The Need for Conservation

Biodiversity quite simply refers to all of the different life forms on our planet, and includes both species diversity and genetic diversity. There are many reasons why we value biodiversity, the most pragmatic being that ecosystems, which maintain life on our planet, cannot function without a variety of species. On a slightly less dramatic note, different species provide us with food (crops, livestock), fibres (wool, cotton), pharmaceuticals (25 % of medical prescriptions in the USA contain active ingredients from plants; Primack, 1998) and entertainment (countryside walks, ecotourism, zoos, gardening, fishing, birdwatching). From a less anthro-pocentric perspective, species may be considered worthwhile in their own right and not simply because they benefit humans, in which case there are important ethical considerations surrounding the predilection of one species, Homo sapiens, to drive numerous other species extinct.

We know from the fossil record that biodiversity has been increasing steadily over the past 600 million years, despite the fact that as many as 99 per cent of species that have ever lived are now extinct (Figure 7.1). Around 96 per cent of all extinctions have occurred at a fairly constant rate, creating what is known as the background extinction rate. This has been estimated from the fossil record as an average of 25 per cent of all living species becoming extinct every million years (Raup, 1994). The remaining 4 per cent or so of all extinctions occurred during five separate mass extinctions, which are identified from the fossil record as periods in which an estimated 75 per cent or more of all living species became extinct. The most recent, and also the most famous, mass extinction occurred in the late Cretaceous (65 million years ago) when approximately 85 per cent of all species, including the dinosaurs, were wiped out.

Many biologists predict that we are now entering a sixth mass extinction (Leakey and Lewin, 1995). Over the past 400 years or so, several hundred species are known

Molecular Ecology Joanna Freeland © 2005 John Wiley & Sons, Ltd.

Millions of years ago

Figure 7.1 Evidence from the fossil record tells us that the total number of living families has increased steadily over the past 600 million years. Numbers of originations and extinctions have fluctuated, but in most time intervals the former outnumbers the latter. Data from Benton (1993)

to have disappeared. Although this might sound like a lot, these recent extinctions actually represent a very small percentage of described taxa and therefore do not suggest anything close to a mass extinction (Table 7.1). Instead, it is the predicted rates of extinctions over the next century that are the main cause for concern. The best estimates of these are provided by the World Conservation Union (IUCN: International Union for Conservation of Nature and Natural Resources), which regularly compiles Red Lists on the numbers of species that are known to be at risk. Several categories are used (e.g. critically endangered, endangered, vulnerable) and these are based on a number of parameters, including current population size,

Table 7.1 The numbers of species extinctions that have been recorded over the past 400 years (adapted from Primack, 1998). Note that the true numbers are undoubtedly higher than this because numerous undescribed species will also have gone extinct, e.g. a large number of plant and invertebrate species were probably wiped out during the destruction of tropical rainforests over the past few decades

Table 7.1 The numbers of species extinctions that have been recorded over the past 400 years (adapted from Primack, 1998). Note that the true numbers are undoubtedly higher than this because numerous undescribed species will also have gone extinct, e.g. a large number of plant and invertebrate species were probably wiped out during the destruction of tropical rainforests over the past few decades

Taxonomic group

Number of extinctions

Percentage of taxonomic group

Mammals

85

2.1

Birds

113

1.3

Reptiles

21

0.3

Amphibians

2

0.05

Fish

23

0.1

Invertebrates

98

0.01

Flowering plants

384

0.2

number of mature adults, generation time, recent reductions or fluctuations in population size, and population fragmentation (see http://www.redlist.org/ for more details).

The Red List that was compiled by the IUCN in 2003 reported that 23 per cent of all mammal species and 12 per cent of all bird species are threatened. We know little about the total proportion of threatened species in other taxonomic groups simply because we lack the relevant information for most species. For example, 49 per cent of fishes that have been evaluated are classified as threatened, but because only around 5 per cent of all fish species have been adequately assessed, this value gives us limited insight into the status of fishes as a whole. Similarly, 72 per cent of evaluated insects have been placed in the threatened category, but <0.1 per cent of insect species have been investigated so far. Few data are available for most groups of plants, with the exception of conifers in which 93 per cent of species have been evaluated, and we know that 31 per cent of these are threatened. Clearly these data are far from complete, but if it turns out that similar proportions of all species in the various taxonomic groups are threatened then the fate of very many species hangs in the balance (Table 7.2). It is for this reason that many people believe that we are currently on the brink of a sixth mass extinction.

So why exactly are so many species threatened with extinction? In most cases, the answer to this is anthropogenic activity. Farming, logging, mining, damming and building have destroyed the habitats of countless species around the world.

Table 7.2 Numbers and proportions of threatened species according to the IUCN 2003 Red List. Note that for most taxonomic groups only a very small proportion of species have been evaluated

Number of Number of threatened threatened

Taxonomic Number of Number of species as % species as %

group described species evaluated species described evaluated

Number of Number of threatened threatened

Taxonomic Number of Number of species as % species as %

group described species evaluated species described evaluated

Vertebrates

Mammals

4842

4789

23

24

Birds

9932

9932

12

12

Reptiles

8134

473

4

62

Amphibians

5578

401

3

39

Fishes

28 100

1532

3

49

Invertebrates

Insects

95 0000

768

0.06

72

Molluscs

70 000

2098

1

46

Crustaceans

40 000

461

1

89

Others

13 0200

55

0.02

55

Plants

Mosses

15000

93

0.5

86

Ferns

13025

180

1

62

Gymnosperms

980

907

31

34

Dicotyledons

199 350

7734

3

75

Monocotyledons

59 300

792

1

65

Many endemic species have suffered from human-mediated introductions of alien species, both deliberate and accidental. Hunting, fishing and trading have led to the overexploitation of many species, whereas countless others have suffered from industrial or agricultural pollution. Although these processes are diverse, a common outcome is a reduction in the sizes of wild populations. When this occurs, species begin to suffer from reduced genetic diversity and inbreeding, and this is where conservation genetics comes into play. In this chapter we will look at some of the most important aspects of conservation genetics by first examining how genetic data can be used to identify distinct species and populations as potential targets of conservation. In subsequent sections we shall build on some of the theory that was presented in earlier chapters by re-visiting genetic diversity, inbreeding, population sizes and relatedness, but this time paying particular attention to how they can be applied to some of the issues surrounding conservation biology.

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