Plants racing the climate
The world is changing. The climate is changing.
Whilst the world’s climate has altered repeatedly and drastically since it formed, its current rate is faster than anything we have observed in the last 12,000 years (the Holocene). This is having varied effects worldwide depending on the location in question (yes; it doesn’t mean everywhere simply gets hotter and drier: the world happens to be a little more complex than that or otherwise we’d consider climate science with the same derision as geography and surf-science).
It is however broadly true that in a lot of areas what we term climatic zones are being shifted towards the poles. The rate of this change varies on the climate zone; in cool areas with steep slopes this change in zones occurs at a speed of around 10m per year, whereas in flat tropical areas climatic zones can be moved by distances over 10km annually.
Such great changes are particularly problematic for plants, as a paper published in Trends in Ecology and Evolution last month shows. Whilst the majority of animals are capable of moving 10km in a year, plants have an obvious disadvantage stemming from being stuck to the damned floor. The overwhelming majority of plants cannot move; so for a species to move they must instead disperse their offspring far and wide. Some of their seeds will be dispersed in the less hospitable direction, some in the preferable one.
So if the survival of the species becomes a race for plants to create offspring and disperse them far and wide, a few factors become absolutely vital.
The first key characteristic in this is how long does it take the plant to reach maturity? In most plant species this period is anywhere between 1 and 30 years, and the end of this spectrum on which a species lies on will seriously affect its longevity as climatic zones shift polewards. It’s known that most seeds are dispersed between 10 and 1500 meters from their parent plant, with only a small proportion travelling further. Clearly in regions where climatic zones may be moving about 10km per year, this presents a serious problem.
The problem is worsened by the fact that species are usually at their least common on the fringes of their habitats, and so the amount of seeds being dispersed from their margins is lessened, further reducing the speed of the species’ movement from the new, inhospitable conditions that their ancestors have been left to.
It is worth remembering however that this has all happened before. The earth’s climate has been changing cyclically for a very long time and so it will surely be pointed out that any plant species alive today will already have survived numerous periods of climatic upheaval. Tropical species will have once lived far closer to the poles before then being confined to small equatorial areas; temperate species will have occupied huge areas before being exiled to relatively few mountain slopes as the earth warmed. If they were able to survive all of that, what’s so different now?
It would take a head lodged firmly in the sand to deny that humanity has changed the makeup of the earth’s surface over the last few millennia. We have altered approximately 75% of the world’s non-icy surface since the dawn of agriculture, and this presents a great challenge for any movement of wild life. If we know animals are struggling to find their way across our altered landscape, then there is a real problem for plants inherently lacking control over their precise movements. Habitats have become tremendously fragmented, making most species’ movements immensely difficult.
As the majority of plants need a bit of assistance with mating, they hire pollinators for the job, and whilst some will take on any old generalist, some have more specific tastes. There are a great many plants that can only be pollinated by a single species of insect, and whose adult forms in turn can only feed on that one type of plant. This gets particularly tricky when the pollinators larvae rely on a specific plant species which is not migrating as quickly, thus damning both players. Life gets even trickier for the majority of plants which rely on specific types of mycorrhizal fungus living amongst their roots for nutrition, but who may be less mobile. Thus generalist plant species are better-able to survive climate change whilst specialised kinds are in a lot more trouble.
Given that a world with many species of plants and animals in is generally more stable, safer and more pleasant for humanity, what can we do about this issue? It is both our responsibility and in our best interests to have a healthy planet and as always prevention is better that the cure, but expressing such sentiments lately is like pissing in the wind. Whilst the best method for tackling this problem is to reduce our carbon emissions, nobody wants to do so.
So in this situation all is essentially damage control. Step one is to monitor the state of our world’s plants to remain alert to which are at the greatest risk (though the paltry funds given to environmental and conservation bodies, combined with our woefully inadequate knowledge of the world’s life makes this tricky)
Wherever possible, tackling habitat fragmentation should be attempted by introducing corridors between habitats. Though not an outright cure to the difficulties of plants migration in a changing climate it at least reduces the difficulties that our plants face, even if only for the most mobile species. Helping some is better than helping none, after all.
In some situations it is desirable to actually move species to new habitat outside of their current range in what’s been termed ‘assisted migration’; perhaps if there is an impassable barrier to the species, such as a large river, a city, or perhaps just running out of mountain as the lower levels become too warm.
The final option is the insurance policy of seedbanks such as the one at Kew, where seeds are kept dormant to be rereleased at a later date. Though again an imperfect protection of the world’s plant diversity, they at least go some way to protecting valuable or vulnerable species.
Ultimately, as with so many environmental problems, the best cure would be for us to reduce our emissions and reduce our demands on the world by reducing population growth. Whilst humanity keeps its eyes firmly closed to these issues however, it falls to scientists and environmentalists to catalogue the impact we are having, and maybe attempt to reduce it a little.
Florida Torreya photo from Wikimedia commons