Chapter 53Community Ecology

ReminderWhat is a community?

Answer: All the populations living together in the same area at one time.

Interspecific InteractionsDefinition: Interactions between organisms of different species.

Include: competition, predation, symbiosis, disease

We look at interspecific interactions in terms of how they affect the different species involved positively or negatively.

Competition (interspecific)1.) When 2 species compete for a resource, both species are negatively affected (-/-).

2.) Competition can lead to the local elimination of one of the 2 competing species called competitive exclusion.

a.) The competitive exclusion principle states that if one species involved in competition has even a slight advantage, the other will eventually be wiped out.

Principle inferred following experiment with paramecium as seen above

Competition (interspecific) contdEcological niche: the sum total of a species use of the biotic & abiotic factors in its environment (where it lives, what it eats, what eats it, abiotic conditions it tolerates)

a.) Fundamental niche: the niche potentially occupied by a species

b.) Realized niche: the niche it actually occupies

c.) According to competitive exclusion principle, no 2 species can occupy the same niche there must be one or more differences in 2 species niches.

Competition (interspecific) contdResource Partitioning: the differentiation of niches that enables similar species to coexist in a community (probably resulting from natural selection among formerly competing species).

Competition (interspecific) contdThe competitive exclusion principle could also explain character displacement: the tendency for characteristics to be more divergent in similar species when they share a habitat.

PredationPredation is a +/- interaction: the predator benefits & the prey is (obviously) harmed.

a.) Predator/prey interactions have refined species adaptations through natural selection.

i.) Predators adapt to hunt or feed on prey

ii.) Prey adapts to escape, confuse or injure predators (even plants have defense against herbivores).

Predation contdDefense mechanisms of prey:1.) Cryptic coloration (camouflage)

Predation contd2.) Aposematic coloration (warning): animals with chemical defenses often have bright warning colors what predators learn to avoid.

Predation contd3.) Batesian mimicry: a harmless (or palatable) prey species mimics a harmful (or unpalatable) one.

Above: Viceroy mimicking monarch

At left: Hawkmoth larva mimicking snake

Predation contd4.) Mullerian mimicry: 2 or more unpalatable or harmful species resemble each other (advantage: the more there are, the more quickly predators learn to avoid)

Symbiotic RelationshipsDefinition: interaction in which 2 organisms of different species live together in direct contact.

Includes mutualism, commensalism and parasitism

ParasitismDefinition: a +/- interaction in which one organism (the parasite) derives its nourishment from another organism, its host, which is harmed in the process.

a.) endoparasite: lives w/in host (tapeworm)

b.) ectoparasite: lives/feeds on external surface of host (lice, ticks)

c.) parasitoidism: lay eggs on or in living hosts. When eggs hatch, young feed on host.

More parasites

MutualismDefinition: a +/+ symbiotic relationship that benefits both species.

Most common example would be plants and their pollinators.

CommensalismDefinition: a +/0 symbiotic relationship in which one species benefits and the other is not affected.

DiseasePathogens are any disease-causing agent virus, bacteria, protists, fungi.

Pathogens are similar to parasites except that they can result in the death of their host.

Concept Check1.) Explain how interspecific competition, predation and mutualism differ in their effects on the interacting populations of two species.

2.) What role do you think natural selection has played in the development of mutualistic relationships?

Community Structure

Species DiversityDefinition: the variety of different kinds of organisms that make up the community. Has 2 components:1.) Species richness: the total # of different species in the community

2.) Relative abundance of the different species

Trophic StructureThe feeding relationships in a community make up its trophic structure.

1.) Can be represented as a food chain which depicts the transfer of food energy as pictured

2.) Each level in food chain/trophic structure is called a trophic level

Food WebsDepict ALL the feeding relationships in a community.

Limits on Food Chain LengthMost food chains are usually 5 or fewer trophic levels. Why?

1.) Energetic Hypothesis: the length of food chains is limited by inefficient energy transfer.

a.) Only 10% of energy stored in the organic matter of each trophic level is converted to organic matter at next trophic level.

Limits on Food Chain Length contd2.) Dynamic Stability Hypothesis: long food chains are less stable than short chains.

Species w/ Large Impact on Communities1.) Dominant Species: those species in a community that are the most abundant or collectively have the highest biomass.a.) Biomass: the total mass of all individuals in a population

b.) Because of their large #, can affect many other biotic & abiotic factors in a community

Species w/ Large Impact on Communities contd2.) Keystone Species: exert strong control on community structure due to their pivotal niche in the ecosystem

a.) Not necessarily the dominant (most numerous) species

b.) Removal experiments are a good way to identify keystone species.These starfish feed on mussels. When removed, mussels take over and kill off algae & other invertebrates. Starfish are keystone species.

Species w/ Large Impact on Communities, contd3.) Foundation Species: cause physical changes in the environment that affect the community structure.

a.) Example: beavers, modification of soil by certain plants, etc

Controls on Community Structure1.) Bottom-up Model: the presence/absence of mineral nutrients controls plant # which control herbivore # which in turn controls predator #.

a.) So, to change the community structure of a bottom-up community, you need to alter the biomass at the lower trophic levels.

Controls on Community Structure contd2.) Top-down Model: Predators limit herbivores which in turn limit plants which in turn limit nutrient levels .

a.) Example in a lake community: removing the top carnivore will increase the # of primary carnivores which will then decrease the # of herbivores, increasing phytoplankton which will lead to a decrease in nutrients.

Community DisturbancesDisturbance: an event such as a storm, fire, flood, drought, overgrazing, human activity that changes a community, removes organisms from it and/or alters resource availability.

a.) Doesnt have to be bad! Many communities are dependent on periodic fires.

b.) Intermediate disturbance hypothesis: moderate levels of disturbance create conditions fostering greater species diversity than low/high levels of disturbance.

Ecological SuccessionDefinition: gradual changes in species in disturbed areas.

1.) Primary succession: succession occurring in a virtually lifeless area where soil has not yet formed (after volcanic eruption)

Ecological Succession contd2.) Secondary succession: occurs when an existing community is cleared by some disturbance that leaves the soil intact.

Contrasting Views on Community Structure1.) Integrated Hypothesis: a community is an assemblage of closely linked species, locked into association by mandatory biotic interactions that cause the community to function as a unit.

2.) Individualistic Hypothesis: A community is a chance assemblage of species found in the same area b/c they happen to have similar abiotic requirements.

Contrasting Views on Community Structure contdWhich one is right???

Wellfor most plant communities that have been studied it seems that the individualistic hypothesis is correctbut what about for animals?

Contrasting Views on Community Structure contdFor animals, the rivet model of community structure goes along with the integrated hypothesis suggest that most animal species in a community are in tight association with other species.

The redundancy model goes against this says most species are not tightly associated and (for example) if you remove one predator, another one will come along and take its place.