Functional extinction of birds drives rapid evolutionary changes in seed size

Galetti et al. Science 340(6136): 1086-1090. DOI:0.1126/science.1233774. Functional extinction of birds drives rapid evolutionary changes in seed size

Birds only disperse what they can carry! From Galetti et al.

Birds only disperse what they can carry! From Galetti et al.

Will Pearse

Will Pearse

Wam-bam, this is a paper I would have loved to put in my undergrad essays. Plants need birds to disperse their seeds, and so when large birds go locally-extinct, plants evolve smaller seeds that smaller birds can carry. This happens really, really fast (within the last 75/100 years!) and so is a great example of rapid evolution.

A nastier man than I would point out that this is somewhat inferred; with no data on what seed size was like 100 years ago there’s a fair bit of supposition going on here. However, their variance decomposition (34% due to birds in forest, 0.1% differences among sites) is really quite striking, so I’m quite happy to go along with this. There’s such a clear link between seed size and probability of being dispersed (figure 2b) that I’m quite happy to accept the smoking gun of a huge selective pressure and observable differences.

Which leaves me with a slight problem, because I always assume that we can ignore both intraspecific variation and rapid evolution when doing ecosystem service work. If trait can evolve this rapidly, treating species’ ecosystem services and traits as fixed is no longer acceptable. Indeed, the situation is doubly problematic because there are going to be a lot of downstream effects of changing seed size, not just on the plant species itself (it’s now shifted on the simplified on the r vs. k selection spectrum), but also other species that interact with that plant. There is a huge literature on how phenology shifts are worse in tri-trophic interaction networks because not every component of the system can keep up with change – I see no reason for this not to be a concern here.

Lynsey McInnes

Lynsey McInnes

This is to all intents and purposes a very neat demonstration of purported rapid evolutionary change in the face of a new selective pressure brough about by human-mediated loss of large-gaped frugivores from forest fragments. One could quibble on whether the frugivore loss is driving the contraction in seed size variation, or whether fragmentation caused the frugivore loss, and so on, but the authors do a thorough job of dismissing other possible correlates….environmental differences among sites, checking the time needed for such a response, and I’m pretty convinced the relationship holds.

This is bad news! It suggests many of those stacks of papers predicting responses to climate change or habitat fragmentation that brush evolutionary responses under the carpet are probably missing key elements of the response game, Similarly, how does this two trophic level result cascade to additional trophic levels. Without big palm seeds and thus big healthy palms, what grows in their place? What effect do these newly dominant plant species have on other pieces of the forest ecosystem.  Ah, its frightening.

What is the next step? Can we rejoin the forest fragments and get the large-gaped frugivores back? Is there enough genetic variation left to get back the large seeds?

This must have been a time-consuming study and its just not feasible to initiate tons of new studies at similar scales to ascertain how pervasive such rapid evolutionary responses are. I would naively guess that it might be better to continue with this system and see if we can work out this change’s effect on additional chunks of the forest ecosystem. Perhaps the authors are already working in that.

The macroecologist in me also ponders the feasibility and merits of expanding the scope of such studies. Perhaps to a mesoscale at least. I am reminded of Phillimore et al‘s very slick mesoscale studies on variation in phenological responses across space in British frogs. Here, the authors were looking to distinguish local adaptation vs. plasticity governing the spatial variation that they saw in order to predict how populations would cope in the face of climate change that will alter the timing of temperature cues. In short, the authors conclude climate change is expected to outpace the frogs’ ability to respond. However, they ignored the potential for microevolutionary change, as the timescales they were thinking of were so short. The challenge now seems to be to incorporate this possible response? Admittedly, easier said than done…


About will.pearse
Ecology / evolutionary biologist

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