In temperate climates, physical extremes produce r selected species - little competition. Constant tropical environs produce K selected species. Competition reduces niche breadth, leading to more species along a resource continuum. More specialists for pollinating flowering plants.
Larger areas support greater species richness.
No real answers, idiosyncratic processes. Effects less in a large community than in a small community. Monocultures more susceptible to pest outbreaks. Pest outbreaks not common in rainforests. Reducing diversity of insect communities with pesticides precipitates pest outbreaks.