SECTION 17.1 COMMUNITY INTERACTIONS

EVOLUTION  Organisms in communities are adapted to live there by natural selection  Natural selection influences other organisms as well – this is coevolution  Coevolution leads to very complex interactions

COEVOLUTION

PREDATION  One organism kills another for food  The predator is the killer, the prey is the food

PREDATION

PARASITISM  One organism feeds on and lives on or in another  Parasites usually do not kill their hosts, but they can make them very sick  Many parasites have complex life cycles to transmit their offspring to another host

PARASITISM

HERBIVORES  Plants cannot escape by moving, so they have evolved other defenses  Secondary compounds are not needed for photosynthesis or to encourage insect pollination  They may be poisonous to insects or bad tasting  Some plants also have thorns or spikes

HERBIVORES

SYMBIOSIS  Two or more species living together long-term  Mutualism is helpful to both  Commensalism is helpful to only one, the other is not helped or harmed

MUTUALISM

COMMENSALISM