sparrows in your community? Although house sparrows tend to be more concentrated in towns and urban areas, they do not stop and turn back at the city limit sign. For zoologists, a fuzzy definition of the space occupied by the population often forces an arbitrary designation of the survey group, such as the "population" of robins nesting on your campus in the spring. Knowing the number of animals in a designated study area is interesting, but we must bear in mind that the ecological population is defined in terms of interactions among organisms of the same species, and not by the ecologist's convenience. After defining the individual and establishing the limits of the population you wish to count, your next task is to choose a counting method. Arctic and prairie habitats lend themselves to accurate survey by aerial reconnaissance. This approach works poorly in forests, at night, underwater, or in soil habitats. If animals can be collected or observed in a standard time or collecting effort, you can get an idea of relative abundance, but not absolute numbers. For example, the number of grasshoppers collected in 50 swings with an insect net through an old field community produces data that could be used to compare relative abundance in different fields, but would not tell you how many grasshoppers were in the population. For estimates of absolute numbers, mark-recapture methods can be very effective. The first step is to capture and mark a sample of individuals. Marking methods depend on the species: birds can be banded with a small aluminum ankle bracelet, snails can be marked with waterproof paint on their shells, butterflies can have labels taped to their wings, large mammals can be fitted with collars, fish fins can be notched, and amphibians can have nontoxic dyes injected under the skin. Marked animals are immediately released as close as possible to the collection site. After giving the animals time to recover and to mix randomly with the whole population, the ecologist goes out on a second collecting trip and gathers a second sample of the organisms. The size of the population can then be estimated from the number of marked individuals recaptured on the second day. The assumption behind mark-recapture methods is that the proportion of marked individuals recaptured in the second sample represents the proportion of marked individuals in the population as a whole. In algebraic terms, R = M S N
Examining Population Density and Dispersion Examine the map of the human population and discuss/brainstorm the following ideas with your table group: Where on the map do you see the greatest population? The least population? What “pattern of spacing,” or dispersion do you notice? Why do you see this type of population distribution and density amongst humans? Now examine the next image presented by your teacher. What density, dispersion, and reproductive strategies do you notice? Essential Concept: Each population has a ______________,
r and K reproductive strategies Organisms that live in stable environments tend to make few, "expensive" offspring. Organisms that live in unstable environments tend to make many, "cheap" offspring. Imagine that you are one of the many invertebrate organisms which existed during the Cambrian or one of their descendents living today. Maybe you live in a tide pool which is washed by waves. A storm appears on the horizon. The waves increase in height. You feel yourself being dashed upon the rocks or into the mouth of a much larger and predatory animal. Finally, you begin to see your brothers and sisters die, one by one, as the forces of nature change your unpredictable environment. If you could design a "strategy" to overcome the problems created by an unpredictable environment, you would have two choices - go with the flow or cut and run to a more stable environment. Suppose you stayed. Then, one thing you could do would be to increase the number of offspring. Make lots of cheap (requiring little energy investment) offspring instead of a few expensive, complicated ones (requiring a lot of energy investment). If you lose a lot of offspring to the unpredictable forces of nature, you still have some left to live to reproductive age and pass on your genes to future generations. Many invertebrates follow this strategy - lots of eggs are produced and larvae are formed but only a few survive to produce mature, reproductive adults. Many insects and spiders also follow this strategy. Alternatively, you could adapt to a more stable environment. If you could do that, you would find that it would be worthwhile to make fewer, more expensive offspring. These offspring would have all the bells and whistles necessary to ensure a comfortable, maximally productive life. Since the environment is relatively stable, your risk of losing offspring to random environmental factors is small. Large animals, such as ourselves, follow this strategy. Plants are also subject to the same sorts of forces as animals. Some live in unstable environments such as a floodplain near a river or a gap in the forest caused by falling trees. Others live in a quite stable environment, such as a climax forest.