Most plants have beneficial associations with soil-inhabiting fungi, called mycorrhizae, which enhance the plant's ability to extract minerals from the soil (see Figure 31.16). Some plants have mutualistic relationships with nitrogen-fixing bacteria of the genus Rhizo-bium (see Figure 37.7).
Animals have important mutualistic interactions with pro-tists, plants, and other animals. Corals and some tunicates gain most of their energy from photosynthetic protists living within their tissues. In exchange, they provide the protists with nutrients from the small animals they capture. Termites have nitrogen-fixing protists in their guts that help them digest the cellulose in the wood they eat. The termites provide the protists with a suitable environment in which to live and an abundant supply of cellulose.
54 2 Types of Ecological Interactions
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