All organisms – be they plants, animals, fungi, or whatever – interact with other species throughout their lives, in relationships that include predation, parasitism, commensalism, and the many and varied forms of mutualism. But when species are transported to a different part of the world, as has happened often during the Anthropocene, these interactions typically break down because usually only one of the participants moves. This loss of ecological relationships can play a role in whether or not a species becomes established in its new home, and has been mostly explored in the “Enemy Release Hypothesis” (ERH) which predicts that, by leaving behind predators or parasites or herbivores, a species becomes more ecologically successful and ultimately invasive in its novel range.

Less well studied, though potentially just as important, is the “Missed Mutualist Hypothesis” (MMH) which in a sense is the twin of the ERH. As well as leaving behind “enemies”, introduced species leave behind “friends” such as pollinators, seed dispersers, mycorrhizal fungi, defensive partners, and other mutually beneficial associates. Negative effects arising from the loss of these relationships could potentially balance the positive impacts arising from the ERH.

In a new quantitative review just published, we review what’s known about the MMH (currently much less than the ERH) and suggest some fruitful lines of enquiry. The study is led by Angela Moles, my collaborator at the University of New South Wales where I spent time as a Visiting Research Fellow in 2019/20 (see my blog posts about that visit starting here). The paper has had a long gestation and gone through several iterations and revisions since we started writing it in late 2019, not least caused by the covid pandemic, but I think that it’s all the better for it.

Here’s the full reference with a link to the paper:

Moles, A.T., Dalrymple, R.L., Raghu, S., Bonser, S.P. & Ollerton, J. (2022) Advancing the missed mutualist hypothesis, the under-appreciated twin of the enemy release hypothesis. Biology Letters 18: 20220220.

Here’s the abstract:

Introduced species often benefit from escaping their enemies when they are transported to a new range, an idea commonly expressed as the enemy release hypothesis. However, species might shed mutualists as well as enemies when they colonize a new range. Loss of mutualists might reduce the success of introduced populations, or even cause failure to establish. We provide the first quantitative synthesis testing this natural but often overlooked parallel of the enemy release hypothesis, which is known as the missed mutualist hypothesis.

Meta-analysis showed that plants interact with 1.9 times more mutualist species, and have 2.3 times more interactions with mutualists per unit time in their native range than in their introduced range. Species may mitigate the negative effects of missed mutualists. For instance, selection arising from missed mutualists could cause introduced species to evolve either to facilitate interactions with a new
suite of species or to exist without mutualisms. Just as enemy release can allow introduced populations to redirect energy from defence to growth, potentially evolving increased competitive ability, species that shift to strategies without mutualists may be able to reallocate energy from mutualism toward increased competitive ability or seed production. The missed mutualist hypothesis advances understanding of the selective forces and filters that act on plant species in the early stages of introduction and establishment and thus could inform the management of introduced species.