Environmental Engineering Reference
In-Depth Information
richness at intermediate productivity (or intermediate
disturbance). This unimodel relationship, also known
as the ' intermediate disturbance hypothesis ' (Connell
1978 ), or ' humped - back model ' (Grime 1979 ), may
result from increasing environmental limitations to the
left and increasing interspecifi c competition to the
right of the peak. Mittelbach
et al
. (2001) , however,
reviewed 171 published studies that revealed positive
and negative relationships between the two as well.
Probably, the entire productivity gradient is not always
covered by the data. Nevertheless, this approach,
which implies that species richness can be regulated by
ecosystem management, has steadily gained attention
in nature conservation (e.g. Bongers
et al
. 2009 , and
references therein). Application of concepts such as
the ' species pool ' and ' assembly rules ' to ecological
restoration help to identify the potential species rich-
ness in a region, and to predict the composition
and interaction webs of a 'target community' following
restoration.
2.5.3
Assembly rules
Which are the environmental fi lters? The term 'assembly
rules' was coined by Diamond (1975), to help explain or
elucidate, through experimentation and observation, the
dynamic structure of stable and rapidly evolving commu-
nities based on niche-related processes. Working with this
concept, Weiher and Keddy (1999) proposed to envisage
two basic kinds of community patterning, with different
causes: (1) environmentally mediated patterns, that
is, correlations between species due to their shared or
opposite responses to the physical environment, and
(2) assembly rules, that is, patterns due to interactions
between species, such as competition, allelopathy, facilita-
tion, mutualism and all other biotic interactions that we
know about in theory, and actually affect communities in
the real world. Currently, all these processes, including
the arrival of propagules, their germination and estab-
lishment, and their interactions with co - occurring species,
are included in the notion of assembly rules.
Cavender - Bares
et al
. (2009) contribute to the clarifi -
cation of the concept of assembly rules, by distinguish-
ing between three perspectives on the dominant factors
that infl uence community assembly, composition and
diversity: (1) the classic perspective that communities
are assembled mainly according to niche-related proc-
esses, (2) the perspective that community assembly is
largely a neutral process in which species are ecologi-
cally equivalent and (3) the perspective that empha-
sizes the role of historical factors in dictating how
communities are assembled, with a focus on speciation
and dispersal rather than on local processes. Note that
these different points of view are not mutually exclu-
sive, and that it is useful to investigate the relative
importance of the different hypothetical processes.
The notion of 'assembly rules' implies that the
species composition of biotic communities can be
explained and predicted. Independent of whether this
claim is justifi ed, the advantage of the search for
assembly rules is that it helps make ecological knowl-
edge about communities and ecosystems explicit in
terms of predictions that can be tested.
2.5.2
Assembly from a species pool
In an ecosystem, biotic communities develop through
a process called community assembly, in which indi-
viduals of species invade, persist or become extinct.
While it is still an open question whether we can really
speak of
assembly rules
as a set of principles or laws
that predict the development of specifi c biological com-
munities, as compared to development that is attribut-
able to random processes, the search for applicability
of assembly rules has opened up fruitful perspectives
in the practice of ecological restoration (Temperton
et al
. 2004 ).
We adopt the approach given by Zobel (1997),
who defi ned
species pools
at three different scales,
with environmental fi lters in between, living or nonliv-
ing. A 'regional species pool' represents the total of
species available for colonization and is defi ned within
a large biogeographic or climatic region, extending
over spatial scales many orders of magnitude larger
than those of local ecosystems. The 'local species pool'
is a selection from the regional pool, defi ned at the level
of a landscape, and a further selection, the 'commu-
nity species pool', is the set of species in a site to be
restored. Assembly rules indicate constraints or envi-
ronmental fi lters determining which species can
occur in the community and which combinations are
irrelevant.
2.5.4
Genetic diversity within species
Within - species genetic diversity is increasingly recog-
nized as an important aspect of biodiversity (Falk
et al
.
2006; Chapter 7). It represents the adaptive potential
of genotypes and the associated phenotypes to their
