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Chapter 53 Population Ecology Population Ecology Population ecology is the study of factors that affect population: • Density • Growth • A population is a group of individuals of a single species that occupy the same general area. • Population ecology focuses on the factors that influence a population’s • Density • Structure • Size • Growth rate Population Density Population density is the number of individuals of a species per unit of area or volume. Examples include : 1. The number of largemouth bass per cubic kilometer (km3) of a lake 2. The number of oak trees per square kilometer (km2) in a forest 3. The number of nematodes per cubic meter (m3) in a forest’s soil • how to measure population density • can be impractical or impossible to count all individuals in a population. • In some cases, estimated by indirect indicators, such as number of bird nests, and rodent burrows. MEASURING DENSITY Density – Number of individuals per unit of area. •Determination of Density •Counting Individuals •Estimates By Counting Individuals •Estimates By Indirect Indicators •Mark-recapture Method N = (Number Marked) X (Catch Second Time) Number Of Marked Recaptures – The dispersion of a population is the pattern of spacing among individuals within the geographic boundaries. • Patterns of dispersion – Within a population’s geographic range, local densities may vary considerably. – Different dispersion patterns result within the range. – Overall, dispersion depends on resource distribution. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings PATTERN OF DISPERSION UNIFORM CLUMPED RANDOM Uniform Dispersion-(pattern) Animals that defend territories often show this Clumped Dispersion-( most common) usually around a common resource Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Random Dispersionunpredictable spacing- not common in nature Fig. 52.2c Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Demography is the study of factors that affect the growth and decline of populations • Additions occur through birth, and subtractions occur through death. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Life Tables and Survivorship Curves A life table is an age-specific summary of the survival pattern of a population. • Track survivorship • Help to determine the most vulnerable stages of the life cycle • Follow a cohort, of individuals of the same age throughout their lifetime. Survivorship curves • Graphically represent some of the data in a life table • Are classified based upon the rate of mortality over the life span of an organism Type I • Shows individuals that have a high probability of surviving through early and middle life but have a rapid decline in the number of individuals surviving into late life. • Most of the individuals will make it to adulthood but the proportion surviving into old age is greatly decreased. • Is plotted as a convex curve on a graph. • Ex. humans Type II • Shows a roughly constant mortality rate for the species through its entire life. • This means that the individual's chance of dying is independent of their age. • Are plotted as a diagonal line going downward on a graph. • Ex. birds Type III • Depicts species where few individuals will live to adulthood and die as they get older because the greatest mortality for these individuals is experienced early in life. • Curve is drawn as a concave curve on a graph. • Ex. o Life History Traits as Evolutionary Adaptations • • • • • • • Age at first reproduction Number and size of offspring Reproductive lifespan and ageing Frequency of reproduction Number of offspring Amount of parental care provided Evolve and represent a compromise of the competing needs for time, energy, and nutrients. Life histories are very diverse, but they exhibit patterns in their variability • Life histories are a result of natural selection, and often parallel environmental factors. • Some organisms, such as the agave plant,exhibit what is known as big-bang reproduction, where large numbers of offspring are produced in each reproduction, after which the individual often dies. Agaves Population Growth Models The logistic and the exponential models are theoretical ideals of population growth. No natural population fits either one perfectly. • Population size fluctuates as individuals are born • Immigrate into an area • Emigrate away • Die – This is also known as semelparity. • By contrast, some organisms produce only a few eggs during repeated reproductive episodes. – This is also known as iteroparity. • What factors contribute to the evolution of semelparity and iteroparity? Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Limited resources mandate trade-offs between investments in reproduction and survival • The life-histories represent an evolutionary resolution of several conflicting demands. – Sometimes we see trade-offs between survival and reproduction when resources are limited. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • For example, red deer show a higher mortality rate in winters following reproductive episodes. Fig. 52.5 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Variations also occur in seed crop size in plants. – The number of offspring produced at each reproductive episode exhibits a trade-off between number and quality of offspring. dandelion Coconut palm The exponential model of population describes an idealized population in an unlimited environment • We define a change in population size based on the following verbal equation. Change in population = size during time interval Births during time interval Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings – Deaths during time interval • Using mathematical notation we can express this relationship as follows: – If N represents population size, and t represents time, then N is the change is population size and t represents the change in time, then: • N/t = B-D • Where B is the number of births and D is the number of deaths Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Population Age Structure • The age structure of a population is the distribution of individuals among age groups. • The age structure of a population provides insight into : a) The history of a population’s survival b) Reproductive success c) How the population relates to environmental factors – We can simplify the equation and use r to represent the difference in per capita birth and death rates. • N/t = rN OR dN/dt = rN – If B = D then there is zero population growth (ZPG). – Under ideal conditions, a population grows rapidly. • Exponential population growth is said to be happening • Under these conditions, we may assume the maximum growth rate for the population (rmax) to give us the following exponential growth • dN/dt = rmaxN The Exponential Growth Model • Exponential population growth describes the expansion of a population in an ideal and unlimited environment. • Exponential growth explains how a few dozen rabbits can multiple into millions • In certain circumstances following disasters, organisms that have opportunistic life history patterns can rapidly recolonize a habitat The Logistic Growth Model • • • • • • • • Limiting factors Are environmental factors that hold population growth in check Restrict the number of individuals that can occupy a habitat The carrying capacity is the maximum population size that a particular environment can sustain. Logistic population growth occurs when the growth rate decreases as the population size approaches carrying capacity. The carrying capacity for a population varies, depending on the species The resources available in the habitat Organisms exhibiting equilibrial life history patterns occur in environments where the population size is at or near carrying capacity. The logistic model of population growth incorporates the concept of carrying capacity • Typically, unlimited resources are rare. –Population growth is therefore regulated by carrying capacity (K), which is the maximum stable population size a particular environment can support. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Example of Exponential Growth Kruger National Park, South Africa POPULATION GROWTH RATE LOGISTIC GROWTH RATE Assumes that the rate of population growth slows as the population size approaches carrying capacity, leveling to a constant level. S-shaped curve CARRYING CAPACITY The maximum sustainable population a particular environment can support over a long period of time. Figure 52.11 Population growth predicted by the logistic model • How well does the logistic model fit the growth of real populations? – The growth of laboratory populations of some animals fits the S-shaped curves fairly well. Stable population Seasonal increase K-Selected Species • • • • • Poor colonizers Slow maturity Long-lived Low fecundity High investment in care for the young • Specialist • Good competitors r-Selected Species • • • • • Good colonizers Reach sexual maturity rapidly Short-lived High fecundity Low investment in care for the young • Generalists • Poor competitors Regulation of Population Growth Density-Dependent Factors • The logistic model is a description of intraspecific competition, competition between individuals of the same species for the same limited resources. • As population size increases competition becomes more intense • The growth rate declines in proportion to the intensity of competition • A density-dependent factor is a population-limiting factor whose effects intensify as the population increases in density. • Density-dependent factors may include accumulation of toxic wastes, stress, predation, limited food supply, limited territory, infectious diseases Density-Independent Factors • Are population-limiting factors whose intensity is unrelated to population density • Include abiotic factors such as fires, floods, sudden unpredictable severe cold spells, earthquakes and volcanoes and catastrophic meteorite impacts • In many natural populations, density-independent factors limit population size before density-dependent factors become important. • Over the long term, most populations are probably regulated by a mixture of both Density-independent and -dependent factors Introduction • Why do all populations eventually stop growing? • What environmental factors stop a population from growing? • The first step to answering these questions is to examine the effects of increased population density. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Population Cycles • A well studied example of boom and bust cycles are the cycles of Snowshoe hares • One of the hares’ predators, the lynx • The cause of these hare and lynx cycles may be winter food shortages for the hares • Overexploitation of hares by lynx • A combination of both of these mechanisms • Density-dependent factors increase their affect on a population as population density increases. – This is a type of negative feedback. • Density-independent factors are unrelated to population density, and there is no feedback to slow population growth. Fig. 52.13 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Negative feedback prevents unlimited population growth • A variety of factors can cause negative feedback. – Resource limitation in crowded populations can stop population growth by reducing reproduction. • Intraspecific competition for food can also cause density-dependent behavior of populations. – Territoriality. – Predation. – Waste accumulation is another component that can regulate population size. • In wine, as yeast populations increase, they make more alcohol during fermentation. • However, yeast can only withstand an alcohol percentage of approximately 13% before they begin to die. – Disease can also regulate population growth, because it spreads more rapidly in dense populations. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Population dynamics reflect a complex interaction of biotic and abiotic influences • Carrying capacity can vary. • Year-to-year data can be helpful in analyzing population growth. Some populations fluctuate erratically, based different factors. •Dungeness crab populations fluctuated hugely over a 40year period. •One key factor causing these fluctuations is cannibalism. •Large numbers of juveniles are eaten by older juveniles and older crabs. Water temperatures and ocean currents. •Small changes in these variables cause large fluctuations in crab population numbers. Fig. 52.18 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Other populations have regular boom-andbust cycles. – There are populations that fluctuate greatly. – A good example involves the lynx and snowshoe hare that cycle on a ten year basis. Applications of Population Ecology • Population ecology is used to • Increase populations of organisms we wish to harvest • Decrease populations of pests • Save populations of organisms threatened with extinction • Conservation of Endangered Species Example of application • A major factor in population decline is habitat destruction or modification. • The red-cockaded woodpecker requires longleaf pine forests with clear flight paths between trees • Suffered from fire suppression, increasing the height of the vegetation on the forest floor • Recovered from near-extinction to sustainable populations due to controlled burning and other management methods The human population has been growing almost exponentially for three centuries but cannot do so indefinitely • The human population increased relatively slowly until about 1650 when the Plague took an untold number of lives. – Ever since, human population numbers have doubled twice • How might this population increase stop? Age Structures • Help predict a population’s future growth. • Population momentum is the continuation of population growth as girls in the prereproductive age group reach their reproductive years. • Age structure diagrams may also indicate social conditions. An expanding population needs • Schools • Employment • Infrastructure Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 52.22 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Estimating Earth’s carrying capacity for humans is a complex problem • Predictions of the human population vary from 7.3 to 10.7 billion people by the year 2050. – Will the earth be overpopulated by this time? Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Wide range of estimates for carrying capacity. – What is the carrying capacity of Earth for humans? – This question is difficult to answer. • Estimates are usually based on food, but human agriculture limits assumptions on available amounts. • Ecological footprint. – Humans have multiple constraints besides food. – The concept an of ecological footprint uses the idea of multiple constraints. Our Ecological Footprint • An ecological footprint is an estimate of the amount of land required to provide the raw materials an individual or a population consumes, including: • Food • Fuel • Water • Housing • Waste disposal • For each nation, we can calculate the aggregate land and water area in various ecosystem categories. • Six types of ecologically productive areas are distinguished in calculating the ecological footprint: • • • • • • Land suitable for crops. Pasture. Forest. Ocean. Built-up land. Fossil energy land. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings – We may never know Earth’s carrying capacity for humans, but we have the unique responsibility to decide our fate and the fate of the rest of the biosphere. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings