As global warming progresses, habitats change in their suitability for various life forms. It may be that moose will not be able to live in Minnesota in the future; Of the two resident moose populations, the one that lives in the area more affected by global warming has pretty much died out probably due indirectly to the effects of increased temperature. There are regions of the rockies where entire forests are dead because of temperature changes. And so on.
Imagine a large flat landscape. As one moves north vs. south, average annual temperature changes, as does the number of days of frost or the number of days of snow cover. As global warming occurs, the cold/snow lines move north. What could happen is that the organisms that do better in a cooler climate move north. That works for animals, but not so much for plants. But, over time, it is possible that as plants disperse their seeds and compete in different, changing habitats, their distribuitons will also change. When we look at geographically spread pollen spectra from ancient deposits we see this. One can track the movement of oak/hickory forests northward following the last glacial maximum in the central and eastern part of the United States, for instance. Global warming can involve just such a transition.
One problem with global warming is that large flat areas of landscape with large contiguous habitats do not exist in many places. A patch of southern forest in a forest reserve in Arkansas can’t “move” north to central Missouri because the space between them is farmland. The breaking up of large areas of the landscape by agriculture and urban zones makes this process impossible.
Even where that is not the case, the change in climate can be more rapid than the movement of habitats. Even though we can track the oak/hickory line moving north in the US from the last glacial maximum, the post glacial shifts of forest types for earlier glacial involved different dominant species. This is because the rapid climate changes that occur before and after glacial periods does not involve a painless movement of habitats, but rather, the widespread destruction of habitats and their subsequent phoenix like rebuilding.
Indeed, during the Last Glacial Maximum, ecological systems were so heavily disrupted that basic, primary productivity was reduced. There was less life on earth for a few thousand years. If ocean acidity and terrestrial biome disruption, including desertification (which is at an all time non-glacial high at present, or so it would seem) proceeds as currently expected, the basic biological productivity on Earth will similarly drop.
At the moment, human civilization uses up an energy budget equivalent to something like 20 percent of the earth’s biological productivity. If global productivity drops considerably, that can’t be sustained. Yes, folks, those of you who have babies at home now can expect that in the future the child you are raising now will not be able to find food, or possibly even sufficient oxygen. I’m not saying that this is going to happen, but it is certainly a possibility on the table. And, by the way, this is why it is time to stop giving anthropogenic global warming deniers any consideration whatsoever.
But, there is one tiny and, in the broad scheme of things pragmatically irrelevant but scientifically interesting bit of good news. Habitats that are currently extant in alpine region may be less affected than lowland habitats, and may serve as refuges.
The mountain refugia hypothesis is as old as the science of biogeography (more or less) but there is a new paper that explores the question of habitat survival in some detail, in the Swiss Alps.
The paper can be summed up in the following two diagrams, one drawn by me to represent the above outlined dismal scenario for terrestrial biomes, and the other provided in the paper for mountain biomes.
In the first, crude, drawing, each of the two bell curves represent the distribution of various habitats across a spectrum of cooler (left side of diagram) to warmer (right side of diagram) conditions. The first bell curve represents the present, and the second the “after warming” conditions. The overlapping area, with the cross hatching, represents the only common range of conditions. In this scenario, there is a total loss of most of the habitats in the present (left side) and an emergence of mostly new habitats, with only a tiny proportion of organisms able to simply survive in situ. This represents utter disaster that would take millenia to recover from. There will be regions of the world where this is what happens to the habitats in them.
Now look at the diagram from the paper, just published, by Scherrer and Korner:
In this diagram, we see that there is more overlap between existing habitats and habitats after a warming period. There is some “total loss” and some “reduced habitat” and some new habitats.
This difference, between larger scale habitat loss across vast flatter landscapes and changes in mountains, is due to the scale of variation. In mountain regions, habitats are more variable, and more importantly, variable in a smaller physical space because of the effects of altitude.
If you start near sea level in a temperate habitat and go north, you’ll reach a boreal habitat in a few thousand miles. If you start in the same exact habitat and go up a mountain side you will reach a boreal habitat in tens of miles or less, but going to a higher elevation. This allows for more different habitats to exist side by side in a small space. This, in turn, allows more shifts of habitats without entirely losing one habitat or another. The paper by Scherrer and Korner defines the extent and nature of this variation, focusing on surface temperature as the main variable.
Our results demonstrate that the topographically induced mosaics of micro-climatic conditions in an alpine landscape are associated with local plant species distribution. Semi-quantitative plant species indicator values based on expert knowledge and aggregated to community means match measured thermal habitat conditions. Metre-scale thermal contrasts signiﬁcantly exceed IPCC warming projections for the next 100 years. The data presented here thus indicate a great risk of overestimating alpine habitat losses in isotherm-based model scenarios. While all but the species depending on the very coldest micro-habitats will ﬁnd thermally suitable ‘escape’ habitats within short distances, there will be enhanced competition for those cooler places on a given slope in an alpine climate that is 2 K warmer. Yet, due to their topographic variability, alpine landscapes are likely to be safer places for most species than lowland terrain in a warming world.
We need to avoid making two mistakes when considering the results of this research. First, variation in habitat across altitude does not clone variation i habitat across latitude. No matter how high you go up on a temperate or tropical-zone mountain, you are not going to reach a place where the sun sets for six months (at the polar circles). Rainfall patterns are important and are determined by a number of factors including several that will not vary across altitude. The mimicking of latitude shift with altitude shift is the product a talented but imperfect mime.
Second, mountain habitats are small and vulnerable. A single pathogen, a major storm, a very bad year for rain, etc. can wipe out a large area which can be re-inhabited from adjoining areas. But isolated mountain habitats do not have adjoining areas from which organisms can migrate or disperse. So if you were thinking that you could relax about global warming, sorry, no dice. Indeed, some of the “lost” habitats in the diagram supplied with the paper may be among the most unique and irreplaceable, such as periglacial biomes in temperate or tropical regions. Such biomes may re-evolve, but they will be very gone and unable to “come back” in any other way once all the glaciers melt off all of the temperate and tropical mountains.
Scherrer, D., & Körner, C. (2010). Topographically controlled thermal-habitat differentiation buffers alpine plant diversity against climate warming Journal of Biogeography DOI: 10.1111/j.1365-2699.2010.02407.x