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Our knowledge of diversity patterns is generally founded on studies performed in the more species-poor part of our globe. Ibanez et al. (2014, this issue) report on patterns based on impressive field efforts in a diversity hotspot, namely New Caledonia. Some intriguing patterns arise from this study offering valuable insights in our quest to understand and manage global biodiversity.
Understanding the drivers of biodiversity is a major task for ecologists in a world that loses diversity at a rate that is higher than ever before. A first step towards gaining a better understanding of the drivers is to obtain a good picture of how diversity varies in time and space and how these variations are related to different environmental variables. This is important both for understanding how environmental changes affect diversity and for identifying the most valuable areas for conservation. We have a good grip on some of these patterns. Species richness generally decreases with distance from the equator (Hillebrand 2004), increases with size of island and decreases with distance to mainland (MacArthur & Wilson 1967). Other patterns are more variable, such as how the diversity varies with substrate, and the relationship between elevation and species richness (McCain & Grytnes 2010). A problem for the generality of these patterns (and other patterns) is that the majority of the really diverse areas of the world are under-studied, and most of our understanding of diversity patterns comes from studies in the temperate and less diverse regions. Studies such as that performed by Ibanez et al. (2014) are therefore of high value in our quest to gain a better understanding of what affects biodiversity at local levels in areas of high biodiversity.
New Caledonia is a global biodiversity hotspot (Myers et al. 2000), with 3371 native vascular plant species within 18 000 km2. A large proportion of the species found (ca. 75%) are endemic to the islands (Morat et al. 2012), and many of them have a narrow range within the island (Wulff et al. 2013). Ibanez et al. (2014) sampled woody vegetation in 201 plots of 0.04 ha distributed in the rain forest of the main island of New Caledonia. In these plots they found 749 different species, which is 46% of all woody species in these forests. This looks like a high proportion of the species pool, considering that many of the species they did not observe will never be able to reach the threshold DBH value of 5 cm. However, their rarefaction curves do not indicate much of a flattening out, suggesting that there are still many more species to be found with higher sampling effort.
The combination of patterns described in Ibanez et al. (2014) shows some intriguing features, as well as some challenging ones. One of the main features of New Caledonia is that there are two main types of substrate: ultramafic and non-ultramafic. Only 25% of the species found in the plots analysed by Ibanez et al. are found on both substrate types. Given the low overlap of species composition between the two substrate types, it is intriguing to observe the similarity in structural characteristics (stem density and basal area) and diversity for the plots on the two substrates. Before looking further into this, just a few words about one of the challenges. Although the work behind the sampling done here is impressive (>28 000 specimens identified), the large area, high diversity and low abundance of many species might cause some patterns to change if more sampling were done. Almost 35% of the species were found in only one or two plots. The low proportion of species found on both substrate types could therefore potentially be an effect of low sampling intensity despite the large effort put into the fieldwork. This is one of the curses and challenges of working in a diversity hotspot. Nevertheless, compared to the similarity of the structure and diversity of the forests on the two substrate types, the floristic dissimilarity between the substrates is striking. Or perhaps it is the similarity in structure and diversity that is really striking?
Pillon et al. (2010) compared the phylogenetic structure of the flora of New Caledonia with the flora of Australia (the flora of New Caledonia is generally thought to have its origin in Australia: Morat 1993), and found a clear disharmony in the phylogenetic structure of the flora of the two regions. They suggest that only some of the phylogenetic groups that made it to the island after its uplift following a period of erosion some 35 million years ago (Morat 1993) have been able to adapt to the ultramafic soil type. This adaptation and subsequent diversification has probably happened after arrival in New Caledonia, as these groups are not overrepresented on similar substrates in Australia or other potential source pools (Pillon et al. 2010). Another pattern is reported from California, where Safford et al. (2005) do not find the same clear difference at family level between species on ultramafic and non-ultramafic soils. It is also interesting to note that the difference becomes less pronounced with elevation in New Caledonia. Anacker et al. (2011) explain the low radiation of ultramafic species in California by the small area and low environmental variation of the areas with ultramafic soils there. In California, ultramafic soils cover 1.5% of the area (Safford et al. 2005), whereas they cover approximately one third in New Caledonia, and this proportion has probably been higher during the history of the island (Pillon et al. 2010).
Ibanez et al. (2014) find differences between stem density and basal area on the ultramafic and non-ultramafic substrates, but, as discussed by the authors, this may be confounded with the much stronger variation along elevation and a skewed sampling of the two soil types along elevation. The structure variables respond much more to factors related to elevation than to substrate type. Hence, despite the difference in the phylogenetic origin of the flora of the two substrate types, they have a remarkable similarity in stem density, basal area and diversity that may indicate that the floras on the ultramafic and non-ultramafic soils contain comparable functional types (although not many characters are investigated). Stem density is not often compared along broad-scale climatic gradients, but a study from China, encompassing a large climatic gradient from cold-temperate to tropical, shows a clear decrease in stem density with temperature (Fang et al. 2012), i.e. the opposite of that found in the study of Ibanez et al. (2014).
A last pattern, or rather a lack of pattern, which we find puzzling in the study of Ibanez et al. (2014), is that there is no relationship between species richness per plot and elevation. However, as mentioned above, there is a clear relationship between number of individuals and elevation. This suggests that if the plots had been standardized to the same number of individuals we would probably have observed a negative relationship between species richness and elevation, and a similar pattern to that described for tree species richness on Mount Kinabalu could have emerged (Grytnes & Beaman 2006). We also note that the number of individuals could have an effect on the observed relationship, where floristic similarity increases with increasing elevation. A higher number of individuals could result in a higher fraction of the total species pool captured, which in turn would result in a lower beta diversity or Bray–Curtis dissimilarity index. Standardizing species richness on number of individuals or size of the sampled area has been discussed before (Oksanen 1996; Gotelli & Colwell 2001), and we guess that agreeing on this is as difficult as agreeing on what biodiversity actually is.
What might be easier to agree on is that efforts to describe the biodiversity in diverse areas, as done in the New Caledonian Plant Inventory and Permanent Plant Network, have an enormous value in our quest to understand and conserve global biological diversity.