Opinion
The use of ‘altitude’ in ecological research

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Altitudinal gradients are among the most powerful ‘natural experiments’ for testing ecological and evolutionary responses of biota to geophysical influences, such as low temperature. However, there are two categories of environmental changes with altitude: those physically tied to meters above sea level, such as atmospheric pressure, temperature and clear-sky turbidity; and those that are not generally altitude specific, such as moisture, hours of sunshine, wind, season length, geology and even human land use. The confounding of the first category by the latter has introduced confusion in the scientific literature on altitude phenomena.

Introduction

Approximately 25% of the land surface of the Earth is covered by mountains, which host at least a third of terrestrial plant species diversity [1], supply half of the human population with water 2, 3 and, by offering steep environmental gradients, represent exciting biological experiments of nature which have stimulated research for centuries 4, 5, 6, 7. As one ascends a mountain, environmental conditions change, and one finds organisms that are commonly well adapted to the local conditions along an altitudinal transect, thus offering ideal conditions for exploring evolutionary adaptation over short spatial distances. However, the interpretation of results obtained from such works becomes difficult when the gradients selected include environmental changes that reflect local or regional peculiarities, such as fire [8], land use [9] or drought [10], not generally associated with altitude above sea level. Much of what has been discussed as being a discrepancy between findings (and supposed mechanisms) from different altitudinal gradients by different researchers, in reality reflects confusion between rather different environmental drivers under the umbrella term ‘altitude’. If the general geophysical phenomena (e.g. temperature, pressure or turbidity) had been separated from coincidental phenomena, the supposed contrasts might not have become an issue and the scientific endeavor would have become much more fruitful.

Here, I summarize the main geophysical drivers along altitudinal gradients from an ecological point of view and contrast these with other drivers not generally associated with altitude. For practical reasons, I address ‘mountains’ as any elevation of land mass from the plains 300 m above sea level [11]. The climatological considerations presented also apply to the vast high-altitude plateaus (e.g. parts of the North American prairie, the Tibetan plateau and the Andean Altiplano), although these are commonly not included in the term ‘mountain’ because of the lack of steepness of the slope 11, 12. For land area-related biological phenomena such as biodiversity and speciation, large plateaus have to be considered as a special case, not covered by the term ‘mountain’. A clear concept of the meaning of ‘altitude’ in an ecological context is essential – and is advocated here – to advance the altitude-related theory of life.

Section snippets

Highlighting the problem

If, for instance, one aims at testing theories of adaptation to altitude but selects a gradient along which the moisture regime varies in a peculiar way (precipitation and evaporative forcing might increase or decrease, depending on region), results are likely to reflect the moisture gradient rather than an altitude gradient in its strict sense, or a combination of the two 13, 14, 15. A functional (mechanistic) interpretation would either require data from altitudinal gradients with ample water

Land area changes

Available land area is a major driver of organismic diversity and its evolution 29, 30. With increasing altitude, land area is shrinking (Figure 1), thereby narrowing opportunities for life 31, 32, 33. The concurrent fragmentation of the land area by geological and gravity-related processes into an ‘archipelago’ of climatic mountain ‘islands’ further reduces the uniform space for any type of habitat conditions [34]. Yet, habitat diversity or ‘geodiversity’ and spatial isolation are also

General climatic trends with altitude

In addition to the altitudinal reduction of land area, there are four primary atmospheric changes associated with altitude: (i) decreasing total atmospheric pressure and partial pressure of all atmospheric gases (of which O2 and CO2 are of particular importance for life); (ii) reduction of atmospheric temperature, with implications for ambient humidity; (iii) increasing radiation under a cloudless sky, both as incoming solar radiation and outgoing night-time thermal radiation (because of

Climatic trends that are not generally related to altitude

In addition to situations where the steepness, but not the general direction, of altitudinal trends varies, other meteorological parameters show regional variation in the direction of change, with precipitation and seasonality exerting the largest influence.

Conclusion

There is a need for a consistent ‘altitude concept’ in comparative ecology. Because there is no ‘standard mountain’, any data collected along altitudinal gradients will reflect the combined effect of regional peculiarities and general altitude phenomena. This distinction becomes crucial when the results of different studies are compared and trends are commented on as being different or similar to those found by others along altitudinal gradients elsewhere. To distill general altitude-related

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