ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Twenty-Four Outline INTRODUCTION Ecology The study of relationships of organisms with one another and the environment Earth is straining to support its largest population of humans fig 24.1 A complex area of biology with important implications Organisms Grouped in Progressively More Inclusive Levels of Organization Community: populations of different organisms living together Ecosystem: community plus its nonliving factors Biome: major collections of land plants, animals and microorganisms POPULATIONS Definition: Individuals of a Given Species that Occur in One Place at One Time Have Characteristic Features Examples: size, density, dispersion and demography Occupies particular place and plays particular role defined as its niche POPULATION SIZE AND DISPERSION Population Size Is an Important Feature Indirectly relates to the ability of a given population to survive Very small populations are more likely to become extinct Inbreeding can be a negative factor Lowers vigor by direct genetic effects Produces reduced levels of variability Extinction is more likely to occur in areas that change radically Population Density Is Very Important With wide spacing, individuals may only rarely interact Related measure is dispersion: way in which individuals are arranged fig 24.2 Randomly spaced Evenly spaced Clumped Clumped distributions are frequent in nature Individuals tend to group within particular microhabitats Microhabitats are not generally uniformly distributed POPULATION GROWTH Key Characteristic of a Population Is Its Capacity to Grow Population numbers remain constant regardless of offspring produced Unchecked, most populations would increase dramatically Under some situations populations can increase rapidly fig 24.3 Must consider circumstances and factors that limit population growth Biotic Potential Intrinsic rate of natural increase dN/dt = riN N = number of individuals within a population dN/dt = rate of change of population number over time ri = intrinsic rate of growth for that population Difficult value to calculate Actual rate of population growth is more readily calculated figure Difference between birth rate and death rate per given number of individuals Actual rate of growth also affected by emigration and immigration Innate capacity for growth is exponential, represented by growth curve Rate of growth remains constant Actual increase in numbers accelerates as population increases Analogous to compounding interest on an investment Such patterns of growth occur for only short periods Carrying Capacity Populations always reach a limit imposed by environmental shortages Size for such stabilization is the carrying capacity A dynamic rather than static value Number of individuals fluctuates around a mean value dN/dt = rN(K-N/K) dN/dt = growth rate of the population r = rate of increase N = number of individuals present at any one time K = carrying capacity As a population grows in size, the rate of increase declines until N=K Competition among individuals for resources increases Build up of wastes Increased ratio of predation Relationship is an S-shaped sigmoid growth curve fig 24.4 As the population stabilizes its rate of growth slows down Density-Dependent and Density-Independent Effects Density-dependent effects Depend on size of population, regulate its growth Accompanied by hormonal changes that alter animal behavior fig 24.5 In general have an increasing effect as population increases Density-independent effects Operate regardless of the population size Include factors such as weather and physical disruption of habitat Agriculture depends on characteristics of a sigmoid growth curve After an area has been cleared, populations grow rapidly Very high net productivity Commercial fisheries exploit populations in rapid growth phase Harvest at the steep, rapidly growing part of the curve Produces optimal yield, maximum sustainable catch from population Over harvesting smaller population can destroy its productiveness r strategists and K Strategists Many species have fast rates of population growth Not a sigmoid curve Growth not effectively controlled by reductions in population size Small populations quickly enter an exponential pattern of growth Population reduction in slow-breeding organisms may cause extinction Populations with sigmoid growth curves limited by carrying capacity (K) Include relatively slow-breeding organisms Tend to live in stable, predictable habitats Called K strategists Other species characterized by exponential growth and sudden crashes Have high intrinsic rate of growth (r) Called r strategists Many organisms are not clearly delineated not pure r or k strategist Have reproductive strategies between the two extremes Change from one extreme to other with environmental conditions Reproduction in r strategists Reproduce early, have many offspring fig 24.6 Offspring are small, mature rapidly, receive little parental care Generations are relatively short, large brood size Examples: dandelions, aphids, mice, cockroaches Reproduction in K strategists Reproduce late, have small broods Offspring are large, mature slowly, receive intensive parental care Generations are relatively long Examples: coconut palms, whooping cranes, whales Many organisms in danger of extinction are K strategists Human Populations Like all other organisms, size is controlled by the environment Humans have expanded populations by technical innovations Early in history controlled by density-dependent and density-independent factors Migration influences adjustment of human populations to particular areas Changes in technology have fostered explosive population growth MORTALITY AND SURVIVORSHIP Intrinsic Rate of Increase Depends on Age and Reproductive Performance Constant environment stabilizes a population`s age distribution Distribution varies by species and regions Sex distribution can also affect population growth statistics Generation time also affects rate of growth Survivorship Curves Express Characteristics of Populations fig 24.7 Survivorship: percentage of original population that survives to a given age Mortality: rate of death Types of survivorship curves Type II Straight curve Individuals are likely to die at any age Example: hydra Type III Produce vast numbers of offspring, few survive to reproduce Once established mortality is low Example: oysters Type I Relatively low mortality when young High mortality in postreproductive years Example: humans Many animal and protist populations are between type II and III Many plant populations are closer to type III DEMOGRAPHY Statistical Study of Populations Measurement of people, therefore the characteristics of populations Helps predict ways in which sizes of populations will alter the future Accounts for age distribution and changing population size over time Stable Population Population with constant size through time Birth + immigration = death + emigration Age structure also remains constant Population Pyramid fig 24.8 Graphical illustration of a population`s characteristics Male and female counts on opposite sides of the vertical age axis Shows population composition by age and sex Can view historical trends of demographic events Examples of human populations fig 24.9 Number of females disproportionately larger than males Females generally have longer life expectancy INTERSPECIFIC INTERACTIONS THAT LIMIT POPULATION SIZE Competition: General Interspecific competition Interaction of individuals of different species Use the same resource that is in short supply Greatest between organisms that obtain food in same manner Most intense between closely similar organisms Intraspecific competition occurs between individuals of a single species Competitive exclusion Two species competing for the same resource One species will use the resource more efficiently That species will eventually eliminate the other species Results of laboratory experiments not readily predictable Example: two species of flour beetle One species would always become extinct Extinct species dependent on environmental conditions, genetics Competition: Examples from Nature fig 24.10 Example: two species of barnacles fig 24.11 One species lives in shallower water, other in deeper water In deeper zone, deep species always outcompeted shallow species If deep species removed, shallow species inhabited deep regions Deep species conversely could not survive in shallow waters Example: five species of warblers fig 24.12 All five initially appeared to be competing for same resources With closer observation, each feeds in different part of tree Each species thus eats different subset of insects Species not truly in competition Predator-Prey Interactions Predation limits size of populations Predation and parasitism are two ends of the same spectrum Predator may exterminate prey, having no food source it dies out fig 24.13 With refuges for the prey, predator-prey populations will cycle fig 24.14 Prey populations driven to low but recoverable numbers Predator numbers subsequently decrease Prey numbers increase Predator numbers increase Such relations are important to biological control Near eradication of prey may cause extinction of predator Prey must survive in small numbers for predator to survive Example: prickly pear cactus in Australia fig 24.15 Became abundant in grazing areas Introduction of moth for biological control Cactus rare, moths still exist to keep them in check Example: American chestnut populations damaged by fungus Organisms producing disease that kills the host are not "successful" Eliminate own source of food Less virulent strains favored by natural selection survive Example: rabbit viral disease, myxomatosis fig 24.16 Rabbits introduced into Australia, soon overpopulated areas Virus introduced, most rabbits died Most virulent strains died along with their rabbit hosts Populations of both organisms now in balance Relationships between large carnivores and grazing animals Moose and wolves on Isle Royale Moose died of other causes, not regulated by wolf population fig 24.17 Intricate interactions between predators and prey Predators control levels of some species, survival of other enhanced Predators greatly reduce competitive exclusion Feedback systems control structure of natural communities THE NICHE Description of a Niche Includes space, food, temperature, conditions for mating, moisture Also takes into account behavior at various seasons or times of day Niche is not synonymous with habitat Realized niche Actual niche of an organism The role the organism plays in a particular ecosystem Fundamental niche Theoretical niche The role the organism would play in the absence of competitors Complex ecosystem can support more species, i.e. rainforest Competition more direct in ecosystem with fewer species, i.e. tundra Niche of an Organism Can Change Over Time Niche is wider if organisms reach a new habitat lacking other organisms Species may become increasingly different as they evolve Possibility for coexistence No longer subject to competitive exclusion Restatement of Gause`s principle of competitive exclusion No two species can occupy the same niche indefinitely Coexist while competing for the same resources One or more features of niche will always differ Niche is a complex concept involving all environmental facets Role of competitive exclusion more obvious when resources are drastically limited Factors defining the niche are difficult to determine