Geoscience Reference
In-Depth Information
C. Discontinuities and the DMR
Traditionally, the study of DMRs has either implicitly or explicitly assumed
that species are continuously distributed across the body size spectrum. This
conception is at odds with the observation of discontinuities in body size
distributions within natural animal communities. These distributions have
often shown a pattern of successive clumps and gaps that appears to be
robust within taxa and across ecosystem types (
Allen and Holling, 2002;
Holling, 1992; Sendzimir, 2008; Skillen and Maurer, 2008
). Many hypoth-
eses, not mutually exclusive, have been proposed to explain this phenome-
non, each one acting on a different spatio-temporal scale (
Allen, 2006;
Scheffer and van Nes, 2006
). All propositions should have connections
with the expected DMRs, but to date, the mechanistic explanations needed
to make appropriate quantitative predictions are lacking (
Allen and Holling,
2008
). The existence of gaps and aggregations within the size spectrum has
been associated to the concentration of resources or its availability to organ-
isms of different sizes, phylogenetic inertia, predation, and emergence of
neutrality when coexistence of species is enhanced by its similar niche
requirements (see
Allen, 2006; Allen and Holling, 2008; Scheffer and van
Nes, 2006
). As for gaps, aggregations and scaling in density are all connected
to the relationship between resource availability and organisms' demands;
the connection between these attributes of the body size distribution within
communities cannot be overlooked.
The textural discontinuity hypothesis (TDH;
Holling, 1992
) makes an
implicit link between the amount of resources a species is able to exploit
and its body size. According to this hypothesis, there are several spatio-
temporal scales at which the ecosystem resources are best exploited, and
hence the body sizes of animals should reflect this fact. Each body mass
clump matches an optimum scale of perception, and hence resource use, for a
given landscape. Considering Eq.
(2) or (4)
, the value of R
T
could depend
directly or indirectly on landscape structure. For example, in a patchy
landscape with patch density f, small species with limited dispersal ability
have access to resources available in the local patch, R
p
—available resources
within a single patch. Larger species with the ability to move between patches
would have access to R
T
¼
R
p
fA
M
being A
M
the area covered while foraging
by an organism of size M, and f the density of patches in the landscape. In
this case two different DMRs will coexist in the ecosystem, one for those
animals limited by within-patch resources and other one for those individuals
exploiting different patches. This discontinuous change in resource percep-
tion determined by landscape structure can lead to clumps and gaps in the
body size distribution (
Szab ´ and Mesz´na, 2006
), and a different DMR for
each clump (see
Figure 3
). The statistical approach to DMR study should
have the potential to detect this kind of pattern.
