Noise-induced phenomena in environmental systems - Noise-induced Phenomena in the Environmental Sciences - page 115

environmental fluctuations limit the ability of the system to support diverse commu-
nities of individuals (i.e., ). In the first case, by favoring biodiversity, noise

plays a “constructive” role in the dynamics of the system, whereas in the second case
it has a “destructive” effect, in that it acts as a source of disturbance resulting in the
noise-induced extinction of some species (e.g., May, 1973). This finding is consistent
with the idea underlying the intermediate-disturbance hypothesis ( Connell , 1978 ;
Huston , 1979 ), i.e., that moderate disturbances can be beneficial to an ecosystem.
According to this hypothesis, disturbances of moderate strength would not allow the
attainment of equilibrium conditions by preventing competitive elimination of the
less-adapted species that leads to an equilibrium with less diverse species composi-
tion than in nonequilibrium conditions. However, Fig. 4.7 shows that environmental
variability may cause diversity enhancement even in the absence of competition. Fig-
ure 4.7 shows also that generalist species (i.e., species with high

< 2 δ

δ ) are better adapted

than specialists (i.e., species with low ) to withstand and benefit from environmental

fluctuations. The effect of these fluctuations depends also on the interplay between the
scales of growth or decay and the intensity of the stochastic drivers ( Loreau , 1992 ).
As noted, the preceding framework assumes that all species have the same

δ

and
do not interact with one another. However, the results presented in Fig. 4.7 are not
affected by these assumptions. In fact, the variability of

δ

δ among species would

not modify the nature of the dependence of σ R of

R . Moreover, interspecies competition and facilitation-cooperation affect vegetation
dynamics by either decreasing (competition) or increasing (facilitation) the species
carrying capacity

/δ on the standard deviation

, which, in turn, affects the potential for biodiversity. To investigate
the effect of competition (facilitation) D'Odorico et al. ( 2008 ) expressed

β

β as a

/δ

decreasing (increasing) function of the number of species (hence of the ratio) and

found that the positive effect of environmental fluctuations on biodiversity is stronger
in the presence of facilitative mechanisms, whereas it is weakened by competition,
consistent with the notion that biodiversity is favored by cooperation and reduced by
competition (e.g., Mulder et al. , 2001 ), though it could be argued that competition
may enhance biodiversity by inducing niche restriction.
The increase of the potential for biodiversity found with low levels of environmental
variance is consistent with the notion presented throughout this chapter that noise may
have a constructive effect and induce order and stability in environmental systems
( Chesson , 1982 , 2000 ; Loreau , 1992 ; D'Odorico et al. , 2005 , 2006b ), whereas the
occurrence of species extinction with relatively strong fluctuations (i.e., high variance)
corresponds to the more intuitive destructive role of noise (e.g., May , 1973 ).
Space-based ecological theories (e.g., Holt , 1984 ; Tilman , 1994 )haveshownthat
the spatial variability of environmental conditions typically found in heterogeneous
landscapes offers better opportunities to maintain higher levels of functionally similar
species. The results in Fig. 4.7 demonstrate that a similar effect may arise from the
temporal variability of resource availability.