50
Basic biogeography: estimating biodiversity and mapping nature
as a monophyletic species (ditto) had evolved independently in the xeric coastal zones of the three islands
(Rees et al ., 2001 ).
Such analyses have contributed to the pattern of
species discovery in the Canaries, whereby, despite
dense human populations and some three hundred
years of scientifi c attention, new species have been
described at the rate of about one species every six days
in recent decades. The discoveries include two large
species of lizards ( Gallotia intermedia and G. gomerana )
and at least two species of trees ( Myrica rivas - martinezii
and Dracaena tamaranae ), and have resulted both from
new fi eld and laboratory work (Izquierdo et al ., 2004 ;
Whittaker & Fern á ndez - Palacios, 2007 ). Many new
fi nds on the Canaries, such as the two species of Gallotia ,
had escaped scientifi c detection because they persist
only in small and endangered populations. Their discovery, as with many recent discoveries around the world,
adds both to the global diversity total and to the total of
threatened species. These examples are indicative of
how diffi cult a business it is to estimate global diversity.
Global estimates of the total species diversity of terrestrial animal and plant species are largely dependent
on the estimated number of arthropods. A classic and
infl uential analysis was undertaken by Terry Erwin
(e.g. Erwin, 1983 ), based on a fi eld study of neotropical
beetles. Erwin took his samples near the city of Manaus
in the heart of the Brazilian Amazon. He used insecticide fumigators to sample beetles from the canopies
of three forest types, collecting the specimens from
a network of collecting trays spaced out along ten
transects, each of 50 m length. He recorded a remarkable 1,080 species (many unknown) from these samples
and, signifi cantly for estimates of species richness, 83
per cent of the species he sampled were restricted to one
type of forest and 14 per cent to two types.
To get from here to an estimate of global arthropod
diversity requires some pretty big suppositions. Erwin ’ s
method was to use an estimate of beetle host specifi city
of 20 per cent (derived from a study of insects on one
tree species, c. 163 beetle species per tree species) and
multiply this by tropical tree species richness ( ≈ 50,000
species). Assuming that beetles comprise about 40 per
cent of canopy arthropod species, and that there are
twice as many canopy species as ground - dwelling
species, it is possible to estimate that there could be as
many as 30 million species. Using similar approaches,
other researchers have arrived at estimates for global
biodiversity of eukaryotes as high as 100 million
species (Groombridge, 1992 ).
taxonomy. The shortfall refers to the enormous discrepancy between the number of species that have
been formally described by taxonomists (around 1.7
million at the last count), and the number of species
that are thought to exist – somewhere between 3 and
100 million (excluding bacteria and viruses). In other
words, it is the disparity between the number of
described species and the total number of species in
existence (Raven & Wilson, 1992 ; Lomolino, 2004 ;
this defi nition from Lomolino et al ., 2010 ). The shortfall is a problematic issue for conservation biogeography, because it can add considerable ‘ noise ’ to any
attempt to map and compare biodiversity.
The depth and breadth of the Linnean shortfall is
clearly illustrated by the number of new species that
are still being discovered – almost at a weekly rate in
some parts of the world. These discoveries are not just
restricted to insects and other small and inconspicuous
inhabitants of tropical rain forests, although these
fauna do represent the bulk of the Linnean shortfall. For
example, 11 of 80 extant species of cetaceans (whales
and porpoises) were discovered only in the 20th century,
one as recently as 1991 (Raven & Wilson, 1992 ).
Higher taxonomic groups are also still being discovered. For example, Raven and Wilson, in their 1992
paper, note that three new families of fl owering plants
were discovered in Central America and southern
Mexico within the previous decade, while two new
phyla were described in the last two decades of the
20th century (Lomolino et al ., 2010 ). However, it is
fair to state that most big and well - studied groups,
such as birds and mammals, have been reasonably well
described, while the shortfall is greatest in smaller, less
charismatic taxa such as fungi and, especially, many
types of arthropods.
Additions to the global list of species come about
both by new collections of voucher material and by
re - inspection of material previously gathered and
residing in museums around the world. In addition to
the traditional systematic taxonomy (whereby specimens are classifi ed based on morphology into different
species), increasingly, as we will discuss later, genetic
data are allowing the identifi cation of morphologically
cryptic species. A nice example comes from studies of
Nesotes beetles in the Canary Islands, where genetic
analyses showed that what had been thought to be
a single species Nesotes fusculus , occurring on the
islands of Tenerife, La Gomera and Gran Canaria,
really represented a paraphyletic group (see Glossary),
in which the fusculus phenotype previously recognized
Basic biogeography: estimating biodiversity and mapping nature
as a monophyletic species (ditto) had evolved independently in the xeric coastal zones of the three islands
(Rees et al ., 2001 ).
Such analyses have contributed to the pattern of
species discovery in the Canaries, whereby, despite
dense human populations and some three hundred
years of scientifi c attention, new species have been
described at the rate of about one species every six days
in recent decades. The discoveries include two large
species of lizards ( Gallotia intermedia and G. gomerana )
and at least two species of trees ( Myrica rivas - martinezii
and Dracaena tamaranae ), and have resulted both from
new fi eld and laboratory work (Izquierdo et al ., 2004 ;
Whittaker & Fern á ndez - Palacios, 2007 ). Many new
fi nds on the Canaries, such as the two species of Gallotia ,
had escaped scientifi c detection because they persist
only in small and endangered populations. Their discovery, as with many recent discoveries around the world,
adds both to the global diversity total and to the total of
threatened species. These examples are indicative of
how diffi cult a business it is to estimate global diversity.
Global estimates of the total species diversity of terrestrial animal and plant species are largely dependent
on the estimated number of arthropods. A classic and
infl uential analysis was undertaken by Terry Erwin
(e.g. Erwin, 1983 ), based on a fi eld study of neotropical
beetles. Erwin took his samples near the city of Manaus
in the heart of the Brazilian Amazon. He used insecticide fumigators to sample beetles from the canopies
of three forest types, collecting the specimens from
a network of collecting trays spaced out along ten
transects, each of 50 m length. He recorded a remarkable 1,080 species (many unknown) from these samples
and, signifi cantly for estimates of species richness, 83
per cent of the species he sampled were restricted to one
type of forest and 14 per cent to two types.
To get from here to an estimate of global arthropod
diversity requires some pretty big suppositions. Erwin ’ s
method was to use an estimate of beetle host specifi city
of 20 per cent (derived from a study of insects on one
tree species, c. 163 beetle species per tree species) and
multiply this by tropical tree species richness ( ≈ 50,000
species). Assuming that beetles comprise about 40 per
cent of canopy arthropod species, and that there are
twice as many canopy species as ground - dwelling
species, it is possible to estimate that there could be as
many as 30 million species. Using similar approaches,
other researchers have arrived at estimates for global
biodiversity of eukaryotes as high as 100 million
species (Groombridge, 1992 ).
taxonomy. The shortfall refers to the enormous discrepancy between the number of species that have
been formally described by taxonomists (around 1.7
million at the last count), and the number of species
that are thought to exist – somewhere between 3 and
100 million (excluding bacteria and viruses). In other
words, it is the disparity between the number of
described species and the total number of species in
existence (Raven & Wilson, 1992 ; Lomolino, 2004 ;
this defi nition from Lomolino et al ., 2010 ). The shortfall is a problematic issue for conservation biogeography, because it can add considerable ‘ noise ’ to any
attempt to map and compare biodiversity.
The depth and breadth of the Linnean shortfall is
clearly illustrated by the number of new species that
are still being discovered – almost at a weekly rate in
some parts of the world. These discoveries are not just
restricted to insects and other small and inconspicuous
inhabitants of tropical rain forests, although these
fauna do represent the bulk of the Linnean shortfall. For
example, 11 of 80 extant species of cetaceans (whales
and porpoises) were discovered only in the 20th century,
one as recently as 1991 (Raven & Wilson, 1992 ).
Higher taxonomic groups are also still being discovered. For example, Raven and Wilson, in their 1992
paper, note that three new families of fl owering plants
were discovered in Central America and southern
Mexico within the previous decade, while two new
phyla were described in the last two decades of the
20th century (Lomolino et al ., 2010 ). However, it is
fair to state that most big and well - studied groups,
such as birds and mammals, have been reasonably well
described, while the shortfall is greatest in smaller, less
charismatic taxa such as fungi and, especially, many
types of arthropods.
Additions to the global list of species come about
both by new collections of voucher material and by
re - inspection of material previously gathered and
residing in museums around the world. In addition to
the traditional systematic taxonomy (whereby specimens are classifi ed based on morphology into different
species), increasingly, as we will discuss later, genetic
data are allowing the identifi cation of morphologically
cryptic species. A nice example comes from studies of
Nesotes beetles in the Canary Islands, where genetic
analyses showed that what had been thought to be
a single species Nesotes fusculus , occurring on the
islands of Tenerife, La Gomera and Gran Canaria,
really represented a paraphyletic group (see Glossary),
in which the fusculus phenotype previously recognized