In 1988 european ships discharged water into lake st. clair, canada. Within 2 years, in all 5 great lakes. Hundreds of millions of dollars of damage to property. Can occur between fishes and plants, birds and plant, fishes and bugs, etc. Competition multiple organism seek the same limited sources. Intraspecific competition when member of same species compete for the same resources in an ecosystem (trees growing close together compete for water, and nutrients from the soil) Interspecific competition members of different species compete for a shared resource (leopard and lion compete for same prey) Species divide shared resources by specializing in different ways. Effects of resource partitioning: character displacement characteristics diverge to lessen competition, disruptive selection. Example: beak depth in galapagos finches: beak depths changed depending on the food supply available due to natural selection. Two or more species benefit from their interactions: species interact all species involved benefit.