Species have been reported to be moving poleward and upward in mountains as a result of climate change. Evidence of this movement is piling up rapidly, and with every passing year the increasing speed at which species are moving up- and poleward is becoming ever so visible. Yet new studies also reveal that this species movement is often not as straightforward as it looks at first sight.
Indeed, one might think: climate warms, so species will follow. The problem is, however, that a species’ reaction to a change in their environment is not always that fast. They often need some time to adjust and move towards where climate is now suitable. This delayed reaction is especially true for sessile species, like plants, that depend almost entirely on seed transportation to travel around.
These so-called lags in species distribution shifts are currently not well understood, and even less well accounted for in our predictions of species distributions for the coming centuries. Which is why the recent review paper (Alexander et al., 2017, arising from a workshop organized by the Mountain Invasion Research Network) provides such timely steps forward for our understanding of these lags.
The paper distinguishes three different types of lags: “dispersal lags” indicate that a species has trouble to spread to higher elevations or towards the poles at the pace of the changing climate, while “establishment lags” result from problems with getting a foothold after arriving in a new environment. The last type of lag, called “extinction lags”, indicate that a species fails to disappear from an environment that might have become inhabitable.
Establishment and extinction lags seem to play a crucial role in shaping species distributions in this dynamic world. For example, closed vegetation in arctic/alpine ecosystems turns out to be very resistant to establishment of upward moving plants species. Thus even when low elevation species arrive at higher elevations, they often find a highly resistant native vegetation that is hard to overtake. At the same time, many species seem to persist at their lower range edge, even through substantial climatic changes. Again, biotic interactions are likely at play, which are said to be more important than climate at the lower range edge of species.
We humans play a critical role here as well. Surprisingly often, human influence is seen to reduce lags in distribution shifts: humans help with transportation of species to colder environments, for example through seeds sticking to clothing, and as such reduce the dispersal lag. Anthropogenic disturbance will reduce the resistance of the receiving community at high elevations as well, making establishment a lot easier for newcomers and thus reducing the establishment lag. Finally, these new invaders might indirectly reduce the extinction lag by outcompeting the native community, forcing them to retreat upward.
So why do we care about these lags? Well, they are critical to understand what will happen to our nature in the coming years, with increased climate warming and unlikely-to-cease anthropogenic disturbances looming at the horizon. Ecosystems are already seen to be changing drastically, with novel communities sprouting up of species that might have never lived together before. Yet as species are likely to be lagging behind to these changing in their environment, much more change might still be on the way. For a species, lagging behind climate change might indicate an inability to keep up with the changes. Good dispersers, on the other hand, might easily be able to track the climate, only to find the novel communties they enter to lack the species they usually interact with and desperately need to survive.