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NPP =
production of organic compounds from CO2, principally through the process of photosynthesis
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Commensalism—
individuals of one species benefit, while individuals of the other species do not benefit and are not harmed (+/0).
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Mutualism—
mutually beneficial interaction between individuals of two species (+/+).
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mycorrhizae,
symbiotic associations between plant roots and various types of fungi.
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Ectomycorrhizae—
the fungus grows between root cells and forms a mantle around the exterior of the root.
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Arbuscular mycorrhizae—
the fungus grows into the soil, extending some distance away from the root; and also penetrates into some of the plant root cells.
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Mammalian herbivores such as cattle and sheep depend on
bacteria and protists that live in their guts and help metabolize cellulose.
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Some positive interactions are highly species-specific, and obligate, meaning?
(not optional for either species).
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Habitat mutualisms—
one partner provides the other with shelter, a place to live, or favorable habitat.
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Trophic mutualisms—
a mutualist receives energy or nutrients from its partner.
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Service mutualisms—
interactions in which one partner performs an ecological service for the other.
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Trophic mutualisms
Example: Leaf-cutter ants and fungus.In mycorrhizae, the fungus gets energy in the form of carbohydrates and the plant gets help in taking up limiting nutrients, such as phosphorus.
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Service mutualisms
Ecological services include pollination, dispersal, and defense against herbivores, predators, or parasites.
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Sometimes the cost is clear in the form of
—a “reward” for a service. During flowering, milkweeds use up to 37% of the energy gain from photosynthesis to produce nectar that attracts insect pollinators.
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Concept 14.1: Positive interactions occur when neither species is harmed and the benefits of the interaction are
greater than the costs for at least one species.
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Concept 14.2: Each partner in a mutualism acts to serve
its own ecological and evolutionary interests.
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Concept 14.3: Positive interactions affect the distributions and abundances of organisms
as well as the composition of ecological communities.
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But when predators are few, the
tree-hoppers may get no benefit from the ants. The interaction may shift from
+/+ to +/0.
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