1 INTERACTIONS IN ECOSYSTEMS: Video & READINGS: Watch the BrainPop video on Ecosystems The two readings below cover many of the same terms and concepts. Read them when you have time. Pay particular attention to terms that are new, or unfamiliar. These readings are a good introduction to ecology and the next BIG section we will cover in class. Endless Interactions You have just finished a bowl of chicken soup for lunch. As you clean up, you wash out the bowl and throw away the can. A garbage truck will haul the can to the dump where it will lie among other trash. A female fly might lay her eggs in the can, and her offspring might fly from the dump, feed on some decayed food, and then rest on top of someone s peanut butter sandwich. Each of these interactions is dependent on the one before it. Most of the time, these interactions go unnoticed. All populations, including human populations, interact with one another in a complex web of relationships. The set of interacting populations present at one time in one place is called a community. In your community there may be dogs, cats, trees, weeds, and humans that interact. When you mow your lawn or your dog bites the mail carrier, for example, the interaction is very direct. Much of the time, however, interactions are indirect. One type of direct interaction is a predator/prey interaction. In this direct interaction, one type of organism (the predator) eats the other (the prey). The predator benefits from the relationship, but the individual prey does not. An animal can spend its life as a predator and then abruptly become the prey. A snake, for example, may prey on ground squirrels and then become prey for a hawk. The snake depends on a large population of ground squirrels. The hawk, then, depends directly on the snake and, in this case, indirectly on the quantity of ground squirrels. The hawk, however, also may prey directly on the ground squirrels. The limiting factor in this case is the food supply.
2 Although the individual ground squirrel does not benefit, the population of ground squirrels may benefit by being held in check by the snake population. There also might be a selective advantage to the ground squirrel population because the snakes would tend to prey on the slower, weaker squirrels. Consequently, the bestadapted ground squirrels would be more likely to escape becoming prey and thus survive to reproduce. Competition is another type of direct interaction. This type of interaction between organisms benefits neither one. Organisms compete for limiting factors, such as space, food, sunlight, nutrients, and water. Competition among organisms may increase as a particular resource becomes scarce. When tadpoles live in densely stocked areas, competition for available space and food increases. As a result, these tadpoles remain tadpoles longer, they suffer higher death rates, and they develop into smaller frogs. In communities of wolves, competition leads to the establishment of social hierarchies. A high position in the pack gives the individual an advantage in terms of obtaining food, mates, shelter, and other resources. Individuals of any species that compete successfully can survive to reproduce and pass on their genetic material to future generations. In this way, populations adapt through time to changes in the environment. Competition also can occur between different species. In the species needs are similar and resources are scarce, then more competition will exist. In New Guinea four species of pigeons rely on a particular fruit tree for their primary food source. The four types of pigeons differ in size, and the smaller pigeons have adapted to feed on the fruit of the lower branches. An experiment involving paramecium demonstrates another example of this form of adaptation. Researchers placed two species of paramecium in the same test tube, which was filled with liquid food. The researchers formulated a hypothesis that one of the species would die out as a result of competition. In fact, both species survived and thrived. One species fed on the food that settled at the bottom of the tube, and the other fed on the food suspended in the liquid. The two species of paramecium, as well as the four species of pigeons, occupied separate niches. The niche of an organism refers to its role in the community: what it eats, what organisms eat it, where it lives, and what indirect relationships it has with other organisms. As a result of occupying different niches, the two species were able to
3 feed and reproduce without interfering with each other even though they lived in the same place, or habitat. Competition is not the only relationship that exists among populations of different species that live in the same habitat. When different species of organisms live in direct, physical contact with one another, the relationship is called symbiosis. For example, certain species of fungi live on the roots of many plants. The fungi also absorb nutrients from the soil and secrete an acid that makes the nutrients available to the plant. At the same time, the fungi are nourished by photosynthetic products from the plant. The fungi also absorb water and protect the plant against various pathogens in the soil. This type of symbiosis that benefits both organisms is called mutualism. Another example of mutualism is the interaction between a certain type of bacteria and the cow in whose intestines the bacteria live. The cow benefits because the bacteria digest the cellulose in the plants that the cow eats, and the bacteria benefit because the cow provides a steady supply of food. A common form of symbiosis is parasitism. In parasitism one organism (the parasite) lives on or in another organism (the host) and uses it as a food source. The host usually remains alive during the interaction, although parasites may weaken a host to the extent that it becomes susceptible to disease or becomes an easier prey for predators. A leech, for example, clings to a turtle s skin and sucks its blood. Other parasitic microorganisms in the turtle absorb food directly from its blood. Interactions such as these eventually may weaken the turtle. In humans, tapeworms absorb food directly from the intestines, where they live. Plants also may have parasites and large microorganisms may have smaller parasitic microorganisms in them. Parasites may be molds, microorganisms, and even other plants such as the mistletoe. All viruses are parasitic because they require a host to reproduce and acquire energy. Theses examples illustrate a few of the many different types of interactions that occur among organisms. Some of these interactions, such as mutualistic relationships, are beneficial for both organisms. Other interactions are not as positive and result in some degree of harm to one member of the interacting pair, as in parasitic relationships. Each of these interactions is influenced by limiting factors such as light, temperature, food supply, and space. Because of these interactions, there is a web of interdependence among living things.
4 Life in a Community All populations, including human populations, interact with one another. These interactions form a complex web of relationships. The set of interacting populations present during one time in one place is called a community. In your community, there may be dogs, cats, oak trees, dandelions, and humans that interact. When you mow your lawn or your dog bites the mailman, for example, the interaction is very direct. Most of the time, however, the interactions are indirect, and people tend to ignore them. Suppose, for example, that you had vegetable soup for lunch and threw away the can. A garbage truck might haul the can to the dump, where it would fill with rainwater. A female mosquito might lay her eggs in the water, and her offspring, once mature, might fly from the dump and bite a person or another animal. Perhaps neither would have been bitten if you had not eaten the soup, but since you did, you have had in indirect relationship with the mosquitoes and their victims. Every community is affected by the abiotic, or physical, environment: sunlight, soil, wind, rain, and temperature. Together, a community and its physical environment make up an ecosystem. Both the terms community and ecosystem can describe the same group of organisms. However, the term ecosystem refers to the community and its abiotic environment. Although the biosphere is too big to study as a whole, it is possible to study a part of it an ecosystem. In Investigation 3.1, you can study two of the abiotic factors in an ecosystem that affect organisms. How do the living components of a particular ecosystem interact? As you read the example below, think of all the factors, biotic and abiotic, that interact to define this ecosystem. An aquatic community lives in the rivers along the west coast of Florida. Among the largest individuals in this community are the river turtles. The adult turtles in the community eat many of the plants that grow in the rivers, including the long, narrow blades of tape grass, which is their favorite food. Consumers that eat only plants are called herbivores. Unlike the adult turtles, young river turtles are carnivores, consumers that eat other animals. The young turtles feast on snails, aquatic insects, and worms. Consumers that eat both plants and animals are omnivores. Because the adult turtles are herbivores and the young turtles are carnivores in this community, they have different roles in the food web.
5 Many other carnivorous animals live in the community. These animals eat turtle eggs and young river turtles. The eggs have the highest mortality rate. Female turtles lay their eggs in holes they dig on land, but they do not guard their eggs. The eggs may be dug up and eaten by skunks, raccoons, or snakes. Unhatched turtles also may be killed by molds that live in the soil and grow through the thin shells into the eggs. If the eggs survive, the young turtles that hatch may be eaten by snakes or raccoons before they reach the river. Even if the young turtles reach the river, they are vulnerable to carnivores. Large fish, alligators, herons, and snapping turtles may eat them. For the young turtles that have survived the trip to the river, mats of floating plants, tangled tree roots, and sunken logs offer some protection from carnivores. When the young turtles have grown large, few organisms can kill them directly. Leeches may become attached to the turtles and suck their blood, but that usually does not kill the turtles. Turtles do die of disease, accidents, and old age. After they die, their bodies become food for the decomposers that return all the matter in the turtle s body to the abiotic world. Each of the relationships described so far is a direct one between river turtles and other types of organisms. Like all organisms, however, river turtles have many indirect relationships with other organisms. For example, plants provide food and hiding places for the turtles and oxygen for the fish that might eat young turtles. Snails eat tape grass and therefore compete with the adult river turtles for the same food supply. Another type of turtle, the musk turtle, eats nothing but snails. With more tape grass, the river turtle population can increase. Thus musk turtles reduce the number of snails that eat tape grass, musk turtles increase the supply of tape grass and thus have and indirect effect on the river turtle population. Because many of the carnivores that eat young river turtles also eat musk turtles, the more musk turtles there are in the river, the less likely it is that young river turtles will be eaten. Adult river turtles and young river turtles have different functions in the community- different niches. The niche of an organism is its role in the community: what it eats, what organisms eat it, and what interactions it has with other living things and with its environment. Where the organism lives is its habitat. The niche of a young river turtle is to eat snails, worms, and aquatic insects and to be a food source for certain fish and turtles.
6 No two types of organisms occupy exactly the same niche within a community at the same time. For example, there are many different types of turtles living in a Florida river. They live in the same habitat, but their niches are different because they have different interactions with other members of the community. Understanding a community requires understanding all the interactions among all the organisms in all their niches. That is not an easy task, even for a small community. There are many other types of relationships that help form the community s web of life. These interactions can be beneficial or harmful. But all the interactions involve the cycling of matter and the flow of energy. Each of these relationships involves at least two different organisms. In a predator-prey relationship, for example, one type of organism (the predator) eats the other (prey). In this interaction, one organism benefits from the relationship, but the other does not. Another type of interaction, competition, benefits neither organism. Consider the Florida river community. Both snails and river turtles eat tape grass. The tape grass that is eaten by a turtle is then not available to be eaten by a snail. In the same way, tape grass eaten by a snail cannot be eaten by a turtle. Therefore, the presence of one of these herbivores would be potentially harmful to the other if there were not enough tape grass to sustain them both. Organisms may compete for such things as food, space, sunlight, nutrients, or water. The competition is always for a limited resource. In the Florida river community, if both tape grass and other plants are scarce, competition for tape grass by herbivores is intense. If plants are abundant, competition for the tape grass is reduced. When different organisms live in direct, physical contact with one another, the relationship is called symbiosis. For example, lichens are leaf-like or crust-like structures that grow on rocks or the bark of trees. Although lichen appears to be a single organism, it is actually two different organisms that live in close association with one another. In the most common lichens, one partner is an alga, a microscopic producer that makes food by photosynthesis. The other partner is a fungus, a consumer that gets its food from the alga but in return provides moisture and nutrients for the alga. Their symbiotic relationship benefits both organisms and is called mutualism.
7 There are many symbiotic relationships. Another is called commensalism. This is where one partner is helped and the other is not affected, neither good or bad. Remoras and a shark are a good example. Using suction disks on the tops of their heads, the remora attach themselves to the shark. The shark is not harmed, and the remoras expend little energy because the shark pulls them around. In addition, the remora eats small bits of the shark s prey as they float by. A form of symbiosis in which one member benefits and one is harmed is called parasitism. In parasitism, one organism (the parasite) lives on or in another organism, using it as a food source. The food source (the host) usually remains alive during the interaction. A leech, for example, clings to a turtle s skin and sucks its blood. Plants also may have parasites. Some molds, microorganisms, and even other plants, such as mistletoe, are parasites. Large microorganisms may have smaller parasitic microorganisms in them. Unlike a predator, which kills its prey outright, a parasite may kill its host indirectly by weakening it. Once it is weakened, the host becomes more susceptible to disease or an easier prey for predators. Organisms may play different roles within their community at different times. For example, recall the difference in roles between young river turtles and adults. Or, an organism may play more than one role in a community. Snapping turtles prey on young river turtles, which make them predators, but they are also scavengers, organisms that eat the flesh of dead animals. They did not kill. You also play more than one role in your community. When you eat a salad, you act as an herbivore. When you eat both a salad and a hamburger, you act as an omnivore. Every community includes many different types of organisms, and an individual can have many types of relationships. Check your understanding by: 1. Giving examples of direct and an indirect interaction between 2 organisms. 2. Distinguishing between a community and an ecosystem. 3. Explaining how an organism s niche differs from its habitat.
8 4. Describing how symbiosis differs from predator-prey relationships. 5. Explaining ways predation and parasitism are alike and how they differ.