Migration of plant species will not compensate for losses in diversity in a warming world

While plants cannot walk, their populations can move. As the world warms species will move to higher latitudes. As some species leave for the poles, others will arrive from the equatorial regions. But will the loss in diversity from species leaving be balanced by the arrival of new species?

Climate change is not uniform and topography can also complicate matters. Rather than moving north, species may move uphill to a cooler climate – at least as long as there’s hill left to move up. As well as temperature, regions will become more arid or damp, adding further stresses for plants to tackle.

Susan Harrison of UC Davis has been examining the evidence for species shifts. In her new article in Philosophical Transactions of the Royal Society B she argues that the complexity of the sudden shift in climate will result in a net loss of biodiversity. This, she states is in contrast to recent studies that have not found a human–induced loss of biodiversity. But there is a problem with assuming this state of affairs will continue.

Climate change velocity is going to increase, but plant adaptation is not. This will affect plants as conditions do not merely heat up, but also alter water availability. Escape for populations will rely on seed dispersal, but can seeds outrun a warming climate? Finally, she argues, a plant’s ability to thrive in a climate is related to certain functional strategies. But these strategies are prone to break down as conditions change. How do you time flowering to benefit pollinators, when all your pollinators have passed? Harrison tackles each of these issues in more depth. 

Will a warming world be dryer?

Harrison starts by examining the change in water availability.  “Over large areas of the terrestrial Earth, warming will make the climate effectively more arid, decreasing plant productivity and exacerbating the role of water as the critical limiting factor,” she writes. Though she notes that aridity is not the only way that the climate can shift. “At high latitudes and elevations where the length and warmth of the growing season are more strongly limiting than water, and in regions where large increases in precipitation are also expected, the effects of warming on potential plant productivity will tend to be positive. However, realization of these potential increases in productivity, unlike the potential decreases in productivity in water-limited climates, will depend on either the presence or the immigration of species capable of thriving under the altered conditions.”

Harrison refers back to research in the specialist literature, including research on an elevated carbon dioxide experiment on grasses by Hovenden and colleagues in Annals of Botany. The survival of grassland in this experiment is influenced by two factors. Warming reduces access to water, raising the possibility of drought. Elevated carbon dioxide in contrast reduces the need for transpiration, so mitigates water stress. Will the two factors balance?

The conclusions Hovenden and colleagues come to is… not really. If there is water, then elevated carbon dioxide helps, but the same carbon dioxide will contribute to failures of the rains. In years when this happens the grasses die. Harrison refers to independent work published in Oecologia and Ecology Letters that shows that plants may die before they can sow seed. Without a seed bank, plants will disappear.

Even in currently snowy regions, an increase in aridity can correlate with reduced biodiversity, so much of the planet could expect to see biodiversity come under pressure. In contrast, the diversity of the polar regions may increase, but balance would need plants to be able to move to colonise these locations. This brings her to the next challenge.

Can plant seeds travel faster than climate change?

How fast is the climate changing? Harrison has a source for that, work by Loarie and colleagues in Nature. They write that climate change has “a global mean of 0.42 km yr-1 (A1B emission scenario). Owing to topographic effects, the velocity of temperature change is lowest in mountainous biomes such as tropical and subtropical coniferous forests (0.08 km yr-1), temperate coniferous forest, and montane grasslands. Velocities are highest in flooded grasslands (1.26 km yr-1), mangroves and deserts.”

Climate has always changed though. Even a couch potato could walk at a rate of a bit over a kilometre a year. Is it such a big deal? How fast was fast climate change in the past? Harrison has a citation for this too, and it’s worrying. Sandel and colleagues distinguished between stable and high-speed climate change in their study of past grasslands. Fast in this scenario was anything over ten (10) metres (not kilometres) per year. Projected speeds for climate change are therefore over a hundred times faster than past conditions in some places.

If plants are going to travel with the climate, they’ll have to do it as seeds. Some plant seeds can travel over long distances, but once they do so they need time to establish a viable population before they’re likely to travel a long distance again. That will cut down the average speed of travel. The distance a seed travels depends in part on how it moves.

Thomson and colleagues have found that getting eaten in a fruit worked best for a seed with wanderlust, with a median distance of 245m. Seed-caching tends to move seeds just 8m and wind barely over two metres on average. If a plant has some sort of ballistic dispersal, then it will move around one metre and if there’s no help at all, then maybe half a metre. If climate change in even the slowest areas is 80m per year, that’s an awful lot of plants needing assistance either with travel or super-rapid reproduction.

If the results are so stark, why don’t they appear more often in papers?

You might be surprised at the difference in the figures between climate speed and dispersal speed. You might not have seen them in recent papers on climate change and migration. A lot of papers I’ve read haven’t seen movement by plants to colonise a location as a difficult problem. On the contrary, many I’ve read have worked on the assumption that if there are plants at lower elevations that can move up to displace species, they will.

Harrison says this is the problem, in that I’m probably not reading articles from a balanced range of regions. A lot of climate change research is in North-temperate alpine zones. There are good reasons for researching the effect of climate change in these zones, but these areas are more easily colonised than elsewhere. This is why climate change may appear to lead to an increase in biodiversity in some places, but alpine zones are not model biomes for the rest of the plant.

How much biodiversity do we need?

The changes that are coming will select for some species over others. As species disappear, so too will different natural tricks and techniques for dealing with adverse conditions. Harrison points to a loss of functional and phylogenetic diversity. This means there will no longer be some plants performing some ecosystem functions, leading to pressures on other species. How many species can we lose before this becomes a serious problem?

Wherever the limit is, we seem to be approaching it faster and faster. As some parts of the world are discovering, the climate can change the environment from shifting over decades to suddenly becoming extremely interesting. The problem is that if you’re in a part of the environment that has just become fascinating, you might be too pre-occupied dealing with it to plan for conservation.