Chapter 8 Community Ecology: Structure, Species Interaction, Succession, and Sustainability

Key Concepts Community Structure Roles of Species Species Interactions Changes in ecosystems Stability of ecosystems

Focus Questions What determines the number of species in a community? How can we classify species according to their roles? How do species interact with one another? How do communities change as conditions change? Does high species diversity increase the stability of ecosystems?

What is Community Structure? 4 characteristics: Physical appearance Species diversity Species abundance Niche structure

1. Physical Appearance Types of plants Relative sizes of plants Stratification of plants and animals Aquatic life zones Edge effects: differences in physical properties (sun, temp, wind) at ecotones

2. Species Diversity Highest biodiversity: rain forests, coral reefs, deep ocean Areas with high biodiversity may have low species abundance (few members of each species) 3 factors affect biodiversity: Latitude (decreases with distance from equator) Depth in aquatic systems (highest at surface and bottom) Pollution Biodiversity increases with: More solar radiation More precipitation Less elevation Seasonal variations

Biodiversity decreases with distance from equator 100 Species Diversity Species Diversity 10 Latitude 80ºN 60 40 20 30ºS

3. Species Abundance Theory of Island Biogeography (aka Species Equilibrium Model): number of species on an island is determined by a balance between the immigration rate of new species and the extinction rate of species already on the island

(a) Immigration and extinction rates © 2004 Brooks/Cole – Thomson Learning High Rate of immigration or extinction Low Equilibrium number Number of species on island (a) Immigration and extinction rates

Theory of Island Biogeography, continued Immigration and extinction rates are affected by: Size of the island Distance from mainland Small islands=lower biodiversity Small islands=higher extinction rate

General Types of Species Native Non-native Indicator Keystone *Scientists apply labels to clarify their niches

Types of Species Native: species that normally live in a particular ecosystem Non-native (exotic, alien): species that migrate into an ecosystem or are accidentally introduced into an ecosystem by humans

Non-native Species Example: 1957 Brazil imported wild African bees to increase honey production (aggressive and unpredictable) Displaced domestic honeybees and reduced honey supply

Indicator Species Species that serve as early warnings of damage to a community or ecosystem Example: birds-found everywhere and respond quickly to change trout and amphibians-indicate good water quality—they require clean water with good oxygen supply

Keystone Species Species with much more important roles than their abundance suggests Critical roles include: Pollination by bees, hummingbirds Dispersion of seeds by fruit-eating animals (bats) Habitat modification (elephants, beaver dams) Predation by top carnivores to control populations (wolf, lion, alligator) Recycling of animal wastes (dung beetle)

Species Interactions Competition Predation Parasitism Mutualism Commensalism

Competition Intraspecific: competition between members of the same species for the same resources Example: territoriality (mark and defend area against their own species) Interspecific: competition between different species for resources Occurs when niches overlap

© 2004 Brooks/Cole – Thomson Learning Number of individuals Species 1 Species 2 Region of niche overlap Resource use Number of individuals Species 1 Species 2 Resource use

Other types of Competition Interference Competition: one species limits another’s access to resources Example: hummingbird defends patches of wildflowers (nectar) so that other hummingbirds may not get to them Exploitation Competition: species have equal access to resources but differ in how fast they exploit it

Competitive Exclusion Principle One species eliminates another species in an area through competition for limited resources Example: Paramecium Can’t occupy the same niche with limited resources

Relative population density Both species grown together High Paramecium aurelia Relative population density Paramecium caudatum Low 2 4 6 8 10 12 14 16 18 Days Both species grown together

Resource Partitioning Species avoid competition by dividing scarce resources amongst them Example: using resources at different times, in different ways, or in different places (warblers)

Predator-Prey Interaction Predation: members of one species (predator) feed directly on another species (prey) -At the individual level, prey is harmed -BUT, predators kill sick, weak individuals (improves genetics) -Remaining prey population has greater access to resources (enough for everyone)

How do prey defend themselves against predators? Ability to run, swim, or fly fast Very developed sense of sight or smell Protective shells (turtle, armadillo) Thick bark Spines or thorns (porcupines, cacti) Chemical warfare (poison) Camouflage/Mimicry (owl butterfly)

Parasitism One organism benefits and the other is harmed. Example: Fleas

Mutualism Both species benefit Example: clownfish and anemone

Commensalism One species benefits and the other is neither harmed nor benefits Example: bird in tree

How do ecosystems respond to change? Ecological succession: gradual change in species composition in a given area Primary succession: establishment of biotic communities on a lifeless ground Secondary succession: reestablishment of biotic community where some type of community is already present

What is Primary Succession? Begins with a lifeless area where there is no soil Examples: bare rock exposed by retreating glacier cooled lava abandoned highway or parking lot newly created shallow pond

Primary Succession, continued First, there must be SOIL Soil formation begins when pioneer species attach themselves to bare rock Trap wind-blown soil particles Secrete acids that break down rock Produce tiny bits of organic matter Perennial plants (live for 2 years without being reseeded) and herbs replace lichens and mosses Early successional plant species grow close to the ground, can tolerate harsh conditions Midsuccessional plant species are herbs, grasses and low shrubs Late successional plant species (mostly trees)

Primary Succession Time Balsam fir, paper birch, and white spruce Exposed rocks Lichens and mosses Balsam fir, paper birch, and white spruce climax community Jack pine, black spruce, and aspen Small herbs and shrubs Heath mat Time

Ecological Succession © 2004 Brooks/Cole – Thomson Learning Early Successional Species Rabbit Quail Ringneck pheasant Dove Bobolink Pocket gopher Ecological succession Midsuccessional Species Elk Moose Deer Ruffled grouse Snowshoe hare Bluebird Wilderness Species Grizzly bear Wolf Caribou Bighorn sheep California condor Great horned owl Late Successional Species Turkey Martin Hammond’s flycatcher Gray squirrel

What is Secondary Succession? Begins in area where community has been disturbed, destroyed, or removed Some soil remains Examples: abandoned farmlands burned or cut forests heavily polluted streams flooded land

Mature oak-hickory forest Secondary Succession Mature oak-hickory forest Young pine forest Shrubs Perennial weeds and grasses Time Annual weeds

How do species replace one another in ecological succession? Facilitation: one set of species makes an area suitable for other species Inhibition: early species hinder the growth of other species Tolerance: late successional plants are unaffected by earlier succession plants (thrive in mature communities without having to eliminate earlier species)

Intermediate Disturbance Hypothesis Communities that experience frequent disturbances have the greatest biodiversity Disturbances create openings for new species

Percentage disturbance Species diversity 100 Percentage disturbance

What is Stability? Complex networks of negative and positive feedback loops that interact to provide stability and sustainability Stability is maintained by constant change in response to changing environmental conditions 3 aspects of stability/sustainability 1. Inertia: ability of a system to resist disturbance 2. Constancy: ability of a system to keep its numbers within the limits (resources available) 3. Resilience: ability of a system to bounce back after a disturbance

Precautionary Principle Alternative view: biodiversity does not necessarily lead to more stability, and if nature is unpredictable, there is no point in trying to manage and preserve old-growth forests and ecosystems. We should convert grasslands to cropfields, drain and develop wetlands, and not worry about extinction Precautionary Principle: when evidence indicates that an activity can harm the environment, we should take precautionary measures to prevent harm, even if the cause-and-effect relationships have not been fully established