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combination of both, determine a species' fundamental niche, which, in
terms of ecological networks, describes all the other species with which a
species can potentially successfully interact (
Shipley et al., 2009
). Interactions
between species that, because of these barriers, are unable to occur are called
'forbidden interactions' (
Oleson et al., 2010
). Ecological networks, however,
actually depict a species' realised niche, which describes the proportion of
other species a species interacts with out of all the possible species with which
it can interact (
Shipley et al., 2009
). Therefore, when we attempt a mechanis-
tic understanding of ecological network structure, we are investigating the
processes that determine who interacts with whom (and who does not) as well
as the relative strength of these interactions.
Importantly, and not surprisingly, network structures are very different
from what would be produced if species interacted at random (
Brose et al.,
2006
). Optimal foraging theory suggests that individuals must maximise
their resource consumption, whilst at the same time minimising the cost (to
fitness) associated with acquiring and consuming the resource (
Hubbard
and Cook, 1978
). Therefore, we expect individuals to consume the most
'profitable' resources in order to forage optimally and thus maximise
individual fitness. Across all network types, species interactions may be
ordered according to trait-pairing characteristics, which impede or facilitate
interactions between individuals; through this process, the relative profit-
abilities of resources change according to these characteristics (
Vazquez
et al., 2009
). For example, in mutualistic pollinator networks, flowers with
longer corollas have stronger interactions with pollinators that have longer
proboscises, as the size of the corolla imposes a minimum size threshold for
any pollination interaction (
Vazquez et al., 2009
). In reference to optimal
foraging theory, one expects that interactions within ecological networks
are structured according to the relative profitabilities of different resources
and, as a result, because of the individual characteristics that determine
resource profitability (
Petchey et al., 2008
), with the strongest interactions
occurring between consumers and their most profitable resources. Further,
trait-pairing characteristics determine the currency by which optimality is
achieved. An example of how different currencies of optimisation effect
network structure can be found in host-parasitoid networks (
Figure 4
). It is
thought that parasitoids forage optimally (see
Section V
), but traits that
determine the fitness gains from parasitizing a particular host species, such
as the foraging or handling efficiency of the parasitoid and the quality or
riskiness of the host, determine whether the host-parasitoid interaction
strengths are structured according to the relative abundance of each host
species (
Figure 4
A) or according to other host characteristics (such as
quality) (
Figure 4
B). The adoption of either of these two strategies can be
explained by whether parasitoids are optimally allocating eggs or time
(
Sections V and VI
).
