Communi t i e s

Objective

• Yesterday: You needed to define and give examples of density dependent and density independent limiting factors

• Today: you need to: identify, describe, and give specific examples of the 3 major population interactions – Feeding relationships– Competition– Symbiosis

What is a Community?

• The interactions between all species living in a given area– The boundaries of the area are defined by the

researcher who is studying the community

• Trophic structure-the feeding relationships between organisms– Food chain-the transfer

of energy between

trophic levels• Decomposers are

a vital part, but aren’t

shown• Arrows point in

the direction of energy

flow

1. feeding relationships

Primary producers

Primary consumers

2o

consumers

3o

consumers

4o

consumers

Autotrophs

Herbivores/omnivores

Carnivores

Carnivores

• Food Web- a bunch of interconnected food chains

Food chains are limited; there are rarely more than 5-7 links from the 1o producer to the top carnivore….

*The 10% rule-only 10% of organic matter at each level is converted into usable energy that is passed to the next level; the other 90% is used by the organism or lost as heat

2. Competition

Competitive Exclusion

Ecological Niches

Character Displacement

Resource Partitioning

competition

1 of 2 competing sps eliminates the other

An organism’s jobin an ecosystem

Adaptation leading to modified niche Sympatric populations

become more divergent in traits

• Intraspecific – same species, often fighting involved to “win” the resource– Can be very intense

• Interspecific – different species, usually no fighting, just better at getting it – Less intense since species have slightly different

niches

Competitive Exclusion Principle

• No two species can occupy exactly the same niche because competition for resources would be too intense

• One species will be better at getting the resources

• The other species must leave, adapt, or die

Solving the Problem

Resource Partitioning

• Two species evolve adaptations that allow them to use the same resource in different ways, at different times, or in different places in order to minimize competition

3. Predation: +/- interaction meaning that one individual (predator) benefits, and another individual (prey) is negatively affected

– Evolutionary connection: Predator-prey interactions may select for unique adaptations• Cryptic coloration- AKA camouflage

• Aposematic coloration-warning colors

• Batesian mimicry- a harmless animal mimics a harmful or unpalatable one

• Mullerian mimicy-two or more unpalatable species resemble each other

Aposematic coloration- poison arrow frog

Batesian mimicry Mullerian mimicry

2 rules about coloration

• Small + beautiful = poisonous• Beautiful + easy to catch = deadly

Coevolution• When two species have lived together and

adapted to each other for a long time such that changes in the gene pool of one species leads to changes in the gene pool of the other species

4. Herbivory: A +/- interaction; an herbivore eats a plant or algae – Evolutionary connection: many plants possess

adaptations that help them avoid herbivory

Strychnos toxifera is a vine that produces strychnine

5. Disease: pathogens (things that cause disease) are another example of a +/- relationship

– Differ from parasites in that they’re usually microscopic

– EX: bacteria, viruses, protists, fungi, & prions

– Like parasites,they can have a large, negative effect on population size

6. Parasitism: a +/- symbiotic relationship between a parasite & its host– Many parasites have complex lifecycles with multiple hosts– Parasite outbreaks can also greatly affect host populations

7.Mutualism: A +/+ symbiotic relationship where both organisms benefit

The Acacia tree provides nectar to these ants and the ants provide protection to the tree

8. Commensalism: A symbiotic +/0 interaction where one organism benefits and the other is unaffected– Example: these egrets follow grazing herbivores to eat

the insects that the herbivores kick up as they travel

Some species have a disproportionate impact on their community

• These species often affect the biodiversity-variety of different kinds of organisms-in the community

• Dominant species-Species in a community which are the most numerous or have the highest biomass

Nonnative species

• Aka alien, invasive, exotic• Accidentally or deliberately introduced

• Not always bad - most crops and livestock are nonnative species

• Sometimes - nonnative species takes over– Often there are no predators for this species,

since it hasn’t evolved alongside them

– This gives the nonnative a competitive advantage

Keystone species

• Have a large effect on the number and type of other species in the ecosystem

Examples:

• Top predators - feed on and help regulate the size of other populations

• Pollinators - control the size of plant populations

Effects of losing a keystone species

• When a keystone species is lost - it leads to population crashes and extinctions of other species

Foundation Species

• Aka ecosystem engineers• Play a major role in creating the

ecosystems in which they live

Examples:

• Elephants pull out or break small trees in the African savannah

• This allows grass to grow and benefits other grazers and open plains predators

Examples:

• Beavers - build dams to create ponds that other species benefit from

What other factors affect the biodiversity of an area?

• Two key factors: location & size of the community– Equatorial-Polar Gradients- The further you move away from the

poles and closer to the equator, the greater the biodiversity of an area.• Why?? 1. Many polar and temperate regions have “started

over” due to glacier movement2. Climate-the tropics receive more direct sunlight for longer

periods » The differences in sun exposure can be measured by measuring evaporation from soil & plants= evapotranspiration

What other factors affect the biodiversity of an area?

• Two key factors: location & size of the community– All other factors being equal, the larger the geographic area of a

community, the greater the number of species• Why?? This is because the greater the area, the greater the

variety of available habitats

A special case: Islands

• In ecology, an island can be in the ocean, or any community that’s otherwise isolated

• Island biodiversity is affected by the rate of species immigration vs. extinction – These two processes are affected by island size and distance

from the mainland • If the colonization of an island is a chance event (for example

the probability that a seed will land on an island), then small islands are less likely to be inhabited in this way.• The closer an island is to the mainland, the more likely it is

that migration will occur between the mainland and the island

When immigration is high, then extinction is low since organisms are populating the island at a higher rate

What is an ecological disturbance?

• Disturbance-storm, fire, drought/flood, overgrazing, or any human activity which can remove organisms or resources from a community– A disturbance isn’t necessarily a negative event…some

communities rely on disturbances for the natural progression of their “life cycle”• Ex-Lodgepole pine trees require periodic fires in order to

release their seeds

Closed cone

Open cone after fire

What happens to a community after an ecological disturbance?

• Ecological succession-the order in which an area is recolonized after a major disturbance has stripped away the existing vegetation

Two main types of succession:

• Primary succession- think of this as a blank canvas… – The first organisms to colonize the area are autotrophic

prokaryotes, followed by lichens & mosses. Soil gradually develop, more plants grow, and animals move in• Community maturation could take 100s or 1000s of years

– When might primary succession occur?

When a new volcanic

island forms or in an area

where a glacier has

receded

Two main types of succession:

• Secondary succession- Occurs after a disturbance, for example a fire, has wiped out a community, but has left the existing soil intact – Often, herbaceous plants colonize the area first,

followed by insects, larger animals, and trees