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Getting the Measure of Biodiversity

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 insight review articles

Getting the measure of biodiversity

Andy Purvis* & Andy Hector†
*Department of Biology and †NERC Centre for Population Biology, Imperial College, Silwood Park, Ascot, Berkshire SL5 7PY, UK

The term ‘biodiversity’ is a simple contraction of ‘biological diversity’, and at first sight the concept is simple
too: biodiversity is the sum total of all biotic variation from the level of genes to ecosystems. The challenge
comes in measuring such a broad concept in ways that are useful. We show that, although biodiversity can
never be fully captured by a single number, study of particular facets has led to rapid, exciting and
sometimes alarming discoveries. Phylogenetic and temporal analyses are shedding light on the ecological
and evolutionary processes that have shaped current biodiversity. There is no doubt that humans are now
destroying this diversity at an alarming rate. A vital question now being tackled is how badly this loss affects
ecosystem functioning. Although current research efforts are impressive, they are tiny in comparison to the
amount of unknown diversity and the urgency and importance of the task.

T
o proceed very far with the study of biodiversity, and no single number can incorporate them both without
we need to pin the concept down. We cannot loss of information. This should not be disappointing;
even begin to look at how biodiversity is indeed we should probably be relieved that the variety
distributed, or how fast it is disappearing, of life cannot be expressed along a single dimension.
unless we can put units on it. However, any Rather, different facets of biodiversity can each be
attempt to measure biodiversity quickly runs into the quantified (Box 1).
problem that it is a fundamentally multidimensional Knowing the diversity (however measured) of one place,
concept: it cannot be reduced sensibly to a single group or time is in itself more-or-less useless. But, as we shall
number1,2. A simple illustration can show this. Figure 1 discuss later, comparable measurements of diversity from
shows samples from the insect fauna in each of two multiple places, groups or times can help us to answer
habitats. Which sample is more diverse? At first sight it crucial questions about how the diversity arose and how we
must be sample A, because it contains three species to might best act to maintain it. We shall see also how the
sample B’s two. But sample B is more diverse in that there usefulness of the answers depends critically on the selection
is less chance in sample B that two randomly chosen of an appropriate diversity measure. No single measure will
individuals will be of the same species. Neither of these always be appropriate (indeed, for some conservation ques-
measures of diversity is ‘wrong’ — species richness and tions, no single measure can probably ever be appropriate).
evenness are two (among many) of biodiversity’s facets, The choice of a good measure is complicated by the frequent

Sample A Sample B

Figure 1 Two samples of insects from different locations, illustrating two of the many different measures of diversity: species richness and species evenness.
Sample A could be described as being the more diverse as it contains three species to sample B’s two. But there is less chance in sample B than in sample A
that two randomly chosen individuals will be of the same species.

212 © 2000 Macmillan Magazines Ltd NATURE | VOL 405 | 11 MAY 2000 | www.nature.com
 insight review articles
Box 1
Parts of the whole: numbers, evenness and difference

Biodiversity has a multitude of facets that can be quantified. Here we Evenness

classify some commonly used measures into three conceptually A site containing a thousand species might not seem particularly
different (although not orthogonal) approaches. diverse if 99.9% of individuals that you find belong in the same
species. Many diversity indices have been developed to convey the
Numbers extent to which individuals are distributed evenly among species2.
The most commonly considered facet of biodiversity is species Most but not all combine evenness with species richness, losing
richness — the number of species in a site, habitat or clade. Species information by reducing two dimensions to one. There are genetic
are an obvious choice of unit when trying to measure diversity. Most analogues of these indices100, such as heterozygosity, that
people have an idea what ‘species’ means and, although their ideas incorporate both allele number and relative frequencies.
differ considerably (reviewed in ref. 96), there is even less
commonality about other levels in the taxonomic hierarchy30 (Fig. 3). Difference
Many other measures are less intuitive, and have arisen only through Some pairs of species (or alleles or populations) are very alike, whereas
appreciation of limitations of measures of species richness. Species others are very different. Disparity101 and character diversity93 are
are also sensible units to choose from a biological perspective: they measures of phenotypic difference among the species in a sample, and
keep their genes more or less to themselves, and to that extent have can be made independent of species number. Some phenotypic
independent evolutionary trajectories and unique histories. The characteristics might be considered more important than others, for
current ‘best guess’7 is that there are around 14 million species, but instance the ecological diversity among species may be crucial for
this is very much a provisional working figure. Regions with many ecosystem functioning. Genetic variability among populations can also
species, especially endemic species, are sometimes called be measured in various ways100. If populations within species differ
hotspots97. enough either genetically or phenotypically, they may be considered to
Species and regions differ in their number of populations. be subspecies, management units or evolutionarily significant units102;
Populations of a given species, if defined on the basis of limited gene numbers of these therefore provide estimates of difference. All these
flow among them, will evolve to an extent independently. Each kinds of difference are likely to be at least partly reflected by the
population contributes additional diversity. The number of genetic phylogenetic diversity103 among organisms, which is estimated as the
populations in the world has been estimated to lie between 1.1 and 6.6 sum total of the branch lengths in the phylogeny (evolutionary tree) linking
billion66. them.
Species or populations differ in the numbers of alleles they have at Sample in different places, and you will find different things. This
given loci. For instance, Mauritius kestrels (Falco punctatus) have lost spatial turnover itself has many facets2 (for example, beta diversity,
over half of the alleles present historically at 12 sampled microsatellite gamma diversity and numbers of habitat types), and important
loci98. consequences for any attempt to conserve overall diversity (see
Moving above the species level, higher-taxon richness is often review by Margules and Pressey, pages 243–253, and refs 104, 105).
used in studies of biodiversity, usually as a less data-demanding Likewise, temporal turnover106 is the extent to which what is found
surrogate for species richness99. changes over time.

need to use surrogates for the aspect in which we are most interest- below the surface; these subsurface lithoautotrophic microbial
ed3,4. Surrogacy is a pragmatic response to the frightening ignorance ecosystems (termed SLiMEs) may have persisted for millions of years
about what is out there. Some recent discoveries highlight just how without any carbon from the surface10. Controversy surrounds
much we probably still do not know. another proposed discovery: whether or not the 100-nm-diameter
nanobacteria found in, among other places, kidney stones are living
The growing biosphere organisms11. At an even smaller scale, genomes provide fossils
Technological advances and the sense of urgency imparted by the rate that indicate great past retroviral diversity12. Genomes have also
of habitat loss are combining to yield discoveries at an incredible rate. been found to provide habitats for many kinds of genetic entity —
This may seem surprising, given that expedition accounts of natural transposable elements — that can move around and replicate
historians from the 18th and 19th centuries conjure up images of dis- themselves. Such elements can provide important genetic variation
covery on a grand scale that seemingly cannot be matched today — to their hosts, can make up more than half of the host’s genome13, and
look in the rocks … a new fossil mammal; look in the lake … a new have life histories of their own14.
fish genus; look on the dinner plate … a new species of bird. Finding There are two other ways in which the biosphere can perhaps be
new large vertebrates nowadays is indeed newsworthy, but a new said to be growing. The first is that the rate at which taxonomists split
species of large mammal is still discovered roughly every three years5 one previously recognized species into two or more exceeds the rate
and a new large vertebrate from the open ocean every five years6. And at which they lump different species together, especially in taxa that
most organisms are much smaller than these are. An average day sees are of particular concern to conservationists (for example,
the formal description of around 300 new species across the whole platyrrhine primates15). Part of the reason is the growing popularity
range of life, and there is no slowdown in sight. Based on rates of dis- of one way of delimiting species — the phylogenetic species concept
covery and geographical scaling-up, it seems that the roughly 1.75 (PSC)16 — under which taxa are separate species if they can be diag-
million described species of organism may be only around 10% of the nosed as distinct, whether on the basis of phenotype or genotype. If
total7. the PSC becomes widely applied — which is a controversial issue17 —
It is not only new species that are discovered. Cycliophora and then the numbers of ‘species’ in many groups are sure to increase
Loricifera are animal phyla (the level just below kingdom in the taxo- greatly18 (although the amount of disparity will barely increase at all).
nomic hierarchy) that are new to science in the past 20 years8. Within A second way in which the catalogue of diversity is growing is that
the Archaea, the discovery of new phylum-level groups proceeds at computer databases and the Internet are making the process of infor-
the rate of more than one a month9. The physical limits of the bios- mation gathering more truly cumulative than perhaps ever before.
phere have been pushed back by the recent discovery of microbial Some existing sites serve to provide examples of the information
communities in sedimentary and even igneous rocks over 2 km already available: not just species lists (http://www.sp2000.org/), but
NATURE | VOL 405 | 11 MAY 2000 | www.nature.com © 2000 Macmillan Magazines Ltd 213
insight review articles
known fossils. The palaeontological record indicates a Cambrian
explosion of phyla around 540 million years (Myr) ago, but
a
sequences suggest a more gradual series of splits around twice as
old23. Likewise, many orders of mammals and birds are now thought
to have originated long before the end-Cretaceous extinction24,25,
which occurred 65 Myr ago and which was thought previously to
have been the signal for their radiation. If the new timescale can be
trusted26, these findings present a puzzle and a warning. The puzzle is
the absence of fossils. Why have we not found traces of these lineages
in their first tens or even hundreds of millions of years? It seems likely
Hominidae that the animals were too small or too rare, with the sudden appear-
ance in the rocks corresponding to an increase in size and rise to
Pongidae ecological dominance27. The warning is that current biodiversity is in
b
a sense greater than we had realized. Major lineages alive today
represent more unique evolutionary history than previously
suspected — history that would be lost with their extinction.
Old-World monkeys
Analysis of the shape of phylogenies has shown that lineages have
New-World monkeys differed in their potential for diversification. Darwin28 had noted that
species in species-rich genera had more subspecific varieties, and
subtaxa within taxa are often distributed very unevenly29, as Fig. 2
melanogaster subgroup
c
illustrates for eutherian species. But these taxonomic patterns can be
taken at face value only if taxa are comparable, which they may not be.
For example, species-rich groups may simply be older, and it is clear
that workers on different groups currently place taxonomic bound-
aries in very different places30 (Fig. 3). Phylogenies allow comparison
of sister clades — each other’s closest relatives — which by definition
are the same age. Time and again, species are distributed too uneven-
ly for simple null models to be tested in which all species have the
same chances of diversifying31,32.
genus Scaptomyza
What are the species-rich groups ‘doing right’? Many explana-
tions fall broadly into two types. Key innovation hypotheses33 posit
40 30 20 10 0 the evolution of some trait that permits its bearers to gain access to
Millions of years ago more resources or be more competitive than non-bearers. Examples
include phytophagy in insects34 and high reproductive rate in mam-
mals35. Other hypotheses focus on traits that facilitate the evolution
Figure 2 Taxonomic boundaries are not comparable among major groups. a, Fourteen of reproductive isolation — speciation — without necessarily
species in nine genera representative of cichlid fish in Lake Victoria. b, Seven species increasing the fitness of bearers. Sexual selection36 and range
representative of several families in anthropoid primates. c, Thirteen species fragmentation37 are examples of this kind. These two types can be
representative of a single genus, Drosophila. Figure reproduced from ref 30, with contrasted as ‘bigger cake’ and ‘thinner slices’ explanations,
permission. although some traits may act in both ways (for example, body
size38,39); another way to split them is to view diversity as ‘demand-
driven’ (niches are waiting to be filled, and differentiation leads to
also maps of the geographical ranges of species (http://www. speciation) or ‘supply-driven’ (speciation occurs unbidden, with
gisbau.uniroma1.it/amd/homepage.html), information on conser- differentiation arising through character displacement). Statistical
vation status of species (http://www.wcmc.org.uk), bibliographies testing of many key innovation hypotheses is hampered by a lack of
(http://eteweb.lscf.ucsb.edu/bfv/bfv_form.html), data on molecular replication — often, the trait in question is unique, and all that can
sequence (http://www.ebi.ac.uk/ and http://www.ncbi.nlm.nih.gov/ be done is to model the trait’s evolution to assess how well it fits the
Genbank/GenbankOverview.html), data on phylogenetic position scenario40. When characters have evolved multiple times in
(http://phylogeny.arizona.edu/tree/phylogeny.html and http:// independent lineages, sister clades provide automatic matched pairs
herbaria.harvard.edu/treebase/), information on the stratigraphic for hypothesis testing (although other phylogenetic approaches are
range of species (http://ibs.uel.ac.uk/ibs/palaeo/benton/ and also available41,42). Comparing sister clades (the procedure used in
http://www.nceas.ucsb.edu/~alroy/nafmtd.html) and much more. most of the examples above) avoids two problems that otherwise
Although the terabytes of information already stored constitute only cloud the issue. First, taxa may not be comparable (Fig. 3), and
a small drop in the ocean, the next two sections show how much can second, they are not statistically independent — related clades
be seen in that droplet about the distribution of biodiversity among inherit their traits from common ancestors, so are pseudorepli-
evolutionary lineages and through time. cates43. Nonetheless, there is ongoing debate about the role and
limitations of phylogenetic tests for correlates of species
Learning from the tree of life richness44,45.
The ongoing explosion of phylogenetic studies not only provides an
ever-clearer snapshot of biodiversity today, but also allows us to make Temporal patterns in biodiversity
inferences about how the diversity has come about19–21. (For an Is biodiversity typically at some equilibrium level, with competi-
ecological perspective, see review by Gaston, pages 220–227.) Phylo- tion setting an upper limit, or do mass extinctions occur so regular-
genies give key information that is not available from species lists or ly that equilibrium is never reached? And, with one eye on the
taxonomies. They detail the pattern of nested relationships among future prospects for biodiversity, how quickly does diversity
species, and increasingly provide at least a rough timescale even recover from mass extinctions? Palaeontologists have addressed
without reliance on a molecular clock22. These new phylogenies are these questions at many scales, from local to global. For the global
pushing back the origins of many groups to long before their earliest view, the data come from huge compendia of stratigraphic ranges of
214 © 2000 Macmillan Magazines Ltd NATURE | VOL 405 | 11 MAY 2000 | www.nature.com
 insight review articles
taxonomic families (see, for example, refs 46, 47), led by Sepkoski’s highlights the difficulties of taking taxonomic patterns at face
ground-breaking efforts, and made possible by the development of value. Neontologists may face much the same problem with species:
computer databases. There are more families now than ever before, taxonomists tend to recognize bird lineages as species if they are
and a model of exponential growth provides a good overall fit to the older than 2.8 Myr but not if they are younger than 1.1 Myr (ref. 52),
numbers of families through time, suggesting expansion without so apparent logistic growth in species numbers through time
limit and no major role for competition in limiting diversity48. But a within bird genera53 might be expected even without a slow-down
significantly better fit is provided by a set of three logistic curves, of cladogenesis.
each with a different carrying capacity, punctuated by mass extinc- The patchy nature of the known fossil record means that some
tion events49. Leaving aside the thorny issue of multiplicity of tests taxa in some places at some times can be studied in much greater
and the big question of why the three carrying capacities are differ- detail than is possible for the biota as a whole. Studies at these
ent, there may be a perceptual problem at play here. Families do not smaller scales can analyse the record at the species level, within a
arise overnight: they are the result of speciation and a lot of time. region or biome, and can better control for problems such as incom-
Consequently, exponential growth at the species level might appear plete and uneven sampling54,55. Such studies find a range of answers:
like logistic growth at higher levels50. This problem of perception is communities may show an equilibrium diversity55,56, an increasing
a recurrent one in palaeontology. For instance, good evidence that geographical turnover57, or radiation punctuated by mass extinc-
biodiversity is often near equilibrium comes from the fact that tion58. This may be a more appropriate spatial scale at which to look
extinction events are commonly followed by higher than normal for equilibrium, as the units have a greater chance of interacting59.
rates of diversification4. However, the peak of origination rates of The temporal pattern of disparity is also of great interest. Does
genera and families is not straight after the extinction peak. Instead, difference accumulate gradually and evenly as lineages evolve their
there is a 10-Myr time-lag throughout the fossil record, implying a separate ways, or is evolutionary change more rapid early in a
lag phase before diversification occurs51. But could the same pattern group’s history, as it stakes its claim to a new niche? Information
arise if speciation rates rose immediately in response to the extinc- from living and fossil species and phylogenies can be combined with
tion, but the new lineages are given generic or familial rank only statistical models41,60,61 to answer this question, although so far rela-
after being around for some time? This scenario would predict tively little work has combined palaeontological and neontological
(incorrectly) that family diversification rates would take longer to data. Rates of morphological and taxic diversification are often
respond than generic rates, so cannot be the whole story, but it incongruent, or even uncoupled61, again highlighting that there is

a
2,000

1,500
Species richness

b 1,000
1,500

500

1,000
Species richness

c 0
Rodentia
Chiroptera
Insectivora
Carnivora
Primates
Artiodactyla
Cetacea
Lagomorpha
Pinnipedia
Xenarthra
Scandentia
Perissodactyla
Macroscelidea
Hyracoidea
Pholidota
Sirenia
Dermoptera
Proboscidea
Tubulidentata

60

500
50
Order

40
Species richness

0
Muridae
Sciuridae
Echimyidae
Heteromyidae
Ctenomyidae
Geomyidae
Dipodidae
Gliridae
Zapodidae
Caviidae
Dasyproctidae
Capromyidae
Hystricidae
Bathyergidae
Erethizontidae
Octodontidae
Anomaluridae
Chinchillidae
Ctenodactylidae
Castoridae
Abrocomidae
Thryonomyidae
Aplodontidae
Pedetidae
Seleviniidae
Hydrochaeridae
Dinomyidae
Myocastoridae
Petromuridae

30

20

Family
10

Figure 3 Subtaxa within taxa are often distributed unevenly. Uneven distribution of
0
species among: a, eutherian orders, with rodents being the dominant group; b, rodent
Genus
families, with murids being dominant; and c, murid genera. Data from ref. 95.

NATURE | VOL 405 | 11 MAY 2000 | www.nature.com © 2000 Macmillan Magazines Ltd 215
insight review articles

Figure 4 Species richness in major groups of organisms. The main ‘pie’

shows the species estimated to exist in each group; the hatched area within
Chordates
each slice shows the proportion that have been formally described. Data
Plants from ref. 7.
Molluscs

Crustaceans

'Protozoa'

Insects

'Algae'

Arachnids

Nematodes

Fungi

Viruses

Bacteria

Others

more to biodiversity than numbers of taxa. At present, it is hard to extinctions, is highly non-random73–76, with related twigs on the tree
tell under what circumstances disparity precedes, or perhaps drives, tending to share the same fate. This selectivity greatly reduces the
species richness, and when the reverse applies. Different models can ability of the phylogenetic hierarchy to retain structure in the face of
give very similar patterns of diversity and disparity over time60, and a given severity of species extinction77,78.
detailed studies at smaller scale62,63 may provide the greatest chance But how much structure is needed? Imagine if the only function of
of an answer. this article was the transfer of information. Many of the words could
be deleted and you would still get the message. It would (we hope) be
The shrinking biosphere less pleasant to read. Similarly, for many people we need biodiversity
What about human impacts on biodiversity? A simple calculation because we like it; it should be conserved just as we conserve Mozart
shows that recent rates of species losses are unsustainable. If there are concertos and Van Gogh paintings79. But how many words could you
14 million species at present7, then each year the tree of life grows by delete before the meaning starts to get lost? Recently, ecologists have
an extra 14 Myr of branch length. The average age of extant species is begun asking similar questions about our environment.
nearly 5 Myr (in primates and carnivores anyway, and species in most
other groups probably tend to be older rather than younger). So the Biodiversity and the stability and functioning of ecosystems
tree can ‘afford’ at most about three species extinctions per year How many species can we lose before we start to affect the way ecosys-
without shrinking overall. There have been roughly this many tems function? Principal environmental factors such as climate, soil
documented species extinctions per year since 160064, and most type and disturbance80,81 strongly influence ecosystem functioning,
extinctions must have passed us by. The rate has been increasing too: but likewise organisms can affect their environment82. Some of the
the last century saw the end of 20 mammalian species alone, a first ideas on how biodiversity could affect the way ecosystems
pruning of the mammalian tree that would take at least 200 centuries function are attributable to Darwin and Wallace28,83, who stated that a
to redress. diverse mixture of plants should be more productive than a mono-
Estimates of current and future rates of loss make even more culture. They also suggested the underlying biological mechanism:
sobering reading. The rate at which tropical forest — probably the because coexisting species differ ecologically, loss of a species could
habitat for most species — is lost is about 0.8% to 2% per year65 (call result in vacant niche-space and potential impacts on ecosystem
it 1% for the purpose of this example). We must expect about 1% of processes. Defining ecological niches is not straightforward, but
the tropical forest populations to be lost with it, a figure that may be Darwin and Wallace’s hypothesis, if correct, provides a general
as high as 16 million populations per year, or one every two biological principle which predicts that intact, diverse communities
seconds66. Most species have multiple populations, so rates of are generally more stable and function better than versions that have
species loss will obviously be much lower. They are most commonly lost species. Recent experimental evidence (reviewed by Chapin et
estimated through species–area relationships65, although other al., pages 234–242, and McCann, pages 228–233), although pointing
approaches are used too67. Wilson68 famously used the species–area out important exceptions, generally supports this idea. Compared
relationship to estimate an annual extinction rate of 27,000 species with systems that have lost species, diverse plant communities often
— one species every twenty minutes. This and similar estimates have have a greater variety of positive and complementary interactions
attracted criticism but recent work67,69,70 has shown that levels of and so outperform any single species84,85, and have more chance of
species endangerment are rising in line with species–area predic- having the right species in the right place at the right time. This last
tions, provided the analysis is conducted at the appropriate scale. ‘sampling effect’ mechanism has prompted much debate on the
What are the implications of such rapid pruning for the tree of life? design, analysis and interpretation of experiments that aim
Simulations in which species are wiped out at random71 indicate that to manipulate biodiversity86. Although the sampling effect is
most of the phylogenetic diversity would survive even a major biological in part — it requires both differences between species and
extinction: up to 80% of the branch length could survive even if 95% an ecological mechanism making some species more abundant than
of the species were lost. This result assumes extinction to befall others — the probabilistic component (more diverse communities
species at random; scenarios of non-random extinction can have have a greater chance of containing a species with particular proper-
very different outcomes72. The current crisis, like previous mass ties) has made it controversial. Nevertheless, loss of species with key
216 © 2000 Macmillan Magazines Ltd NATURE | VOL 405 | 11 MAY 2000 | www.nature.com
 insight review articles
Box 2
Plant diversity and productivity at different scales

Diversity
Latitudinal gradient

Productivity

Diversity
Biomass gradient

Productivity
Productivity

Diversity
Experimental diversity gradient

Diversity Productivity

For plants, the relationship between diversity and productivity productivity increases. Experimental manipulations of plant diversity
changes with scale107,108. At global scales (panel a in the figure within habitats (c) reveal that, although relationships vary, productivity
above), from high latitudes to the tropics, plant diversity in large areas tends to increase with diversity owing to increasing complementary or
may be positively related to increasing productivity. At regional scales positive interactions between species and the greater likelihood of
(b), plant diversity in small plots is frequently negatively related to diverse communities containing a highly productive species. In
increasing productivity, often as part of a larger unimodal ‘hump- manipulation experiments, biodiversity is the explanatory variable and
shaped’ distribution of diversities. Numbers of species correlate with productivity the response, whereas in observational studies the
several factors including the size and hence number of individual relationship is usually viewed the other way round as illustrated here
plants sampled, spatial heterogeneity, and competitive exclusion as for all three cases.

traits, as in the sampling effect, is not restricted to ecological experi- agreement and differences in experimental results. Nevertheless, this
ments: logging, fishing, trapping and other harvesting of natural work represents only a first general approach to the subject; many
resources frequently remove particular organisms, often including issues remain outstanding and other areas are as yet uninvestigated.
dominant species. First, do these short-term and small-scale experiments in field plots
Although 95% of experimental studies support a positive reveal the full effects of diversity, and how do we scale up in time and
relationship between diversity and ecosystem functioning, many space92? Second, although we know that local extinction is often not
have found that only 20–50% of species are needed to maintain most random, many recent experiments compare the performance of
biogeochemical ecosystem processes87. Do the other, apparently communities differing in the presence or absence of a random set of
redundant, species have a role to play over longer timescales, provid- species. How adequate is this model? Third, how will species loss
ing insurance against environmental change? We need to know. interact with other components of global change such as rising CO2?
Biodiversity can also impact ecological processes such as the inci- Darwin and Wallace observed that niche differentiation could cause
dence of herbivory and disease, and the resistance of communities to changing diversity to have consequences for ecosystem processes, but
invasion. Once again, although exceptions exist, in experiments the magnitude of these effects could depend crucially on the exact
which manipulate diversity directly, communities with more species mechanism of coexistence. Finally, how do we integrate these new
are often more resistant to invasion88,89, probably for the same reason within-habitat relationships between diversity and ecosystem
that they are more productive. Diversity of one group of organisms processes with large-scale patterns in biodiversity and environmen-
can also promote diversity of associated groups, for example between tal parameters, as reviewed by Gaston on pages 220–227 of this issue?
mycorrhizas and plants90 or plants and insects88. Box 2 suggests one way in which the relationship between plant
The study of the relationship between biodiversity and ecosystem diversity and productivity could vary with scale.
processes has made rapid progress in the past decade, and is proving
an effective catalyst for linking the ecology of individuals, communi- Challenges and prospects
ties and ecosystems. Some general, although not universal, patterns Recent years have seen exciting advances in our knowledge of biodi-
are emerging as theory and experiment progress together91. We have a versity, our identification of factors that have shaped its evolution
good understanding of the underlying causes, where we see both and distribution, and our understanding of its importance. But we
NATURE | VOL 405 | 11 MAY 2000 | www.nature.com © 2000 Macmillan Magazines Ltd 217
insight review articles
24. Bromham, L., Phillips, M. J. & Penny, D. Growing up with dinosaurs: molecular dates and the
can see only a small, probably atypical, part of the picture (Fig. 4). A mammalian radiation. Trends Ecol. Evol. 14, 113–118 (1999).
detailed view is emerging of birds, mammals, angiosperms, and shal- 25. Cooper, A. & Penny, D. Mass survival of birds across the Cretaceous-Tertiary boundary: molecular
low-sea, hard-bodied invertebrates, but much less is known about evidence. Science 275, 1109–1113 (1997).
most of the rest of life. How far are we justified in generalizing from 26. Foote, M., Hunter, J. P., Janis, C. M. & Sepkoski, J. J. Evolutionary and preservational constraints on
the origins of biological groups: divergence times of eutherian mammals. Science 283, 1310–1314
the groups we know well to biodiversity as a whole? This is a crucial (1999).
question, for instance in the choice of protected areas (see review by 27. Cooper, A. & Fortey, R. Evolutionary explosions and the phylogenetic fuse. Trends Ecol. Evol. 13,
Margules and Pressey, pp. 243–253). There is no short cut — we need 151–156 (1998).
28. Darwin, C. On the Origin of Species by Means of Natural Selection (Murray, London, 1859).
more basic information about more groups; and not just species lists, 29. Dial, K. P. & Marzluff, J. M. Nonrandom diversification within taxonomic assemblages. Syst. Zool.
but who does what and with whom. 38, 26–37 (1989).
A related point is that biodiversity cannot be reduced to a single 30. Avise, J. C. & Johns, G. C. Proposal for a standardized temporal scheme of biological classification
number, such as species richness. This is a real problem for biologists, for extant species. Proc. Natl Acad. Sci. USA 96, 7358–7363 (1999).
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