Examine the reasons for the existence and localities of biodiversity hotspots.

Part II Biogeography – Biodiversity Hotspots

Louise Sherwin - Girton College

Supervision 2: _ Examine the reasons for the existence and localities of biodiversity hotspots

“I have never experienced such intense delight... such a plenitude of forms, colours, behaviours—such a magnitude of Life! What explains the riot?”

- Darwin (1851)

“The current massive degradation of habitat and extinction of species is taking place on a catastrophically short timescale, and their effects will fundamentally reset the future evolution of the planet's biota.”

- Novacek & Cleland (2001)

Biodiversity may be regarded as ‘the number, variety and variability of living organisms’ (MacDonald, 2003: 406). Whilst global variation in biological diversity has long been a source of fascination, it has recently been the basis for increasing concern (Tilman, 2000). Awareness of the extent and rate of the current biodiversity crisis (or the ‘sixth extinction’, as termed by Leakey & Lewin, 1996) has led to a significant re-assessment of the role of biodiversity in ecosystem functioning and the scope for policy intervention to enable its preservation, exemplified by the international ratification of the Convention on Biodiversity in 1992. The identification and analysis of spatial patterns of biodiversity has been central to conservation goals of maximum preservation at the least cost (Reddy & Davalos, 2003). The hotspots approach is one of many methods for delimiting areas of priority conservation, although it has proven to be one of the most popular. ‘Biodiversity hotspots’ were first introduced by Myers in 1988 and may be defined as:

“areas featuring exceptional concentrations of endemic species and experiencing exceptional loss of habitat” (Myers et al., 2000:853).

By applying parameters to this definition, Myers et al. (2000) identified twenty-five hotpots that predominately represent tropical forests and Mediterranean-type habitat zones (numbering fifteen and five hotpots respectively). In this essay, the reasons for the existence and the localities of biodiversity hotspots will be examined. It should be noted at this stage that biodiversity hotpots do not represent a neutral scientific observation, but are implicitly influenced by human values. The approach encompasses decisions on which aspect of biodiversity is worth conserving (for biodiversity itself is an inconsistently and ill-defined concept according to Gaston, 1996) and the best way to do this from within a framework of economic rationality and efficiency. Perhaps more importantly given the human basis for the rise and power of the concept, the hotspots approach will then be critically considered in terms of how far it adequately meets the needs behind the reasons for its adoption. Following on from this, a number of issues may be identified that undermine the sustainability of the hotpot approach. A critical analysis of the biodiversity hotpot approach raises a number of broader questions for conservation science which lies at the science-action interface, including the need for clear conceptualisation of concepts such as ‘biodiversity’, for modesty when data is incomplete or skewed and a realisation of the impact such suggestions can have on local lives. However, it is first necessary to briefly outline the main characteristics of the hotspots approach.

To identify biodiversity hotpots, Myers et al. (2000) used the dual criteria of over 0.5% plant endemicity (1,500 of the world’s 300,000 plant species) and the loss of 70% or more of its primary vegetation through habitat loss. Twenty-five hotspots were thus categorized, representing up to 44% of vascular plant species and 35% of mammals, birds, reptiles and amphibians in just 1.4% of the Earth’s land surface (ibid., 2000). In terms of primate and carnivore evolutionary history, hotspots maybe even more important, representing almost 70% of evolution for these groups (Sechrest et al., 2002). The premises of the hotpots approach are explicitly outlined by Mittermeier et al. (1998), who state that ‘the biodiversity of… every nation is critical to its survival’, that ‘many areas of high-biodiversity exhibit very high levels of endemism’ and ‘to achieve maximum impacts with limited resources, we must concentrate heavily on those areas highest in diversity and endemism and most severely threatened’. As it was alluded to earlier, the reasons for the existence of biodiversity hotpots are separate and distinct from the reasons for spatial variations of biodiversity. The former represents a conservation approach that utilises the latter as a partial mode to achieve its aims. This is important for the explanation of the reasons for the existence of biodiversity hotpots because ‘conservation only has meaning in the context of human intention’ (Jepson & Canney, 2001: 225). Thus, as a value-based conservation approach, the underlying reasons for the existence of biodiversity hotpots are two fold. Firstly, there has been growing concern of mans’ impact upon the number and extent of species and the conservation of biodiversity is a central tenet of sustainable development theory. Secondly, there is a desire to meet this challenge within the confines of limited resources and demands for economic efficiency. Biodiversity hotpots offer a scientifically based method for policy intervention that meets these dual demands.

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Figure 1 The 25 hotspots. The hotspot expanses comprise 30–3% of the red areas.

(Source: Myers et.al., 2000:853)

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Figure 1 The 25 hotspots. The hotspot expanses comprise 30–3% of the red areas.

(Source: Myers et.al., 2000:853)

The localities of the biodiversity hotpots are a function of the parameters applied to the definition of the term, to reflect the reasons for their existence. As illustrated by Figure 1 above, the localities of the biodiversity hotpots are skewed towards the mid to low latitudes. Sixteen of the hotspots are located in the tropics and virtually all tropical islands are included (Myers et al., 2000). Hotspot boundaries were determined according to their biogeographical units, using natural breaks in the case of real or ecological islands, recognized divisions (for example Wallace’s line between Sundaland and Wallacea) or best judgement by experts (ibid., 2000). The localities of the biodiversity hotpots are controlled by the two aforementioned parameters. For example, through the nature of the parameter of 0.5% or more plant endemicity, the hotpots do not include freshwater or marine ecosystems (although others have subsequently applied the approach to these habitats). For the 0.5% of plant endemicity, there are a number of latitudinal and other spatial gradients in biodiversity that may explain the resulting pattern (Gaston, 2000). Terrestrial areas of high biodiversity also tend to be areas of high endemism and the hotspot approach functions through this linkage (Mittermeier et al., 1998). An increase in species diversity may generally be observed for a given area as one moves from high to low latitude, although there is reason to suggest that peaks do not necessarily lie exactly on the Equator and that the gradient is asymmetric with a greater decline in the Northern hemisphere (Gaston, 2000). Although this pattern of diversity has been broadly accepted, the mechanisms that could explain this feature remain contested. Gaston (2000) notes that more than twenty-five different and non-exclusive processes have been invoked, for example based on chance, historical perturbation, productivity, interspecific interactions, environmental stability and habitat heterogeneity. Explanations for biodiversity gradients can be grouped into historical and equilibrium theories, the former considering current species diversity to be out of synch with environmental conditions and the latter the opposite (MacDonald, 2003). The amount of ambient environmental energy is thought to be one of the most important characteristics in explaining biodiversity and it may operate on a number of spatial axis, including latitudinal, elevational and depth gradients (Gaston, 2000). However, it is likely that a number of separate mechanisms may be acting to produce spatial gradients in biodiversity and it is perhaps unhelpful to look for a singular explanation across all species (ibid., 2000). Islands are well represented within the hotspots approach, although this is not surprising given the levels of endemism facilitated through such geographic isolation (MacDonald, 2003). For the endemicity determinant of the localities of biodiversity hotspots therefore, explanatory reasons are difficult to discern, complex and far from understood. It is quite possible that a number of mechanisms may be operating together and their importance varies according to the scale at which biodiversity is examined (Gaston, 2000).

The localities of biodiversity hotspots cannot solely be explained by spatial gradients of biodiversity however, for an ‘exceptional threat of habitat loss’ is also an important part of their definition (Myers et al., 2000). Myers et al. (2000) use a qualifying factor of 70% or more of primary vegetation loss, justifying this figure on the grounds that most large scale endemic species concentrations are included within this limit and that lowering the figure to 60% would not substantially increase the percentage of endemic species contained within hotpots. Digitized forest cover data from the World Conservation Monitoring Centre and reference material were used to delimit this aspect (Mittermeier et al., 1998). Through the inclusion of a ‘threat’ factor in the definition of biodiversity hotspots, the approach includes a number of human reasons for their respective locality. Although population density and growth rates can provide an indication of the threat to biodiversity, they are not necessarily reasons in themselves for habitat loss (Cincotta et al., 2000). For example, disturbance can occur without any increase in population via over-logging and increased access to forest resources (ibid, 2000). Furthermore, aggregate figures do not represent any spatial distribution of population nor reflect differences in culture, affluence and technology all of which may influence the severity of the threat to the immediate habitat (ibid., 2000)

In summary therefore, the existence of biodiversity hotpots may be attributed to a perceived need to preserve maximum species richness in an economically efficient manner. The localities of biodiversity hotpots represent the application of parameters to fulfil these aims. However, although it has been relatively easy to delimit these parameters and hotpots areas (even if the basis for inclusion / exclusion may appear relatively arbitrary), it is more difficult to explain the underlying mechanisms and reasons for these patterns. Spatial patterns of endemicity and biodiversity are complex and not fully understood. Likewise, the variety of human factors that can lead to habitat loss are diverse, multi-faceted and are in many aspects specific to local contexts.

Whilst the reasons for the existence and localities of biodiversity hotpots have briefly been summarised, it is also important to examine the validity of these reasons in terms of the soundness of the premises, assumptions and data used in the hotpots approach. In terms of the reasons for existence of biodiversity hotspots, Jepson & Canney (2001) have criticised the approach for failing to fully represent social values pertaining to species conservation. Biodiversity hotpots may be seen to embody a particularly functional approach for the preservation of the greatest number of species at the least cost. Jepson & Canney (2001) identify four culturally inflected meanings of biodiversity that are not covered within the hotspots approach. Firstly, it preserves biologically rather than aesthetically spectacular landscapes and does not include many symbols of cultural nationalism, such as the North American national parks. Secondly, all species are considered equal within the hotspots framework and therefore it has a limited capacity to protect charismatic species such as mega-fauna in Africa. Thirdly, the approach focuses on preserving unique species but this does not necessarily correlate with species of direct value to humanity, for example, the hotspots only coincide with two of the centres of origin of cultivated plants. Lastly, the hotspots approach may conserve the maximum amount of biodiversity on a global scale, but this does not necessarily correlate to where they are used on a regional or local level and biodiversity conservation will be an issue and policy consideration for all. From a more scientific perspective, the hotspots approach may also be queried in terms of its ability to preserve other measures of biodiversity, such as species evenness, genetic diversity and species difference. Such criticisms directly challenge the existence of the hotpots approach, because they question the validity of the central assumption of preserving maximum species richness.

The robustness of the reasons behind the localities of biodiversity hotspots has also been questioned and this analysis will draw attention to three possible pitfalls of the hotpots approach. Firstly, although using endemism as a proxy for high biodiversity works well in terrestrial ecosystems, this link has been queried in the case of coral reefs (Hughes et al., 2002). For corals and reef fishes, high biodiversity arises primarily from the combined contributions of numerous widespread species, rather than large numbers of species with small ranges (ibid., 2002). In such circumstances, delimiting biodiversity hotpots may be considered inappropriate for conservation and a strategy of the preservation of connectivity and genetic diversity of widely dispersed species may prove more fruitful (ibid., 2001).

Secondly, the bias and lack of data used to justify the hotspots approach within terrestrial ecosystems has also been queried. In an analysis of the passerine species in sub-Saharan Africa, Reddy & Davalos (2003) found that the biodiversity information contained a significant spatial sampling bias. Firstly, the data was considerably skewed towards rivers, cities and roads than would be found in a random distribution. Secondly, species were overwhelmingly more sampled in existing priority conservation areas. This has considerable implications for the accurate definition of biodiversity hotpots localities because:

‘setting biodiversity priorities means comparing areas with one another, and valid comparisons cannot be made unless the same relationship between sample and observed richness can be assumed to hold for all areas being compared.’

(Reddy & Davalos, 2003:1725)

The numbers of species in areas of low-sampling intensity will be underestimated and therefore the data must be adjusted using statistical techniques (such as rarefaction, modelling and extrapolation) or through more extensive surveying to correct for this bias (ibid., 2001). The lack of a complete data base is not denied by Myers et al. (2000) and in fact they suggest that there are a number of potential hotpots which they have been unable to include for this reason, for example the Ethiopian Highlands, south-eastern China, Taiwan and northern Rwanda. A bias within the type of species data is also apparent in the analysis of species endemicity congruence of biodiversity hotpots. The hotspots approach is based on plant endemicity but this does not necessarily translate into endemicity within other taxa, and this was found to be the case on a continental scale by on a Gaston & Williams (1996). Furthermore, the concentration on birds, mammals, angiosperms and shallow-sea, hard-bodies invertebrates may represent an atypical view of global biodiversity for life as a whole, and this could have considerable repercussions for the success of the hotpots approach (Purvis & Hector, 2000).

The impact of global warming is also important in questioning the validity of the hotspots approach. For example, the Fynbos biome is likely to lose 65% of its area by 2050 according to climate simulation models (Midgley et al., 2002). Range dislocation is another important factor, and this may well occur at a rate greater than the ability of species to track the change and migrate (ibid., 2002). This analysis suggests a rather different view of the biodiversity crisis and the necessary conservation responses. It is possible that the spatial limits of biodiversity hotspots will change dramatically within the next century. As such, the approach must be more flexible if it is to be sustainable into the long term future than current discussions might suggest. Although it is beyond the confines of this essay, such potentialities raise a number of ethical and moral dilemmas in terms of how such flexibility can be combined with sustainable development and participation for local populations. Smith et al. (2001) has argued that hotpot criteria should be weighted more towards ecological transitional zones than they are as present, because such areas preserve adaptive responses to climate change. However, this causes further problems as we must then address questions of the varying importance of different transitions and of core or periphery areas (Araujo, 2002).

In conclusion, the existence of the hotpots approach has been shown to be a function of human needs for a conservation strategy that can resolve the tension between imminent and substantial species loss and limited funds and resources for this cause. However, within such an all-encompassing strategy it is difficult to represent the diverse viewpoints that an emotive subject such as species conservation provokes. The reasons behind the localities of biodiversity hotpots are complex and not fully understood. A number of theories and mechanisms could possibly explain biodiversity gradients and habitat loss and each locality may represent a different interaction between these explanatory mechanisms. Furthermore, separate processes may be seen to be important on different geographical scales. A number of criticisms may be levelled at the scientific basis of the hotspots approach and those of a limited and incomplete data base and the challenge of global warming should not be taken lightly. Although the hotpots approach provides a useful method of conserving biodiversity it should not be seen as a catch-all solution and complementary approaches, such as habitat preservation in all biomes should parallel this approach.

References

Araujo, MB (2002) Biodiversity hotspots and zones of ecological transition Cons. Biol., 16, 1662-1663

Cincotta, R et al (2000) Human population in the biodiversity hotspots Nature, 404, 990-992

Gaston, KJ & Williams, PH (1996) Spatial patterns in taxonomic diversity, in Biodiversity A biology of numbers and difference

Gaston, KJ (2000) review article: Global patterns in biodiversity Nature, 405, 220-227

Hughes, T.P, Bellwood, D.R., & Connolly, S.R. (2002) Biodiversity hotspots, centres of endemicity, and the conservation of coral reefs, Ecology Letters 5: 775–784

Jepson, P & Canney, S (2001) Biodiversity hotspots: hot for what? Global Ecol. Biogeog., 10, 225-227

Leakey, R. & Lewin, R. (1996) The Sixth Extinction, Weidenfield and Nicolson: London

MacDonald, G (2003) Biogeography – Introduction to Space, Time and Life. Wiley: New York.

Midgley, G et al (2002) Assessing the vulnerability of species richness to anthropogenic climate change in a biodiversity hotspot Global Ecol. and Biogeog., 11, 445-451

Mittermeier, R et al (1998) Biodiversity hotspots and major tropical wilderness areas: approaches to setting conservation priorities Cons. Biol., 12, 516-520

Myers, N et al (2000) Biodiversity hotspots for conservation priorities Nature, 403, 853-858

Novacek, M.J. & Cleland, E.E. (2001) The current biodiversity extinction event: Scenarios for mitigation and recovery PNAS vol. 98, no. 10, 5466-5470

Purvis, A & Hector, A (2000) Getting the measure of biodiversity Nature, 405, 212-219

Reddy, S. & Davalos, L.M. (2003) Geographical sampling bias and its implications for conservation priorities in Africa Journal of Biogeography, 30, 1719–1727

Sechrest, W et al (2002) Hotspots and the conservation of evolutionary history Proc. Nat. Acad. Sci., 99, 2067-2071

Smith, T.B., Kark, S., Schneider, C.J., Wayne, R.K. & Moritz, C. (2001) Biodiversity hotspots and beyond: the need for preserving environmental transitions Trends Ecol., Evol., 16, 431

Tilman, D (2000) Causes, consequences and ethics of biodiversity Nature, 405, 208-211

Teacher Reviews

Here's what a teacher thought of this essay

Rebecca Lewis

23rd of Mar 2012

Overall this is an excellent essay, showing a high level of comprehension. It has a clear structure, covers not just the definition of hotspots, but problems with that definition and why they might arise. It is largely articulate and demonstrates a wide reading base. To make it even better, the author should concentrate on her writing style. At times she allows her sentences to get very long, and loses coherency as a result. She is also overly dependent on a few sources that she unnecessarily quotes rather than paraphrasing in her own words. In a few places the writing is repetitive or convoluted, and as a result the essay is very long, despite lacking detail in a few key areas. At my (Russell group) university, this would score 5 stars.

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