Geoscience Reference
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depending on the level of organisation of the empirical data analysed. Defin-
ing interacting entities within a food web as individuals or as aggregations
into species or size classes will clearly affect our perception of the size
structure of ecological communities. A key question that remains to be
answered is: to what extent can this approach be extended to terrestrial
systems? It seems reasonable to suggest that for many true predator-prey
interactions similar size constraints will apply. Some important obvious
exceptions spring to mind, however, when we consider large land carnivores
that hunt in packs and that eat prey considerably larger than themselves. This is
analogous to orcas that specialise on other cetaceans, but this type of pack
hunting of larger prey is evidently a far less common feedingmode inwater than
it is in land. Also, parasitoids (which are especially prevalent in terrestrial
systems) and parasites appear to be potential exceptions to size-based rules of
trophic interactions (see
Henri and vanVeen, 2011
), although we can only
speculate in the absence of comparable data (but see
Cohen et al., 2005
). The
interactions at the base of the web between primary consumers and modular
organisms are challenging to view from an individual-based perspective, but it
might be that by focusing on the modules that are actually interacting (e.g. the
leaf rather than the tree as the prey item) as the entities of interest, such an
approach could be usefully applied.
Another important task for future research is to investigate how complex,
size-structured food webs might respond to large permanent perturbations
like species losses (
Rudolf and Lafferty, 2011
). Are webs with certain dis-
tributions of body sizes more robust to species loss than webs with other
distributions? For instance, in terrestrial ecosystems, herbivores are often
much smaller and have shorter generation times than their resources (e.g.
small insects feeding on large trees), while the opposite is true in aquatic
ecosystems (e.g. large zooplankton feeding on small phytoplankton) (
Shurin
et al., 2006
). What are the dynamic consequences of this major difference in
the size structure between aquatic and terrestrial ecosystems? How will it
affect the risk and extent of cascading extinctions following the loss of a
species? Moreover, how does the life cycle of a species affect its keystone
status? Would loss of species in which individuals pass through a broad range
of body sizes during their life cycle have more far-reaching consequences
than the loss of species in which individuals do not grow in size after the age
of independence? Developing a new generation of size-structured food web
models (
Hartvig et al., 2011
) in combination with gathering new data on the
size structure of aquatic and terrestrial food webs (based on individual
feeding events) offers a promising means of shedding new light on these
and other related questions.
Finally, if we can gain better insights about the correlation between
different dimensions of size structure (
Box 1
;
Riede et al., 2011; Yvon-
Durocher et al., 2011
) and where and when intraspecific and interspecific
