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Definitions Population ecology • Habitat • Population • Community • Ecosystem • Biotic Factors • Abiotic Factors Characteristics of Populations Age Structure Diagrams • Size Shows age distribution of a population • Age Structure • Density • Distribution 45.1 Rapid Growth Slow Growth Zero Growth Negative Growth Distribution Distribution clumped • Different species occupying the same area compete… • Clumped populations: – Suitable physical, chemical, and biological conditions are patchy, not uniform. – These interactions influence the density and dispersion of individuals and populations. – Social groups – Offspring not mobile 45.1 45.1 1 Distribution Distribution • Rare in nature uniform random • Result of fierce competition for very limited resources • Uniform environmental conditions • Members are neither attracting nor repelling each other 45.1 45.1 Capture-Recapture Method Determining population size • Direct counts are most accurate but seldom feasible Total population size Total captured in sampling 2 Marked individuals in sampling at time 1 Marked individuals in sampling at time 2 • Sample an area, then extrapolate • Capture-recapture method is used for mobile species 45.2 45.2 Survivorship Curves • represent age-specific patterns of death for a given pop in a given environment • Each species has a characteristic curve. • Three types of curves are common in nature. 45.5 Fig. 45-10a, p.805 2 Fig. 45-10c, p.805 Fig. 45-10b, p.805 Reproductive Strategies Life History Patterns • Patterns of timing of reproduction and survivorship • Vary among species • Different environments and population densities can favor different reproductive strategies • r-selection favors traits that maximize number of offspring • Summarized in survivorship curves and • K-selection favors traits that improve offspring quality life tables Life History and Reproductive Strategies-Certain combinations of traits tend to occur together: Trait r-selected K-selected Lifespan Short Long Growth rate Fast Slow 1st Reproduction Early Late Reproductive effort High Low Parental care Less More Offspring Many, small Few, large Reproductive episodes all at once more than once Kind of environment Variable Stable r- or K- selected? • • • • • • Mouse Mushroom Oak tree Human Elephant Dandelion 3 r • Net reproduction per individual per unit time Population Growth r = b - d • Can be used to calculate rate of growth of a population 45.3 45.3 Exponential Growth (G) Per capita growth rate x Total population (r) (N) POPULATION SIZE Population growth per = unit time G=r x N Exponential Growth: Any quantity that is growing at a rate proportional to its size. TIME 45.3 45.3 Biotic Potential • Biotic potential: Maximum rate of increase Limits on Growth of Populations per individual (r) under ideal conditions • Varies between species • In nature, biotic potential is rarely reached Limiting factor: any essential resource that is in short supply 45.3 4 Carrying Capacity (K) Actual rate of increase is influenced by environmental conditions: • nutrient supply • competition for space • Pollution • floods • etc. • Maximum number of individuals that can be sustained in a particular habitat • Logistic growth occurs when population size is limited by carrying capacity 45.4 45.4 Logistic Growth Logistic Growth Population growth per unit = time (G) Net reproduction per indiv. per unit time x # Individuals (N) x Proportion of resources not yet used (r) 45.4 45.4 Overshooting Capacity • Population may temporarily increase above carrying capacity POPULATION SIZE Carrying capacity Resources become limiting Births = Deaths • Overshoot is usually followed by a crash; dramatic increase in deaths Fastest growth of pop. Resources abundant Births > Deaths Reindeer on St. Matthew’s Island TIME 45.4 Fig. 45-9, p.803 5 Density-Dependent Factors Density-Independent factors • Logistic growth equation • Factors unaffected by population density. deals with densitydependent controls • Natural disasters, climate changes, etc. • Limiting factors become more intense as population size increases 45.4 Human Populations World population density Human Population Problems Side-Stepping Growth Controls • World pop reached 6 billion in 1999 current population • About 2 billion live in poverty • Most resources are consumed by the relatively few people in developed countries • Expanded into new habitats • Agriculture increased carrying capacity; use of fossil fuels aided increase • Hygiene and medicine lessened effects of density-dependent controls 6 Future Growth Human Population Growth • Exponential growth cannot continue forever • Breakthroughs in technology may further increase carrying capacity • Eventually, density-dependent factors will slow growth Fertility Rates • Worldwide, average annual rate of increase is 2.6% • Total fertility rate (TFR) is average number of children born • Highest in developing countries, lowest in developed countries Fig. 45-17b, p.815 7