116 CHAPTER 5 How Ecosystems Work
- Distinguish among mutualism, commensalism,
and parasitism. - Define predation and describe predator–prey
relationships. - Define competition and distinguish between
intraspecific and interspecific competition. - Discuss an example of a keystone species.
N
o organism exists independently of other
organisms. The producers, consumers, and
decomposers of an ecosystem interact with
one another in a variety of ways, and each
forms associations with other organisms. Three main
types of interactions occur among species in an eco-
system: symbiosis, predation, and competition.
Symbiosis
In symbiosis, one species usually lives in or on another
species. The partners in a symbiotic relationship may ben-
efit, be unaffected, or be harmed
by the relationship. Symbiosis is
the result of coevolution, the in-
terdependent evolution of two in-
teracting species. Flowering plants
and their animal pollinators are
an excellent example of coevolu-
tion (see Chapter 6 for more on
evolution). Bees, beetles, birds,
bats, and other animals transport pollen from one plant to
another. During the millions of years over which these as-
sociations developed, flowering plants evolved several ways
to attract animal pollinators. One of the rewards for the
pollinator is food— nectar (a sugary solution) and pollen.
Plants have a variety of ways to get the pollinator’s atten-
tion, most involving showy petals and scents.
As plants acquire specialized features to at-
tract pollinators, animals coevolve specialized
body parts and behaviors to aid pollination
and obtain nectar and pollen as a reward. Co-
evolution is responsible for the hairy bodies of
bumblebees, which catch and hold sticky pol-
len for transport from one flower to another.
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This Hawaiian honeycreeper uses its gracefully curved bill to sip
nectar from the long, tubular flowers of the lobelia.symbiosis An
intimate relationship
or association
between members of
two or more species;
includes mutualism,
commensalism, and
parasitism.Interactions Among Organisms
LEARNING OBJECTIVES
Coevolution is also responsible for the long, curved beaks
of certain Hawaiian birds that insert their beaks into tu-
bular flowers to obtain nectar (Figure 5.16).
The thousands, or even millions, of symbiotic
associations that result from coevolution fall into three
categories: mutualism, commensalism, and parasitism
(summarized in Figure 5.17a). One example of mutual-
ism, an association in which both organisms benefit, is
the interaction between acacia ants and the bull’s horn
acacia plant ( Figure 5.17b). The ants make hollow nests
out of thorns at the base of the plant’s leaves and gain
special nutrients from the leaf tips. In return, the ants
effectively protect the plant from invertebrate and verte-
brate herbivores and clear away competing plants. Both
ant and acacia depend on this association for survival.
Commensalism is a symbiotic relationship in which
one species benefits and the other is neither harmed nor
helped. One example of commensalism is the relation-
ship between a tropical tree and its epiphytes, smaller plants
such as mosses, orchids, and ferns that live attached to the
bark of the tree’s branches ( Figure 5.17c). An epiphyte
anchors itself to a tree but typically doesn’t obtain nutri-
ents or water directly from the tree. Its location enables
it to obtain adequate light, water (as rain dripping down
the branches), and required nutrient minerals (which rain
washes out of the tree’s leaves). The epiphyte benefits from
the association, whereas the tree is apparently unaffected.
Parasitism is a symbiotic relationship in which one
species (the parasite) benefits at the expense of the other
(the host). Parasitism is a successful lifestyle; more than 100
parasites live in or on the human species (Figure 5.17d).Photo Resource Hawaii/Danita Delimont