Chapter 3 Ecosystems: Interactions, Energy and Dynamics Lesson 1 Relationships in Earth s Environments Main idea: Ecosystems contain both living and nonliving parts. Within these ecosystems there are a variety of relationships between the organisms. Next Generation Science Standard -Performance Expectation MS-LS2-1. Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem. MS-LS2-2 Construct an explanation that predicts patterns of interaction among organisms across multiple ecosystems. What will you learn? Evaluate biotic and abiotic factors in an ecosystem. Sequence the different levels of biological organization. Analyze how ecology and the environment are related. Why is it important? Abiotic and biotic factors interact to make up an ecosystem. Organisms have interactions that create patterns which repeat across many ecosystems. Vocabulary ecosystem sample counts ecology populations spacing habitat limiting factors niche carrying capacity biotic biotic potential abiotic exponential growth population pattern community natality biome mortality biosphere symbiosis population density mutualism trap-mark-release parasitism Earth s Ecosystems Earth s biosphere, is composed of ecosystems. An ecosystem is the living and nonliving components of an environment and the way they interact, see Figure 1. Ecology is the study of the interactions among organisms and their environment. Often, ecosystems are confused with another word- habitat. Habitat is the place a population of organisms normally occurs. Another word you know that is frequently confused with niche. A niche is the role an organism is adapted to and fills in its environment. For example the earthworm s habitat is the soil, its niche includes aerating, moving and enriching the soil. While all these words are related, an ecosystem is much more than a niche or habitat. Ecosystems include interactions among all of the living and nonliving parts of an environment. The living things are the biotic parts of the ecosystem, the prefix bio comes from a Greek word that means life. The non-living things are the abiotic factors. The a in abiotic means not, so abiotic means not living. Figure 1. Can you pick out the biotic and abiotic parts of this ecosystem?
Abiotic Factors Water All living things need water to survive. Living things are made of cells, and water accounts for 70% or more of a cell s total mass. In fact, most organisms are 50-95 percent water. Respiration, digestion, photosynthesis, and other life processes require the presence of water. More than 95 percent of Earth s surface water is found in the oceans. The ocean (saltwater environment) and rivers, lakes and ponds (freshwater environments) are home to a wide variety of organisms. Light and Temperature Light and temperature affect the environment. The availability of sunlight determines where plants and other photosynthetic organisms live. By the process of photosynthesis, energy from the Sun is changed into chemical energy that is used for life processes. Most green algae live near the water s surface where sunlight can penetrate. In dense forests where little sunlight penetrates to the forest floor, very few photosynthetic plants grow. A region s temperature also determines which plants and animals survive there. Some areas of the world have a consistent temperature year round, but other areas have seasons during which temperatures change drastically. Water environments throughout the world also have widely varied temperatures, with organisms living in the freezing cold Arctic, extremely hot water near ocean vents, see Figure 2, as well as almost every temperature in between. Figure 2. Ecosystems exist in almost all environments on Earth., including the Hydrothermal vents.
Air Air is all around you, and has an impact on lives of most species. Air is composed of a mixture gasses including mostly nitrogen, oxygen, and carbon dioxide. Living things depend on the gases in air for respiration. The atmosphere is the layer of gasses and airborne particles that surround the Earth. Polluted air, can cause the species in an area to change, move or die off. Clouds and weather occur in the atmosphere. Species are affected by weather in the areas where they live. The ozone layer, in the atmosphere, above the Earth s surface, protects organisms from harmful radiation from the Sun. Soil From one location to another, soil can vary greatly. Soil is more important than you might think. Soil type is determined by the amounts of sand, silt, and clay it contains. Various kinds of soil contain different amounts of nutrients, minerals and moisture. Figure 3. Soil is an important abiotic factor. Because soils are composed of different materials, soil type determines what kinds of plants will grow in a certain location. Different plants need different kinds of soil, nutrients and minerals to grow. The types of plants in an area help determine which other organisms can survive in that area, so soil affects every organism in an environment. Biotic Factors Abiotic factors provide only some of the things an organism needs to survive. Organisms also need other organisms, also known as biotic factors, for food, shelter, protection, and reproduction. How organisms interact with one another and with abiotic factors can be described in an organized way.
Levels of Organization The living world is highly organized. Atoms are arranged into molecules, which might be organized into cells. Cells form tissues, tissues form organs, and organs form organ systems and organ systems for m organisms. Biotic and abiotic factors also can be arranged into levels of biological organization, as shown in Figure 4. Figure 4 The living world is organized into the levels seen here.
Populations All the members of one species that live together make up a population. For example, all of the humans living on Earth at the same time make up a population, there are populations of elephants, maple trees, grasshoppers and dandelions. Every organism you can think of is a member of a population. Members of a population compete for food, water, mates, and space. The resources of the environment and the ways the organisms use these resources determine how large a population can become. Communities Most populations of organisms do not live alone. They live and interact with populations of other types of organisms. Groups of populations that interact with each other in a given area form a community. Populations of organisms in a community depend on each other for food, shelter, and other needs. Ecosystems In addition to interactions among populations, ecologists also study interactions among populations and their physical surroundings. An ecosystem is made up of a biotic community and the abiotic factors that affect it. Scientists categorize ecosystems into three main types, terrestrial (land), fresh water (lakes, streams, rivers) and ocean(shallow waters, coral reef, deep ocean). You will learn more about the interactions that occur in ecosystems later in this chapter. Biomes Scientists divide Earth into different regions called biomes. A biome (BI ohm) is a large region with plant and animal groups that are well adapted to the soil and climate of the region. Many different ecosystems are found in a biome. Examples of biomes include tundra, as shown in Figure 5, tropical rain forests, and grasslands. Figure 5 Biomes are collections of ecosystems The Biosphere Where do all of Earth s organisms live? The part of Earth that supports life is the biosphere (BI uh sfihr). It includes the top part of Earth s crust, all the waters that cover Earth s surface, the surrounding atmosphere, and all biomes.
Populations. in Ecosystems As scientists study organisms in their natural environments, they are able to learn things. They can sometimes make generalizations about, and see patterns that are similar, in these populations. They learn about population needs, limits, densities and spacing as well as interactions of populations with one another. Competition Food and Space Organisms in the wild do not always have enough food, water or living space. Competition occurs when two or more organisms seek the same resource at the same time. Take the example of the Gila woodpecker, which lives in the desert ecosystem on the Sonoran Desert (located in Arizona and Mexico). This woodpecker makes a nest in a hole that it drills in a Saguaro cactus. If an area has too many Gila s or too few Saguaro cacti, the woodpecker must compete with each other for nesting spots. Organisms may also compete for mates. Growth Limits Competition limits population size. If the amount of available nesting space is limited, some woodpeckers will not be able to raise young. Gila woodpeckers eat cactus fruit berries and insects. If food becomes scarce some woodpeckers might not survive to reproduce. Competition for food, living space or other resources can prevent population growth. In nature, the most intense competition is usually among individuals of the same species, because they need the same kinds of food and shelter. Competition also takes place among individuals of different species. For example, after the Gila woodpecker abandons its nesting hole, other organisms such as owls, flycatchers, snakes and lizards compete for the shelter that the Gila woodpecker created, as shown in Figure 6. Figure 6 Competition for resources in the desert ecosystem.
Population Size Sometimes ecologists need information on whether a population is healthy, growing or in danger of disappearing. In order to make these determinations, populations are counted. Some populations are easy to measure. If you were raising mice it would be easy enough to count each one. In the wild figuring out how many of a certain organism there are, is more complicated. The size of a population that occupies a specific area is called population density. There are several methods used to measure populations. Measuring Populations One method used to measure populations is called trap-mark-release, or sometimes capture-recapure. If you wanted to count wild rabbits you need to locate them when they are visible, that is when they feed at dusk and dawn. Otherwise they are underground in their burrows. So ecologists set traps that capture rabbits without injuring them, see Figure 7. Each captured rabbit is marked and released. Later another sample of rabbits is captured. Some of these rabbits will have marks, but many will not. By comparing the number of marked and unmarked rabbits in the second sample, ecologists can estimate the population size. Figure 7 Humane trap for trap-mark-release method. If you wanted to count the rabbits over a large area, say 100 acres, ecologists would use a method called sample counts. With this method, the rabbits in one square acre would be counted, then that number would be multiplied by 100 to estimate the population size. This method is hard to use with fast moving animals. Sometimes aerial photography is used and animals are counted from the photo, see Figure 8. Sometimes scientists are interested in populations spacing, or how organisms are arranged in a given area. They will look to see if organisms are evenly spaced, spaced randomly or clumped together. Random spacing of plants usually occurs when the wind or birds disperse seeds. Clumping occurs when animals or plants gather in groups. Figure 8 Sample count shown below is useful for populations spread out over large areas. Ecologists use this method to estimate populations.
Figure 9 The animal population, maintains a fairly consistent carrying capacity over the year, with mortality and natality resulting in neither a noticeable net gain or loss. Limiting factors are biotic or abiotic factors in the ecosystem that restrict the number of individuals in a population. Some obvious limiting factors are food, water, space, predators disease and hunting. Remember our example of the Gila woodpecker and the Saguaro cactus? If there are few cacti in the ecosystem, that will limit the number of Gila woodpeckers which can live in the area because they need the cactus plants for shelter. In any ecosystem, the availability of food, water, living space, mates, nesting sites and other resources is often limited. A limiting factor can affect more than one population in a community. Suppose the Saguaro cacti succumbs to a disease and there is a massive die off. There will be a smaller Gila woodpecker population for lack of shelter (area to build a nest). This might in turn reduce the size of the population of the organism who feed on the Gila including bobcats, coyotes, snakes, and foxes. Carrying Capacity is the term for the largest number of individuals of one species that an ecosystem can support over time. For example a population of robins lives in a grove of trees at a local park. Over the years you notice the number of nests increases, and the number of robins increases. Soon nesting space is the limiting factor that prevents the population from getting any larger. This ecosystem has reached its carrying capacity for robins. If the population begins to exceed the environment s carrying capacity, some individuals will not have enough resources. They might die or be forced to move to another location. As you can see in Figure 9, the carrying capacity is maintained (after a brief spring surplus), by a variety of factors including starvation, disease, hunting, predation etc. Biotic Potential refers to the size a population would be if there were no limiting factors. The maximum rate at which a population increases when plenty of food and water re available, weather is ideal and not diseases or enemies exist, is its biotic potential. Most populations never reach their biotic potential, or they may, but just for a short period of time. Eventually, the carrying capacity of the environment is reached and the population stops increasing.
Figure 10 Organisms will follow a similar pattern of beginning growth and exponential growth, until they reach a somewhat level carrying capacity (also see Figure 11).
Changes in Populations Moving Around Most animals can move from place to place, and their movements can affect population sizes. For example, a male mountain sheep might wander miles in search of a mate. Then when he finds a mate, they and their offspring might establish a completely new population of mountain sheep far from the male s original population. Many organisms exhibit similar behavior patterns when they reach maturity. You might also know that many bird species move from one place to another during their annual migrations. In Southern California the Cliff Swallows migrate every year in March to the Mission at San Juan Capistrano. Thousands of orange tailed birds swoop in to reclaim their mud nests in the eaves at the mission and other areas in town. In October they migrate back to Argentina. When March rolls around again, the birds migrate back to San Juan Capistrano. Even plants and microscopic organisms can move, carried by wind, water or animals. The tiny spores of mushrooms and ferns float on the air. The seeds of dandelions, maple trees and other plants can be carried by the wind as well. Other plants have spine covered seeds that hitch a ride by clinging to animal fur or peoples clothing, or by rivers or an ocean current. Exponential Growth Imagine what might happen if a pair of coyotes moved into a valley where no other coyotes live. If resources are plentiful, the population will grow quickly. This pattern of growth is called exponential growth, which means that the larger a population becomes the faster it grows, see point B on Figure 11, Figure 10. Figure 11 The pattern of beginning growth (a), rapid growth (b), leveling off (c), reaching carrying capacity (d) and fluctuations(d) is common for many types or organisms in ecosystems as shown below. After several years, the population becomes so large that the coyotes begin to compete for food and den sites. Population growth slows, and the number of coyotes remains fairly constant and reached equilibrium. This ecosystem has reached its carrying capacity for coyotes. This pattern (particular way something happens) of growth is common in many types of organisms.
Natality and Mortality Natality, in the study of ecology is the term for birth rate and mortality is the term for death rate. These rates are studied to calculate the changes of a population --is it increasing, decreasing or staying the same in size? Natality and mortality influence the size of a population and its rate of growth. A population gets larger when the number of individuals born is greater than the number of individuals that die. When the number of deaths is greater than the number of births, populations get smaller. For example if 200 bunnies are born but 250 die in one season the population has decreased by 50. If 300 bunnies are born but only 250 die, the population has increased by 50. Organisms can die by a variety of means as shown in Figure 9. Some reasons include old age, predation, hunting, starving and disease. Can you think of other reasons animals might die in the wild? Symbiosis and Other Interactions In ecosystems, many species of organisms have close relationships with other types of species that are necessary for their survival. Symbiosis (sihm Bee OH sus) is any close interaction between two or more different species. Symbiotic relationships can be identified by the type of interaction between organisms. Mutualism Mutualism is a symbiotic relationship between organisms from two different species in which both benefit from the relationship. The oxpecker is a bird that has a mutualistic relationship with a rhino or a zebra, see Figure 12. In this relationship, this bird lives on the zebra or rhino, sustaining itself by eating all of the bugs, ticks and parasites on the animal. The bird benefits by having a readily available source of food. The zebra or rhino benefits from having the bugs removed. Also, when there is a danger to the zebra or the rhino, like a predator, this bird flies high and makes noise in order to alert nearby animals to the impending danger. Figure 12 Mutualism-- both organisms benefit from this relationship.
Commensalism Commensalism is a form of symbiosis that benefits one organism without affecting the other organism. For example, Anemonefish also known as clownfish, below, live amid the tentacles of the anemones which protects them from predators. Predators trying to get these fish can be poisoned by the tentacles of the anemones. Remember the Gila woodpecker we discussed earlier in this lesson. It creates a hole in a Saguaro cactus for a nest. When it abandons its nest other desert animals such as a lizard a snakes or an owl might move into the abandoned hole. This relationship is another example of commensalism. Parasitism Parasitism is a symbiotic relationship between two species in which one species benefits and the other species is harmed. Some species of mistletoe are parasites because their roots grow into a tree s tissues and take nutrients from the tree. Aphids are a type of insect parasite that feed on the sap of the host plant. On the preceding page, we discussed the oxpecker bird and his relationship with the zebra and the rhino. Remember the bird is eating bugs and tics off of the zebra and rhino, these organisms are parasites. So the bugs and tics have a parasitic relationship with the zebra and rhino. Figure 13 Mistletoe and aphids are parasitic to the plants on which they feed, stealing nutrients from the plant.
Predation Predation is another type of interaction. It is a natural way that population size is regulated. Predation is the act of one organism hunting, killing and consuming another organism. Owls are predators of mice, and mice are their prey. Lions are predators and zebra and gazelles (among other organisms) are their prey, see Figure 14. Predators are biotic factors that limit the size of the prey population. Availability of prey is a biotic factor that can limit the size of the predator population. Predators are more likely to capture old, ill or young prey, so the strongest individuals in the prey population are the ones that manage to reproduce. This improves the prey population over several generations. Figure 14 Predators are biotic factors that limit the size of the prey population. Lesson 1 Review Summary Populations can be described by size, density and spacing. Limiting factors affect population size. The number of individuals an environment can support and maintain over time is called the carrying capacity. The biotic potential is the rate a population would increase without limiting factors. A close interaction between two or more different species is called symbiosis. Mutualism, commensalism and parasitism are types of symbiotic relationships that can exist between organisms. Predators are biotic limiting factors of prey. Self Check 1. Describe how limiting factors can affect a population. 2. Explain the difference between a habitat and a niche. 3. Describe and give an example of two symbiotic relationships that occur among populations in a community. 4. Explain how sound could be used to relate the size of the cricket population in one field to the cricket population in another field. 5. Think critically: A parasite obtains food from its host. Most parasites weaken but do not kill their hosts. Why?