Ecological Equivalence: Niche Theory as a Testable Alternative to Neutral Theory - Recent Advances and Issues in Environmental Science - page 3

a simulation that for the fi rst time spans across dispersal-limited neutral to stochastic
niche scenarios without fi xing the total abundance of individuals.
The neutral simulation developed here is consistent with the models of Solé et al.
[17] and Allouche and Kadmon [18] in having total species, S , abundance of individu-
als, N , and zero-sum dynamics as emergent properties (in contrast to refs. [1, 11, 12,
19]). The S species are identical in all respects including interspecifi c interactions
equal to intraspecifi c (in contrast to refs [13, 16]). Non-neutral simulations developed
here extend the model of Chave et al. [11] by allowing competitive differences to vary
stochastically on a continuous scale, as in Purves and Pacala [12]. They extend both
these models by allowing pre-emptive recruitment and emergent zero-sum dynamics,
and the model of Calcagno et al. [20] by adding dispersal limitation. They are consis-
tent with Tilman's niche theory [21, 22] in their population abundances being a func-
tion of species-specifi c vital rates.
These simulations confi rm the previously untested prediction [12] that coloniza-
tion-competition trade-offs with stochastic colonization will exhibit zero-sum ecologi-
cal drift and produce rank abundance curves that resemble neutral drift. Truly neutral
dynamics should nevertheless sustain a lower total density of individuals at density-
dependent equilibrium. This is because intrinsically identical species must interact as
strongly between as within species. They therefore experience no competitive release
in each others' presence, contrasting with the net release to larger populations obtained
by segregated niches. The simulations demonstrate this fundamental difference, and
I discuss its use as a signal for dynamic processes when predicting species-area rela-
tionships.
Analysis of Abundance Patterns for Two-niche Communities
Species characterized by extremely different intrinsic attributes can achieve ecological
equivalence in a zero-sum game played out at dynamic equilibrium. Take for example
a two-species community comprising a dominant competitor displacing the niche of
a fugitive (e.g., [23]). The fugitive survives even under complete subordination, pro-
vided it trades competitive impact for faster growth capacity [24]. Figure 1 illustrates
the equal fitness, zero-sum outcome at density-dependent equilibrium under this most
extremely asymmetric competition. The carrying capacity of each species is a function
of its intrinsic lifetime reproduction (detailed in Materials and Methods Equation 1),
and equilibrium population sizes are therefore a function of the species-specific vital
rates. Regardless of variation in the ratio of dominant to fugitive carrying capacities,
0 ≤ k D / k F ≤ 1, the system density of individuals is attracted to the stable equilibrium
at N = n F + n D = k F . Knocking out the fugitive reduces N to the smaller k D , but only
until invasion by another fugitive. This may be expected to follow rapidly, given the
fugitive characteristic of fast turnover. The steady-state scenario is effectively neutral
by virtue of the dominant and fugitive realizing identical vital rates and constant total
density at their co-existence equilibrium despite contrasting intrinsic (heritable) rates.
The reality that species differ in their life history traits therefore underpins the as-
sumption of ecological equivalence, which then permits fitting of intrinsically neutral
models with vital rates set equal to the realized rates. In the next section, these predic-
tions are extended to simulate the drift of species invasions that sustains the dynamics