(i) Preamble

The basic question that I pose in this section is: if we step back from the relatively narrow confines of the study of biogeography for its own sake (and it is really a very broad field), how can an understanding of biogeographic pattern and process help in dealing with one of the most important issues facing humanity today – the conservation of biodiversity and the processes that sustain and influence it?

As we have appreciated, extinctions of species have been a regular occurrence during the history of life on earth (recall the great Permian extinction). Estimates of extinction rates are tricky, but there is evidence that anthropogenically-based extinctions have experienced a surge in the last 100 or so years. After accounting for a baseline rate of natural extinction, Pimm et al. (1995) estimated that recent extinction rates may be 20-200 times the natural background rate. This is the basis for suggestions that we are now in the midst of a modern "extinction crisis".

As a direct consequence of human activities, it is estimated that one species of bird goes extinct every four years whereas the estimated rate of extinction during the Quaternary Period was one extinction every 83 years (Brown and Lomolino 1998). At least 120 bird species have gone extinct over the last 400 years, and a recent global estimate for extinctions of all taxa is about 100 species per year. In Canada alone, 27 species (plants and animals) have become extinct in the last 150 years.

The estimates of extinctions may, in fact, understate the problem. It is estimated that there may be as many as 50 million species of life on earth, yet only less than 5 million have been described, i.e. we could be losing species we don’t even know we have. In addition, what about "population diversity"? It has been estimated that there are up to 3 billion populations of life on earth (i.e., semi-discrete aggregations of species) and that up to 16 million populations go extinct every year in tropical rainforests alone (see Hughes et al. 1997). For instance, extensive range contractions of many species is occurring worldwide (see figure below) and even though a species may be recorded as present from year to year, range collapse eliminates individual populations and may presage imminent species extinction.

Probably the biggest factor in extinction, other than perhaps overexploitation, is habitat loss and degradation. Consider the figure below which shows the loss of forest cover in various areas of the world.

How can biogeographic study contribute to biodiversity conservation?

(ii) Biodiversity "hotspots"

Faunal surveys of species ranges and biogeographic pattern can reveal areas where geographic ranges of many species overlap. These are known as biodiversity "hotspots". Particularly important are hotspots of endemism. If the probability of extinction is increased for species with limited range, then hotspots of endemism are regions where a certain degree of habitat loss could result in the greatest loss of biodiversity per unit area. Biogeographic surveys that identify such hotspots, therefore, can help prioritize areas for conservation attention.

 The figures below illustrate a latitudinal gradient in bird endemism showing the concentration of hotspots in tropical areas. The lower panel shows that islands, despite constituting only 10% of the total land area on the earth, contain almost 50% of the endemic bird areas worldwide. Biogeographic surveys clearly identified these kinds of habitats as "hotspots" of biodiversity.

Myers et al. (2000, Nature 403: 853-858) indentified 25 such hotspots that accounted for 44% of all plant species and 35% of all species in four vertebrate groups that were found in only 1.4% of the earth's surface (see below). There was broad concordance in many of these hotspots for high endemism in plants and animal groups surveyed. Note, however, that the study did not include invertebrates, among which insects may comprise 95% of all animal species (Myers et al. 2000).

See also Roberts et al. (2002) for marine systems and degrees of concordance with terrestrial systems identified in Myers et al.

The figure below depicts the degree of concordance for hotspot areas for birds, herps, and butterflies in Central America. Note greater concordance between birds/herps than between either of these taxa and butterflies.

Some areas may also be hotspots of diversity in terms of the depth of evolutionary distinctiveness, i.e., an area may have low endemism, but the species that are endemic may be representative of very old lineages (and hence be less "replaceable" than more recently derived taxa).

The figure below show phylogenetic relationships among genera in four groups of fishes. Note how the most basal lineage in each group is found in Madagascar. Not only does this island have high general endemism (see Myers et al.), but (at least for fish), it also has representatives of very old lineages!

(iii) Phylogeography and the identification of "ESUs"

The term "evolutionarily significant unit" (ESU) was coined in the mid-1980s to try and develop an empirical definition of intraspecific units of diversity that likely represented historically isolated groupings that should form the most important units for conservation below the species level (see Ryder 1986). These ESUs not only represented the "bioheritage" of a species, but also may represent groups with distinct historical tendencies and future fates. The actual criteria used to define ESUs have been the subject of much debate. One definition that has been adopted in conservation of Pacific salmon below the species level is that of Waples (1995): a population, or group of populations, that is substantially reproductively isolated from other such groups and which represent an important part of the evolutionary legacy of a species.

By "important" is usually meant a population or group of populations of occuping an unusual habitat (i.e. high altitude lakes, desert lakes) or which displays an unusual phenotype (e.g. ability to survive at high tempertaures, unusal life history or colouration). The pheotypic traits are usually suspected of being of significant adaptive value. Alternatively, proxy measures of phenotypic distinctiveness are often attempted through molecular analyses (i.e. they are typically cheaper and faster to accomplish for a braod array of geographic samples). One "molecular-based" definition is that of Moritz (1994) which uses molecular data to define ESUs as groups of populations that are reciprocally monophyletic for mtDNA lineages and that differ significantly in allele frequencies at nuclear loci.

The development of molecular techniques has facilitated the development of the field of phylogeography (see lectures on Analytical Biogeography I. Section (vi)) and of the identification of putative ESUs at least in terms of mtDNA variation.

An example is the work of Joseph et al. We discussed these examples before and a summary is given in the figure below. Recall that these workers identified concordant "breaks" in mtDNA phylogroups north and south of the Black Mountain Barrier in southeastern Australia in a variety of vertebrates. The reciprocally monophyletic lineages in the some of these species (e.g. skink, robin) could form the basis for ESU designation in these species. In addition, the concordant breaks across the same geographic area argue for the region being a hotspot of biodiversity at the within species level.

Another example is the striking concordance among vertebrates and invertebrates in the southeastern USA and distinct "Atlantic" and "Gulf Coast" lineages resolved with mtDNA. Review the work and publication by Avise (1992, Oikos 63: 62-76) for an excellent further example of the application of analytical biogeography (phylogeography) to conservation.

(iv) Island Biogeography: designing nature reserves and predicting species losses

Recall in our discussions of island biogegraphy the issue of the "SLOSS" (single large or several small) debate with respect to the design of nature reserves (see Analytical Biogeography II, section (iv). The effects of area, isolation, and interconnectedness (all features of island biogeographic theory and observation) were (are) important components of reserve design and the most efficient use of scare resources in conservation. For reasons that should be clear to you now, the refuge design on the left is "better" than the one on the right.

In addition, the species-area relationship has been applied in attempts to predict the loss of species under different scenarios of habitat loss (either through climate/biome change or land use changes by humans).

The schematic below illustrates the possible shifts in relative areas of three vegetation types in montane habitats of the Great Basin area of the US under a scenario of a 3 degree C climatic warming. Note the complete loss of the conifer forest and the great reduction in the juniper woodland .

If one knows the species-area relationships for such habitats, then it is possible to predict the species number change as area changes. An example is shown below. The degree of change, of course, depends on the magnitude of the slope of the species-area relationship as shown in the two example species-area lines below.

Finally, so-called "gap analysis" can identify gaps in protected area strategies. Basic zoogeographic data on animal distributions are overlain on maps of proposed protected areas (see figure below). Large gaps (see botton graphic) may identify other areas that should received protection (especially if a distinct "ESU" may be found there).

(v) Predicting the effects of climate change

Recall our earlier discussions of the "climate space" of species and how such information can be used to predict possible range changes from predicted changes in temperature or precipitation (see The Physical Setting I: section (iv)). The consequences of some possible range changes can be quite serious. For instance, Rogers and Randolph (2000) used basic biogeographic analyses to make predictions about the global spread of malaria under several global warming scenarios. Temperature and moisture levels in various areas were modelled with respect to occurrence of both the parasite Plasmodium falciparum (in terms of a lower temperature limit below which development of the parasite stops) and its mosquito hosts (temperature dependent blood-feeding intervals and longevity). Several other diseases could change in global distribution owing to climate-based changes in parasite/host distribution (e.g., yellow fever, lyme disease, sleeping sickness).

(vi) Exotic species introductions

Introduction of exotic species represents one of the most significant causes of historical extinctions of native fauna (along with habitat loss and overexploitation) as well as one of the most important causes of current endangerment.

The figure below shows the extent of the problem. The bars on the right show variation in the numbers of native plant species across geographic regions and countries. The far right column shows the extent of exotic species introductions as a percentage of the total species number. Note that Canada has a relative high percentage at almost 24%! Also, many island regions have been impacted greatly by exotic introductions. Exotics in New Zealand make up almost 50% of the total plant fauna. Also, the numbers of introduced fish species in New Zealand (30) exceeds the total numbers of native species (27)! The introductions do not include the transplantation of species within a country, but outside their natural geographic range which are even more numerous. A classic example of the latter is the introduction (both intentional and accidental) of various fish species (coho, chinook, and pink salmon, sea lampreys) to the Great Lakes from both the Pacific and Atlantic drainages, see Kelso et al. 1996 for a recent discussion).

Invasive species may cause or contribute to the extinction of native species via habitat destruction, competitive displacement, disease introduction, predation, or genetic assimilation.

Clearly, the study of dispersal, colonization, "climate space", etc., all components of basic zoogeography, can play key roles in understanding the processes and consequences of the establishment and spread of exotic species in various ecosystems. A classic example involves efforts to understand and stem the spread of zebra mussels (native to Europe) that have become established in eastern North American waterways (introduced via ship ballast water).

Zebra mussels encrusting a native crayfish (yeech!).

Zebra mussels encrusting a water intake pipe.

A more local example involves the invasion of the European green crab into northeast Pacific waters.

Two age classes of green crab.

In particular, insular ecosystems appear to be particularly vulnerable to invasions from exotics as the figure below suggests. Note how the percentage (A) or absolute number (B) of invasive species tends to decline as the number of native species increases. Island areas with lower diversity of native birds and mammals appear to be at higher risk of invasions.

Islands also tend to show greater extinction rates than mainland areas. The right bars in the figure below represent numbers of extinctions on islands, bars on the left those on mainland areas.

Such elevated extinction on islands is often a consequence of introduced species. Recall our example of the brown tree snake and extinctions of native birds on Guam.

The graph below shows a depression in the slope of the species-area relationship for native reptiles on New Zealand islands that have introduced rat species (which prey on native reptiles).

In all these examples, the principles of island biogeography come into play here and one could have predicted the greater negative effects of introductions on islands, but in most cases it was too late (the introductions in many cases occurred decades ago).

These examples should illustrate that zoogeography, far from being simply of academic interest, can play clear, important, and diverse roles in very prqactical issues of resource management, human health, and biodiversity conservation.

References:

Avise, J.C. 1992. Molecular population structure and biogeographic history of a regional fauna: a case study with lessons for conservation biology. Oikos 63: 62-76.

Brown, J.H. & M.V. Lomolino. 1998. Biogeography. 2nd Edition. Sinauer Assoc., Inc. Chap. 17

Hughes, J.B. et al. 1997. Population diversity: its extent and extinction. Science 278: 689-692.

Joseph, L., Moritz, C. & Hugall, A. 1995. Molecular support for vicariance as a source of diversity in rainforest. Proc. Royal Soc. Lond. B, 260: 177-182.

Kelso, JRM, Steedman, RJ & Stoddart, S. 1996. Historical causes of change in Great Lakes fish stocks and the implications for ecosystem rehabilitation. Can. J. Fish. Aquat. Sci., 53 (supp1,1)

Mack, R.N. et al. 2000. Biotic invasions: causes, epidemiology, global consequences, and control. Ecol. Appl. 10: 689-710.

Moritz, C. 1994. Applications of mitochondrial DNA analysis in conservation: a review. Mol. Ecol. 3: 401-411.

Myers et al. 2000. Biodiversity hotspots for conservation priorities. Nature 403: 853-858.

Pimm, S.L. et al. 1995. The future of biodiversity. Science 269: 347-350.

Roberts, C.M. et al. 2002. Marine biuodiversity hotspots and conservation priorities fir tropical reefs. Science 295: 1280-1284.

Rogers, D.J. and S.E. Randolph. 2000. The global spread of malaria in a future, warmer world. Science 289: 1763-1766.

Ryder, OA. 1986. Species conservation and systematics: the dilemma of subspecies. Trends Ecol. Evol. 1: 9-10. (Origin of the term "ESU").

Sala, O.E. et al. 2000. Biodiversity – Global biodiversity scenarios for the year 2100. Science 287: 1770-1774 (changes in biome distribution owing to climate change, land use, etc).

Spellerberg, I.F. & Sawyer, W.D. 1999. An introduction to applied biogeography. New York: Cambridge University Press.

Waples, R.S. 1995. Evolutionarily significant units and the conservation of biological diversity under the endangered species act. Am. Fish. Soc. Symp. 17: 8-27.