Community Ecology

Chapter 53

•Community - group of species living close enough for interaction.

•Species richness – # of species a community contains; relative abundance - # of members of each species.

• 2 theories on community development:

• 1Individualistic hypothesis - species arrive in community by chance because of abiotic needs.

• 2Interactive hypothesis - species are in community because of similar biotic needs and interactions between species.

INDIVIDUALISTIC HYPOTHESIS

Fluctuations are independent of one another and constantly changing.

INTERACTIVE HYPOTHESIS

Fluctuations are dependent upon one another.

Relationships

• Interspecific interactions - relationships between species in community.

• Defined by how they affect other species.

• 1Interspecific (between species) competition - resources are limited.

• Organisms own ecological niche (specific role in environment).

• If two organisms have same niche, cannot coexist (competitive exclusion principle)

Both species occupy different niches because they require the same biotic needs.

• Resource partitioning allows two similar species to coexist.

• Done by differentiating species’ niches.

Same genus of lizards, but the differentspecies occupy different areas in the treesto allow the resources to be shared.

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• 2Predation - 1 species hunts, kills, eats other (the prey).

• AParasitism - 1 species lives off other species (may/may not kill host)

• Predators and prey have own survival adaptations.

• Predators - claws, fangs, etc.

http://janoelofsesafaris.com/images/update11-01/cheetah1.jpg

• Plants – toxic chemical compounds - protect against herbivores.

• Prey - defenses (i.e. blending in to environment)

• Cryptic coloration - prey uses coloration to deceive predators.

• aAposematic coloring - present as warning sign to predators.

• Prey - odors and toxins as defense against predators.

Frog shows toxicity by presenting with bright colors.

• bMimicry - organism resembles another species.

• 1Batesian mimicry - harmless organism mimics harmful one.

• 2Mullerian mimicry – 2+ species resemble each other (both species are not tasteful)

Batesian mimicry

Mullerian mimicry

• Parasites - live on host (ectoparasite) or in host (endoparasite).

• Parasitoidism - parasite eventually kills host.

http://www.sierrapotomac.org/W_Needham/Pictures/Wood_Tick_040307.jpg

• 3Mutualism - both species benefit.

• 4Commensalism - 1 species benefits, other is unharmed but does not benefit (barnacles that attach to side of whales)

Acacia tree and ants – the ants feed off the tree and receive shelter; in return, the ants protect the tree from any harmful insects that may kill the tree.

• Coevolution - 1 species evolves, puts pressure on another species to evolve.

• 2nd species relies on 1st species; must adapt to continue to benefit (selective pressure)

http://www.fs.fed.us/wildflowers/pollinators/pollinator-of-the-month/images/rubythroat/ruby-throated_hummingbird_lg.jpg

• Trophic structure - based on feeding patterns in community.

• Food chain - flow of energy from producers up to carnivores.

• Trophic level shows where organism fits in food chain.

• Food web - more accurate way to look at feeding.

• Feeding relationships shown in interrelated way.

• Food chains have limits on length (2 hypotheses):

• 1Energetic hypothesis - length of food chain limited by inefficiency of energy transfer along chain.

• 2Dynamic stability hypothesis - long food chains less stable than short chains.

• Dominant species - most abundant or highest biomass.

• Biomass - sum weight of all individuals in population.

• Keystone species - species that have regulating effect on entire community.

• If removed, can affect entire community.

• 2 models at how food chain is controlled.

• 1Bottom-up model - producers are the controlling factor of the population.

• 2Top-bottom model - consumers are controlling factor of population.

http://www.iarc.uaf.edu/highlights/2005/coupled_marine_ecosystem/food_web.jpg

Disturbances

• Stability - ability of community to persist through disturbances.

• Fire, weather, or human activities can alter communities.

Before disturbance

After disturbance

Disturbance in marine community

• Some disturbances necessary for community to thrive.

Fire is necessary for this community to bringnew wildlife.

Succession

• Succession - sequence of community changes after disturbance.

• 1Primary succession- no life, soil not formed yet.

• Mosses and lichens first; cause development of soil.

• Example - after glacier retreats.

• 2Secondary succession - after major disturbance.

• Soil left intact.• Grasses first, then trees and

other organisms.

• Community’s diversity (biodiversity) controlled by two factors; size and biogeography.

• Remember: species richness - different species living in a community; relative abundance - # of those species living in the area.

Community 1 has higher species richness

Community 2 has higher relative abundance

• Species - richer towards equator, poorer towards the poles. (due to climate and evolutionary history of species)

• Greater the geographic area, more abundant the number of species.