2.1.9—Population interactions

Mutualism is +/+, competition −/−, and predation, herbivory, parasitism and disease generally harm one partner; the mechanism predicts population and selection effects.

Translate label to fitness: food loss, injury, infection or benefit. Then ask which traits are favoured over time.

Predators lower prey survival; prey defenses become more valuable, while parasites may favour host resistance without rapidly killing the host.

No. The parasite benefits while the living host is harmed.

Parasitism is not the same as predation; a living host is normally maintained.

Apply each relationship with a named mechanism: grazing deer reduce plant biomass (herbivory, +/−); sea stars consume mussels and can limit mussel abundance (predation, +/−); ticks gain food while reducing host fitness (parasitism, +/−); flowering plants gain pollination while bees gain food (mutualism, +/+); a transmissible pathogen can lower host survival and spread faster at high density (disease, pathogen +/host −); and two plant species using the same limited light or nutrients can reduce each other's growth (competition, −/−). Then predict population feedback and selection, such as defence, resistance, resource partitioning or traits that improve capture or cooperation.