Module 3 Community Theory

why study pop and higher order units?

indivs act in space, it is not interesting on its own unless interacting with other orgs

• interaction is whats important and interesting

what happens above the indiv level

most interesting dynamics concerning biota in a spatial context

community

an assemblage of interacting orgs or pop within an ecosystem

ecological community

a group of trophically similar, sympatric sp that actually or potentially compete/facilitate/predate/mate in a local area for the same or similar resources

ecological community examples

• plants

• invertebrate

• fish

• bird

are communities static

no because of co-occuring processes

what are some co-occuring processes that affect communities

• dispersal - moving around

• disturbance - hurricanes, fires, flood

community assembly

an unceasing process of sp arrivals, persistance, increase, decrease and extinctions played out in an individualistic (indiv sp) way

• dynamics is essential to this def

niche assembly

communities are groups of interacting sp whose presence/absence and realtive abundance can be deduced from assembly rules that are based on ecological niches or functional roles of each sp

assembly rules

• ecological laws build in

• have to follow a certain order to get the community you observe or restoration in a given area

• based on ecological niches

what does niche assembly predict

stable or equilibrium assemblage of particular sp

dispersal assembly

communities are open, non-equilibrium assemblages of sp thrown together by chance

• ever changing and come and go at random

• more based on chance than small scale things

• no distinct rules

• presence/absence is dictated by random dispersal, speciation, and stochastic local extinction

examples of dispersal assembly

island biogeography

unified neutral theory of diversity

competition

occurs when 2 or more pop compete for common resource (food or territory) and is limited

intraspecific

same species compete

interspecific

indivs of diff sp compete

competitive exclusion

no 2 sp can exist on same limiting resource in same place, one will cause extinction of another

scramble competition

occurs when 2 or more pop use the same limited resources, competition is indirect or accidental

contest competition

direct interaction btwn indiv , often aggressive

• allelopathy or territorial

coexistence

rule in nature

species coexistence depends on avoiding competition by

• use a range of resources

• switching diets

• living in a diverse enviro with various patches and hiding places

through methods of diffusing competition, what happens

• becomes more diffuse

• not directly competing all the time

• each resource represents a small portion of total resource requirements

resource partitioning

sp avoid niche overlap by partitioning available resources according to size and form, chemical composition, and seasonal availability

example of resource partitioning

gap partition hypothesis

• conspecific indiv like certain diff available conditions

  • others take adv of gap

spacial compeition

in dynamic and patchy enviro, the good dispersers can move quickly into disturbed or poor environment and avoid competition with superior competitors that dominate a good habitat

character displacement

competition btwn 2 sp can be avoided by evolutionary changes

island biogeography theory

equilibrium number of sp on an island is the outcome of a balance btwn immigration of new sp from nearest mainline and extinction of sp on island

• try and predict diversity

two key assumptions of island biogeography theory

• rate of sp immigration drops

  • initial high rate of rapidly dispersing sp then drops

  • slow colonizers dimish over time

• rate of extinction rises

  • more sp, ability to have higher extinction rate

equilibrium number of sp

immigration and extinction cross

• dynamic bc sp composition changes through time

• changes only a little

• sp richness is that equil, not niche-assembly

turnover rate and area relationship

• higher area = less turnover

• smaller area = higher turnover

island biogeography revision

• immigration rates decrease with increasing distance from source area

• extinction rate varies inversely with island size

  • all else being equal, equil number of sp should be higher on near islands

  • extinction greater on smaller than larger, equil lower on smaller

species area effect

as area increases the number of sp increases

• true for islands and mainlands

  • greater on islands

• number of sp doubles when area increases by a factor of 10 on islands

• rate of increase in sp richness is lower on mainland

• islands isolated so colonization is more difficult

habitat diversity hypothesis

larger areas have more habitats = more sp

• tries to explain species area effect

area-alone hypothesis

larger areas should carry more sp regardless of habitat diversity

island biogeo - simberloff and wilson

• florida keys

• mangrove island

• high arthropod species richness on large nearer island

  • low SR on smaller farther islands

• methyl bromide - fogged islands and killed all

• SR returned to almost same before fogging

• implies some equilibrium number

• dispersal into islands was rapid initially and evens out over time

island biogeo - diamond

• bird surveys channel island california

• 1917 and 1968

• almost same SR in both surveys

• sp composition was diff (30% diff btwn surveys)

• equilibrium number of SR but turnover in composition

• sp change

island biogeo - krakatau

• 1883 volcano explosion

• new island

• increase number of sp, lower rate, higher extinction rate

autochthonous speciation

evolution on the island

criticisms of island biogeography

• autochthonous speciation may be significant

• immigration rate may depend on island area as well as distance from the course

  • migrating sp are more likely to encounter a larger island

• extinction may depend on distance from source because dwindling sp may be rescued by migrants

• habitat diversity will affect sp richness

  • enviro factors influence diversity

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  • equilibrium number will change with varying enviro conditions

    • enviro changes occurs as fast as colonization and speciation

relaxation

decrease in number of sp that can be supported in the enviro as the enviro changes

lomolino 2000 proposal

• sp richness on small islands is dominated by chance enviro

  • hurricanes

• crossing ecological threshold on larger island

  • habitat diversity, carrying capacity , extinction, immigration

• way over threshold

  • autochthonous evolution of island endemic bolsters sp numbers

latitudinal sp gradient

high diversity plateau in tropics and drops when going toward both poles

small island effect

diversity governed by stochastic events and lack of habitats

• small island - less sp numbers

unified neutral theory of biodiversity

• constructed on foundation of island biogeo theory

• neutral theory

  • ex. ecological communities are structured entirely by ecological drift, random migration, and random speciation

• dispersal assembly

ecological drift

variability in pop growth rates arising from random diff among indivs in survival and reproduction

neutral theory

treats orgs in the community as essentially identical in their per capita probabilities of giving birth, dying, migrating, and speciating

• all indivs follow same rules the same way in the model