Noise-induced phenomena in environmental systems - Noise-induced Phenomena in the Environmental Sciences - page 109

with increasing noise intensities (i.e., with increasing CV). The existence of noise-
induced (partly) vegetated states for values of lower than R 1 indicates that rainfall

fluctuations prevent the occurrence of desert conditions. Conversely, for

R

R > R 2 ,

rainfall fluctuations act as a disturbance on vegetation in that the mode of v is smaller

than the stable deterministic state v = 1. The disappearance of bistability for R 1 <

<

R 2 reduces the likelihood of catastrophic shifts to the desert state and enhances
the resilience of dryland ecosystems.
These results are consistent with the findings obtained by Zeng and Neelin ( 2000 )in
the study of the coupled atmosphere-biosphere dynamics of the African continent by
use of tropical circulation models coupled with interactive vegetation dynamics. These
authors investigated the effect of interannual fluctuations in sea-surface temperatures
(SSTs) on the stability of vegetation-ecosystems in Africa. Simulations with no
interannual variability in SSTs showed that the coupled vegetation-climate dynamics
are bistable in the savanna regions of sub-Saharan and southern Africa [Figs. 4.4 (a)-
4.4 (c)]. In these regions the system converges either to a vegetated or unvegetated
steady state, depending on the initial conditions. Figure 4.4 (c) shows the areas where,
in the absence of SST fluctuations, the vegetation-climate dynamics are bistable.
However, when the system is forced by interannual fluctuations in SSTs the spatial
extent of these bistable areas significantly decreases [Fig. 4.4 (d)] and the variability
of the SSTs stabilizes the coupled system in an intermediate state. In fact, Fig. 4.5
clearly shows that, although in the absence of SST fluctuations the system exhibits
the alternative stable states of desert and forest, interannual fluctuations in SSTs
allow the system to converge to a state with intermediate vegetation cover. In this case
the dynamics are not bistable but converge to this intermediate state regardless of the
initial conditions. Moreover, interannual rainfall variability resulting from oscillations
in the SSTs enhance vegetation cover in areas that would be desert in the absence
of climate fluctuation. A similar effect has also been found in the dynamics of other
ecosystems ( Rennermalm et al. , 2005 ).

R

4.2.3 Noise-induced biodiversity
In the previous subsection we showed how random climate fluctuations may convert
bistable deterministic dynamics into a system with only one stable state. This effect
of noise-induced stability enhances ecosystem resilience, i.e., the ability to recover
after a disturbance (see Box 4.1). Here we focus on another possible mechanism of
noise-enhanced resilience, which is based on the increase in biodiversity, as explained
in Box 4.2.
Recent studies ( Hughes et al. , 2007 ) investigated the two-way interaction be-
tween species diversity and disturbances. Biodiversity has been related to the way
ecosystems respond to environmental fluctuations ( Yachi and Loreau , 1999 ). At
the same time, the effect of environmental variability on biodiversity has been