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: 1 Individualistic hypothesis - species arrive in community by chance because of abiotic needs. 2 Interactive 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.

1 Interspecific (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 different species occupy different areas in the trees to allow the resources to be shared.

2 Predation - 1 species hunts, kills, eats other (the prey). A Parasitism - 1 species lives off other species (may/may not kill host) Predators and prey have own survival adaptations. Predators - claws, fangs, etc.

Plants – toxic chemical compounds - protect against herbivores. Prey - defenses (i.e. blending in to environment) Cryptic coloration - prey uses coloration to deceive predators.

a Aposematic coloring - present as warning sign to predators. Prey - odors and toxins as defense against predators.

Frog shows toxicity by presenting with bright colors.

b Mimicry - organism resembles another species. 1 Batesian mimicry - harmless organism mimics harmful one. 2 Mullerian 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.

3 Mutualism - both species benefit. 4 Commensalism - 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. 2 nd species relies on 1 st species; must adapt to continue to benefit (selective pressure)

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): 1 Energetic hypothesis - length of food chain limited by inefficiency of energy transfer along chain. 2 Dynamic 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. 1 Bottom-up model - producers are the controlling factor of the population. 2 Top-bottom model - consumers are controlling factor of population.

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 bring new wildlife.

Succession Succession - sequence of community changes after disturbance. 1 Primary succession- no life, soil not formed yet. Mosses and lichens first; cause development of soil. Example - after glacier retreats.

2 Secondary 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.