Our world is organized by a large number of borders . Imaginary lines that serve to delimit our territories, established for political, administrative and even arbitrary reasons.
Sometimes these borders are defined by a river or other natural feature, such as the border between Argentina and Brazil and Uruguay, formed by the Uruguay River , or the Mississippi River , in the United States, which separates the states of Wisconsin, Illinois, Kentucky Minnesota, Tennessee and Mississippi, Iowa, Missouri, Arkansas and Louisiana. But this is not usually the case, political borders are generally not related to real natural barriers.
There are a large number of barriers that delimit certain areas for living beings, called biogeographic regions . These regions and barriers are studied by biogeography .
Although in nature there are many forms of delimitation of territories, they are not usually fixed and fully defined borders. Some of them are diffuse: they distinguish areas clearly different from each other, but not in a defined limit, but through a gradient, a kind of gray zone of gradual transition between one zone and another. These kinds of gradual boundaries are very common in biology.
Other barriers are physically defined by a specific agent that delimits one side from another, although they are not usually fixed borders, but rather change in the short, medium or long term.
The coastline: an example of a changing barrier
One of the clearest examples of a changing natural border is the coastline , which forms a clear natural border between terrestrial and marine ecosystems defined by a very clear boundary —so far there is no water, from here there is—, and yet it is a constantly changing border.
In the short term, in minutes or hours, waves and tides change the coastline; in fact, the intermediate area through which this border moves, that area that is sometimes emerged and sometimes flooded, is called the intertidal zone, and forms its own ecosystem, generally very rich and biodiverse.
In the medium term, which in natural terms can be from months to centuries, it can change, expanding the emerged land due to the deposition of sediments from the ground, due to the arrival of a lava flow, as happened on the island of La Palma in 2021 , or by a drop in sea level; or by entering the sea inland, due to an increase in its level or due to the effect of erosion.
In a very long term, of millions of years, tectonic movements can clearly alter the coastline, through folding of the earth’s crust or displacements due to plate tectonics .
Other barriers caused by water
Just as the coastline is a barrier between two ecosystems, one terrestrial and one marine, a body of water can act as a barrier between two terrestrial ecosystems. On a small scale, this is the case of streams, rivers, lagoons and lakes, and on a large scale, seas and oceans. The larger a barrier, the greater the effect in preventing species colonization. That is why island ecosystems are so vulnerable to the entry of exotic species: they have evolved for a long time with a very specific set of endemic species, which have become highly specialized, and any interference can be ecologically catastrophic.
Certain bodies of water function not only as barriers, but also as corridors. A river, for example, can be an important barrier to prevent the passage of a plant species from one side to another, but it can make it easier for it to colonize regions far from the same bank . The winds channeled by the river are longitudinal, and usually prevent the seeds from crossing it; but, they can carry them down the river and reach new territories without difficulty.
Just as water functions as a barrier to land, land also functions as a barrier to water. The orography of the basins keeps the masses of river water separated, preventing, or at least making it difficult, for the organisms present in one basin to reach neighboring basins. The sleeves keep large masses of water isolated from the ocean that feeds them, as is the case with the Mar Menor. And the continents themselves separate large bodies of water, such as seas or oceans, from one another. In this sense, endorheic basins are isolated from the rest of the water masses, inside emerged territories and function, for these purposes, in the same way as islands. In Spain we find endorheic basins such as the Laguna de Fuente de Piedra in Malaga, the Laguna de Gallocanta in Aragón or the Laguna de Salinas, in Alicante. The famous —and misnamed— Caspian and Dead Seas are also endorheic.
Other types of natural barriers
Not all barriers are defined by water. There are other factors, or combinations of factors, that when they occur, barriers are formed for certain living beings.
The canyons are some of the most obvious. Although they are usually formed by the action of rivers, if they are deep enough they can have their own value as a border, even if the river that formed them dries up or changes course. They can be difficult for animals to cross, except for animals that fly or are capable of climbing steep walls, and airborne plants often encounter the same problem as rivers: longitudinal winds prevent reaching the other. side. However, they also serve as corridors, either for plants, thanks to those same winds, or for animals, which can travel below from one region to another, crossing mountainous areas without difficulty.
And it is that mountain ranges can also function as borders. Not only do they present a high altitude that many living beings are unable to overcome, but they are often associated with differences in the local climate, drier and colder environments that can prevent the entry and maintenance of living beings that inhabit the valleys. In this way, consecutive valleys isolated by mountainous regions can also function as islands, whose borders are not rivers but mountains. A clear example of this event are the Pisuerga, Carrión, Cea, Tuéjar, Esla, Porma, Curueño, Torío, Bernesga, Luna, Omañas and Sil rivers, which each form their own valley in the mountainous regions of northern Palencia and León. .
Borders are also the mountain ranges that separate the two Iberian plateaus or those that isolate the Ebro or Guadalquivir valleys.
Although there are still more abstract barriers . For many living things, climatic differences are themselves barriers. The Iberian Peninsula is dominated by two bioclimatic regions : the Euro-Siberian, dominated by a temperate climate, which occupies the entire Cantabrian coast and most of the Pyrenees; and the Mediterranean, characterized by its homonymous climate and which covers the rest of the Iberian extension. These regions are in turn divided into smaller areas, which are defined by climatic variables such as annual or summer precipitation, the level of ambient humidity, average, maximum, minimum temperatures, the number of days a year that frosts occur , or the difference between summer and winter temperatures.
These climatic differences have a strong effect on the vegetation, so that the different bioclimatic areas are well defined by their plant composition, and as a consequence, their communities and ecosystems.
Bioclimatic barriers are not easy to define, on the one hand, because the climate does not usually change drastically from one area to another, but gradually, so we would be facing diffuse borders. But mainly because anthropogenic climate change is displacing these barriers , forcing ecosystems to move with them or disappear.
Andrade, C. et al. 2020. Climate change projections for the Worldwide Bioclimatic Classification System in the Iberian Peninsula until 2070. International Journal of Climatology, 40(14), 5863-5886. DOI: 10.1002/joc.6553
Rahel, F. J. 2007. Biogeographic barriers, connectivity and homogenization of freshwater faunas: it’s a small world after all. Freshwater Biology, 52(4), 696-710. DOI: 10.1111/j.1365-2427.2006.01708.x
Rivas-Martinez, S. et al. 2017. Bioclimatology of the Iberian Peninsula and the Balearic Islands. In J. Loidi (Eds.), The vegetation of the Iberian Peninsula: Volume 1 (pp. 29-80). Springer International Publishing. DOI: 10.1007/978-3-319-54784-8_2
Satler, J. D. et al. 2017. Do ecological communities disperse across biogeographic barriers as a unit? Molecular Ecology, 26(13), 3533-3545. DOI: 10.1111/mec.14137
Simberloff, D. S. 1969. Experimental Zoogeography of Islands: A Model for Insular Colonization. Ecology, 50(2), 296-314. DOI: 10.2307/1934857