Page 656 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

620 part IV Soils, ecosystems, and Biomes
▲Figure 19.11 Black-legged Kittiwake in nesting habitat, northern coast of Iceland. [Bobbé Christopherson.]
forest habitats, especially in deciduous forests (those that drop their leaves in winter). Like other nuthatches, this species occupies a particular ecological niche by foraging for insects up and down tree trunks, probing into the bark with their sharp bills and often turning upside down and sideways as they move (Figure 19.12). This behaviour enables them to find and extract insects that are overlooked by other birds. They also jam nuts and acorns into the bark, and then bang on them with their bill to extract the seeds. Although nuthatches and woodpeckers occupy a similar habitat, the nuthatch’s distinctive foraging behaviour causes it to occupy a spe- cific niche that is different from a woodpecker’s.
The competitive exclusion principle states that no two species can occupy the same niche (using the same food or space) because one species will al- ways outcompete the other. Thus, closely related spe- cies are spatially separated either by distance or by species-specific strategies. In other words, each species
operates to reduce competition and to maximize its own reproduction rate—because, literally, species sur- vival depends on successful reproduction. This strat- egy, in turn, leads to greater diversity as species shift and adapt to fill different niches.
species Interactions
Within communities, some species are symbiotic—that is, have some type of overlapping relationship. One type of symbiosis, mutualism, occurs when each organism ben- efits and is sustained over an extended period by the rela- tionship. For example, lichen (pronounced “liken”) is made up of algae and fungi living together (Figure 19.2c). The alga is the producer and food source for the fungus, and the fungus provides structure and physical support. Their mutualism allows the two to occupy a niche in which nei- ther could survive alone. Lichen developed from an earlier parasitic relationship in which the fungi broke into the algal cells. Today, the two organisms have evolved into a supportive and harmonious symbiotic relationship. The partnership of corals and algae discussed in Chapter 16 is another example of mutualism in a symbiotic relationship (Figure 19.2d).
Another form of symbiosis is parasitism, in which one species benefits and another is harmed by the associ- ation. Often this association involves a parasite living off a host organism, such as a flea living on a dog. A parasitic relationship may eventually kill the host—an example is parasitic mistletoe (Phoradendron), which lives on and can kill various kinds of trees (Figure 19.13).
▲Figure 19.12 White-breasted Nuthatch in its ecological niche. [Cally/alamy.]
▲Figure 19.13 Dwarf mistletoe growing on a Douglas fir in the Rocky Mountains, Colorado. Dwarf mistletoes (Arceuthobium) are parasitic plants found on conifers throughout western north america. The host trees suffer reductions in growth and seed production, and are prone to infectious diseases. at least five species occur in the region. [USFS.]