EX 4 [2/6] species richness/diversity, closed vs open communities

discuss the 5 geographical trends in species diversity noted by G. G. Simpson

[1] there are larger #s of species in all phylogenic lineages in the tropics
[2] increased changes in elevation (topography) = increased diversity
[3] more # of species in the west than in the east
[4] more species in ecotones (areas with abrupt physical changes)
[5] less species in peninsulas

discuss how to add species to a community without pushing others to extinction

-add more resources to the ecosystem
-add species with overlapping niches (which increases competition)
-increase specialization among species (so niches become narrower for everyone)
-exploit existing resources more fully

compare the 8 hypotheses about species richness in the tropics

[1] TIME: tropics have a longer geological history & more time for development
[2] SPATIAL HETEROGENEITY: tropical communities are physically more complex (ground, shrubbery, under canopy, canopy, overcanopy/emergent)
[3] COMPETITION: greater competition = niche partitioning and character displacement & release
[4a] PREDATION: more intense predation in the tropics reduces competitive exclusion (and thereby extinction of weak predators)
[4b] PEST PRESSURE: more trees = greater specialization of insects/vertebrates that feed on seeds and fruits
[5] CLIMATE: tropics have greater temp stability (both between seasons and years)
[6] PRODUCTIVITY: tropics have higher biomass production -> reduces food limitation
[7] DISTURBANCE: moderate disturbances lead to max productivity (too much or too little leads to excessive competition)
[8] GEOGRAPHIC AREA: there is simply more land in the tropics than in temperate & polar zones

differentiate between "closed communities" and "open communities"

-"closed communities" have ecological limits just like an individual organism
-"open communities" do not have discrete boundaries

describe the controversy between supporters of "closed communities" vs "open communities"

PRO CLOSED: Shelford & Clements
PRO CLOSED: Shelford and Clements
-argued for a very organismal/holistic view of communities
-both orgs and communities (1) are made of interacting parts, (2) develop throughout time, and (3) have a "physiology"
PRO OPEN: Gleason, Curtis and Whittaker
-argued that ecological succession was not "autogenic" in its progression (just because a community was growing didn't mean it was approaching any climax)
-communities must be open because new species could enter and invade at any time

what do scientists today think of open vs closed communities (Gleason vs Clements)?

-most back Gleason (open) and abandoned Clements (closed)
-communities are thought of as being "mostly open", though ecotones act as natural boundaries between communities (though ecotones can be easily crossed by some species)
-argued that species evenness would always be zero if communities were truly closed