Chapter 17: Species Interactions and Community Structure

community food web

a summary of the feeding interactions within a community

strong interactions

feeding interactions of a few spp. that have a dominant influence on community structure

pros of emphasizing strong interactions in food webs

simplified, allows determination of spp. with most significant influence

cons of emphasizing strong interactions in food webs

potential to overlook important interactions, doesn't provide the complete picture of

Tscharntke established food web associated with wetland reed

identified the strength of feeding interactions

direct interactions

involve positive or negative effects of one spp. on another without intermediary spp. ex: competition, predation, herbivory, mutualism

indirect interactions

one spp. affects another through an intermediary spp. ex: apparent competition, indirect commensalism

indirect commensalism

activity of one spp. benefits another spp. through an intermediary spp.

indirect commensalism example

beavers have a positive effect on beetles through cottonwood trees, chemical changes on tree leaves that have been resprouted by beaver-fallen stumps-> better nutrition for beetles compared to intact tree leaves

apparent competition

results of two spp. sharing predator or herbivore OR one spp. facilitating predators or herbivores of another spp.

apparent competition example

black mustard (Brassica nigra, exotic) shelters small mammals which feed on surrounding vegetation which leads to increased herbivory on bunchgrass (Nassella pulchra, native)

keystone species

spp. whose feeding activities have an inordinate influence on community structure, have high impact on community structure despite low biomass

robert paine's keystone species hypothesis

keep prey pop. below carrying capacity

reduce likelihood of competitive exclusion which leads to increased spp. diversity

dominant (foundation) species

substantial influence on community structure due to high biomass

Paine's experiment that led to his proposing the keystone species hypothesis

Paine removed top predator (Pisaster) from intertidal food web, diversity of control plot remained, diversity removal plot decreased, removal of Pisaster resulted in community collapse

trophic cascade

effects of predators on prey can alter more than one trophic level

ecosystem engineers (keystone modifiers)

create or extensively modify physical environment (ex: beavers create dams and habitats for other species

mutualistic keystone species

cleaner wrasse fish remove ectoparasites from fish, act as mutualistic keystone spp. on coral reefs (when wrasses removed, 24% decrease fish diversity)

seed dispersal mutualists

native ant disperse 30% of seeds in fynbo trees of South Africa, ant dispersal critical to survival of fynbos