Page 98 - Environment: The Science Behind the Stories

Just as predators and prey evolve in response to one
another, so do parasites and hosts. Coevolution describes a
long-term reciprocal process in which two (or more) types
of organisms repeatedly respond by natural selection to the
other’s adaptations. This seesawing process can occur with
any type of species interaction. Hosts and parasites often
become locked in a duel of escalating adaptations, known
as an evolutionary arms race. Like rival nations racing to
stay ahead of one another in military technology, host and
parasite may repeatedly evolve new responses to the other’s
latest advance. In the long run, though, it may not be in a
parasite’s best interest to do its host too much harm. A para-
site might leave more offspring in the next generation—and
thus be favored by natural selection—if it allows its host to
live longer.

Herbivores exploit plants
Figure 4.6 A parasite benefits at the expense of its host. In herbivory, animals feed on the tissues of plants. Insects that
Parasitic sea lampreys gain nourishment by sucking blood from fish feed on plants are the most common type of herbivore; nearly
in the Great Lakes.
every plant in the world is attacked by insects (Figure 4.7).
Herbivory generally does not kill a plant outright but may
affect its growth and reproduction.
Parasites exploit living hosts Like animal prey, plants have evolved an impressive

arsenal of defenses against the animals that feed on them.
Organisms can exploit other organisms without killing them.
Parasitism is a relationship in which one organism, the Many plants produce chemicals that are toxic or distasteful to
parasite, depends on another, the host, for nourishment or herbivores. Others arm themselves with thorns, spines, or irri-
some other benefit while doing the host harm. Unlike pre- tating hairs. In response, herbivores evolve ways to overcome
dation, parasitism usually does not result in an organism’s
immediate death.
Many types of parasites live inside their hosts. For exam-
ple, tapeworms live in their hosts’ digestive tracts, robbing
them of nutrition. Other types of parasites are free-living
and come into contact with their hosts infrequently. Cuckoos
of Eurasia and cowbirds of the Americas lay their eggs in
other birds’ nests and let the host species raise the parasite’s
young. Still other parasites live on the exterior of their hosts,
such as fleas that bite their hosts, or ticks that attach them-
selves to the skin. The sea lamprey is a tube-shaped verte-
brate that grasps the bodies of fish with a suction-cup mouth
and a rasping tongue, sucking their blood for days or weeks
(Figure 4.6). Sea lampreys invaded the Great Lakes from the
Atlantic Ocean after people dug canals to connect the lakes
for shipping, and the lampreys soon devastated economi- CHAPTER 4 • S PEC i ES i n TERA CT i on S A nd Co mmuni T y E C ology
cally important fisheries of chubs, lake herring, whitefish,
and lake trout. Fisheries managers have since reduced lam-
prey populations by applying chemicals that selectively kill
lamprey larvae.
Many insects parasitize other insects, killing them in the
process, and are called parasitoids. Parasitoid wasps lay eggs
on caterpillars, and when the eggs hatch, the wasp larvae bur-
row into the caterpillar’s tissues and slowly consume them.
The wasp larvae metamorphose into adults and fly from the
body of the dying caterpillar.
Parasites that cause disease in their hosts are called
pathogens. Common human pathogens include the protists
that cause malaria and amoebic dysentery, the bacteria that Figure 4.7 In herbivory, animals feed on plants. This larva
cause pneumonia and tuberculosis, and the viruses that cause (caterpillar) of the death’s head hawk moth is feeding on leaves in
hepatitis and AIDS. Europe. 97

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