Visualizing Environmental Science

120 CHAPTER 5 How Ecosystems Work

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a. Adult meerkats stand guard at their burrow. If one of the

sentries spies a predator such as an eagle or a hawk, it will alert

the other meerkats, and all will scramble into their burrows.

Photographed in the Kalahari Desert, South Africa.

b. When at rest among forest leaves, the Indian leaf butterfly is

difficult for a predator to spot. The species is found in tropical Asia.

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school of anchovies, or flock of pigeons. This social be-

havior decreases the likelihood of a predator catching

an individual unaware because the group has so many

eyes, ears, and noses watching, listening, and smelling for

predators (Figure 5.19a).

Chemical defenses are common among animal

prey. The South American poison arrow frog has poi-

son glands in its skin and bright warning colors that

experienced predators avoid. Some animals blend into

their surroundings and so hide from predators. Certain

insects resemble twigs, tree trunks, or leaves so closely

you would not guess that they are animals until

they move (Figure 5.19b).

Plants possess adaptations that protect

them from being eaten. The presence of spines,

thorns, tough leathery leaves, or even thick wax

on leaves discourages foraging herbivores from

grazing. Other plants produce an array of protec-

tive chemicals that are unpalatable or even toxic

to herbivores. The nicotine found in tobacco is so

effective at killing insects that it is an ingredient

in many commercial insecticides.

Competition

Competition occurs when two or

more individuals attempt to use

an essential common resource

such as food, water, shelter, living

space, or sunlight. Resources are

often in limited supply in the en-

vironment, and their use by one individual decreases the

amount available to others. If a tree in a dense forest grows

taller than surrounding trees, it absorbs more of the in-

coming sunlight. Less sunlight is available for nearby trees

that the taller tree shades. Competition occurs among in-

dividuals within a population (intraspecific competition) and

between different species (interspecific competition).

Competition isn’t always a straightforward, direct

i nteraction. Consider a variety of flowering plants that

live in a young pine forest and compete with conifers for

such resources as soil moisture and soil nutrient minerals.

Their relationship is more involved than simple competi-

tion. The flowers produce nectar that some insect species

consume; these insects also prey on needle-eating insects,

reducing the number of insects feeding on pines. It is

therefore difficult to assess the overall effect of flower-

ing plants on pines. If the flowering plants were removed

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How might behaviors that animals use to avoid

predators be the same around the world?

What predator avoidance behaviors might

animals in your environment show that are

different from those in animals in the African

desert?

GLOBAL

LOCAL

competition The

interaction among

organisms that vie for

the same resources

in an ecosystem

(such as food or living

space).

Many potential animal prey, such as woodchucks, run

to their underground burrows to escape predators. Oth-

ers have mechanical defenses, such as the barbed quills of

a porcupine and the shell of a pond turtle. Some animals

live in groups—a herd of antelope, colony of honeybees,