Types of Interactions

Most living things produce more offspring than will survive.A female frog, for example, might lay hundreds of eggs in asmall pond. In a few months, the population of frogs in thatpond will be about the same as it was the year before. Whywon’t the pond become overrun with frogs? An organism,such as a frog, interacts with biotic and abiotic factors in itsenvironment that can control the size of its population

Types of ecological interactions

competition

predationsymbiosis

parasitism

mutualism

commensalism

Limiting Factors Populations cannot grow without stopping, because the

environment contains a limited amount of food, water, living space, and other resources. A resource that is so scarce that it limits the size of a population is called a limiting factor. For example, food becomes a limiting factor when a population becomes too large for the amount of food available. Any single resource can be a limiting factor to a population’s size.

In Japan, green herons make interesting use of the biotic and abiotic parts o ftheir environment. They will drop sticks and even bread crumbs into the water to attract fish. Sometimes, they wil lcatch a fish within 2 or 3 seconds of dropping the bait

Keystone Species: Although all members of an ecosystem have importantroles, some members are more important than others to the overall integrity of theecosystem. Such species are called keystone species. In the same way thatremoving an arch’s keystone causes the arch to collapse, removing a keystonespecies can cause the whole ecosystem to collapse. This occurred when the seaotter was hunted to near extinction in the kelp forests of the Pacific Ocean.With the otters gone, nothing was left to eat the sea urchins, which multipliedand ate all of the kelp. The kelp was home to dozens of animals, all of whichdisappeared, as did most of the sea urchins. When the otter was reintroduced,populations of all of these organisms returned!

Carrying capacity

The largest population that an environment can support is known as the carrying capacity. When a population grows larger than its carrying capacity, limiting factors in the environment cause individuals to die off or leave. As individuals die or leave, the population decreases. For example, after a rainy season, plants may produce a large crop of leaves and seeds. This large amount of food may cause an herbivore population to grow. If the next year has less rainfall, there won’t be enough food to support the large herbivore population. In this way, a population may become larger than the carrying capacity, but only for a little while. A limiting factor will cause the population to die back. The population will return to a size that the environment can

Competition – two species share a requirement for alimited resource reduces fitness of one or both species

When two or more individuals or populations try to use the same resource, such as food, water, shelter, space, or sunlight, it is called competition. Because resources are in limited supply in the environment, their use by oneindividual or population decreases the amount availableto other organisms.Competition happens between individuals within apopulation. The elks in Yellowstone National Park areherbivores that compete with each other for the samefood plants in the park. This competition is a big problemin winter when many plants die.Competition also happens between populations. Thedifferent species of trees in at the right are competingWith each other for sunlight and space.Some of the trees in thisforest grow tall to reach sunlight, whichreduces the amount of sunlight availableto shorter trees nearby.

Predation – one species feeds on another enhancesfitness of predator but reduces fitness of prey

herbivory is a form ofpredation

Predators and Prey

• Predator Adaptations

To survive, predators must be able to catch their prey. Predators

have a wide variety of methods and abilities for doing so.

The cheetah, for example, is able to run very quickly to catch

its prey. The cheetah’s speed gives it an advantage over other

predators competing for the same prey.

Other predators, such as the goldenrod spider, shown at the

right, ambush their prey. The goldenrod spider blends in

so well with the goldenrod flower that all it has to do is wait

for its next insect meal to arrive.

Predators and Prey• Prey Adaptations

Prey have their own methods and abilities to keep from being

eaten. Prey are able to run away, stay in groups, or camouflage

themselves. Some prey are poisonous. They may advertise their

poison with bright colors to warn predators to stay away. The

fire salamander sprays a poison that burns.

Predators quickly learn to recognize its warning coloration.

Many animals run away from predators. Prairie dogs run

to their underground burrows when a predator approaches.

Many small fishes, such as anchovies, swim in groups called

schools. Antelopes and buffaloes stay in herds. All the eyes,

ears, and noses of the individuals in the group are watching,

listening, and smelling for predators. This behavior increases

1. Chemical Defense

There are two main ways animals can use chemicals to defend themselves.

Animals can synthesize toxin using their own metabolic processes, or they can accumulate toxin from the food they eat.

1. Chemical DefenseMany animals accumulate toxin from their food rather than synthesizing it from scratch.

For example, the larvae of Monarch butterflies accumulate toxins from the plants they inhabit.  Birds that eat the Monarchs vomit and learn to avoid them in the future. Their bright coloration allows birds to remember and avoid them. 

Photo courtesy of T. W. Davies, Cal. Acad. of Sciences.

1. Chemical Defense• Animals which

synthesize their own toxin are able to convert chemical compounds in their body to a poison.

► There are many amphibians that produce skin toxins. The skin toxins are produced by special poison glands, usually located on the animal's back or throughout the skin.

The poison dart frog has poison glands scattered all over its body.

Photo courtesy of Dr. John Daly

2. Camouflage

Animals that camouflage themselves pretend to be something they are not. Either their coloration, marking patterns, or entire body resembles something else in their environment, here a leaf, an owl.

2. Camouflage

Here an aptly named walking stick pretends to be a twig, in an attempt to avoid being seen by a bird or other predator. This is an example of cryptic coloration.

Photo courtesy of Dr. Lloyd Glenn Ingles, Cal. Acad. of Sciences.

2. Camouflage

In this picture, a four-eyed butterfly fish uses deceptive markings. The large spot near the tail resembles an eye. When predators attack the wrong end, the butterfly fish can swim away in the other direction!

Symbiosis – two species live together can includeparasitism, mutualism, and commensalism

Parasitism – one species feeds on another Benefits one species but hurts the other

Mutualism – two species provide resources or servicesto each other BOTH benefit

Commensalism – one species receives a benefit fromanother species Benefits one species does nothing for the other.

Exploring Relationships: In India, the chital, a small deer, has trouble finding enough grass to eat during the dry season. This deer relies on a certain type of monkey to get enough to eat.• Can you guess how the monkeysbenefit the deer?• Which species benefit fromthis relationship? These deer have keen eyesight, hearing, and sense of smell. They therefore warn the monkeys of predators. Which species do you think benefit?

The honeyguide, a small African bird, lives up to its name. The purpose of its songis to lure nearby creatures to a nest of honeybees it has found. Many animals have learned to listen for this bird! Baboons, mongooses, ratels (or honey badgers),and even people will follow the bird in order to claim the honey. Out of harm’s way, the bird waits for the leftovers: bee larvae. The bird’s unique digestive system allows it to eat wax as well.

• Which animals in this storyare in a mutualistic relationship?

• Which animals are prey, andwhich are predators?

1. Acacia ants live on the bullhornacacia tree, which providesthe ants with food andshelter. The ants deter grazinganimals from eating the tree.What type of relationship is this?2. Plants called epiphytes, such ascertain orchids, live on otherplants, which provide only aplace to grow. Epiphytesabsorb sunlight, water, andnutrients from their surroundingsand make their ownfood.What type of relationship is this?3. There is a tiny wasp that laysits eggs in a variety of otherinsects, such as caterpillars,spiders, aphids, and flies. Thewasp larvae feed on the hostinsect and eventually kill it.What type of relationship is this?

Section Quiz: Match the correct word to the definition

a.preyb.symbiosisc.mutualismd.commensalisme.parasitismf.predator

1.a relationship between two organisms in which one benefits and the other is not affected2.an organism that eats all or part of another organism3.a relationship in which two different organisms live in close association with each other4.an organism that is killed and eaten by another organism 5.a relationship where one organism benefits and the other is harmed6. a relationship between two species in which both species benefit

Write the letter of the correct answer.7.The largest population an environment can support is itsa. carrying capacity. c. population.b. limiting factor. d. symbiosis.8. One type of competition involves individuals competing for resources. The other involves competition between differenta. organisms. c. environments.b. populations. d. relationships.9. Young wasps are eating the tomato hornworm that is their host. What is this an example of?a. commensalism c. parasitismb. mutualism d .competition10. A bird eats a worm. Who is the predator?a. the wormb. the birdc. both the bird and the wormd. neither the bird nor the worm