Home on the Range... Shifts - Part Two: No Place to Go

Today I will be presenting to you the long-awaited next part of the “Home on the Range… Shifts” blog mini-series. First however, I think it’s important we briefly recap last week’s blog, and remind ourselves what a range shift is.

To define a range shift, it is a change in the distribution of a species, wherein the area inhabited by the metapopulation grows, shrinks or migrates. Last week, Parmesan et al.(1999) kindly informed us that a range shift occurs on a population, not an individual level. In essence then, it is not individual migrations, but rather net colonisations or extinctions of communities at distribution boundaries that result in what we call “range shifts”.

This Panda, for instance, is doing range shifts wrong.

After wading through a veritable swamp of evidence last week, this week we will be turning our attention to the sad truth behind what initially appears like a strong adaptational response to climate change, and look at those that are getting left behind.

  

We will start with what has become essentially a “cult classic” of range shift research papers. Cited 891 times according to Nature, Warren et al.’s (2001) study into the changes in distribution of 46 butterfly species paints a bad picture for the future of some unfortunate butterflies. The paper examines a range of generalist and specialist species, all of whom find the northern boundaries to their distribution in the UK.

Warren et al. begin by informing us of what we would expect to see from British butterflies; that they should be taking full advantage of losing the shackles of climate and readily colonising newly available habitats. This assumption is also supported by their relatively short generational turnover and high general mobility. Whilst this is true for some, Warren et al. note that between 1970 and 1999, 3/4s of the species analysed actually declined in distributional area.

This decline in range has affected species in a disproportionate manner. Out of 18 generalists (those suited to a wide range of environments), only 9 declined in area compared to 26 of 28 specialist species (that is, species that occupy a habitat niche, or are considered “fit” only in a small area). Mobile species also fared better, with only 11 of 20 declining compared to 24 of 26 sedentary. As 26/28 specialist species are also sedentary, whereas 18/18 generalists are mobile, this means that stark differences exist between the successful expansion of generalists, and, as Warren et al. warn, we may soon see biological communities become dominated by  these mobile “jack of all trades”.

Here is a graph taken from the Warren et al. (2001) study, that shows the changing distribution of habitat specialist Argynnis adippe, which has shown the most dramatic decline in range. Black circles show records where distribution has stayed the same between 1970-1999, green shows apparent extinctions (where the animal was recorded between 1970-1982, but not for 1995-1999), pink shows new records (not recorded between 1970-1982, recorded between 1995-1999) and white shows pre-1970 extinctions. As yoi can see, although there are expansions in the range, the vast majority of circles show evidence of extinction.

 

This mass decline in butterfly range however is surprising. Although it is logical that in times of transition such as these mobile generalists take the lead,  the extent of decline in sedentary specialists here is of a far greater magnitude than expected, seeing as many specialist and sedentary species had relatively little trouble negotiating the rapid climate change (such as the Dansgaard-Oeschger events) seen in the Quaternary. Something, then, has changed. There is a new factor at play, and it goes by the name of habitat fragmentation (or the breaking up of natural habitats through human land-use change).

Habitat fragmentation, according to Warren et al., has prevented the spread of sedentary specialists as habitat patches have become simply too isolated to colonise, and on the rare occasion that a specialist does reach a newly available suitable habitat, it is unlikely that sufficient others will also arrive for the population to begin to grow. They go on to note that, due to this fragmentation, it is unlikely that we will see patterns of expansion that match those of similar species in the quaternary. Since 1940, the UK has seen losses of up to 70% of semi-natural habitats; this perhaps explains why we see sedentary specialists such as Plebejus argus (with low mobility and a liking for lowland heathland) declining by 28% whilst mobile generalists such as Polygomia c-album expand their ranges uninhibited by around 30%.

This endangering of specialists by climate change and habitat fragmentation is not unique to butterflies, or even to fauna. Honnay et al.’s (2002) study on the effects of climate change on the ranges of 112 forest plant species in Belgium also finds that habitat fragmentation is hampering the spread of species that really need to get up and go. According to Honnay et al. (2002), most forest plant species are indeed specialists themselves, who are adapted to what are considered to be environmentally stable forests.

Alongside being specialists, the very nature of most forest plant species puts them at a significant disadvantage in times of rapid warming. Long generational times, for instance, prevent in situ adaptation to new conditions, as genes that may provide advantages in warmer conditions cannot be passed through generations quickly enough to match warming. The way such species cope then, is through the tracking of changing environs and the colonisation of newly available/suitable forested environments.

This, however, seems to be becoming impossible. Honnay etal. (2002) study change in two landscapes, one fragmented and one of high-connectivity, using (due to long generation times making a direct study impossible) snapshots of forests of different ages and using them as a make-shift time-series. They found that in the fragmented landscape, 30% of species had a colonisation success of 0%, showing no colonisation over a period of 42 years, and 77% of species in this habitat had a colonisation success rate of less than 20%. Only 15% of species were able to colonise suitable habitats within the study area. The species within the high-connectivity landscape fared a little better, with only 17% of species having a colonisation success rate of more than 81%, and 66% with a success rate lower than 20%.

This graph, taken from Honnay et al. shows the colonisation success of species in both the fragmented landscape (left) and high-connectivity landsacpe (right). Dark grey represents colonisation success of >5%, light grey those that have a negative occupancy vs. distance-to-source population relation (essentially meaning that the further away the less change of colonisation), and white the occurence of colonisation independent of distance. It is clear from this graph the effect that habitat fragmentation has on the ability of species to expand their distributions, however both landscapes still seem to be heavily disadvantaged

 

Again we see fragmented landscapes having a dramatic effect on the ability to colonise. However, and this is important, even in a high-connectivity landscape, colonisation rates and speeds are not faring much better. Honnay et al. theorise that to migrate sufficiently to match projected warming (of between 2.1 and 4.6^oC by 2080 in the region), the plants need to expand their ranges by between 3000 – 5000 metres per year; at present they are achieving rates of an average of just a few metres.

So what else is holding back these plants? On top of fragmented landscapes making colonisation a far harder task, there is also a recent marked absence of large migrating herbivores that might carry seeds in their fur or dung. These factors dramatically limit the ability of plants to disperse and colonise, and combined, make it seemingly impossible to match Holocene migration rates of 1000 – 2000km.

This figure, also taken from Honnay et al. shows the probabity of occurance of a plant species in a selected new suitable habitat for both fragmented (triangles) and high-connectivity (circle) landscapes. It is done for four speces, Anum maculatum (a), Geum urbanum (b), Adoxa moschatellina (c) and Primula elatior (d). You can see that for all four species, occurences of colonisation are far greater in high-connectivity landscapes, however both have great difficultly colonising distances greater than ~600m; a factor that may be due to the absence of large migratory herbivores.

The only way forward for these plants, as seen by Honnay et al., is now through human intervention. Supposedly, plant seed transplantation or artificial seed transport are the only ways in which these specialist species will be able to migrate. Not only have humans forced them into a corner with global warming, but, through habitat fragmentation and the removal of large herbivores, have rid them of the tools to deal with it.

 

Here then, I have presented two very shocking examples of the dangers of range shifts. In a pure world, untouched by humans, it is all but possible that many of the species described here would have little trouble keeping pace with climate. But we don’t live in that world. Our world has become a jigsaw of human biomes, or “anthromes”, where we have not only transformed the landscape but removed some of its vital inhabitants. Species now aren’t merely facing the challenge of climate change, one that they have shown that they are able to conquer numerous times in the past, but are also facing the challenge of finding their way through a world that is, essentially, broken.

We have seen that species cannot match previous rates of migration. Now, with human induced climate change being arguably the most rapid and intense climate event we have seen for millennia, it does make you question whether Thomas et al.’s (2004) prediction of 52% of species committed to extinction seems so far-fetched after all.

Until next time. 

Just in case you are feeling a bit macabre after that rather depressing post, here is an article from The Economist that shows how wolves, when faced with a hard time in North America, have adapted by forming what some are calling a new species, dubbed as the "coywolf" - see here - this animal is actually faring better than both its wolf and coyote bretheren, hence "greater than the sum of its parts"