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Unit 3 Ecosystems Topic 6: Population ecology Populations • All of the individuals of a species in a given area at the same time Clumped • Characteristics of populations – – – – – Size (N) Density Distribution Sex ratio Age structure Uniform Random Population Size is affected by: • Additions: – Birth – Immigration • Subtractions – Death – Emigration Change in population size: N=(birth+immigration)-(death+emigration) Population growth Change in population size: N=(birth+immigration)-(death+emigration) • Growth occurs if inputs are greater than outputs. • Under ideal conditions, the intrinsic growth rate is observed. • This is the maximum potential for growth of a population. • It is essentially the maximum amount of offspring that a population could have, if all females of reproductive age give birth to the maximum number of offspring they are capable of. Population growth: Exponential • Exponential growth is defined as a rate greater than 1. • Occurs in populations… – That are introduced to a new area – That are rebounding from a catastrophe • Cannot be sustained indefinitely. Why? Logistic rate of growth • So, can populations continue to grow exponentially? K = carrying capacity Max. size that environment can support with no degradation of habitat. Varies with changes in resources. Exceeding Carrying Capacity • Members of populations which exceed their resources will die or move to an area with more resources. Figure 8-6 Population growth general patterns Predator/prey population cycles Predator/prey population cycles What do you think the primary producer graph looks like? Predator/prey population cycles Population regulation • Limiting factors control population growth • Density independent – Abiotic (precipitation, temperature, acidity, salinity, etc.) – Catastrophic abiotic (flood, fire, volcanic eruption, tsunami, earthquake) • Density dependent – Biotic (food supply, disease, parasites, competition, predation) Regardless of pop. density, these factors are the same for all individuals. The effects of these factors are influenced by population density. Organisms that are more crowded: •Compete more for resources •Are found easily by predators •Spread disease and parasites more readily What determines reproductive capacity of a population? – sex ratio (how many females vs. males?) – generation time (at what age do females reproduce?) – age structure (how many generations reproduce?) Reproductive strategies • K-selected – late reproduction – few offspring – invest a lot in raising offspring • primates • coconut • r-selected – early reproduction – many offspring – little parental care K-selected • Invertebrates • many plants r-selected More generalized characteristics of r and K selected species Survivorship curves What do these graphs tell about survival & strategy of a species? (Generalized strategies) Survival per thousand 1000 Human (type I) I. High death rate in post-reproductive years Hydra (type II) 100 II. Constant mortality rate throughout life span Oyster (type III) 10 1 0 25 50 75 Percent of maximum life span 100 III. Very high early mortality but the few survivors then live long (stay reproductive) Populations of r and K strategists Pop. Number size of individuals Populations of k strategists tend to grow exponentially when initially established and then become stable around a carrying capacity. Populations of r strategists tend to r strategist k strategist have periods of rapid growth (births) followed by sharp declines (most young do not survive). Which is which? Time Populations over time • • • • Evolution occurs at the level of the population. What is survival of the fittest? How can community dynamics create biological diversity? How can climatic changes create biological diversity?