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Ecosystem Change Life constantly undergoing change Populations change or evolve Ecosystems evolve also Change in populations size: population dynamics (more detail later) Changes may include increase and decreases in population size Growth Curves Populations inherent ability to exponentially increase population size They don’t Why Populations Change in size (total number of individuals) Change in density (number of individuals in a certain space) Change in age distribution (proportion of individuals in each age group) Change in habitat distribution Clumps Resources may vary greatly from place to place Better protection against predators Some predator species increase chances of securing a meal—wolf pack Temporary clumps for mating and care for young Population Measurement Count directly Mark the presence of individuals in a representative area and extrapolate to a larger area Estimated based on sign of presence such as droppings, birdcalls, etc. Change in Population Size Increases are exponential, not arithmetic Increase is proportional to number of individuals already present J-shaped growth Exponential growth: slow at first, followed by increasingly faster rates of growth, also called Geometric growth: growth follows a geometric pattern of increase (2,4,8,16) Rule of 70: divide 70 by annual percentage yields approximate doubling time in years S-shaped Growth Growth model starts off like J-shaped form, but rate of growth slows and ceases altogether Stabilizes at certain level, denoted as K Environmental resistance limits growth in S-shaped growth Logistic growth Biotic Potential Capacity for growth “r”=intrinsic rate of increase if the population has unlimited resources Most populations grow at rate less than this Environmental Resistance All factors that act to limit growth Abiotic: light, temperature, critical nutrients Biotic: specialized niche, too many competitors, drought, low repro. rate, inability to fight disease or migrate or adapt Carrying Capacity Biotic potential and environmental resistance determine carrying capacity “K” Maximum # of individuals of given species that can be sustained indefinitely in a given space (area or volume) Growth rate decreases as size nears K, resources begin to dwindle Real World Population growth varies in complex ways May show J- or S-shaped growth at various times Growth rates may hover around K K Carrying capacity not fixed Stable in stable environments, population can hover above K or below K depending on environmental variables Patterns Rate of population decrease is faster than increase=population crash or dieback Population explosion followed by dieback is called irruptive of Malthusian growth Growth Rate Difference between birth and death rate over a period of time birthrate=# of individuals born in a given time expressed as a proportion of the total population=r Growth Rate Death rate=# of individuals who die in a given time Growth rate (r)=birth rate-death rate r<0: population shrinks r>0: population grows r=0: population at zero population growth Reproductive Strategies r-selected or Malthusian: Opportunistic species in highly variable environments High growth rates—take advantage of infrequent favorable conditions Reproduction is rapid, with little care of offspring r-selected High growth rates—take advantage of infrequent favorable conditions Pressures are density-independent: physical (abiotic) forces and predation limit growth Smaller size r-selected Reproduction is rapid, with little care of offspring, many offspring Sheer numbers and effective dispersal Reproductive Strategies K-selected or Logistic: relatively stable environment, numbers fluctuate within narrow range around K Population size limited by carrying capacity Reproduction slower, considerable care of offspring K-selected Larger size Pressures are density-dependent: as population increases, food and living space limit growth Live longer, mature slowly, fewer offspring/generation Examples r-selected: rodents, many insects, marine invertebrates, weedy annual plants K-selected: whales, wolves, elephants, primates What about humans? +/- Population Size Emigration—movement of individuals out of population Dispersal Take genes with them—protect species in case of catastrophe Factors that Regulate Growth Intrinsic—within organism Extrinsic—outside the population Biotic Abiotic Density-dependent Density-independent Density-independent Abiotic-weather, climate Extremes at wrong time of year Without regard to the number of organisms present Density-dependent Decrease natality, increase mortality Interactions between populations Some within population Interspecific—predator-prey population levels, peaks and valleys slightly offset Cyclical Density-dependent Intraspecific—compete for resources As carrying capacity is reached, resources are limited First, quickest, clever, lucky Territoriality Fighting equipment Attractiveness-song and dance Density-dependent Stress—”stress shock” causes physical and psychological and behavioral changes Combinations—OK food and water, space is lacking, lab animals, humans, chickens Humane livestock practices—not crowding chickens, livestock, “slow food” movement Population Size Maintenance Conservation biology: maintain populations for long term viability Maintain biodiversity: Number of species and Genetic variation