Noise-induced phenomena in environmental systems - Noise-induced Phenomena in the Environmental Sciences - page 111

Box 4.2: Biodiversity and ecosystem resilience
Recent experimental (e.g., Steneck et al. , 2002 ; Bellwood et al. , 2004 ; Folke et al. , 2004 )
and theoretical studies (e.g., Chapin et al. , 1997 ; Walker and Salt , 2006 ) determined a
number of ecosystem attributes able to enhance resilience. Among them, biodiversity
was found to have the potential to increase resilience by increasing species redundancy
( Lawton and Brown , 1993 ), i.e., the coexistence of species that have a similar effect on
the ecosystem function ( Chapin et al. , 1997 ; Folke et al. , 2004 ) but different sensitivities
to disturbances. By definition, a redundant system could function with a lower number
of species. However, systems that minimize their diversity tend to be less resilient
( Chapin et al. , 1997 ; Walker and Salt , 2006 ). In fact, in the absence of disturbances they
have the same functioning as their redundant counterparts. However, in a redundant
system, if environmental stressors cause the extinction of some species, other species
with similar functional traits but different sensitivity to disturbance would be able to
provide the same service to the ecosystem. This effect, or insurance hypothesis
( McNaughton , 1977 ; Naeem and Li , 1997 ), indicates that redundancy provides the
system with a “buffer” that favors postdisturbance recovery.
This effect of resilience enhancement is due in particular to the response diversity
( Elmqvist et al. , 2003 ), which exists when the organisms of the same functional group
have different response traits, i.e., different sensitivities to disturbance ( Bengtsson et al. ,
2003 ; Folke et al. , 2004 ). Although the functional diversity ensures the existence of
groups of organisms that perform different functions in the ecosystem, the response
diversity ensures that the system is redundant and therefore resilient.
Some authors have shown that a more diverse system is more resilient, i.e., is able to
respond better to environmental fluctuations ( McNaughton , 1977 ; Yachi and Loreau ,
1999 ). In Subsection 4.2.3 we show that random environmental fluctuations may, in
turn, augment the resilience of the system by enhancing diversity.
investigated both experimentally (see Mackey and Currie , 2001 ) and theoretically
(e.g., May , 1973 ; Chesson , 1994 ). For example, May ( 1973 ) studied the effect of
environmental fluctuations on niche overlap and species packing and showed how
environmental variability allows for the existence of higher degrees of niche over-
lap, thereby favoring biodiversity. Tree-grass co-dominance in savannas has been
explained as the result of nonequilibrium dynamics induced by interannual climate
variability or random fire occurrences ( Higgins et al. , 2000 ; D'Odorico et al. , 2006b ),
whereas in riparian ecosystems the coexistence of species with different water tol-
erances has been related to random water-table fluctuations ( Ridolfi et al. , 2007 ).
All these studies suggest that random environmental variability may favor the coex-
istence of species with different functional traits and demonstrate how biodiversity
can be enhanced by environmental variance (e.g., Chesson , 1994 ). This notion of
noise-enhanced biodiversity is supported by some experimental observations: