August 15, 2013

When animal and plant populations move up and out

Plants are not always as static as they seem. While most individual plants stay relatively still compared to most animals (no, not all), plant populations can shift laterally over time in an analogous way to animal populations. The main difference is the rooted nature of plants means that they move slowly, across generations. The cause of this is usually environmental: the species is shifting as the climate changes or as competition or another form of pressure encroaches it.

Richard Brusca and colleagues have published the results of a plant survey in Arizona which shows that species distributions have changed in the last 50 years. Since a survey carried out by Robert Whittaker in 1963, the vegetation on an area of the Catalina Mountains has crept its way higher up the mountain. Given the difference requirements and tolerance range of different plant species, the entire vegetation does not shift simultaneously or equally; this means that community composition has changed as some plants "move" upwards.

This is important evidence of the effect climate change may be having or will have worldwide: as temperatures get warmer, plant communities move higher in an attempt to remain in cooler conditions. Sometimes, species are left with nowhere to go, which happens to high altitude species with nowhere higher to go or species that don't have populations with nearby montane retreats.

It isn't the first time this has happened to the world's vegetation. During the Pleistocene (and probably late Pliocene) there were numerous climate fluctuations. In the northern hemisphere, this phenomenon manifested itself through glacial and interglacial periods. Glacial periods are what we commonly call "ice ages". While high and temperate latitude species retreated and spread with the advance and diminishment of ice, in the tropics things were quite different. At lower latitudes, the Pleistocene climate fluctuations took the form of drier and wetter periods. Though it may not sound as extreme as ice ages coming and going, these changes did have drastic effects on tropical habitats.

In the South American lowlands, vast areas of lush rainforest were broken up by savannahs during the drier periods. Along the Andean mountain range, many species retreated upwards. During more humid periods, organisms would retreat back down to a comfortable altitude. It is actually thought that these population advances and retreats across neotropical landscapes may have played an important role in engendering the extremely high levels of biodiversity found there today. Sticking with populations creeping up mountains, some authors (see sources) believe that these continual altitudinal shifts may have split populations of some species long enough for them to become to genetically distinct to interbreed when they came into contact again (see Figure 1). Overtime, this would lead to species' diverging. This explanation is a gross oversimplification of the process, but that is the basic idea behind it.

Model proposed by Adams (1977) for diversification of montane taxa during glacial periods, species would be pushed down with the shifting environmental conditions. This would enable horizontal dispersal to other peaks. During the warm interglacial periods, however, populations would be forced upward once more, resulting in once contiguous populations now residing in isolation. Adapted by O'Reilly (2012) from Prance's (1987) adaptation of Adams (1977).

Darwin proposed the same may have happened in Africa – I used South America as an example because some of it's ecosystems contain the highest levels of biodiversity on Earth.

However, I'm not suggesting that we needn't worry about this altitudinal retreat of plant populations in Arizona. Quite the contrary. The fluctuations during the Pleistocene may have influenced species divergence in the tropics, but it's important to remember that the epoch also ended in a series of big extinction events! What's more, this time we as humans are responsible for a very rapid climate change which in the long run may not give species time to adapt to or the environment to easily recover from. I just thought the ascent-differentiation-descent of tropical species during the Pleistocene was an interesting phenomenon to bring up!



Adams, M.J. (1977). Trapped in a Colombian Sierra. Geographical Magazine, 49: 250—254.

Brusca, R., Wiens, J.F., Meyer, W.M, Eble, J., Franklin, K., Overpeck, J.T., Moore, W. (2013). Dramatic response to climate change in the Southwest: Robert Whittaker's 1963 Arizona Mountain plant transect revisited. Ecology and Evolution, DOI:10.1002/ece3.720

Bush, M. B., Colinvaux, P. A., Wiemann, M. C., Piperno, D. R., & Liu, K. (1990). Late Pleistocene temperature depression and vegetation change in Ecuadorian Amazonia. Quaternary Research, 34 (3): 330—345.

     — (1994). Amazonian speciation – a necessarily complex model. Journal of Biogeography, 21: 5—17.

Darwin, C. (1859). Geographical Distribution (I). In The Origin of Species. 344—374. Penguin Books Ltc, Middlesex, UK.

Hooghiemstra, H. (1989). Quaternary and Upper Pliocene glaciations and forest development in the
tropical Andes: evidence from a long high-resolution pollen record from the sedimentary basin of Bogota, Colombia. Palaeogeography, Palaeoclimatology, Palaeoecology, 72: 11—26.

Van der Hammen, T. (1974) The Pleistocene changes of vegetation and climate in tropical South America. J. Biogeogr. 1: 3—26.

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