Elevational range size patterns of vascular plants in Himalaya contradict Rapoport’s rule

Martin Macek discusses his recent article Elevational range size patterns of vascular plants in Himalaya contradict Rapoport’s rule. Find out more about this research in the Himalayas and what makes Ladakh the perfect place to test this rule.

The Rapoport’s rule, formulated by Stevens in 1989, for latitudinal gradients of range size – and later extended to include elevational gradients – quickly established in the ecological literature. However, soon, it became obvious that the supporting evidence for Rapoport’s rule was heavily confused by geometric constraints and other methodological pitfalls. After revisions to the methodology and respective range-size patterns, mixed evidence was found for different regions and species groups, so the ‘rule’ could no longer be considered as generally valid. But the question of why range-size differs between species along latitudinal or altitudinal gradients and what drives these differences remained. Where better to study elevational range limits than in the world’s highest mountain range – the Himalayas?

Here, on the steep ascends from the valley bottoms to mountain passes with heavy backpack on your back, you can clearly see how the flowering plants gradually disappear, and only lichen-covered stones remain. Then, on the way back to the valley, some beautiful flowers like Waldheimia tridactylites (Fig. 1) follow the streams that are fed by melting glaciers down the valley. Some other plants (such as the Ladakiella klimesii shown in Fig. 2 below) can be found only at the summits.

*Figure 1: Pink flowers of Waldheimia tridactylites can be found in Ladakh along a broad elevational gradient starting at 3600m, and reaching 6150 m above sea level.* *Photo: Martin Macek*

There’s a vast collection of primary plant occurrence data for this area. We owe this incredible amount of work mostly to Dr Leoš Klimeš, who criss-crossed the region of Ladakh during ten years of hard work. He recorded almost one hundred thousand plant presences at four thousand localities distributed all over the region. Unfortunately, Dr Klimeš went missing during his last fieldwork in Ladakh in 2007, but his name is forever commemorated by the tiny plant Ladakiella klimesii, which he discovered here at elevations reaching 6000m asl.

*Figure 2: Ladakiella klimesii, a tiny plant from Brassicaceae family, can be found at the highest elevations supporting plant life, reaching 6150m a.s.l. Photo: Martin Macek*

The second part of this research – defining random expectation for hypothesis testing – turned out to also be a bit tricky. If the elevational range placement was completely random, then some species would surpass elevational limits of our study area. For such species, we would know only part of their full elevational range. Moreover, some species would be placed completely outside the domain and such species would not be recorded at all. Further, statistical uncertainty in average range size is coupled with number of observations (species). Therefore, species richness gradient needed to be preserved to get realistic estimate of expected variability in average range size along the elevational gradient. Non-uniform probabilities for range placement can make this job easier, but that also comes with decisions to make (it matters if you choose low-limit, midpoint, or high-limit as definition point for random sampling).

*Pronounced elevational gradients predetermine Ladakh to be an ideal ecological laboratory. Cremanthodium elisii with sacred mountain Gumburanjon in background.* *Photo: Martin Macek*

This all sounds like it’s getting complicated, it allows us to find out how the elevational Rapoport’s rule performed in Ladakh. The answer is ‘honestly, not very well’. Patterns allegedly supporting Rapoport’s rule turned out to be artefacts of range truncation and richness gradient. Species with narrow range sizes, like Ladakiella klimesii, were surprisingly frequent on the top of the roof of the world. This evidence suggests that elevational Rapoport’s rule does not really work – at least for plants.