Elevational Gradients in Species Richness

Christy M McCain, University of Colorado, Boulder, Colorado, USA
John‐Arvid Grytnes, University of Bergen, Bergen, Norway

Published online: September 2010

DOI: 10.1002/9780470015902.a0022548

The abiotic and biotic gradients on mountains have enormous potential to improve our understanding of species distributions, species richness patterns and conservation. Here we describe how abiotic factors change with elevation, how flora and fauna respond to these changes and how elevational species richness patterns have been studied to uncover drivers of biodiversity. There are four main trends in elevational species richness: decreasing richness with increasing elevation, plateaus in richness across low elevations then decreasing with or without a mid-elevation peak and a unimodal pattern with a mid-elevational peak. We discuss the history of elevational richness studies and overview the various hypotheses thought to be important in richness trends, including climatic, spatial, biotic and evolutionary factors.

Key Concepts:

  • Elevational gradients exhibit complex variation in abiotic conditions over short distances.
  • Patterns of elevational species richness follow four common patterns: mid-elevation peaks, decreasing, low-elevation plateaus and low plateaus with mid-elevation peaks.
  • Patterns of elevational species richness vary between taxonomic groups.
  • A combination of water availability and temperature is often found to be related to elevational species richness patterns.
  • No consistent support is found for the importance of area or mid-domain effects for elevational species richness patterns.
  • Support for the various mechanisms underlying elevational richness patterns tends to be related to the ecology and evolutionary history of the taxonomic group of interest.
  • Elevational gradients are valuable in our task to disentangle the causes behind broad-scale patterns in biodiversity, and in our quest to understand threats to biodiversity with climatic change.

Keywords: climate; biotic interactions; diversity; elevation; environmental gradient; mountains; precipitation; productivity; species–area relationship; temperature

Figure 1. Two examples of elevational gradients: a tropical mountain (left column; e.g., Venezuela) and a temperate mountain (right column; e.g., SW USA). (a) Temperature generally decreases linearly with elevation. Both mountains have similar wet adiabatic lapse rates of 5.68 and 5.24°Celsius per 100 m, respectively, but with much cooler average annual temperatures at the higher latitudes. (b) Precipitation varies greatly along elevational gradients. The tropical mountain has overall wetter conditions with peaks in precipitation at the lowest and mid-elevations, whereas the arid-based, temperate mountain shows the typical pattern of increasing precipitation with elevation. The combination of temperature and precipitation values result in habitat zonation with elevation. The tropical gradient changes from lowland tropical rainforest to premontane rainforest to montane rainforest to cloud forest, and finally elfin forest and alpine grasslands. The arid, high latitude gradient changes from desert scrub or grassland to chaparral to pinyon-pine forests to mixed hardwood-pine forests to ponderosa pine forests, and finally fir forests and alpine grasslands.
Figure 2. The percentage of the four main elevational richness patterns demonstrated on mountain gradients across the globe, including decreasing, low-elevation plateau, low-elevation plateau with a mid-peak (LPMP) and midpeak for nonflying small mammals (McCain, 2005), bats (McCain, 2007a), birds (McCain, 2009), reptiles (McCain, 2010) and plants (Rahbek, 2005 Figure 3f3). Preliminary results for salamanders and frogs are very similar to small mammals and birds respectively. A few studies of plants and frogs have found increasing richness with increasing elevation, but these appear to be quite rare.
Figure 3. Studies of elevational species richness can be strongly influenced by methodological issues of scale, sampling and disturbance; here we show several examples. The scale of sampling falls into two broad categories: local transect studies which are sampled ideally at equal intervals from the base to the peak of the mountain usually within a 1–2 years (a1); and regional compilations of sampling from many researchers, slopes and years (a2). The regional compilations can be heavily influenced by the greater area at the base of mountains, thus potentially leading to more greater estimated richness at lower elevations. The distribution of sampling effort can influence the estimates of species richness by not spreading the effort evenly over the gradient (b1), which can lead to higher richness in areas of high sampling and low richness in areas of low sampling (e.g., compare a1 and b1). If sampling is only distributed over a portion of the gradient (b2), this truncation can lead to the identification of a very different pattern of species richness (e.g., compare a1 and b2). Reduced sampling at the highest elevations tends to have less influence on species richness estimates, since diversity is generally reduced at these elevations (b3). Habitat disturbance, particularly widespread and concentrated within a zone of elevation (e.g., lowlands, c2) can lead to reduced estimates of species richness in disturbed areas (e.g., compare c1 and c2).
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Article Title: Elevational Gradients in Species Richness
 
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McCain, Christy M, and Grytnes, John‐Arvid(Sep 2010) Elevational Gradients in Species Richness. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0022548]