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• In looking at nature…never forget that every single organic being around us may be said to be striving to increase its numbers. – Charles Darwin, 1859 Major Characteristics of a Population • Population dynamics is a study of how populations change in: • size (total # of individuals) • density (# of individuals in a certain space) and • age distribution (the proportion of individuals in each age in a population) in response to changes in environmental conditions. • Dispersion patterns in nature Density Clumped • Why clumping? 4 reasons: • 1. The resources a species needs vary greatly in availability from place to place. • 2. Living in herds, flocks, and schools can provide better protection from predators. • 3. Living in packs gives some predators a better chance of gaining a meal (hunting advantage). • 4. Some animal species form temporary groups for mating and caring for their young. Dispersion Pattern: Uniform • Some species maintain a fairly constant distance between individuals. • Creosote bushes in the desert have this pattern so that there is better access to scarce water resources. Uniform Dispersion Pattern: Random • Organisms with a random distribution (dandelions) are fairly rare. The world is mostly clumpy. Entrances and Exits on the Global Stage • Populations increase through births and immigration and decrease through deaths and emigration. • Population change= (births + immigration) – (deaths + emigration) • These variables depend on changes in resource availability and other environmental changes. Age Structure • A population’s age structure can have a strong effect on how rapidly its size increases or decreases. • Usually described in terms of organisms that are: – not mature enough to reproduce (the prereproductive stage) – and those that are capable of reproduction (the reproductive stage), – and those that are too old to reproduce (the postreproductive age) Survivorship Curve • The size of a population that includes a large proportion of young organisms is likely to increase. • In contrast, the size of a population dominated by individuals past the reproductive stage (older) is likely to decrease. • If there is a fairly even distribution between these stages, the population will remain stable (births = deaths). 3 types of survivorship curves: • Early loss, constant loss, late loss Intrinsic rate of increase (r) • The rate at which a population would grow if it had unlimited resources. • Individuals in populations with a high rate of growth typically reproduce early in life, have short generation times, reproduce many times, and have many offspring each time they reproduce. • Ex: houseflies. Descendents of a single female housefly could total 5.6 trillion within 13 months. • This is exponential growth. – That would suck. Environmental Resistance • Fortunately, no population can grow indefinitely. There are always limits to growth. • Decrease factors: too much or too little light, temperature too high or low, unfavorable chemical environment, low reproductive rate, specialized niche, inadequate food supply, poor habitat, too many competitors, inadequate defense from predators, diseases and parasites, inability to migrate or adapt to change. Carrying Capacity (K) • Together, biotic potential and environmental resistance determine the carrying capacity. • This is the maximum # of individuals of a given species that can be sustained indefinitely in a given amount of space. • The growth rate of a population decreases as it nears the carrying capacity of its environment because resources (food and water) begin to dwindle. Carrying Capacity Our man Edward O. Wilson (and Robert H. MacArthur) • These two suggested that species could be classified into 2 fundamental reproductive patterns: • r-selected species • And • K-selected species r- selected species • • • • • • • • • Cockroach & dandelion Many small offspring Little or no parental care and protection of offspring Early reproductive age Most offspring die before reaching reproductive age Small adults Adapted to unstable climate and environmental conditions High population growth rate (r) Population size fluctuates wildly above and below carrying capacity (K) • Generalist niche • Low ability to compete • Early successional species K- selected species • • • • • • • • • Elephant & Saguaro cactus Fewer, large offspring High parental care and protection of offspring Later reproductive age Most offspring survive to reproductive age Larger adults Adapted to stable climate and environmental conditions Lower population growth rate (r) Population size fairly stable and usually close to carrying capacity (K) • Specialist niche • High ability to compete • Late successional species Review: • Population dynamics is a study of how populations change in: • Dispersion patterns: • 3 ways that age structure is broken down: • Some limiting factors: • What is carrying capacity? • Explain r- selected species vs. K- selected species. Homework: • Find 2 graphs on-line: • A J- curve graph and an S-curve graph • Explain in a paragraph what is going on ecologically and why • Monday we will discuss graphing and practice it!