Elevational diversity gradient
Elevational diversity gradient (EDG) is an ecological pattern where biodiversity changes with elevation. The EDG states that species richness tends to decrease as elevation increases, up to a certain point, creating a "diversity bulge" at middle elevations. There have been multiple hypotheses proposed for explaining the EDG, none of which accurately describe the phenomenon in full.
A similar pattern, known as the latitudinal diversity gradient, describes an increase in biodiversity from the poles to the equator. While the EDG generally follows the LDG (i.e., high elevations in tropical regions have greater biodiversity than high elevations in temperate regions), the LDG does not account for elevational changes.
Origin
The first recorded observation of the elevational diversity gradient was by Carl Linnaeus in his treatise On the growth of the habitable Earth. In this document, Linnaeus based his predictions on flood geology, assuming most of the world was at one point inundated, leaving only the highest elevations available for terrestrial life. Since, by Linnaeus’ hypothesis, all life was concentrated at high elevations, a higher species diversity would be observed there even as life re-populated lower elevations.
In 1799, Alexander von Humboldt and Aimé Bonpland described elevational changes along the Andean slopes, noting how climatic changes impacted plant and animal communities. These observations contributed to Leslie R. Holdridge's "life zone concept" (1947). Climatic variables shaping life zones include mean potential temperature, total annual precipitation, and the ratio of mean annual evapotranspiration to mean annual precipitation. These variables, most notably precipitation and temperature, vary along an elevational gradient, resulting in the distribution of different ecosystems.[1]
Much of the current literature correlates elevational diversity to gradients in single climatic or biotic variables including "rainfall, temperature, productivity, competition, resource abundance, habitat complexity, or habitat diversity".[2]
Implications
Mid-domain effect
A pattern in species richness is also observed as one moves along an elevational gradient; generally, species richness is thought to decline with increasing elevation. Whether this decline is monotonic or if it assumes different shapes based on the
For certain taxa and regions, there is a mid-elevational peak in species richness. This pattern has received empirical support for small mammals,
This elevational pattern, however, was less consistent for species with small ranges, suggesting that environmental factors may be more clearly accounted for when constraints on domain boundaries are loosened. In cases where geometric models fail to explain the location of the midpoint or the trend in species richness, other explanations need to be explored. An example of this can be seen with microbes, which have been shown to exhibit monotonically decreasing diversity when moving from low to high elevations.[10]
Mountain-mass effect
The mountain-mass effect (also known as the Massenerhebung effect or mass-elevation effect) was proposed in 1904 by A. de Quarvain. This phenomenon recognizes the correlation between mountain mass and the occurrence of physiognomically similar vegetation types; similarity in vegetation type is observed at higher elevations on large mountain masses.[13] Furthermore, under a climatically driven mountain–mass effect, there is a “positive linear trend observed in the elevation of highest diversity with mountain height”.[2] This trend is most evident on isolated mountain peaks.
Hypotheses
Area hypothesis
Another hypothesis that is cited to explain the upper limit of the elevational diversity gradient is the area hypothesis, which states that larger areas can support more species. As elevation increases, total area decreases; thus, there are more species present at middle elevations than at high elevations.
However, this hypothesis does not account for differences between
Rainfall hypothesis
This hypothesis states that diversity increases with increasing
Resource diversity hypothesis
The resource diversity hypothesis states an increase in diversity can be seen when an increase in the diversity of available resources such as soil and food is present.[15][16] In this hypothesis diversity increases in an area of higher resource diversity even when resource abundance is constant. However resource diversity, especially in food, could be a result of other influences, such as rainfall and productivity; as such, it may be inappropriate to consider the resource diversity hypothesis as a mechanism acting independently of other factors influencing diversity gradients.[17]
Productivity hypothesis
The productivity hypothesis states that diversity increases with increased productivity.[14] There is some contradiction to this as other research suggests that after a certain point increasing productivity correlates with a decrease in diversity.[18]
It is generally thought that productivity decreases with an increase in elevation, however, some research shows a peak in productivity at mid-elevation which may be related to a peak in rainfall within the same area.[14]
Temperature hypothesis
The temperature hypothesis correlates increasing temperature with an increase in species diversity, mainly because of temperature's effect on productivity.
Competition
There are conflicting views on the effect of competition on species diversity. Some hold the view that an increase in interspecies competition leads to local extinction and a decrease in diversity.[14] Others view competition as a means of species specialization and niche partitioning, resulting in increased diversity.[20]
In other studies, the competition between plant species at high elevations has been shown to facilitate the movement of plant species into high-stress environments. The competition between plant species leads to hardier species spreading into the high-stress environment. These founder species then provide shelter and facilitate the movement of less hardy species into the area.[10][21] This may result in the movement of plant species up a mountainside.
State of research
Current research illuminates a variety of mechanisms that can be used to explain elevational diversity gradients. No one factor can be used to explain the presence of diversity gradients within and among taxa; in many cases, we must consider more than one hypothesis or mechanism to fully understand a pattern in elevational diversity. The emerging macroecological experiments along environmental gradients (for example, mountain elevation gradients) are an important tool in ecological research because they allow for the disentangling the effects of individual environmental drivers on biodiversity, the independent effects of which are not easily separated due to their covariance in nature.[22] For instance, microcosm experimental setups in subtropical and subarctic regions (China and Norway, respectively) showed clear segregation of bacterial species along temperature gradients, and interactive effects of temperature and nutrients on biodiversity along mountain elevation gradients.[22] A more expansive research program for mountain biogeography may be extremely beneficial for conservation biologists seeking to understand factors driving biodiversity in known hot spots.[23] Further research and reviews are also needed to address contradictions in the scientific literature and to identify the extent of interactions between current explanations and hypotheses.
References
- ^ Kricher, J. C. (2011) What and where are the tropics? Tropical Ecology, pp. 7-25. Princeton UP, Princeton, NJ.
- ^ a b McCain & Christy M. (2005) Elevational gradients in the diversity of small mammals. Ecology, 86.2, 366-72.
- ^ a b c Rahbek, C. (1995) The elevational gradient of species richness – a uniform pattern. Ecography, 18, 200–205.
- ^ McCain, C. M. (2004) The mid-domain effect applied to elevational gradients: species richness of small mammals in Costa Rica. Journal of Biogeography, 31, 19-31.
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- ^ Sanders, N.J. (2002) Elevational gradients in ant species richness: area, geometry, and Rapoport's rule. Ecography, 25, 25-32.
- ^ Grytnes, J. A. (2003) Species-richness patterns of vascular plants along seven altitudinal transects in Norway. Ecography, 26, 291-300.
- ^ Grytnes, J. A., & 0. R. Vetaas (2002) Species richness and altitude: a comparison between null models and interpolated plant species richness along the Himalayan altitudinal gradient, Nepal. American Naturalist, 159, 294-304.
- ^ a b c Bryant, J.A., Lamanna C., Morlon H., Kerkhoff A.J., Enquist B.J. & Green J.L. (2008) Microbes on mountainsides: contrasting elevational patterns of bacterial and plant diversity. Proceedings of the National Academy of Sciences, 105, 11505–11511.
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- ^ Colwell, R.K. & Lees, D.C. (2000) The mid-domain effect: geometric constraints on the geography of species richness. Trends in Ecology and Evolution, 15, 70–76.
- ^ Flenley, J. R. (1994) Tropical montane cloud forests. Cloud forest, the Massenerhebung effect, and ultraviolet insolation (eds L. S. Hamil-ton, J. O. Juvik & E N. Scatena), pp. 150-155. Springer-Verlag, New York, New York, USA.
- ^ a b c d e Rosenzweig, M.L. (1992) Species diversity gradients: we know more and less than we thought. Journal of Mammalogy, 73.4, 715-30.
- ^ a b Gentry, A.H. (1988) Changes in plant community diversity and floristic composition on environmental and geographical gradients. Annals of the Missouri Botanical Garden, 75.1, 1-34.
- ^ SÁnchez-Cordero, V. (2001) Elevation gradients of diversity for rodents and bats in Oaxaca, Mexico. Global Ecology and Biogeography, 10, 63–76.
- ^ Heaney, L. R. (2001) Small mammal diversity along elevational gradients in the Philippines: an assessment of patterns and hypotheses. Global Ecology and Biogeography, 10, 15–39.
- ^ Sanders, N.J., Lessard, J.P., Fitzpatrick, M.C. & Dunn, R.R. (2007) Temperature, but not productivity or geometry, predicts elevational diversity gradients in ants across spatial grains. Global Ecology and Biogeography, 16, 640–649.
- ^ Pounds, J. A., Martín R. Bustamante, Luis A. Coloma, Jamie A. Consuegra, Michael P. L. Fogden, Pru N. Foster, Enrique La Marca, Karen L. Masters, Andrés Merino-Viteri, Robert Puschendorf, Santiago R. Ron, G. Arturo Sánchez-Azofeifa, Christopher J. Still, and Bruce E. Young. "Widespread Amphibian Extinctions from Epidemic Disease Driven by Global Warming." Nature 439.7073 (2006): 161-67. Print.
- ^ Menge, B. A. & Sutherland, J.P. (1976) Species diversity gradients: synthesis of the roles of predation, competition, and temporal heterogeneity. The American Naturalist, 351-69.
- ^ Choler, P., Michalet R. & Callaway, R.M. (2001) Facilitation and competition on gradients in alpine plant communities. Ecology, 82, 3295-3308.
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- ^ Lomolino, M. V. (2001) Elevation gradients of species-den-sity: historical and prospective views. Global Ecology and Biogeography, 10, 3-13.