01Species interactions
Competition (−/−), predation and herbivory (+/−), parasitism (+/−), mutualism (+/+, pollinators), commensalism (+/0, barnacles on whales).
Competitive exclusion: two species can't share the exact same niche forever. Resource partitioning lets them coexist (warblers feeding at different tree heights).
02Structure and change
Keystone species have outsized effects (sea otters control urchins, protecting kelp forests). Trophic cascades ripple through food webs.
Simpson's diversity index measures diversity: D = 1 − Σ(n/N)². Higher D = more diverse.
Succession: primary (bare rock, no soil — after lava flows; pioneers like lichens) vs. secondary (soil remains — after fire or farming).
03Notebook box
Worked example: Community with 10 A and 10 B (N = 20). D = 1 − (0.5² + 0.5²) = 0.5.
Symbiosis signs: mutualism ++, commensalism +0, parasitism +−.