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Population Ecology Chapter 55 1 Outline • • • • • • • • • Population Ecology Population Dispersion Metapopulations Demography Cost of Reproduction Biotic Potential Influence of Population Density Population Growth and Life History Models Exponential Growth 2 1 Ecology • The distribution & abundance of an organism • & its interactions with other organisms (biotic) • & its interactions with its environment (abiotic) Behavior Genetics Physiology Ecology Evolution 3 Is ecology & environment the same? Environment: immediate surroundings of an organism macro-environment micro-environment Abiotic factors temperature salinity H2O-dehydration/hydration humidity sunlight photoperiod O2/CO2 atmospheric pressure pH wind 4 2 Environmental Variation • • Key elements of an organism’s environment include: – temperature – water – sunlight – soil Many organisms actively employ mechanisms to maintain homeostasis, while others conform to their environment. 5 Responses to Environmental Change • • Individual responses – physiology – morphology – behavior Evolutionary responses – Shorter ears and limbs (Allen’s Rule) – Larger ears (Bergmann’s Rule) 6 3 Morphological and Behavioral Adaptation 7 Populations • • A population consists of group of interbreeding individuals of a given species living together at the same place at the same time Population distributions – Most species: limited geographic range – Population ranges change through time. § environment changes § organisms circumvent previously inhospitable habitat 8 4 Altitudinal Shifts 9 Population Dispersion • • • Randomly spaced – Individuals are randomly spaced within a population. Uniformly spaced – Uniform spacing within a population often results from resource competition. Clumped spacing – Individuals clump into groups or clusters in response to uneven distribution of resources in their immediate environment. 10 5 Population Dispersion 11 Population Dispersion 12 6 Population Dispersion • • Human effect – By altering the environment, humans have allowed some species to expand their ranges, although the overall effect has most often been detrimental. Dispersal mechanisms – light seeds (wind) – hooks and hairs (animal fur) – fruits (animal digestive systems) 13 Seed Adaptations 14 7 Metapopulations • • Metapopulations are a network of distinct populations interacting with each other by exchanging individuals (genes). – degree of interaction depends on dispersal – Rate of extinction in habitats is balanced by the rate of colonization of empty habitats. – prevent long-term extinction Source-sink metapopulations occur in areas where some habitats are suitable for long-term maintenance, while others are not. 15 Demography • • Demography - statistical study of populations Factors affecting growth rates – sex ratio § effect also determined by mating habits – generation time § average interval between birth of an individual and the birth of its offspring 16 8 Demography • Age structure – cohort - individuals of the same age § fecundity - number of offspring produced in a given period of time § mortality - number of deaths in a given period of time – Age structure determined by the relative number of individuals in each cohort. 17 Demography • • Life tables – constructed by following the fate of a cohort from birth until death Survivorship curves – Survivorship is the percentage of an original population that survives to a given age. § Type I - full life span § Type II - mortality unrelated to age § Type III - early susceptibility 18 9 Survivorship Curves 19 Cost of Reproduction • A life history is the complete life cycle of an organism. – all involve significant trade-offs § Due to limited resources, increased reproduction may decrease survival and chances of future production. v cost of reproduction Ø Natural selection will favor the life history that maximizes lifetime reproductive success. 20 10 Cost of Reproduction • Investment per offspring – Key reproductive tradeoff concerns the amount of resources to invest in producing any single offspring. § number of offspring versus size of each offspring v In many species, offspring size critically affects chances of survival. 21 Cost of Reproduction • • Reproductive events per lifetime – semelparity - organisms focus all reproductive efforts on a single, large event – iteroparity - organisms produce offspring several times over many seasons Age at first reproduction – Longer-lived animals tend to reproduce later, and provide more parental care than shorter-lived animals. 22 11 Biotic Potential • Exponential growth model – Rate at which a population of a given species will increase when no limits are placed on the rate of growth. § Innate capacity for growth of any population is exponential. v Even when rate remains constant, actual increase in number accelerates as the population size grows. 23 Biotic Potential • • Carrying capacity – Carrying capacity (K) is the maximum number of individuals the resources in a given area can support. Logistic growth – As a population approaches carrying capacity, its growth rate slows as resources become depleted. § sigmoidal growth curve 24 12 Population Growth 25 Influence of Population Density • Density-dependent population controls – As populations approach their carrying capacity, competition for resources can be severe, leading to decreased birth rate and increased risk of mortality. § In some cases, growth rates increase with population size. v Allee effect 26 13 Influence of Population Density • • Density-independent population controls – Growth of a population at any instant is limited by some factor other than population size. § external environmental conditions Population cycles – snowshoe hares § Food plants (willows) and predators (lynx) both control the population. 27 Linked Population Cycles 28 14 Population Growth and Life History Models • • K-selected species tend to produce relatively few, large offspring. – large investment in parental care r-selected species tend to produce many, small offspring. – small investment in parental care 29 Exponential Growth • Throughout most of human history, human populations have been regulated by food availability, disease, and predators. – resemble K-selected species – Due to the easing of environmental restraints, the human population has grown explosively over the last 300 years. 30 15 31 Advent of Exponential Growth • Population pyramids – bar graph displaying number of people in each age category § stable - rectangular § rapid growth - triangular § decreasing - inverted pyramid 32 16 Population Pyramids 33 Population Pyramids • • Uncertain future – Earth’s rapidly growing human population is possibly the greatest challenge facing facing the biosphere. – uneven resource distribution § population distribution § wealth gap § ecological footprint Population growth rate on the decline 34 17 Ecological Footprints 35 Summary • • • • • • • • • Population Ecology Population Dispersion Metapopulations Demography Cost of Reproduction Biotic Potential Influence of Population Density Population Growth and Life History Models Exponential Growth 36 18