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).