Wednesday, August 31, 2016

The Trouble with the Plankton

Earlier this summer, I went to a Gordon Research Conference on Ocean Global Change Biology at the invitation of Sinead Collins. I don’t typically work on oceans, and so the fit might not see obvious, but the relevant part was that the field has taken on a distinctly evolutionary flavor. It turns out that many ocean biologists are now focusing on adaptive responses of marine organism to climate change, especially ocean acidification. It was a wonder to sit through talk after talk of studies assessing the potential for (usually) plankton to adapt to either increased acidity or warmer water. Even the talks that didn’t focus on evolution almost always referred to it in an informed and considered manner. I had previously been to a similar conference 8 or so years earlier, and that time I saw only the barest hint of evolution – so this was an exciting change. Yet it isn’t in my nature to be complementary without qualification (or critical without qualification) – and the same will apply here.

I couldn't decide my favorite photo so here are Google's favorites.
In 1961, G. Evelyn Hutchinson wrote a paper titled The Paradox of the Plankton, in which he discussed the apparent paradox that so many species of plankton coexist even though they compete for similar nutrients – ostensibly in contradiction to the principle of competitive exclusion. I wish to here introduce – by way of verbal analogy – The Trouble with the Plankton, which is somewhat related to the Paradox of the Plankton in its emphasis on variation.

Stated simply, I suggest that Ocean Global Change biologists should stop worry about whether or not plankton will evolve in response to climate change – they will! Unlike many other organisms, evolution is not normally going to be a problem for phytoplankton (or even zooplankton) – for four main reasons.

I couldn't decide my favorite photo so here are Google's favorites.
1. Most species of plankton are extremely abundant, which means that standing genetic variation will be huge, as will be mutational inputs. In short, genetic variation – the raw material for evolution – should be massive for essentially all marine plankton.

2. Most species (and indeed many populations and even individuals) of plankton experience dramatic fluctuations in environmental conditions across space (vertical and horizontal) and time (hourly, daily, seasonally). This past variation in environmental conditions means that past selection will have tested (and sometimes favored) adaptive genetic variants for a wide variety of conditions – again maintaining high genetic variation in adaptively-relevant variants.

3. The rate of abiotic environmental change in the ocean is very modest not only in relation to the above-noted past and present spatiotemporal variation in selection but also in relation to the generation time of phytoplankton (and zooplankton). As a result, the per-generation shift in the environment owing to climate change will be tiny in relation to the potential evolutionary speed of plankton.

4. Many plankton show adaptive plasticity in response to different abiotic conditions, including acidity and temperature. This plasticity should buffer the immediate negative effects of environmental change and thus allow further time for evolution.

Fitting these expectations, every study at the conference showed strong evolution in response to dramatically altered environments (often much more so than projections for climate change), despite often extremely limited starting genetic variation. Many studies of freshwater plankton have similarly shown that evolution in even small experimental populations can accomplish – in only a single summer – full adaptation to environmental changes projected to take place over decades. And “resurrection” studies that bring past zooplankton to life also show rapid responses to all sorts of environmental changes. So I suggest that we don’t need more studies asking “can plankton adapt to climate change” – they can – simple as that.

However, I do think that further evolutionary studies are critical for Ocean Global Change Biology – I merely suggest that their focus should be a bit different.

1. Studies could profitably ask “what are the consequences of the evolution of plankton for communities and ecosystems.” I image that the evolution of plankton in response to climate change could dramatically alter their relationship with other species in the community. Some of those species, especially those with longer generation times, such as planktivorous fish, might have trouble responding adaptively. Thus, it would be fascinating to take those experimentally evolved lines of plankton and see how they interact with other key species in the community.

Here you can find more arguments for considering the ecological effects of evolution.  
2. Although most (maybe all) plankton will have no trouble adapting to abiotic changes associated with climate change, they might have trouble adapting to some correlated biotic changes. For instance, planktivorous fish might dramatically change in abundance with climate change, which might then impact plankton populations in ways that are strongly modified by evolution.

Of course, the general statements above are not intended to imply that all marine invertebrates will easily adapt to climate change. Corals for instance seem to be near their physiological (and evolutionary) limits already and might have no suitable genetic variation to respond to selection. Of course, changing their symbionts might be another way to adapt – although that too will have limits. Also, species in already extreme conditions (e.g., the hottest or most acidic water) might not be able to persist locally as those conditions change. Indeed, acid rain caused the extirpation of many (but not all) plankton species – and very warm (or cold) temperatures could do the same.

Regardless of whether or not I am correct that plankton will have little trouble adapting, I do think evolutionary studies are extremely informative. I can’t wait to be invited back for the next Gordon Research Conference – or perhaps I won’t be given this post. 

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