|Herb-rich woodlands in western Victoria are dominated by grasses, |
herbs and geophytes. (Photo: John Morgan)
High species richness also fascinates biologists because the coexistence of large numbers of species is of theoretical importance. How can so many species occur in same area when no two species can occupy the same niche?
A new paper by Bastow Wilson et al. in the Journal of Vegetation Science examines the world records for species at all the spatial scales at which biologists quantify species richness. They searched the published literature on species richness in quadrats (no small task) and found that plants can really pack it in.
At small scales, the mountain grasslands in central Argentina have been recorded to have up to an astonishing 89 species per m2 . Several meadows in Romania and the Czech Republic were nearly as rich. At larger scales, the world record for the number of species was 942 species in 1-ha in a tropical rainforest from Ecuador. This makes the mind boggle!
What I found most fascinating was thinking about why such high numbers of species can coexist. Wilson et al. provide an interesting mechanistic explanation. While it is clear that grasslands have smaller plants than rainforests and hence, can pack more species into a smaller area ('the species packing effect'), the two communities are intrinsically different in another key way.
|A typical grazed mountain grassland from Argentina.|
(Photo courtesy of Jodi Price)
The highest richness grasslands across the globe all share a common feature - they are all subject to repeated disturbance (by mowing, grazing or fire). This leads to more symmetric competition between dominant and subordinate species and hence, slower competitive exclusion. In a local sense, we see exactly the same ecological phenomenon in frequently burned C4 grasslands (where richness is very high at small scales) compared to unburned grasslands (where richness can be very low).
Tropical rainforests, by comparison, have a more stable environment, the disturbances being mainly patchy treegap formation. Their richness has been explained by continous speciation in a stable ecosystem that favours subtle differences in life history.
This paper highlights that we still have a long way to go if we want to understand the mechanisms that underpin co-existence. It also highlights that simple biological surveys are paramount if we want to identify biodiversity 'hotspots'. Could we, for instance, readily identify where the global records for bryophyte diversity would be? And at what sampling scale would this occur?