Population and Community Ecology

Complexity

POPULATION ECOLOGY

Population Change = (births + immigrations) – (deaths + emigrations)

Population Characteristics

• Population Size (N) is the total number of individuals within a defined area at a given time

• Population Density is the number of individuals per unit area (or volume)–Used to determine rarity/abundance, impact– Important for wildlife managers

Population Characteristics

• Population Distribution is a description of how individuals are distributed with respect to one another; 3 types:–Uniform–Random–Clumped

Population Characteristics

• Population Sex Ratio is the ratio of males to females (duh); usually 50:50

• Population Age Structure is a description of how many individuals fit into particular age categories

Factors Influencing Population Size

• Density-Dependent Factors influence an individual’s probability of survival and reproduction that depends on size of the population

• Limiting resource is a resource a population cannot live without; food, water nutrients

• Carrying capacity (K) is the limit to how many individuals the food supply can support

Factors Influencing Population Size

• Density-Independent Factors influence an individual’s probability of survival and reproduction regardless of population density

• Factors include tornadoes, hurricanes, fires, volcanic eruptions and other climatic events

GROWTH MODELS

The Exponential Growth Model

• Growth Rate = No. of births – No. of deaths• Intrinsic Growth Rate (r)is the maximum

potential for growth of a population under ideal condition

• J-shaped Curve results when rapid growth of a population not limited by resources occurs

The Exponential Growth Model

• Mathematically, The exponential growth model is:

Nt = N0ert • Nt = future population

• N0 = current population• r = intrinsic growth rate• t = time• e = natural log

The Logistic Growth Model

• Logistic Growth Model describes a population whose growth is initially exponential, but slows as the population reaches carrying capacity

• S-shaped curve is the graphical result• Variations on the Logistic Model:• Overshoot occurs when a population

temporarily exceeds its carrying capacity (ex. – spring births); die-off occurs

REPRODUCTIVE STRATEGIES

K-Selected Species

• Abundance determined by carrying capacity• Low intrinsic growth rate• Typically are large organisms• Late reproductive maturity• Produce few, large offspring• Provide substantial parental care

r-Selected Species

• High intrinsic growth rate (r)• Rapid population growth followed by

overshoots and die-offs• Tend to be small organisms• Early reproductive maturity• Reproduce frequently, producing many offspring• Provide little or no parental care

Survivorship Curves

SPECIES INTERACTIONS

Competition

• The struggle of individuals to obtain a limiting resource

• Competitive exclusion principle states that two species competing for the same limiting resource cannot coexist

• Resource partitioning can result if two species divide a resource based on differences in behavior and morphology

Competition

• Temporal partitioning different species hunt at different times

• Spatial partitioning different species reduce competition by using different habitats

• Morphological partitioning over time, species evolve morphologically to reduce competition (Darwin’s finches)

Predation

• True predators kill their prey and consume most• Herbivores consume plants• Parasites live on or in the organism (host); rarely

kill the host; a type of symbiosis• Pathogens are parasites that cause disease• Parasitoids lay eggs inside other organisms

Mutualism

• Will benefit two interacting species by increasing both species’ chances of survival

• Example – pollinators• A type of symbiosis

Commensalism

• Relationship in which one of the interacting species benefits but the other is neither harmed nor helped

• A type of symbiosis

Keystone Species

• Species that plays an important role in a community

• Typically exist in low numbers• May be predators, sources of food, mutualistic

species, or providers of some other essential service

• Ecosystem engineers

Invasive Species

• Invasive Species

COMMUNITIES CHANGE OVER TIME

Primary Succession

• Occurs on surfaces initially devoid of soil• Examples: abandoned parking lot, rock after

glacial retreat, newly cooled lava• Type of community that develops depends on

climate

Secondary Succession• Occurs in areas that have been disturbed but

have not lost their soil• May follow events such as hurricanes,

tornadoes, etc• Pioneer species colonize new areas rapidly

Aquatic Succession• Most common example: freshwater lakes

FACTORS AFFECTING SPECIES RICHNESS

Latitude• Number of species declines as one moves

northward/southward of the equator• Southern U.S.: 12,000 species, Canada: 1,700

species

Time• The longer a habitat exists, the more the

colonization, speciation, and extinction can occur

Habitat Size and Distance• Theory of island biogeography: dual

importance of habitat size and distance from colonizing species determines species richness

• Larger habitats can support larger populations• Oceanic islands farther from continents tend to

have fewer species