# A b exponential growth predict a growth curve for a

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scale, suburbanites “dust” their lawns. (a) (b) Exponential Growth: Predict a growth curve for a population.

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500 C HAPTER 15 Population Ecology: How Do Organisms Interact to Form Populations? Number of individuals Time (months) Figure 15-9 Some populations grow exponentially, at least for a time.After a period of relatively slow growth, the population size soars. is still 0.4 (although not necessarily constant, a population’s “little r” may remain so) and that the gardener has left them alone (probably not a reasonable assumption). Now, in just two weeks, the population increases by 40. By late October, the population is approximately 1000 (under the same assumptions). In just two weeks, the population increases by 400.In just one year,our slug population could,if left to its own devices,jump from 10 to 45,000! The flowers never had a chance. Note: With exponential growth, where the larger the population, the faster it grows. Growth of Most Populations Follows an S-shaped Curve Obviously, in nature, populations are seldom “left to their own devices.” If the gardener hadn’t discovered the slugs and interfered with their exponential growth, something else would have—snakes or shrews perhaps (Figure 15-10). Predators are one of the factors Chapman had in mind when he spoke of environmental resistance. 1 dN/dt 2 rN , Figure 15-10 Shrews are mouse- sized friends to gardeners.These voracious predators can eat an amount of slugs and other garden pests equal to their own weight each day. Emerald Ash Borer: Observe the effect of this exponential growth.
15-3 How Do Populations Grow? 501 Logistic Population Growth Number of individuals Time (months) Logistic Population Growth Figure 15-11 Most populations do not grow exponentially for long.After a period of such growth, environmental resistance limits further growth. What would have happened to the population of Salvia in our garden if there had been no slugs? We can guess that, through the first few growing seasons, their numbers would have behaved as the slug numbers did.Then, after a period of little change, num- bers would increase rapidly, as predicted by the exponential equation. But gardens fill. There is, after all, only so much space for plants. For these flowers, environmental resis- tance is measured in terms of available space. When all spaces are filled, a population can increase no further. Similarly, although factors other than space may be key, a gar- den filled with Salvia can support only so many slugs. Notice that the growth of both of these populations, when graphed, would follow an S-shaped curve (Figure 15-11). Rapid growth would be followed by a flattening off. Numbers would not increase beyond an upper limit. In other words, each population has a carrying capacity , defined as the number of individuals the environment can support.

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