Page 99 - Environment: The Science Behind the Stories

You, too, are engaged in a symbiotic mutualism. Your
digestive tract is filled with microbes that help you digest food
and carry out other bodily functions—microbes that you are
providing a place to live. Without these mutualistic microbes,
none of us would survive for long.
Not all mutualists live in close proximity. Pollina-
tion (Figure 4.8), an interaction vital to agriculture and our
food supply (p. 272), involves free-living organisms that
may encounter each other only once. Bees, birds, bats,
and other creatures transfer pollen (containing male sex
cells) from flower to flower, fertilizing ovaries (containing
female sex cells) that grow into fruits with seeds. Most
pollinating animals visit flowers for their nectar, a reward
the plant uses to entice them. The pollinators receive food,
and the plants are pollinated and reproduce. Various types
of bees pollinate 73% of our crops, one expert has esti-
mated—from soybeans to potatoes to tomatoes to beans to
cabbage to oranges.
Figure 4.8 In mutualism, organisms of different species
benefit one another. Hummingbirds visit flowers to gather nectar,
and in the process they transfer pollen between flowers, helping
the plant to reproduce. Ecological Communities

these defenses, and the plant and the animal may embark on A community is an assemblage of populations of organ-
an evolutionary arms race. isms living in the same area at the same time (as we saw in
Some plants recruit certain animals as allies to assist Figure 3.11, p. 78). Members of a community interact with
in their defense. Such plants may encourage ants to take one another in the ways discussed above, and these species
up residence by providing swelled stems for the ants to interactions have indirect effects that ripple outward to affect
nest in, or nectar-bearing structures for them to feed from. other community members. The strength of interactions also
In return, the ants protect the plant by attacking insects varies, and together species’ interactions help determine the
that land or crawl on it. Other plants respond to herbivory structure, function, and species composition of communities.
by releasing volatile chemicals when they are bitten or Community ecology (p. 78) is the scientific study of species
pierced. The airborne chemicals attract predatory insects interactions and the dynamics of communities. Community
that may attack the herbivore. Such cooperative strate- ecologists study which species coexist, how they interact,
gies—essentially trading food for protection—are exam- how communities change through time, and why these pat-
ples of mutualism. terns occur.

Mutualists help one another
Energy passes among trophic levels
Unlike exploitative interactions, mutualism is a relationship
in which two or more species benefit from interacting with Some of the most important interactions among community
one another. Generally each partner provides some resource members involve who eats whom. As organisms feed on one
or service that the other needs. another, matter and energy move through the community from
Many mutualistic relationships—like many parasitic one trophic level, or rank in the feeding hierarchy, to another
relationships—occur between organisms that live in close (Figure 4.9).
physical contact. Physically close association is called
symbiosis, and symbiosis can be either mutualistic or para- Producers Producers, or autotrophs (“self-feeders”),
sitic. (Indeed, biologists hypothesize that many mutualis- comprise the first trophic level. Terrestrial green plants,
tic associations evolved from parasitic ones.) Thousands cyanobacteria, and algae capture solar energy and use pho-
of terrestrial plant species depend on mutualisms with tosynthesis to produce sugars (p. 50). The chemosynthetic
fungi; plant roots and some fungi together form symbiotic bacteria of hot springs and deep-sea hydrothermal vents
associations called mycorrhizae. In these relationships, the use geothermal energy in a similar way to produce food
plant provides energy and protection to the fungus while (p. 51).
the fungus helps the plant absorb nutrients from the soil.
In the ocean, coral polyps, the tiny animals that build coral Consumers Organisms that consume producers are
reefs (p. 449), share beneficial arrangements with algae known as primary consumers and comprise the second
known as zooxanthellae (p. 449). The coral provide hous- trophic level. Herbivorous grazing animals, such as deer
ing and nutrients for the algae in exchange for a steady and grasshoppers, are primary consumers. The third trophic
supply of food that the algae produce through photosyn- level consists of secondary consumers, which prey on pri-
98 thesis (p. 50). mary consumers. Wolves that prey on deer are considered

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