Balance in Other Mutualistic Ecological Processes
Pollination is one of a number of ecological processes, that often
involve mutualism, in which one group of organisms carries out an
important function which helps sustain the ecosystem. Is pollination
unique in that natural selection can, as we argue, cause an imbalance in
supply and demand? Or could similar phenomenon occur in other mutalistic
groups?
Frugivory is perhaps the closest to pollination, especially as animal
frugivores also play an important and often mutalistic role in the
reproduction of their plant partners by dispersing their seeds. Fruit
production, like nectar production, has a non-zero cost to the plant
(Encinas‐Viso et al., 2014). The majority of frugivores are highly
mobile and eat fruit from multiple species, which often varies
seasonally in terms of which fruit is consumed (Herrera, 2002; Corlett,
2011) and the proportion of fruit in their diet. Plant-frugivore
interactions often involve many species, forming complex networks of
mutualistic partners (Vidal et al., 2014). Frugivores also differ in
their effectiveness as seed dispersers (Calviño-Cancela et al., 2009).
Therefore, if the supply of fruit is seasonal and limiting (Milton,
1980), imbalances in this system would seem likely. However, this area
remains poorly understood (Calviño-Cancela et al., 2009). Nevertheless,
we would expect phenological supply and demand fluctuations to be more
likely in habitats in which seed dispersers show pronounced seasonal
variation in diet, habitat use, or foraging behaviour (Campos-Arceiz,
2008).
Another very important ecosystem process is provided by the many
decomposer organisms which break down organic matter and in doing so
release plant nutrients into the soil. Whether or not it is considered a
mutualism, this system is much less likely to result in positive
feedback causing imbalance for several reasons. First, decomposer
organisms lack the mobility (Rantalainen et al., 2004) of pollinators
and so are not as free, for example, to serve particular trees that
provide more dead leaves in the autumn. Second, their resource is not as
ephemeral. Nectar does not persist in the environment. By contrast, dead
leaves and other organic matter may persist for a long duration thereby
buffering temporal imbalances.
Mycorrizal-plant relationships also have parallels with pollination.
Multiple fungi interact with individual plant hosts (Johnson et al.,
2012) and vice-versa (Weremijewicz and Janos, 2013). As with the
interactions between plants and nitrogen fixing bacteria, the partners
will generally lack mobility. In these mutualisms, and also in
brood-site pollination mutualisms such as those between fig plants and
fig wasps, there may also be processes that control the mutualism to the
benefit of one or both partners (e.g., Jander & Herre 2010; Wang et al.
2013). Selection dynamics of these communities has been likened to human
economic markets (e.g. Werner and Kiers, 2015; see also Noe &
Hammestein, 1995).