Chapter 53 Population Ecology Population Ecology Study of

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/m 3 – Trees - 5, 000/km 2 – Deer - 4/km 2

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 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 = r. N • N = population size • t = time • r = rate of increase

From Calculus • The equation DN/Dt = r. N becomes: • d. N/dt = rmax N • rmax = intrinsic rate of increase

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

Logistic Growth • d. N/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 densityindependent growth factors.