Mutualisms – a little help from friends?

It’s been a while since we’ve last blogged but we’re hoping to pick it up more regularly again over the next few months now that the group is quickly growing.

For now, I thought I’d comment on an interesting study published this month in Applied Vegetation Science by Laura Burkle and Travis Belote.  It links up nicely to work we’re doing on priority effects, which is the idea that species that arrive earlier into a habitat influence the interspecific interactions of later arriving colonists.  Priority effects are hardly surprising. Succession theory long predicted that communities develop along different trajectories depending on the species that are initially present.  But generalizing the direction and strength of priority effects remains challenging.  This is where Sgt. Pepper’s Lonely Hearts Club Band can offer some advice:

In their study, Burkle & Belote experimentally show that colonists can minimize inhibitory priority effects from pioneer species that are strongly competitive in the presence of their soil mutualist ‘friends’: mycorrhizae.  The mycorrhizae literally let later-arriving colonists “get by”, “get high” and “gonna try” by providing access to below-ground resources despite being initially slower to access resources than pioneer species.

Even more intriguing, Burkle & Belote find that diversity–productivity associations become less negative in the presence of soil mycorrhizae.  In the absence of mycorrhizae, inhibitory priority effects can restrict local species diversity and increase primary production, especially if the pioneers are more productive, e.g. faster growth rate, leading to strong negative slopes (see panel a below).  By contrast, a less competitive or facilitative pioneer, or colonists that are more different from pioneers in their niches than their competitive abilities, may favour species co-existence and result in functional complementarity that enhances both diversity and productivity:

Figure 1. Hypothesized responses of primary production to changing species richness. Gradient of species richness can be thought of as ‘time’ in the context of an assembling community. Primary production (a) decreases with richness if pioneer species are more productive, or (b) increases if they are less dominant and allow later-arriving colonists to co-exist. Grey curve in (a) might occur when competitive differences among species decline.

Figure 1 from Tanentzap (2015). Hypothesized responses of primary production to changing species richness.  Gradient of species richness can be thought of as ‘time’ in the context of an assembling community. Primary production (a) decreases with richness if pioneer species are more productive, or (b) increases if they are less dominant and allow later-arriving colonists to co-exist. Grey curve in (a) might occur when competitive differences among species decline.

The findings of Burkle & Belote now suggest that biotic interactions that change environmental conditions, such as through predation and mutualisms, can also modify the strength of priority effects (grey line in panel a above).  We’ve commented more upon this here.  At the very least, we’re pleased to get the words of Lennon and McCartney into Applied Vegetation Science and hope to have kicked off a new lyrical competition.

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