Chapter 53 Population Ecology

Population Ecology Study of the factors that affect population size and composition.

Population Individuals of a single species that occupy the same area.

Population Dynamics

Important Characteristics 1. Density 2. Dispersion

Density Number of individuals per unit area or volume. Ex: Diatoms - 5 million/m3 Trees - 5,000/km2 Deer - 4/km2

Dispersion Pattern of spacing among individuals. Types: 1. Clumped 2. Uniform 3. Random

Clumped Dispersion May result form a patchy environment. May increase chances for survival. Ex: Schooling behavior Flocks of birds

Uniform Dispersion Often the result of antagonistic interactions between individuals. Ex: Territories Spacing between desert plants

Random Dispersion Often the result of the absence of strong attractions or repulsions between individuals. Not a common pattern.

Demography The study of the vital statistics that affect population size. Ex: Birth and Death rates

Demographic Factors Age structure of the population. Birth and Death rates. Generation time. Sex ratio and reproductive behavior.

Life Tables Mortality summary for a cohort of individuals. First developed from life insurance studies.

Life Tables Show Mortality rate per year. Life span of the organism. Fecundity (birth rate).

Survivorship Curve Plot of the numbers of a cohort still alive over time. Curve Types: Type I Type II Type III

Type I Low early deaths. High late deaths. Ex: Humans Other large mammals

Type II Constant death rate. Ex: Annual plants Many invertebrates

Type III High early deaths. Low late deaths. Ex: Trees Oysters

Comment Curve type may change between young and adults. Ex: Nestlings - Type III Adult Birds- Type II

Life History Strategies Are the product of natural selection. Can favor Maximize survival Maximize reproduction

Examples Semelparity – one shot reproduction with many offspring. Ex. – Salmon, Agave Iteroparity – repeated reproduction events with a few offspring each time.

Life History Strategies 1. "r" or Opportunistic species 2. "k" or Equilibrial species

"r" Species Increase fitness by producing as many offspring as possible. Do this by: Early maturation Many reproductive events Many offspring

Result Maximize reproduction so that at least a few offspring survive to the next generation. Most offspring die (Type III curve).

"k" Species Increase fitness by having most offspring survive. Do this by: High parental care Late maturation Few reproduction events Few offspring.

Result Maximize survivorship of each offspring. Few offspring, but most survive (Type I curve).

What is the strategy For a weed? For an endangered species? For Garden Pests?

Population Growth DN/Dt = b - d Where: N= population size t = time b = birth rate d = death rate

Rate of Increase r = difference between birth rate and death rate. r = b - d

Assignments Read Chapter 53 or 43 in Hillis Chapter 52 – today Chapter 53 – Mon. 4/28 Labs – Transpiration, Aquatic Productivity

Equation: DN/Dt = rN N = population size t = time r = rate of increase

From Calculus The equation DN/Dt = rN becomes: dN/dt = rmax N rmax = intrinsic rate of increase

Exponential Growth dN/dt = rmax N Characteristic of "r" species. Produces a “J-shaped” growth curve. Only holds for ideal conditions and unlimited resources.

Logistic Growth dN/dt = rmax N K-N K K = carrying capacity

Result “S-shaped” growth curve. Characteristic of “k" species. Common when resources are limited.

Comment K is not a constant value. Populations often oscillate around “K” as the environment changes.

Additional Comments Populations often overshoot “K”, then drop back to or below “K”. The equations are now on the AP formula sheet. Be prepared to use them.

Regulation of Population Size 1. Density- Dependent Factors 2. Density- Independent Factors

Density-Dependent Affect is related to N. As N increases, mortality increases. Ex: Food, nesting space, disease

Density-Independent Affect is not related to N. Mortality not related to population size. Ex: Weather and climate

Population Cycles Cyclic changes in N over time. Often seen in predator/prey cycles. Ex: Snowshoe Hare - Lynx

Causes Density dependent factors. Chemical cycles. Saturation strategy to confuse predators.

Age Structure Diagrams Show the percent of a population in different age categories . Method to get data similar to a Life Table, but at one point in time.

Importances Can be used to predict future population growth trends, especially for long lived species.

Exponential Growth Produces age structures that are a triangle or pyramid shape.

Logistic Growth Produces age structures that have even sizes between most age categories.

Declining Populations Produce age structures with a narrow base and wider middles.

Summary Know density and dispersion patterns. Know Life Tables and survivorship curves. Be able to contrast and compare “r” and “k” strategies.

Summary Know exponential and logistic growth curves and equations. Know density and density-independent growth factors.