Ecological Equivalence: Niche Theory as a Testable Alternative to Neutral Theory - Recent Advances and Issues in Environmental Science - page 5

Figure 2. Equilibrium co-existence of a sexually reproducing parent population n P invaded by
an asexual mutant, n M . With the mutant having identical vital rates except for twice the intrinsic
propagation rate per capita: b M = 2 α PM < k P / k M . (A) Phase plane. (B)

Equilibration of abundances over time given by Equation 1, with a 50% drop in the parent's intrinsic
death rate imposed at t = 3 to illustrate approximate constancy of N = n M + n P .

b P , the parent species persists if

⋅

The above examples of dominant versus fugitive and sexual versus asexual were
illustrated with models that gave identical realized rates of both birth and death at co-
existence equilibrium. Fitness invariance and zero-sum dynamics, however, require
only that species have identical net rates of realized birth minus death. The simulations
in the next section show how neutral-like dynamics are realized for communities of
coexisting species with trade-offs in realized as well as intrinsic vital rates.

Comparison of Simulated Neutral and Multi-niche Communities with Drift
Figure 3 illustrates the SADs and species-area relationships of randomly assembled
S-species systems under drift of limited immigration and new-species invasions (pro-
tocols described in Simulation Methods). From top to bottom, its graphs show con-
gruent patterns between an intrinsically neutral community with identical character
traits for all species (equivalent to identically superimposed isoclines in Figures 1 and
2 models), and communities that trade growth capacity against competitive domi-
nance increasingly starkly. The non-neutral communities sustain more total individu-
als and show greater spread in their responses, reflecting their variable life-history
coefficients. Their communities nevertheless follow qualitatively the same patterns
as those of neutral communities. For intrinsically neutral and niche-based communi-
ties alike, Figure 3 shows SADs negatively skewed from log-normal (all P < 0.05,
every g 1 < 0), and an accelerating decline in rank abundances of rare species (cf.
linear for Fisher log-series) that is significantly less precipitous than predicted by
broken-stick models of randomly allocated abundances among fixed S and N ; Figure
4 shows constant densities of total individuals regardless of area (unambiguously
linear), and Arrhenius relationships of species richness to area (unambiguously linear
on loglog scales).