1) niche breadth mechanism. Change niche breath, niche overlap remains the same = increased specialization. The community becomes more susceptible to fluctuation.
2) niche overlap mechanism. Niche breath remains the same, overlap increases.
3) increase resource base mechanism. Make bottom line of graph longer.
Most evidence points to number 3, although my dissertation indicated there may be maximum packing (2) as well. The tropics have more resources than the temperate zone. There is also a stability of resources and increased habitat diversity. Large insects are disproportionately more abundant. There is also a direct correlation between BSD (= bird species diversity) and foliage height diversity
Ultimately abiotic factors affect species diversity in the tropics: 1) increased energy from sun; 2) higher average temperature; 3) decreased seasonality; 4) increased rainfall--all of these affect plants and animals