Population Control

Populations Population Group of interbreeding individuals of the same species Population Dynamics = Changes in populations (demography) Can change in size, density dispersion or age distribution Population Ecology = number of individuals of a species in an area AND how/why the number changed over time. Effected by Resource competition Predation Disease

Population Size Number too small Number too large Better chance of extinction Too much inbreeding Can’t adapt (not enough genetic diversity) Number too large Starvation Increased predation Decreased fertility Parasitism & disease are likely

Population Density Number at a certain time in certain space Ecological population density = individuals per unit of habitat Area for terrestrial Volume for aquatic

Population Density Population Density = # of individuals area or volume Determined by external factors Habitat Resources What is the population density of 200 birds in 20 square miles? A: 200/20 = 10

Most Populations Live in Clumps Population distribution Clumping Species cluster for resources/better chance of finding resources Protection from predators (allows some predators to get prey) Ability to hunt in packs Social

Most Populations Live in Clumps Uniformly dispersed Rare pattern Resources may be scarce-defend resources by physical/chemical means Randomly dispersed Resources evenly spread out Fairly rare

Populations Can Grow, Shrink, or Remain Stable Population size governed by: Births and immigration (b and i) Deaths and emigration (d and e) Population change = (births + immigration) – (deaths + emigration) Age structure Pre-reproductive age Reproductive age Post-reproductive age

Some Factors Can Limit Population Size Density dependent vs density independent factors Range of tolerance Variations in physical and chemical environment Individuals may have different tolerance ranges Trout tolerance of temperature

Some Factors Can Limit Population Size Limiting factor principle Too much or too little of any physical or chemical factor can limit or prevent growth of a population, even if all other factors are at or near the optimal range of tolerance Precipitation, nutrients, sunlight

What Limits the Growth of Populations No population can grow indefinitely because of limitations on resources and because of competition among species for those resources Change in response to environmental stress or change in environmental conditions Growth rate (r) r = (b-d) + (i-e) If r (+), population size If r (-), population size If r = 0, stationary population size

No Population Can Grow Indefinitely: J-Curves and S-Curves Biotic or reproductive potential (r) Maximum rate at which a population can grow if it has unlimited resources! High potential if: Reproduce early in life Has short generation times Can reproduce many times Produce many offspring each time There are always limits to population growth in nature Environmental resistance – factors that limit population growth Carrying capacity (k) Maximum population of a given species that a particular habitat can sustain indefinitely

No Population Can Grow Indefinitely: J-Curves and S-Curves “r” + environmental resistance = “k” Populations can exceed their carrying capacity When this happens The corrective negative feedback loop does not take effect immediately A dieback (population crash) results

No Population Can Grow Indefinitely: J-Curves and S-Curves Exponential growth When populations are growing at biotic potential At a fixed percentage per year Logistic growth Population faces environmental resistance

When a Population Exceeds Its Carrying Capacity It Can Crash A population exceeds the area’s carrying capacity Members will die unless they adapt or move to another area with more resources Reproductive time lag may lead to overshoot Subsequent population crash Damage may reduce area’s carrying capacity

Minimum Viable Population (MVP) Estimate of smallest # of individuals necessary to ensure the survival of a population If below MVP = extinction likely Individuals can’t find mates Genetically related individuals breed = weak or malformed offspring Genetic diversity = too low

Case Study: Exploding White-Tailed Deer Population in the U.S. 1900 – deer habitat destruction and uncontrolled hunting 1920s–1930s – laws to protect the deer Current deer population explosion Spread Lyme disease Deer-vehicle accidents Eating garden plants and shrubs

Different Species Have Different Reproductive Patterns “r-strategists” Life based on r (growth rate) Rapidly increase numbers Below carrying capacities for long periods of time “k-strategists” Life based on carrying capacity (k) Live in a state of equilibrium Close to carrying capacity

Reproductive Strategies: “r-strategists” Live in disturbed environments Generalists Have populations that fluctuate rapidly in size Do not compete well with other species Are widely distributed Slow to respond to ecological opportunities but live in a wide variety of environments Short-lived Have many relatively small young Have short periods of embryonic development Reach adulthood rapidly Small sized adults Invest little or no parental care in young Reproduce once per lifetime Go through regular boom and bust cycles

Reproductive Strategies “k-strategists” Live in stable environments Specialists Have populations stable in size Compete well against other species Restricted in distribution and where they can live Take advantages of ecological opportunities but live in specific types of environments Are long-lived Have few, relatively large young Have long periods of embryonic development Reach adulthood slowly Large sized adults Invest intensive parental care in young Reproduce throughout lifetime Do well when population size is near “k”

Survivorship Curves Many organisms have reproductive patterns between the extremes of “r” & “k” Some change from one to the other depending on environment R-strategists: early loss K-strategists: late loss In between = constant loss

Humans Are Not Exempt from Nature’s Population Controls Ireland Potato crop in 1845 Bubonic plague Fourteenth century AIDS Current global epidemic