Chapter 5: Community Ecology

HOW DO SPECIES INTERACT?

Interspecific Competition

Members of two or more different species interact to gain access to the same limited resources

• Niches overlap; the greater the overlap, the more intense the competition

• No two species can occupy the same niche for very long; Competitive Exclusion

• Both species may suffer

Intraspecific Competition

Members of the same species interact to gain access to the same limited resources

Predation

• Predator – prey relationship; 1 species feeds directly on all or part of another species; usually on live species

• Help sustainability (ex. – kelp-urchin-otter)• Carnivores use either ambush or pursuit to

capture prey• Ambush: camouflage is used; adapted by prey

as well

Predation

• Chemical warfare: used by spiders, snakes, to paralyze prey

• Prey use adaptations such as speed, alert systems (senses) , avoidance (ex. – shells), mimicry and scare tactics

• Role in natural selection: weeding out the weak, aged, sick in a population

Predation

• Coevolution may occur; changes in one species gene pool leads to adaptive changes in another’s

Parasitism

One species (the parasite) feeds on the body of, or energy used by, another organism

• Parasite is much smaller than host• May weaken but rarely kills host• Some live in host (tapeworms) some attach to

outside of host (lampreys)• Some have little contact with hosts (cowbirds)• Coevolution can happen (malaria)

Mutualism

Interaction that benefits both species by providing food, shelter, or some resource

• Pollinators• Birds that eat parasites off skin of other

animals, also act as alarm system• Clownfish – anemone• Gut-inhabitant species

Commensalism

Interaction that benefits one species while other is not affected

• Epiphytes such as orchids and bromeliads

NATURAL SELECTION REDUCES COMPETITION

Resource Partitioning

When species competing for similar resources evolve traits that allow them to share resources at different times, ways, or places

• Examples: warblers and honeycreepers

LIMITS OF POPULATION GROWTH

Characteristics

Populations differ in:• Distribution• Numbers• Density• Age structurePopulation Dynamics studies changes in all the

above in response to environmental changes

3 Patterns of Distribution

• Clumping – example; desert vegetation around springs; location & size varies with availability of resources; offers advantages

• Uniform Distribution• Random Distribution

Numbers

Numbers vary cyclically• Population Change = (births + immigration) –

(deaths + emigration)• Age Structure: proportions of individuals in

various age groups– Pre-reproductive– Reproductive– Post-reproductive

Indefinite Population Growth? No!

• Biotic Potential: large animals at a disadvantage; low biotic potential

• Intrinsic Rate of Increase (r): rate at which population would increase if unlimited resources are available

• High r value: reproduce early, often, short generation time, produce many offspring

• Scientific Principles of Sustainability – always limits population growth

Environmental Resistance

Combination of all factors that limit population growth

• With biotic potential, it determines the Carrying Capacity (k); the maximum population a habitat can sustain

• Exponential growth: 1-2% increase; when graphed, produces a “J” curve

• Logistic Growth: rapid exponential growth followed by a leveling off; when graphed, produces an “S” curve

Population Crash

• No logistic curve is achieved• Brought on by a reproductive time lag in rates

of births and deaths• Die-back occurs• Sometimes when a population exceeds k, it

causes damages that reduce k (over-grazing in the US)

Reproductive Patterns

• r-selected species: have many offspring, give little or no parental care, are opportunists, susceptible to population crash

• K-selected species: reproduce later in life, produce small numbers of offspring, matire slowly, longer life spans, parental protection, logistic pattern

Genetic Diversity

Affects smaller populations:• Founder Effect: a population colonizes a new

habitat• Demographic Bottleneck: few individuals survive

a catastrophe• Genetic Drift: random changes in gene frequency

→ unequal reproductive success• Inbreeding: increases frequency of defective

genes

Population Density

Number of individuals per unit area or volume• Density-Dependent Controls: predation,

infectious disease, competition• High Density: successful reproduction, leads to

increased competition• Abiotic Controls are density independent (ex. -

Types of Population Change

• Stable: population fluctuates slightly above and below its carrying capacity

• Irruptive: occasional population explosion followed by a crash to stable level; algae, insects (summer – winter)

• Cyclic: follow a top-down or bottom-up regulation

• Irregular: no recurring pattern

Humans not Exempt

• Irish potato famine• Bubonic plague• AIDS/HIV

ECOLOGICAL SUCCESSION

Primary Succession

A gradual establishment of biotic communities in lifeless areas where there is no soil in terrestrial ecosystems and no bottom sediment in an aquatic ecosystem

• Examples: bare rock from glacial retreat, newly cooled lava, parking lot or highway, newly created pond or reservoir

• Involves a pioneer species

Primary Succession

• Slow process• Pioneer species begin soil formation (ex –

lichens and mosses)• Followed by mid-successional plants (shrubs,

grasses, herbs)• Late-successional plants (trees)

Secondary Succession

A series of communities or ecosystems with different species develop in places containing soil or bottom sediment

• Occurs where an ecosystem has been disturbed, removed, or destroyed

• Include abandoned farmland, burned or cut forests, heavily polluted streams, flooded land

Secondary Succession

• Both types of succession increase biodiversity and thus the sustainability of communities and ecosystems

• Environmental disturbances can set both processes back

• Succession does not follow a predictable path

Stability in Living Systems

• Inertia (Persistence): the ability of a living system to survive a moderate disturbance

• Resilience: the ability of a living system to be restored through secondary succession after a moderate disturbance

• Ecosystems are one or the other• Tipping point comes into play (systems dealing

with multiple stresses)