Environmental Engineering Reference
In-Depth Information
A
B
C
D
(a)
(a)
(a)
(a)
Competition
Realized
niche
Recruitment
limitation
Facilitation
0
Predation
LOW
HIGH
Fundamental
niche
(b)
Disease and parasitism
Productivity
LOW
HIGH
LOW
HIGH
Environmental
stress
HIGH
LOW
(b)
(b)
(b)
Resource enhancement
LOW
HIGH
Realized
niche
(c)
Predation
refuge
Recruitment
enhancement
0
Fundamental
niche
Competition
Habitat amelioration
LOW
HIGH
LOW
Productivity
HIGH
LOW
HIGH
Resident species richness
Intensity or frequency of
disturbance or predation
Environmental
stress
HIGH
LOW
Figure 6.4 Four fundamental models of ecology, with and without facilitation, illustrating paradigm shifts if facilitation
is included. (A) The realized niche can be larger than the fundamental niche due to facilitation; (Aa) without facilitation,
(Ab) with facilitation. (B) Facilitation may affect the competitive abilities of species along an environmental gradient;
(Ba) facilitation weak, constant, (Bb) facilitation strong, variable. (C) Facilitation may have an impact on the success of
invaders; (Ca) without facilitation, (Cb or Cc) with facilitation. (D) Facilitation may change the relationship between species
richness and ecosystem productivity; and (Da) without facilitation, (Db) with facilitation (lower line, primary space holders;
upper line, secondary space holders). (From Bruno et al . 2003. Reproduced by permission of Elsevier.)
Brooker et al . 2008). Figure 6.4 depicts how funda-
mentally an ecological theory or model can change
when facilitation is included along with competition.
Here we focus on changes in the relative importance of
competition and facilitation in plant communities as
proposed in the so-called stress - gradient hypothesis (see
Maestre et al . 2009 ). The notion of ' stress ' in the latter
paper is derived from the work of Grime (1979), who
defi ned stressful environments as those in which pro-
ducers are limited in their ability to convert energy to
biomass. While in Grime's approach only the relative
importance of competition changed along a productiv-
ity gradient, Maestre and coworkers elaborated on this
idea, now also proposing a gradient in facilitation.
Central to the stress-gradient hypothesis is the idea
that facilitation should be especially common in com-
munities under high physical stress or with high con-
sumer pressure, whereas competition should be the
dominant structuring force wherever the physical
environment is relatively benign and consumer pres-
sure is less severe. Indeed, overall, the relative fre-
quency of facilitation and competition varies inversely
across gradients of physical stress, also known as gra-
dients of ecosystem productivity, whether in space or
in time, but there are interesting exceptions (see also
Chapter 11). Facilitation may, for instance, affect the
competitive abilities of species along an environmental
gradient in such a way as to keep it at a low level all
along a gradient (Bruno et al . 2003 ), and - adding to
the complexity - different facilitative mechanisms may
function in a nested hierarchical manner (e.g. VerdĂș &
Valiente - Banuet 2008 ). For example, experiments with
intertidal smooth cordgrass ( Spartina alternifl ora ) com-
munities have shown that there exists an hierarchical
organization of facilitation among species. The
cordgrass provides shade and reduces wave action in
these harsh environments, which results in a cascade
of facilitative effects on other plant and animal species
(e.g. ribbed mussels, barnacles, snails and seaweeds)
(Altieri et al . 2007 ).
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