Community Ecology

Chapter 53

Community

• More than one species living close enough together for potential interaction

Community Interactions

• Interspecific (between species) interactions link species in a community

• There are different combinations of benefit/detriment in different interactions

• Competition, predation, herbivory, symbioses, and disease

Interspecific Competition

• two species compete for a resource• result is detrimental to one or both species• competitive exclusion - strong competition

can lead to the local elimination of one of the two competing species

• Invasive species• Phragmites grass

outcompetes and excludes native cattails

Ecological Niches

• “Profession” of a species• Resource acquisition, behaviors, tolerable

limits of environment (temp, size, position), • Overlapping niches can lead to character

displacement – characteristics diverge in sympatric populations to specialize in particular part of resources (resource partitioning)

Predation

• Advantage to one species/disadvantage to the other

• One species captures and eats part or whole of another species

Preventing Predation

Preventing Predation

Herbivory• Special case of predation• Animals eating plants• Generally seen as advantage for animal and

disadvantage for plant

Preventing Herbivory• Plants cannot move• Produce defenses (chemicals, mechanical) or

develop mutualistic relationships that protect themselves

Parasitism

• One organism benefits by taking nutrients from another organism who is harmed

Disease

• Pathogens similar to parasites• Pathogens usually lethal/parasites usually not• Pathogens microscopic/parasites multicellular

Mutualism

• Interspecific interaction where both species benefit

Commensalism

• Interaction where one species benefits and the other species is neither helped nor harmed

• Doesn’t really exist – everything is a spectrum

Species Diversity

• Species richness - total number of different species in the community

• Relative abundance - proportion each species represents of the total individuals in the community

• species diversity of the community is dependent on both species richness and relative abundance

Trophic Structure

• feeding relationships between organisms• Food chain - transfer of food energy up the

trophic levels from its source in plants and other photosynthetic organisms (primary producers) through herbivores (primary consumers) to carnivores (secondary and tertiary consumers) and eventually to decomposers

Limited Energy

• 10% transfer of energy from one trophic level to the next limits food chain length

Dominant Species

• species in a community that are the most abundant or that collectively have the highest biomass

• dominant species exert a powerful control over the occurrence and distribution of other species

Keystone Species

• not necessarily abundant in a community• exert strong control on community structure

not by numerical might but by their pivotal ecological roles, or niches

Foundation Species• organisms exert their influence not through

their trophic interactions but by causing physical changes in the environment that affect the structure of the community

• alter the environment through their behavior or by virtue of their large collective biomass

Disturbance

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

Ecological Succession

• disturbed area may be colonized by a variety of species, which are gradually replaced by other species, which are in turn replaced by still other species

• Primary succession – process begins in lifeless area without soils

• Secondary succession – process begins when previous life removed by disturbance; soils present

Biogeographic Factors

• community’s species diversity correlated to geographic location and geographic size

• Equatorial – Polar gradient – more species near equator due to climate

• Species – area – the larger the area, the more species can exist