reflect There is a saying, No man is an island, which means that people need one another in order to survive. Everyone on Earth is interconnected in some way. This is not only true of human beings, but of all living and many nonliving things on Earth. In an ecosystem, individual organisms, populations, and entire communities interact with each other and with their environments. In fact, much of how living things behave is in response to changes in their environment. What are some of these environmental factors that cause change? How do living things respond to these changes? Ecosystems Earth is made up of several important parts, such as air, soil, and living creatures. These parts are called spheres, such as the atmosphere or the lithosphere. The sphere containing life is called the biosphere because bio means life. The biosphere contains all the living things on Earth in various ecosystems. Ecosystems are composed of all the biotic (living) and abiotic (nonliving) factors within a particular environment. The organisms in an ecosystem are organized into levels based on the number of individuals present and how they interact. The fi rst and smallest level of an ecosystem is an individual organism. One human is an organism. A lone wolf is also an organism. In a prairie ecosystem, a single buffalo is an organism. What are the abiotic factors of this ecosystem? What are the biotic factors? Organisms are rarely found alone. More often than not, several organisms live together. More than one organism of the same species in an area is called a population, which is the second level of organization in an ecosystem. A population is made up of organisms of the same species that are able to interact and mate with one another. All of the buffalo living together in the prairie are an example of a population. Populations are not isolated from other groups of organisms. In the prairie ecosystem, buffalo live among populations of birds, insects, grasses, and trees to name a few. All of these populations living together in the same area make up a community, which is the third level of organization in an ecosystem. 1
The abiotic factors of an ecosystem include climate, water, living space, inorganic nutrients, and food. These abiotic factors often serve as limiting factors to biotic components of the ecosystem. When a limiting factor, such as water, is in short supply, then the productivity of the ecosystem, including the organism s ability to survive, is limited. External Factors That Affect Organisms, Populations, and Communities The abiotic and biotic factors in an ecosystem are interconnected. When biotic and abiotic factors change, they affect the other parts of the ecosystem. The response to change made by an organism affects the population, which in turn, affects the whole community. Organisms: Changes to an ecosystem, especially changes that cause a decrease in the supply of a limiting factor, affect an organism s ability to survive. Thus, organisms respond in a way that attempts to counteract the effect of the external factor. An organism s goal is always survival. It responds to external factors in its environment in order to maintain homeostasis. Humans do this all the time without thinking about it. If a person gets too cold, he or she puts on a jacket or wraps up in a blanket. If a person s body becomes dehydrated, signals are sent to the brain to indicate thirst. In response, a person gets a drink of water. homeostasis: a steady, stable internal state Temperature is an external factor that affects all organisms. Responses vary among different species, but maintaining homeostasis, including a suitable internal body temperature, is a common goal among all organisms. In cold temperatures, some animals may begin to shiver involuntarily. Shivering is characterized by small muscles contractions in the core of an organism s body. These tiny contractions use up energy in the form of ATP. When the molecular bonds in ATP are broken, heat is generated. The result is an increase in body temperature. Ectotherms, or animals that use external sources to regulate their body temperature, respond to decreasing temperatures in a variety of ways. They may bask in the Sun or on a hot rock, burrow into an insulated nest, or curl up to minimize the amount of surface area that is exposed to cold. This lizard is basking on hot, sunlit rocks in order to keep its body warm. Even plants respond to temperature changes. Many plants enter a dormant, or resting, state in winter where they do not grow or produce food. In preparation, they lose their leaves and their stems become weaker. This allows the plants to conserve energy and survive colder temperatures. 2
what do you think? In an effort to survive, organisms respond to the various external factors that affect their ecosystems. Look at the examples of external changes in the table below. For each one, list a possible response that an organism might have to maintain homeostasis. External change Temperature increase Drought New predator New competition for food Organism response Populations: Populations respond to external factors in ways that refl ect the efforts of the individual members to survive and reproduce. Populations can change in size, distribution, and range based on the responses of its organisms. For example, consider a population of deer living in a forest. Suppose a disease affects the plants that the deer use as a food source. In response to this external factor, the population may shrink as some members die from starvation. Birth and death rates also determine the size of a population. When birth rate exceeds death rate, the population grows and vice versa. Birth and death rates change in response to external factors. Low predation, abundant food and water, ample living space, and low competition for resources contribute to high birth rates and population growth. The abundance or scarcity of nutrients not only affect population size, but its distribution and range in the ecosystem. Organisms will go where resources are available. This can cause populations to travel together (migrate) to new areas or spread out in smaller groups to decrease competition. Zebra migrate in groups in order to find an ample supply of food. look out! Even in ideal living conditions, the size of a population can reach a limit. Once a population becomes too large, it outgrows the availability of food, water, and living space. Resources are not unlimited, and can serve as limiting factors to population size. As the population grows, competition for resources increases. Those organisms that cannot successfully compete for resources will die. 3
The death rate begins to equal the birth rate and instead of experiencing growth, the population becomes stable. The population may still experience short periods of growth, but they will be balanced out with short periods of decline. At this point, the environment has reached its carrying capacity for that population. The carrying capacity is the maximum number of members of a population that an ecosystem can support. Communities: Every species has a unique way of life called a niche. A niche includes a set of abiotic and biotic factors a species needs to survive and reproduce. This can include the type of food it eats, light and water needs, ideal climate conditions, living space requirements, and survival and reproduction strategies. Species in a community typically occupy different niches. For example, in a seashore community, snails and barnacles are able to live in the high tide zone. During low tide, this area is not covered with water. In contrast, algae and sea stars could not survive here for long without drying out. Although these species live in the same ecosystem, they do not share the same niches. Fish, coral, seaweed, sea turtles, and many other aquatic species occupy a variety of niches in a reef community. Interactions between species often revolve around the different niches they occupy, which can result in changes to the community as a whole. Communities respond to changes in biotic and abiotic factors in ways that reflect the responses of the individual organisms and populations they contain. When confronted with a changing environment, an individual organism responds in a way that enables it to survive. These responses affect the population the organism belongs to. Cumulative responses of a population affect the structure and biodiversity of the community as a whole. Predation is an example of a factor that can affect the structure of a community. For example, in a forest community, a population of hawks feeds on rabbits, squirrels, and other small mammals. The relationship regulates the population size of the rabbits and squirrels. However, if an external factor such as disease or competition kills individual hawks and decreases the hawk population, then the rabbit and squirrel populations will grow because of decreased predation. The external factor was a detriment to the hawk population, but benefi ted the rabbits and squirrels. As a result, the structure and species distribution of the community changed. A community may also change when a new species immigrates into the community or one emigrates out. 4
what do you think? Clownfish and sea anemones have a mutualistic relationship. The clownfi sh are protected from predators by the sea anemone s stinging tentacles. In return, the anemone benefi ts from scraps of food dropped by the clownfi sh. Predict what role external factors affecting the sea anemone population will play in the clownfish population and the reef community as a whole. Looking to the Future: Human Activity and Habitat Destruction An organism s habitat not only provides shelter, but also the food resources it needs to obtain nutrients. Habitat is particularly important to the giant pandas of China. These large bears survive almost exclusively on the bamboo plant, which makes up 99% of their diet. Bamboo is not rich in nutrients, so the pandas must eat large quantities of the plant to adequately nourish themselves. Pandas spend about 12 hours a day eating up to 38 kg (84 lb) of bamboo. Unfortunately, this food source is becoming more difficult to fi nd in their habitat. Too often, human activity is the external factor affecting populations of plants and animals in their natural environment. The pandas bamboo forest habitat is being destroyed as the demands of the farming and timber industries grow. Although bamboo is a fast-growing plant, it only grows at specific altitudes and pandas require a lot of bamboo to survive. Two breeding pandas need approximately 30 square kilometers to support them, which is equal to about 15,000 football fi elds combined. The destruction of the bamboo forests decreases the shelter and food resources available to the panda population in China. As a result, the panda population is dwindling. Scientists estimate that the wild panda population is fewer than 2,500 individuals, which makes this unique creature an endangered species. 5
What Do You Know? Response to changes in an ecosystem can vary among different organisms in the same community. An external factor that is advantageous to one population may be harmful to another. This affects the diversity and structure of the community that the populations live in. Each scenario below describes a change in an ecosystem. Read each scenario and predict the most likely outcome of the change by fi lling in each blank with either increases or decreases. 1. The birth rate of fi eld mice exceeds death rate. This the mouse population and the grass population on which the mice feed. 2. A population of bluebirds emigrates from a forest ecosystem. This the size of the community within the ecosystem and competition for resources among other bird populations that eat the same foods as the bluebirds. 3. A grassland ecosystem experiences a drought. This competition for water among all organisms and plant populations in the grassland. 6
connecting with your child Response to the Ecosystem To help them learn more about ecosystem responses, have students monitor their own responses to external factors over several days. Encourage students to pay attention to things they naturally do without realizing that they are responses to the ecosystem. For example, students could observe their responses to changes in the temperature, noise level, presence of food, amount of light, and presence of other people in their surroundings. Have students record how they responded to each situation and why they made their choice. Most often, it will be because the response benefited them in some way. Have them explain how the benefi t could relate to survival in the wild. Next, have students take care of a small houseplant, aquarium, or terrarium over several weeks. Direct them to introduce changes to the external conditions in order to observe how the organisms respond. Suggestions include varying the amount of food, water, light, and heat available to the houseplant, aquarium, or terrarium. Have students record their observations and encourage them to make predictions each time they introduce a new external factor. Ask students to record how each organism s response affects other members and parts of the community. Be sure that students make every effort to treat the organisms with respect. They should not introduce external factors that unnecessarily cause distress or harm to the organisms. Here are some questions to discuss with students: Why must organisms respond to changes in their ecosystem? How do you think these responses affect the long-term survival of species? How do you think responses of a community affect an ecosystem? Biome? Planet? 7