May 22, 2014

Why is the world's richest biodiversity hotspot so biodiverse? Part 1

Today, 22nd of May, is the International Day for Biodiversity. This time last year, I was living in one of the most biologically rich places of the planet, somewhere on the eastern edge of Ecuador's share of the Tropical Andes biodiversity hotspot (figure 1). As the name suggests, this hotspot englobes the tropical portion of the Andes mountain range, along with its immediate foothills as they spill out into the Amazon rainforest.

Figure 1 - Tropical Andes hotspot shaded in dark grey. © X O'Reilly
According to some criteria, the Tropical Andes hotspot ranks as the most biodiverse of the biodiversity hotspots. As far as science has thus far revealed, this area holds around 45,000 plant species, nearly half of which are endemic – that is, not native anywhere else. This is both the greatest number of absolute plant species and endemic plant species found in any one biodiversity hotspot. The Tropical Andes also boast more species of terrestrial vertebrate animals than any other hotspot, nearly half of which are also endemic. Within vertebrate animals, birds and amphibians are more diverse and endemic here than anywhere else, and mammal and reptile diversity is amongst the highest in the world. The figures for invertebrates are likely to be similar, given the frequent correlation in vert and invert diversity. (Myers et al., 2000)

In short, in terms of biodiversity the Tropical Andes hotspot gets top marks and, as such, its eligibility for being considered a "hyper-hot" conservation priority seems pretty well justified, as emphasised by authors such as Myers et al. (2000). To be honest, the area probably gets pretty high marks in terms of cultural diversity too, with countless indigenous nations scattered throughout the Andes and the Amazon. This may not be coincidence, however I shan't go into correlations between historical human population centres and regions of high biodiversity, but for further reading on the subject I recommend Fjeldså (2007) and the literature he cites.

Only about 25% of the Tropical Andes' original primary vegetation remains (unfortunately, there are many biodiversity hotspots doing much worse), of which about 25% is protected on paper (there are hotspots with a much higher percentage of protected land; Myers et al. 2000). I say protected on paper because in this part of the world, a protected area is a place where indigenous communities are made to vacate or strict rules are imposed upon them if they are to continue living in the area. While these rules are usually in the longterm interests of the community if they wish to protect the land they live off, apparently these rules are not for oil or mining companies to abide by, whether national or international.

Figure 2 - Terrestrial biodiversity hotspots, © Myers et al. 2000
Politics aside, a truly fascinating question is why this particular area of the world isn so biodiverse. Figure 1 is a map of the world's biodiversity hotspots (please note that the 25 biodiversity hotspots as defined by Myers et al., 2000 are the most biologically diverse terrestrial areas – marine systems are harder to quantify but many areas are no less diverse than these bits of land!). Note how many of these regions are distributed in tropical and subtropical regions. Indeed, Charles Darwin and many others noticed what seemed to be an increasing gradient of biodiversity as one travels from the poles to the tropics (Darwin, 1859; Dobzhansky, 1950).

Biologists have been attempting to explain this apparent gradient for over a century, and still there is no clear consensus on the matter (Pianka, 1966). A long standing debate regarding the tropics is whether they are “cradles” or “museums” of biodiversity (Stebbins, 1974): do places like the Tropical Andes engender more species or do they simply retain more? Various arguments have been put forward to justify how the tropics may favour the evolution and diversification of more species than temperate regions, or why fewer extinctions may occur. The time hypothesis (Wallace, 1878; Fischer, 1960) is perhaps the simplest in principle. The idea is that the tropics host older communities, which have had more time to diversify. The time hypothesis has been explained both in terms of evolutionary history and climatic history.

First, we’ll consider the time hypothesis in terms of evolutionary history. Many authors argue that the diversity gradient may have formed due to many clades originating in the tropics before moving to higher latitudes. In other words, that lineages in the tropics have had more time to diversify because that is where they originated.  For instance, in evolutionary terms, temperate frogs are more derived from their ancestral lineages than their tropical counterparts, which are far more diverse (Wiens et al., 2006). It may be that the potential of taxa to spread and adapt to new conditions is constrained by their geographical origin, and so taxa which originated in the the tropics are less likely to diversify as much in temperate regions (Buckley et al., 2010).

What about in terms of climatic history. The idea is that, free of the temperamental climatic history of higher latitudes (namely the Pleistocene glaciations), the tropics have either retained more species or tropical species have had more time to diversify. Now, as attractive an idea this may seem, there are problems with it:
  1. The effects of glaciation cycles for Pleistocene flora and fauna were indeed dramatic and lead to some species extinctions, but the main consequences were habitat shifts and biogeographical changes (Pianka, 1966; R6). Maybe if species hadn’t have had to move around so much they could have diversified more? Fair point, but…
  2. the tropics were not immune to the Pleistocene glaciation cycles and suffered vast habitat shifts as well (van der Hammen, 1974). Perhaps climatic stability prior to the Pleistocene could explain the greater biodiversity of the tropics, except that…
  3. higher latitudes have enjoyed long periods of climatic stability during which its communities could have diversified (Simpson, 1964), and
  4. there is evidence which suggests the latitudinal gradient goes back long before the Pleistocene glaciations, possibly as far back as the Mesozoic (think dinosaurs) or the Palaeozoic (think vertebrates crawling onto land) (Mittelbach et al. 2007).

In other words, although there is evidence that suggests the Pleistocene glaciation cycles affected both species distributions and diversification, it is unclear that this affected temperate regions more than tropical regions.

What if we applied the time hypothesis to a smaller scale – for example that of a year. One of the features which distinguishes higher latitudes from lower latitudes, is seasonality. Though many tropical regions have rainy seasons and dry seasons, overall they enjoy a more constant climate. The biggest fluctuations are likely to be between night and day rather than seasons. Temperate regions, on the other hand, undergo considerable (or in some cases, extreme) seasonal variation. The climatic stability hypothesis explains that species in stable climatic conditions (tropics) can afford to specialise, whereas species subjected to annual or periodic changes in temperature and humidity need to be able to adjust to a wider range of conditions, thus precluding extreme specialisation. The more specialised species are, the narrower the niche they fill, which means there are a greater number of niches available to be filled by other species (see Pianka, 1966). Climatic stability certainly seems like a very logical reason for increased diversity in the tropics, but it is difficult to prove. Tropical species do generally tend to have a narrower thermal tolerance than temperate species (Pianka, 1966), but it is difficult to say whether this is due to relative climatic stability or, for instance, consequence of interactions within already diverse communities of organisms.

Along similar lines as the climatic stability reasoning, but with a focus on biotic interactions, it has been argued that competition between organisms is a relatively stronger evolutionary driving force in the tropics than at lower latitudes, where seasonal variability exerts a greater pressure on organisms than biotic interactions do (Schemske, 2002) (competition hypothesis). The idea behind this competition hypothesis is that increased niche overlap drives ecological divergence and ultimately leads to greater speciation (Dobzhansky, 1950; Emerson and Kolm, 2005). Decreased niche overlap due to biotic interactions may also increase biodiversity: for instance, a greater abundance of predators and parasites in the tropics might keep prey and host populations restricted to narrower niche breadths, potentially dampening competition between prey species and opening up new niches for species to adapt to. Some empirical studies, mainly in rock pool ecosystems, have shown that removing a predator species can lead to an overall reduction in biodiversity (Paine, 1966; O’Connor and Crowe, 2005; O’Connor et al., 2013). Studies on marine zooplankton show an increasing abundance and diversity of predators as one moves toward the tropics (Grice and Hart, 1962). However, while predator-prey interactions are thought to have been an important evolutionary driving force in animal evolution, the predation hypothesis still begs the question of why predators and parasites should be more abundant in the tropics to begin with. The productivity hypothesis boils down to a similar chicken-and-egg question – it attributes the greater diversity of the tropics to greater productivity (Currie et al., 2004). But, surely the tropics are more productive because they are more biodiverse?

There are different variations of the spatial heterogeneity hypothesis. All share that they associate greater diversity with increased habitat or geographical complexity. The idea is that the subdivision and complexity of the space occupied by species allows for greater specialising and divergence. Both abiotic and biotic elements can bring about spatial heterogeneity. One variant of this hypothesis ascribes the increasing diversity of animal life towards the tropics, to the diversity and abundance of plant life, since it seems reasonable to assume that the two correlate. Then again, who is to say it is not animal herbivores driving plant speciation, in a way analogous to the predation hypothesis? More importantly why should plant life be more diverse in the tropics to begin with? Regardless of this hypothesis, animal diversity cannot be flawlessly predicted based on plant diversity. Amphibian and bird species richness correlate reasonably well with plant diversity, but the same is not true for reptiles, for instance (R15).

Another take on the spatial heterogeneity hypothesis I will explain in further detail in a future post (Why is the world's richest biodiversity hotspot so biodiverse? Part 2), but simply, it suggests a varied relief is behind the impressive biological diversity of the world’s biodiversity hotspots. There is substantial evidence supporting the role of geographic variation in the diversification of species, by providing barriers to gene flow (Coyne and Orr, 2004), creating new dispersal routes (Antonelli et al., 2009), and generating environmental gradients (Endler, 1973). Needless to say, geographical heterogeneity is not a feature restricted to the tropics and as such is not used as a stand-alone explanation behind the latitudinal biodiversity gradient on Earth. However, as will be explained in more detail in the next post, be combined with other hypotheses to help explain the global pattern of biodiversity. This brings us to an important point: hypotheses which claim ecological interactions explain the diversity gradient ultimately need other extrinsic factors to justify the greater variety or intensity of these interactions in the tropics; yet we know that biotic interactions are a significant evolutionary driving force, and so it seems naive to attempt to explain biodiversity patterns as products of exclusively abiotic factors. Consequently, there is still no consensus on what determines the latitudinal biodiversity gradient, though it seems likely to be result of a combination of the hypothesis described and other mechanisms.

Returning to our Tropical Andes biodiversity hotspot and to make matters more complicated, biodiversity doesn’t only increase towards the Neotropics following the global diversity gradient, it is higher than anywhere else (Myers et al.). The incredible biological diversity of South America cannot simply be down to the vast extent of tropical territory, because similar latitudes elsewhere in the world don't hold a candle to the Neotropics in terms of say, plant diversity (Gentry, 1982). For instance, Africa and South America share some closely related plant groups that have diversified considerably more on the latter continent than the former since their split (Gentry, 1982). Similarly, while African mammals have diversified steadily over time and the old world continent is phylogenetically diverse, South America has fewer higher taxa diversity but is more speciose (Davies and Buckley, 2011). Clearly, different forces must be afoot. 

So, what is it that the Neotropics have got that's so special? Well, they've got a very interesting geological history, including something very very big that nowhere else has: the Andes, the largest mountain range above sea level. The complex history behind the rise of the Andes and its influence on biodiversity will be the focus of the next post. For now, I should really focus on preparing for my exams…

Cited literature:

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Buckley, L. B. et al. (2010) Phylogeny, niche conservatism and the latitudinal diversity gradient in mammals. Proc. R. Soc. B 277, 131–2138.

Coyne, J.A. & Orr, H.A. (2004). Allopatric and Parapatric Speciation. Speciation. 83—123. Sinauer Associates, Sunderland, MA.

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