Interactions Among Living Things

Because all living things within an ecosystem interact with each other and with nonliving things in their environment, ecosystems are complex and always changing. Some scientists who study ecosystems focus on specific relationships between organisms. A well-studied example is the relationship between the Canadian lynx (a species of wildcat) and the snowshoe hare. Two hundred years ago, Canadian trappers observed changes in population between the two animals. As the population of lynx rose, the population of hares fell. The lynx population then fell and hare populations rose. For a long time, scientists believed that the number of lynx determined the fluctuating numbers of hares. But today, scientists know that the density of hares changes regardless of the lynx population. The question is why.

Scientists study other kinds of interactions between animals, too. You may be familiar with the word symbiosis. The prefix sym- means “together,” and the base word bias means “life.” Symbiosis, then, means “living together.” But all organisms within an environment could be said to be living together. The term symbiosis is used in science to describe three special relationships.

Mutual Symbiosis

You may have watched bees buzzing from flower to flower in a summer garden. The bees hover over flowers, extending their long tongues into the heart of flowers to soak up nectar to take home to feed the hive. As bees gather food, pollen sticks to their hairy legs. The tiny grains of pollen contain cells that are necessary for a plant to reproduce.

As bees move from flower to flower, the pollen attached to their bodies travels, too. Pollen rubs off of the bee’s body and falls into flowers. Inside the flowers, changes in the pollen play a special part in the plant’s reproductive process. Thus two things happen. First, flowers feed bees, and bees help flowering plants reproduce. The relationship between the two organisms is mutually;;beneficial-the relationship benefits both species. This is an example of mutual symbiosis, or mutualism. A relationship that is mutual is one that is directed toward and received by each other equally.

There are many examples of mutualism in the natural world. Some include organisms too small for us to see.

Termites and Bacteria

The cell walls of plants are stiff and strong. This strength comes from the fibers of cellulose that form the cell walls. Cellulose is a carbohydrate that most animals-even animals that eat only plants-cannot digest. That’s because their bodies don’t produce the enzyme to do the job. However, some plant-eating organisms, like termites, have a mutualistic relationship with microscopic organisms that can do the job for them.

Termites are the host for microscopic bacteria and protists. The host is the organism on which another organism lives or feeds on. Most protists are single-celled, but there are multicellular protists also. These organisms, which are like plants, animals, and fungi in some ways but not in others, are independent organisms that interact with their environments as other living things do.

At first, scientists thought that both bacteria and protists helped termites digest cellulose. Now they know that the protists themselves have a mutualistic relationship with the bacteria. Like the termite, protists can’t digest cellulose without assistance. They benefit from their relationship with bacteria but seem to have no effect on termites.

Acacia Trees and Ants


Some plants produce chemical defenses, or substances that hungry insects and mammals find distasteful. Other plants, like the acacia tree, do not. Although the trees form nasty thorns, the thorns alone aren’t enough to deter hungry pests. The trees attract animals that strip leaves from their branches, slowing tree growth. Slower growth gives a competitive advantage to other vegetation, which can grow faster and taller, blocking sunlight for the acacias.

Acacias, however, have a mutually beneficial relationship with ants that serve as effective defenders. These ants live inside the thorns that grow at the base of a tree’s leaves. The thorns provide a home for ants. The ants bite animals that attempt to eat a tree’s leaves. Their bites are annoying enough that even elephants avoid acacias that serve as hosts to ants.

Ox-peckers, Rhinos, and Zebras


Imagine having a personal pest-control service. An ox-pecker is an African bird that feeds on ticks and other parasites, or harmful organisms. It lands on rhinos and zebras, where it feasts on any parasites it finds. This pest-control service is one benefit for rhinos and zebras, but there is another. When it senses danger, an ox-pecker screams a warning as it flies upward.

Humans and E. coli

Your body has its own mutualistic relationship with bacteria called Escherichia coli, or E. coli, which live in your digestive system. As a host, you provide these microbes with protection, nutrients, and a means of moving. In return, the bacteria help you digest food and absorb vitamins. They make the walls of your small intestines slightly acidic, which slows or prevents harmful bacteria from colonizing there. E. coli also stimulate your immune system to produce antibodies. These are proteins that kill harmful microbes or prevent them from colonizing.

Parasitic Symbiosis

The word predator may lead you to think of animals you’ve seen on television sprinting across African savanna’s to capture helpless prey. But not all predators are so large or even able to run. Some are microscopic parasites that depend on other organisms for their survival in a kind symbiotic relationship called parasitism. These parasites meet their nutritional requirements through their hosts. However, in parasitism, what benefits the parasite harms the host.

There are two kinds of parasites. Endoparasites live inside an organism’s body. Ectoparasites live on the host’s skin. There is also a third kind of parasitic relationship, in which an insect lays its eggs on a living host. The eggs hatch, and the young parasites devour the host. Consider the following examples of parasitic symbiosis.

The Tick

Ticks are not insects but arachnids, tiny invertebrates related to spiders. They have biting mouth parts that help them attach firmly to a host’s skin, sucking blood. A tick may feed for several days. In that time, a variety of pathogens, or disease-causing organisms, can pass from the tick into the host’s bloodstream.

Ticks cannot run, fly, or jump, but they can crawl. As animals walk through grasses or near shrubs, ticks waiting on the tips of the plants crawl from the plants to the unsuspecting hosts. The American dog tick, also called a wood tick, feeds on humans and a variety of medium-to-large mammals.

The Tapeworm

There are many different kinds of tapeworms that infect both vertebrate and invertebrate species-animals with and without a spinal column. Among vertebrates, including humans and domestic animals, these endoparasites infect the liver and digestive tract.

A tapeworm has no mouth, but its head has suckers and sometimes hooks, which it uses to attach to the host. Food is absorbed through the worm’s outer covering.

The worm, which is self-reproducing, can produce as many as 40,000 unhatched young, or embryos. In a human, these embryos leave the body through waste, or feces . If the infected waste reaches water supplies, other organisms, including humans, ingest the embryos. Once in an organism’s digestive tract, the embryos transform into larvae. The larvae bore through the host’s intestinal wall and enter a blood vessel. The blood carries it to muscle tissue, where the larvae form a cyst, or protective outer covering. If that cyst is eaten, the larvae attach to the new host’s intestine and quickly develop into adults. Tapeworm cysts sometimes find their way into the digestive tracts of humans and animals by way of infected or poorly cooked meat.

Not all people infected by tapeworm cysts show symptoms of disease. Some, however, experience many of the same symptoms associated with flu, including vomiting, headache, and weakness. About 70 percent of people with the disease also develop seizures, which are temporary electrochemical changes in the brain.

Commensal Symbiosis

Commensal symbiosis, better known as commensalism, occurs when one species benefits from a host, but the host is unaffected. In other words, the host is neither helped nor harmed by the relationship. Here are some examples of commensalism.

The Pseudo-scorpion and Beetles


The pseudo-scorpion (soo-doh-SKOR-pe-uhn) is a small eight-legged invertebrate, like its distant relative, the scorpion. But unlike a scorpion, the pseudo-scorpion has no tail. To move from place to place, this tiny animal attaches itself to the underside of a beetle’s wings. When the beetle flies, the pseudo-scorpion hitches a ride to new territory.

Cattle Egrets and Livestock

Cattle egrets are small white birds that follow grazing cattle, horses, and even farm tractors. As livestock or machinery move through grassy fields, they disturb insects. Cattle egrets, which move among the feet of livestock or perch upon their backs, catch these insects. The egret gets food, but the livestock receive no benefit.

Sharks and Remoras

A remora is also called a sucker-fish. It has a flat appendage, or external structure, on the top of its head. The appendage works somewhat like a vacuum, allowing the remora to attach itself snugly to the underside of a swiftly moving shark. Although a remora is capable of swimming, hitching a ride with a shark or other fish larger than itself has benefits. The remora uses its tiny, sharp teeth to eat whatever food debris falls from its host’s mouth without causing any harm or benefit to the host.