Population Ecology

A few terms to get us started… Natality: birth rate (the number of births in a given time as related to the total population) Mortality: death rate (the number of deaths in a given time as related to the the total population

Properties of Populations A population is a group of organisms that belong to the same species and live in a particular place at the same time. Populations can be measured in terms of size, density, dispersion, growth rate, age structure, and survivorship.

Properties of Populations, continued Population Size A population’s size is the number of individuals that the population contains. Population Density Density is a measure of how crowded the population is. It is expressed as the # of organisms/given area.

Properties of Populations, continued Dispersion Dispersion describes the distribution of individuals within the population and may be random, uniform, or clumped.

Population Dynamics Age Structure Patterns of Mortality A population’s age structure indicates the percentage of individuals at each age. Patterns of Mortality Populations show three patterns of mortality or survivorship curves: Type I (low mortality until late in life) Type II (constant mortality throughout life) Type III (high mortality early in life followed by low mortality for the remaining life span).

Survivorship Curves

Two ways to study populations Survey—A random portion of the population is counted, and represents that population + easier to conduct - not necessarily accurate Census—Every individual in the population is counted/studied - Takes more time, more difficult or impossible + Results are more accurate

Population Growth Rate Demographers, scientists who study population dynamics, define the growth rate of a population as the amount by which a population’s size changes in a given time. Population Growth (Birth rate + immigration) – (death rate + emigration) = growth rate

The Exponential Model The exponential model describes perpetual growth at a steady rate in a population. The model assumes constant birth and death rates and no immigration or emigration. A population could reach its biotic potential (exponential growth rate) if these conditions are met: --environmental conditions are ideal --there are no restrictions on reproduction --mortality rate is kept extremely low

The Logistic Model In the logistic model, birth rates fall and death rates climb as the population grows. When the carrying capacity is reached, the number of individuals the environment can support is reached and population growth becomes stable.

Two models of population growth

A few definitions… Carrying capacity: this is the maximum number of individuals in a population that occupy a given habitat.  It is symbolized by the letter "K“.  At K, mortality equals natality. Limiting factor: any element or factor that slows or inhibits the growth of a population

Limiting Factors Population-limiting factors, such as competition, are density-dependent because the effect on each individual depends on the number of other individuals present in the same area.

Limiting Factors Population-limiting factors, such as bad weather and fires, are density-independent because the effect on each individual does not depend on the number of other individuals present in the same area. Perils of Small Populations Small populations have low genetic diversity and are subject to inbreeding, so they are less likely to adapt to environmental changes.

Population Fluctuation These graphs show the numbers of snowshoe hares and Canada lynx over a thirty-year period. Note how the lynx population reaches its peak one or two years after the hares have reached theirs. Why do you think this is the case?

A little story… St. Matthew Island in the Bering Sea

A little story… 6,000 reindeer lived here on St. Matthew Island in the Bering Sea in 1963. By the 1980’s, zero Reindeer remained.

Reproductive Strategies When studying populations, organisms are divided into two categories based on how they reproduce. The two categories are based on a complex equation shown on the next slide. You do not need to know this equation for this class, just the characteristics of the two reproductive strategies.

Reproductive Strategies ∆ N/ ∆t = r N((K-N) / K) = rate of growth ∆ means “change in” N is the number of individuals in a population t is a span of time r is the realized intrinsic rate of population growth K is the carrying capacity *r-selected species—capable of very rapid population growth, exponential growth pattern followed by a crash in the adult population *K-selected species—have more or less stable populations adapted to exist at or near carrying capacity in relatively stable habitats

Reproductive Strategies r- Selected (maximum growth rate, below carrying capacity) Early reproduction Short life span High mortality rate Little or no parental care Large investment in producing large numbers of offspring Below carrying capacity Examples: Frogs Grasshoppers Fish K-Selected (maximizes population size near carrying capacity) Late reproduction Long life span Low mortality rate Extensive parental care Greater investment in maintenance and survival of adults At or near carrying capacity Examples: Bears Humans Elephants

Population Ecology… Helps determine certain hunting/fishing limits and regulations Allows us to study affects of various factors (pollution, other organisms) on the population of a species Helps us predict future population numbers based on current trends Allows us to see how populations are related and even depend on each other (like St Matthew’s Island)