Noise-induced patterns in environmental systems - Noise-induced Phenomena in the Environmental Sciences - page 245

In landscape ecology, cooperative and synergistic short-range effects are usually
associated with positive feedbacks resulting from the ability of some species or
functional types to create environmental conditions that favor plant establishment,
growth, and survival. These feedbacks typically operate within a short range. For
example, cooperation among neighboring individuals may lead to the concentration
of resources in vegetated areas where plant individuals find more favorable conditions
for establishment and survival ( Charley and West , 1975 ; Schlesinger et al. , 1990 ;
Greene , 1992 ; Wilson and Agnew , 1992 ; Bhark and Small , 2003 ; D'Odorico et al. ,
2007a ). The aerial parts of plant individuals that have already been established in
a certain patch may favor the growth of other plants in the same area by limiting
soil-moisture losses associated with evapotranspiration ( Vetaas , 1992 ; Thiery et al. ,
1995 ; Lejeune et al. , 2004 ; Zeng et al. , 2004 ; D'Odorico et al. , 2007a ) either through
a mulching effect (i.e., soil evaporation limited by wilted leaves and litter) or shading
(i.e., when the foliage shades the ground surface, thereby limiting evaporation) ( Zeng
and Zeng , 1996 ; Scholes and Archer , 1997 ; Zeng et al. , 2004 ; Caylor et al. , 2006 ;
Borgogno et al. , 2007 ). Moreover, the formation of physical and biological crusts
on bare soil may further reduce the infiltration of surface water (e.g., Fearnehough
et al. , 1998 ). Physical crusts, typically 1-3 mm thick, are generated by rain splashing
( Esteban and Fairen , 2006 ), whereas biological crusts are formed by microorganisms
such as cyanobacteria, which exude mucilaginous secretions that bind together soil
grains and organic fractions (e.g., Meron et al. , 2004 ). These crusts greatly reduce
the soil-infiltration capacity, thereby decreasing the soil moisture available in the
underlying soil layers, with consequent limitations on the establishment and growth
of perennial vegetation ( Fearnehough et al. , 1998 ). Because soil crusts, on the other
hand, seldom develop beneath vegetation canopies because of the reduced raindrop
impact ( Boeken and Orenstein , 2001 ; Meron et al. , 2004 ; Borgogno et al. , 2007 )and
the limited light available to the photosynthetic activity of biological crusts ( Wa lker
et al. , 1981 ; Greene , 1992 ; Joffre and Rambal , 1993 ; Greene et al. , 1994 , 2001 ), a
positive feedback exists between presence of vegetation and absence of crusts.
In vegetated areas the protection against evapotranspiration and soil-crust formation
enhances surface-water infiltration which, in turn, favors vegetation growth. The
associated increase in root density, in turn, enhances the soil infiltration capacity
( Walker et al. , 1981 ; Greene , 1992 ; Joffre and Rambal , 1993 ; Greene et al. , 1994 ,
2001 ; HilleRisLambers et al. , 2001 ; Okayasu and Aizawa , 2001 ; Gilad et al. , 2004 ;
Yizhaq et al. , 2005 ; Borgogno et al. , 2007 ). Moreover, a dense canopy of established
plants provides protection against herbivores (e.g., birds), thereby favoring plant
reproduction and growth ( Lejeune et al. , 2002 ) in densely vegetated areas (propagation
by reproduction effect) where higher rates of seed production and germination occur
(e.g., Lefever and Lejeune , 1997 ; Lejeune et al. , 1999 ; Lejeune and Tlidi , 1999 ;
Lefever et al. , 2000 ; Couteron and Lejeune , 2001 ).
Species able to modify the abiotic environment, redistribute resources, and facilitate
the growth of other species as well as their own are known as ecosystems engineers