Download Insect diversity: facts, fiction and speculation

Biological Journal ofthe Linnean Society (1988), 35: 321-337. With 8 figures
Insect diversity: facts, fiction and speculation*
NIGEL E. STORK
Department of Entomology, British Museum (Natural History), Cromwell Road, London
S W7 5BD
Biologists are still trying to grasp the global dimensions of the phylum Arthropoda and its major class
the Insecta, in spite of the fact that over a million species of arthropods have been described. T h e
canopy of rain forest trees is believed by many to hold the key to the immense diversity of insects. In
recent years the use of knock-down insecticides to sample insects from rain forest canopy has revealed
information on the canopy’s arthropod inhabitants and community structure. T h e sampling
techniques involved are outlined and data reviewed on taxonomic and guild structure, species
abundance, body size and biomass ofinsects, and the faunal similarity of trees. Calculations by Erwin
(1982), based on knock-down insecticide studies ofthe beetle fauna ofone species ofcentral American
tree, suggest there may be 30 million species of tropical forest arthropods. Reanalysis of these
calculations, using additional data, produces a range of possible estimates from about 10 to 80 million.
T h e unknown range of plant host-specificities of tropical insects is the main weakness of this method of
calculation. Assessment of the faunal importance of the canopy in relation to that of other rain forest
biotopes requires comparative quantitative studies. T h e preliminary results of one such simple study
suggest that over 42 million arthropods may be found in a hectare of Seram rain forest ( a t the time of
study), and that 70% occur in the soil and leaf litter and 14% in the canopy. They also suggest that
Collembola and Acarina are the dominant groups in this hectare, and that there are as many ants as
all the other insects (excluding Collembola),
KEY WORDS:-Insect diversity - canopy soil - leaf litter
insecticide fogging - arthropod community structure.
~
tree trunks
-
-
global estimates
CONTENTS
Introduction .
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Knockdown insecticide methods
. . . .
Structure of the canopy arthropod community .
Taxonomic composition .
. . . .
Guild composition.
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Body size and biomass.
. . . . .
Temporal, altitudinal and other variation .
Number of arthropods in the five principal biotopes
How many species of arthropods are there? . .
Acknowledgements
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References. .
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INTRODUCTION
I t is now over 230 years since Linnaeus first started describing species of plants
and animals using the binomial system. For some groups, such as the birds, the
*This paper was originally read at a symposium meeting on I4 January 1988 and entitled “An inordinate
fondness for beetles; problems arising from the abundance of insects”, held jointly by the Royal Entomological
Society of London and the Linnean Society and which formed part of the Linnean Society’s Bicentenary
celebrations in 1988.
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0024-4066/88/120321+ 17 $03.00/0
01988 T h e Linnean Society of London
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N. E. STORK
task he initiated is almost complete, 9018 species having been described. Each
year an average of three ‘new’ species of bird are described from hitherto
inaccessible or poorly known parts of the tropics (Diamond, 1985). But for other
groups, an unknown and almost certainly high proportion of species await
discovery and description. It is appropriate that the present discussion should
focus on Haldane’s alleged comment on the Creator’s ‘inordinate fondness for
beetles’ since there are more described species of insects, let alone the unknown
number of undescribed species, than of all the other animals and plants combined.
At the 1977 Royal Entomological Society’s Symposium on Insect Diversity,
Southwood presented a pie chart representing the relative contributions of the
then 1.40 million described species of plants and animals (Southwood, 1978) and
he noted the striking dominance of the insects (57% of the described species). His
pie chart provides fuel for Haldane’s comment with the beetles representing 25%
of all described species. Today the relative proportions of described animal and
plant species remain largely the same, as shown in Fig. 1, but the total now stands
at more than 1.82 million (N. M. Collins, Conservation Monitoring Centre,
IUCN, personal communication). I n reality the figure of one million plus for
insects represents the number of species names available and not the number of
described species. Some may have been described accidentally more than once
while others may comprise a complex of barely distinguishable species. The
crucial question being asked by many entomologists, biologists in general and,
significantly, by conservationists, is how many species of insects and other
Described living species:
Total 1.8 million
invertebrates
Figure I . A pie-chart representing all of the 1.82 million described species of animals and plants
divided into the major groups (data supplied by Dr N. M. Collins, Conservation Monitoring Centre,
IUCNi.
INSECT DIVERSITY
323
arthropods are there? Until recently most believed 3-5 million species to be a fair
estimate (May, 1986) but in one well-publicised attempt to answering this
question, Erwin (1982) suggested that there could be as many as 30 million species
of tropical arthropods. The validity of this figure is discussed in the present paper.
It is clear that even if there are five million, or as many as 30 million, species of
tropical and other arthropods, the centre of diversity is in tropical rain forests.
Many reasons for this have been suggested (Price, 1984). One particular biotope,
the high canopy, is widely believed to hold the key to the immense species richness
of the insects. The inaccessibility of the high canopy, which for so long has
hindered many attempts at biological studies of its fauna and flora, led Erwin
(1983b) to term this biotope “the last biological frontier”. However, in the last ten
years the use of knockdown insecticides has revolutionized the study of canopy
insects and their community structure. In this paper the methodology and
protocol behind insecticide sampling in trees is summarized and an overview
presented of the structure of canopy insect communities as revealed by this
technique, in an attempt to summarize our present position in this expanding field
of biology.
Too many biologists have concentrated on the arthropod fauna of one
particular biotope such as the soil, leaf litter or canopy, and have failed to relate
their findings to the other component biotopes of rain forest; even the relative
abundance of arthropods in different forest biotopes has largely been ignored. If
the canopy does play such a vital role in arthropod diversity it is important that its
contribution is related to those of other forest biotopes. T h e preliminary results of
a recent study (Stork & Brendell, unpublished observations) investigating this
problem are presented and discussed in the light of data from similar studies.
KNOCKDOWN INSECTICIDE METHODS
Prior to the use of knockdown insecticides for sampling insects from trees,
information on the community structure of the canopy arthropod fauna was
usually anecdotal or derived from sampling methods which measured the activity
rather than real abundance of insects. Use of knockdown insecticides produces
samples that are largely independent of insect activity, and which can be related
directly to particular trees, known parts of trees or volume of canopy. Since the
technique is largely unaffected by changes in climatic conditions, samples
collected from different sites or at different times can be compared directly.
Two main kinds of sampling with knockdown insecticides dominate the study
of canopy arthropods: spraying and fogging. With the former, a fine spray of small
droplets is produced by injecting the insecticide solution into the air blast of a
large motor-driven fan. The whole unit is carried on the experimenter’s back and
the spray directed about 5-10m into the canopy. This technique has been
employed with considerable success in Britain and S. Africa (Southwood, Moran
& Kennedy, 1982a, b; Moran & Southwood, 1982). Fogging produces much
smaller droplets as the insecticide is broken up by injection into the exhaust fumes
produced by a small engine. Since the exhaust fumes are hot, the fog produced is
warm and rises through the canopy, given relatively still air conditions. The
longer ‘throw’ of the fog and its greater mobility, make this technique more
suitable than spraying for the study of taller trees. The additional use of ropes and
pulleys to hoist foggers into the canopy, and the use of radio-control devices for
324
N. E. STORK
Figure 2. A rloud of insecticide fog, produred by a fogger suspended from a branch by ropes and
pulley, rising through a 70 m high Bornean tree.
releasing the insecticide fog, have meant that sampling from the tops of trees over
70 m in height has been possible (Fig. 2) (Erwin, 1983a, b; Stork, 1987a, b;
Morse, Stork & Lawton, 1988; Stork, unpublished observations).
Synthetic pyrethroids are the most widely used insecticides for canopy sampling
(Martin, 1966; Gagnit & Martin, 1968; Gag&, 1979; Southwood et al., 1982a, b;
Moran & Southwood, 1982; Erwin & Scott, 1980; Erwin, 1982, 1983a, b; Stork,
1987a, b). These insecticides affect the nervous systems of insects so that usually
few insects fail to fall from the trees unless parts of the canopy are inadequately
fogged. Certainly some insects such as mealybugs or some psyllid nymphs die still
attached to leaves and some leaf miners, leaf rollers and insects burrowing in bark
or dead wood are rarely sampled by fogging. However, there is little evidence of
much loss due to insects flying away from fogged foliage and visual searches of the
latter suggest few surface insects remain.
STRUCTURE O F THE CANOPY ARTHROPOD COMMUNITY
In recent years many authors have looked at various aspects of the arthropod
fauna in trees. The results of Southwood and others (Southwood et al., 1982a, b;
Moran & Southwood) for temperate trees and Erwin (Erwin, 1982, 1983a, b;
Erwin & Scott, 1980) and Stork (Stork, 1987a, b; Barnard, Brooks & Stork, 1986;
Paarmann & Stork, 1987; Morse et al., 1988) for tropical trees are particularly
important in this context. This is not the place for a full review of the literature
325
INSECT DIVEKSITY
but rather a n opportunity to highlight the main areas of progress and I shall
therefore relate the results of some of these authors to those from my own study of
arthropods in Bornean trees. Here, ten trees representing five species and four
families were fogged using ropes and pulleys with samples being collected on
plastic sheets (Stork, 1987a, b, unpublished observations; Morse et al., 1988).
Taxonomic composition
More than 2,800 species were represented in the 24,000 arthropods collected
from the ten Bornean trees, the major groups for both individuals and species
being Hymenoptera, Coleoptera and Diptera (Fig. 3). I n other tropical and
temperate samples from trees these three groups and the Hemiptera are usually
the commonest groups. Ants dominate the Bornean samples with 4489 individuals
but only 99 species. In contrast, 1455 Chalcidoidea are represented by 739
species, 437 of which were singletons with only eight species having more than ten
individuals-the
commonest species having 19 individuals. No other species
analyses for complete tropical samples have been attempted by other authors but
data is available for numbers of individuals. The figure of 18.7% for the
Species
Individuals
20
15
10
5
5
10
15
20
25
30
Coleoptero
Diptero
For rn ic id a e
O t h e r Hyrnenoptera
Herniptera
Psocoptero
Thysonoptera
Orthopteroids
Arachnoids
Lepidoptera
Collernbola
Neuropteroids
Figure 3 . Histograms showing the perccntage of different arthropod groups in terms of species and
individuals in fogging samples from ten Bornean trees.
35
326
N. E. STORK
proportion of arthropods made up by ants in the Bornean samples seems low
compared with others of43.4% to 53.4% for various forest types in Brazil (Adis,
Lubin & Montgomery, 1984; Adis & Schubart, 1985; Erwin, 1983a) and 42.5%
for similar forest in Seram (see below and Stork & Brendell, unpublished
observations). In temperate samples from Britain and S. Africa ants usually
account for less than 10% of the fauna except on one tree species sampled (Robinia
pseudoacacia) in S. Africa, where they amounted to 58% (Southwood et al., 1982b).
Similarly, the total number of all arthropods collected per m 2 varies from 119.9 in
Borneo to 32.1--160.9 in Brazil, and an amazing 1200.6 in Seram. Temperate
samples also show similar variation.
Guild composition
A perhaps more informative view of the canopy arthropods can be gained by
looking at them in terms oftheir feeding habits or guilds (Root, 1967, 1973). T h e
guilds used here are phytophages (divided into ‘chewers’ and ‘suckers’),
parasitoids, ants, scavengers, epiphyte grazers, insect predators, other predators,
and ‘tourists’. In Fig. 4 the relative guild composition of samples from Borneo
(Stork, 1987a) are compared by means of pie-charts with pooled data from
Britain and S. Africa (Moran & Southwood, 1982). T h e remarkable feature of
these pie-charts is the similarity in overall composition of these tropical and
temperate samples for the species and dissimilarity for the individuals. T h e
similarity ofguild composition for species also extends, to some extent, on trees of
different species. Moran & Southwood found that some guilds (those with a clear
outer band on the pie-charts) have a significantly constant proportion of the total
species in temperate trees of different species. T h e same is true for the tropical
samples but not necessarily with the same guilds (Stork, 1987a).This rather neat
picture of relatively constant proportions of species in particular guilds for
different tree species, and across widely different latitudes, is tempered by Erwin’s
data on changes in the species composition of different guilds of beetles with
season for samples from trees of Luehea seemannii Triana & Planch (Erwin & Scott,
1980), as shown in Table 1. Although the relative contributions of the guilds are
similar in the early and late rainy season, the proportions of herbivore and
predator species are considerably different in the dry season. Without further data
is is impossible to say whether this is a consistent pattern for the complete guilds
including the other arthropods.
Body size and biomass
After looking at the abundance and species composition, perhaps the next stage
in an analysis of canopy communities might be to examine the body size
distribution. This can give an indication ofthe way in which the species utilize the
canopy in terms of species-packing and also the biomass of the fauna since the
latter is highly correlated with body-length (Rogers, Hinds & Buschbom, 1976;
Rogers, Buschbom & Watson, 1977; Schoener, 1980). As Lawton (1986) noted,
the idea that plant architecture has a large role to play in determining the
diversity and abundance of insects is now well established. He and Morse,
Lawton, Dodson & Williamson (1985) suggested that one way of predicting the
abundance ofdifferent sized insects on a plant was to use a measure ofthe surface
INSECT DIVERSITY
327
.
.
. .
- .
- .
.
.
.
.-.
Temperate
Tropical
Figure 4. Comparisons of the guild composition of the arthropod fauna in fogging samples from
temperate and tropical trees: A, species; B, individuals. 'Temperate data for broad-leaved S. African
and British trees from Moran & Southwood (1982) and tropical data for Bornean trees from Stork
(1987a). C-chewers, S-suckers ( C S = phytophages), T-'tourists', A-ants, P-parasitoids, IPpredators that are insects, OP-predators other than insects (arachnids), SC-scavengers, E-epiphyte
feeders. [Note the labels were accidentally transposed for insect predators and other predators in
Fig. 1 of Stork, 1987al.
+
TABLE
1. T h e numbers of beetle species and their percentage contribution t o
t h e total beetle species in different guilds as represented i n insecticide fog
samples from Luehea seemannii 'I'riana & P l a n c h (Leguminosinae) collected a t
different times of t h e y e a r ( D a t a from E r w i n & Scott, 1980)
Season
Early rainy
Guild
Herbivores
Fungivores
Predators
Sravengers
Late rainy
Dry
No. spp.
"/o
No. spp.
yo
No. spp.
:L
324
48
167
70
53.2
7.9
27.4
11.5
217
35
100
33
56.4
9.1
26.0
8.6
93
12
99
28
40.1
5.2
42.7
12.1
N . E. W O R K
328
Log abundance
per species
E
:
Log body size
0
Figure 5. A three-dimensional graph plotting number ofspecies against each log body length class and
against total log abundance for all beetles (from Morse, Stork & Lawton, 1988).
complexity of the plant known as its fractal dimension. To illustrate their point
they used fogging data on species abundance of beetles found on L. seemannii
(Erwin & Scott, 1980) and fog sample data from birch trees. How this relates in
practice to the number of species was illustrated in a three dimensional figure by
examination of the relationship between body size and species abundance of the
beetles from the Bornean fog samples discussed above (Fig. 5) (Morse et al., 1988).
The surface of this 3-D figure is relatively simple, the commonest species being of
medium size and low abundance. T h e theoretical basis for this figure has not been
postulated and further studies are required. A number ofauthors (Damuth, 1981;
Peters, 1983; Peters & Raelson, 1984; Peters & Wassenburg, 1983; Brown &
Maurer, 1986), have noted a simple linear relationship between population
densities of different species and body-weight of various groups such as mammals
and invertebrates (not including insects). In other words small species live at
higher population densities than larger ones. For the Bornean fog samples no such
relationship was found between insect species abundance and body-size (which
are equated with population density and body-weight). Since no (or only weak)
linear relationships were found between population density and body-weight for
birds, it is probable that in this group and winged insects the ability to fly means
that small species can occur in low population densities.
Temporal, altitudinal and other variation
The main drawback of much of the data on community structure discussed
above is the exclusion of factors such as temporal and altitudinal variation. We
know that considerable fluctuations in population sizes of insects occur on
temperate and tropical trees, often coinciding with leaf flush. The data from
INSECT DIVERSITY
329
Figure 6 . Relative proportions of individuals in insecticide fog samples from ten similar oak trees
(Quercus robur L.) in Richmond Park, U.K. for the following groups: Coleoptera (a), Diptera ( b ) ,
Heteroptera (c), Homoptera (d), and Hymenoptera (e).
studies in Brunei, Brazil, S. Africa and Britain represent the canopy fauna at
particular unrelated points in time and it is difficult to compare them when the
trees sampled have different forms and seasonality. Excluding such major
problems as seasonal, altitudinal and latitudinal variation, what faunal variation
can be observed between individual, apparently identical, trees of the same
species? Within one tree species, factors such as tree age, height, canopy depth,
breadth and volume, trunk girth and proximity to the next tree of the same
species, may all have considerable bearing on the faunal composition. I n an
analysis of some of the factors contributing to the faunal similarity of the ten
Bornean trees it was found that, for many groups (even some phytophagous
groups), taxonomic relatedness of the trees was not an important factor, but that
the distance between trees or the similarity in amount ofvines and other epiphytes
was more important (Stork, 1987b).
With so many factors influencing the composition of the canopy fauna it is
important to establish a base level for further comparisons by examining the
degree of variation between trees of one species which are, to all intents and
purposes, virtually identical in age, height, form and more or less evenly spaced.
One such set of ten oak trees (Quercus robur L.) was fogged in H a m Cross Wood in
Richmond Park (U.K.) (18 August 1983, fog time 18.45-19.15, drop time 1.52.0 h ) , with samples from each tree being collected on two 2.5 X 4.0 m plastic
sheets placed on the short grass beneath the trees (Stork, unpublished). The
number of the arthropods collected for each tree varied considerably, the
difference between the trees with the highest and lowest numbers of individuals
being a factor of about six. The same was true for biomass (calculated as dry
weight). Much of this variation must be due to differences between the trees in the
amount of canopy above the sheets. In spite of this variation in abundance, the
relative proportions of most arthropod groups in each tree are similar as is shown
for five groups (Coleoptera, Diptera, Heteroptera, Homoptera and
Hymenoptera) in Fig.6. However, the numbers of some groups, such as the
Psocoptera, are considerably influenced by the position of the plastic sheets in
relation to the tree trunk. Similar analyses for species were not made except for
330
N. E. S’I’ORK
the Coleoptera where the number of species per tree sample varied from 7 to 24.
However, in spite of such large variation in the number of species and also
number of individuals, the diversity is fairly similar for the different samples. For
instance, the mean Simpson Index of diversity for the beetles in the tree samples of
0.203 has a low standard deviation of 0.093.
The preference of some species for tree trunks, alluded to above, is only one of a
number of factors that affect the distribution of species within trees. Others, such
as preference for different canopy levels, exposed or concealed parts of the canopy,
sunny or shaded parts, or preference for different parts of the tree at various times
of the day or year, may all contribute to the differences noted in sampling (Stork
& Hammond, unpublished observations).
In further studies (Stork & Hammond, unpublished observations) no clear
differences were discernible in the species composition for the beetles on oak trees
of different age (trees compared of 250+ and 100 years old) although the sample
size was low (total of four trees). However, the crepuscular activities of many
beetle species contributed to the presence of many more ‘tourist’ species (sensu
Moran & Southwood, 1982; Stork, 1987a) in fog samples taken in the evening
than those taken in the morning.
NUMBER O F ARTHROPODS IN T H E FIVE PRINCIPAL BIOTOPES OF A LOWLAND RAIN
FOREST
The importance of the canopy, in terms of its contribution to the arthropod
fauna of a forest as a whole, has been emphasized, either explicitly or implicitly,
by several authors (Erwin, 1983a, b; Southwood, 1978) and yet few studies have
established the true dimensions of this contribution. Of recent authors, only Adis
& Schubart (1985) have investigated this question. In their study they compared
the arthropod faunas of several biotopes in seven Amazonian forest types. Soil
arthropods were extracted from soil cores using a modification of the Kempson
extraction technique (Kempson, Lloyd & Ghelardi, 1963), and canopy
arthropods were collected by fogging. In addition, they used ground and arboreal
photo-eclectors to determine the relative activity of arthropods.
A more extensive survey was carried out in the lowland rain forest of Seram
(Indonesia)in 1987, and included the five major biotopes of that particular forest:
the canopy, tree trunks, ground and low vegetation, leaf litter, and soil (Stork &
Brendell, unpublished observations). Canopy samples were obtained by fogging
using a suspended tray system to collect the samples (Stork & Hammond,
unpublished; Stork, unpublished) and arthropods were collected from separate
leaf litter and soil core samples with Tullgren funnels. Arthropods were collected
from the low vegetation (from ground level up to 2 m) using a DVac suction
device, and from tree trunks using insecticide spraying, with samples from the
latter being collected on trays folded around the bases of the tree trunks. The
samples were sorted to Order and counted. T h e body size distribution and
biomass of the arthropods in the five biotopes are considered in a separate analysis
and are not discussed in the present paper (Stork & Brendell, unpublished
observations).
In Fig. 7 the relative numbers of arthropods in the different samples are
displayed as pie charts with three groups being highlighted; Formicidae (ants),
Collembola (springtails) and Acarina (mites). T h e predominant features of these
I N S E C I DIVERSITY
Tree trunks
33 1
Ground vegetarlon
ranea and Opiliones
Psocoptera'
Leaf l i t t e r
sol1
Figure 7. Pie-charts far tree trunks, ground vegetation, canopy, leaflittcr and soil showing the d a t i v e
contributions of the major arthropod groups, in terms of numbers of individuals, for a n area of
lowland rain forest in Seram, Indonesia in August 1986. (See text for further details.)
N . E. STORK
332
A
B
Thysonoptera
Collernbolo/
Ground v e c l e t o t i o w
Tree trunks
I// /
/\-Coleoptera
Acorina
Opiliones
Figure 8. Pie-charts for the relative proportions of the estimated 42.3 million arthropods in a hectare
of Seram rain forest (calculated from data resulting from soil, leaflitter, ground vegetation, tree trunk
and canopy samples): A, the contributions of the five major biotopes: B, contributions of the major
arthropod groups.
pie-charts are clear. T h e Collembola dominate the soil and litter samples with the
mites forming the major component of the remaining arthropods. These two
groups are also important in the ground vegetation and tree trunks but form only
a minor portion of the canopy fauna. Almost the reverse is true of the ants which
are particularly dominant in the canopy but also comprise an important
component in the other four biotopes. The significance of this data is shown more
clearly in Figs 8A-B. Since all the sampling methods are quantitative it is possible
to estimate the number of arthropods each biotope contributes to a given area of
the tropical forest being sampled. T h e number of canopy arthropods in a hectare
of forest (10,000 m 2 ) is equal to the mean number of arthropods per fogging tray
multiplied by 10 000. Similar calculations were carried out for the soil, leaf litter
and ground vegetation arthropods. For the tree trunk fauna a guess of 800
equivalents to the tree trunk spray samples per hectare was used in the
calculations. All of these sampling methods miss a part of the arthropod fauna of
the biotopes they sample and so their contributions to the forest arthropod fauna
may be underestimated. By these calculations the total number of arthropods
contributed by these biotopes to a hectare of forest is 42.3 million comprising (in
millions) : soil-23.7, leaf litter-6.0, ground vegetation-0.1, tree trunks-0.5,
canopy-12.0. What is perhaps most surprising about these figures is the small
contribution of the ground vegetation and tree trunks and the large contribution
of the canopy. Fig. 8B, which gives a breakdown of the arthropod composition in
the hectare of forest, shows that the springtails and mites make the largest
contribution and are the underlying reason for the dominance ofthe soil and litter
fauna. Most startling of all is that these estimates indicate that the ants virtually
outnumber all the other insects (excluding Collembola) combined and are as
numerous as the mites. In spite of the notable absence of Isoptera (termites),other
data suggest that addition of arthropods from other forest biotopes, such as fallen
trees, would not seriously affect the overall picture (Stork & Brendell,
unpublished observations).
It should be noted that these estimates are from a small data set, for one small
area of forest and again only represent the picture at one point in time. T h e
INSECT DIVERS I TY
333
density of arthropods in the canopy samples is 10 times that of the Bornean and
Brazilian samples mentioned above but this may not be typical for rain forests. Do
the soil and leaf litter arthropods show similar extreme fluctuation in numbers
and do the relative contributions of the different biotopes to the total arthropod
fauna of rain forest change with season? T o answer these sorts of questions, it is
important that similar but long-term studies are carried out, comparing, at both
the Order and species level, the faunal contribution of arthropods of different sites
at different times of the year.
HOW MANY SPECIES OF ARTHROPODS ARE THERE?
Although most general texts on entomology give round estimates for the
number of described species in the different insect orders, few are bold enough to
estimate the number of undescribed species. Sabrosky ( 1952) suggested that
estimates range from 2.5 to 10 million and Arnett (1985) considered that there
was probably double the number of described species at that time. I n the main
these are purely intuitive guesses which often have as much value as some of the
more mathematical approaches to this problem. Frank & Curtis (1979) showed
that trend line analysis, based on the rates of description of taxa through time, is a
poor method of estimating numbers of species. They suggested that using numbers
of known species before and after a number of revisionary studies can produce
realistic figures. Such methods are acceptable for groups where considerable
taxonomic effort is being applied but not for the others. May (1986) summarized
a number of approaches to the problem of global estimates. He suggested that the
majority of the million described species of insects are from the temperate region
and that if there are three times as many tropical species as temperate species, as
found in birds, for instance, then there are 3-5 million species of insects
worldwide.
One incredible estimate of 30 million species (Erwin, 1982) has become wellpublicised but as yet has not been critically analysed by other authors. Although
May (1986) expressed some reservations about the various assumptions made by
Erwin, he noted his admiration for Erwin’s work: “Erwin’s work is important in
focusing concern on this huge class of animals that has so long been neglected”
and “his work (which) has advanced us to the point where we can begin
systematic investigation of each of the links in his chain of argument; in
comparison, the earlier estimate of 3-5 million is pure hand-waving”. Since I am
one of the few people with equivalent data to that of Erwin, I am perhaps as well
placed as any to tackle the question of global diversity and to appraise Erwin’s
estimate.
Erwin’s calculations are simplified in Table 2 and as follows. He sampled the
arthropods from 19 trees of the species Luehea seemannii Triana & Planch by
fogging, with the tree foggings being carried out in the three main seasons
recognised in that part of the Panamanian forest. After sorting the beetles to
1200+ species and assigning them to guilds, he used guesses of host specificity
levels for the herbivores, fungivores, predators and scavengers of 20, 10,5 and 50/,
respectively, to arrive at an estimate of 163 beetle species specific to L. seemannii in
the fog samples. He suggests that there are an estimated 50,000 species of tropical
trees and that all may have the same number of host specific beetles. Erwin
further suggests that beetles comprise 40% of the canopy arthropod species and
334
N. E. STORK
TABLE
2. Summary of Erwin’s method of estimating 30 million species of tropical arthropods
(Erwin, 1982)
..........
I . Number of beetlc species fogged from 19 L . seernannii trees. . .
1200
.........................
163
..............................................
8 150000
..............................................
20000000
5. Canopy twice as rich as ground in specks of arthropods, therefore total number tropical rain
fbrest arthropods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30000000
that the canopy fauna is at least twice as rich in species as the ground fauna. By his
estimation then, there are a total of 30 million species of tropical arthropods.
There are a great many points that could be made about this estimate and some
of the more important ones are discussed here. Erwin produced no evidence to
indicate that he had reached a plateau for the number of beetle species to be
collected from L. seernannii. If he had continued to sample further trees the beetle
species total might have been considerably higher. It should be remembered that
trees have their own plant communities of lichens, vines, bromeliads, ferns and
other epiphytes, and that some insects may be associated with these latter plants.
For the ten Bornean trees discussed above, for instance, the similarity of their
chrysomelid beetle fauna was related more to similarity in the amount of vines
and epiphytes on the trees than to the taxonomic relatedness of the trees (Stork,
198713). Furthermore, legumes are probably common trees in Central American
forests and are favoured as hosts by at least some insect groups. Some moths are
more frequently recorded from Leguminosinae than almost all other tree families
combined (Table 3) (Holloway, 1987). Perhaps the most contentious part of his
calculation is that concerning host-specificity. T h e concept of host-specificity he
has used is misleadingly simple and, since it is not an ‘either/or’ phenomenon, all
possible gradations of host-specificity are likely to exist. Not all insects are specific
to just one species of plant; some may be specific to a species group, genus, tribe, or
family of plants, or be non-specific. Ideally it would be gratifying to be able to
apply a sliding scale of insect host-specificity for the different taxonomic levels of
plants but information leading to such does not exist. However, since insect
species specific to single tree species arguably comprise the largest portion of all
host-specifics, manipulations of Erwin’s simplified concept of host specificity may
provide at least some insight into possible upper and lower limits. Reducing the
host-specificity level of the herbivores has a much more dramatic effect on the
ABLE 3. ‘I’hc number of species of some moth groups recorded on difkrcnt families of trees (from
Holloway, 1987)
‘l‘rcc families
Lcgumiriosiriar
Avcragr no. spp for 1 1 other plant hmilies
Bornco
Lasiocampidae
Costa Rica
Non-feeding big moths
15.0
4.5
34.0
4.8
INSECT DIVEKSI'I'Y
335
TABLE
4. Estimates of numbers of beetle species specific to Luehea
seemannii with various levels of host-specificity for different
guilds. H-% host-specificity, S-number of species specific to
L. seemannii
No. of
species
H
S
H
H
S
~
Herbivores
Predators
Fungivores
Scavengers
Total
682
20
136
296
5
15
6 9 1 0
7
9
6
5
5
1143
163
10
1
1
1
S
~~
68
3
1
1
73
5
1
1
1
34
3
1
1
39
TABLE
5. Estimates (in millions) of the number of beetle
species specific to tropical trees with the three different
levels of host specificity used in Table 4
Herbivore specificity
Beetle spp. specific to:
L. seemannii
50 000 Tropical tree spp.
163
8.15 m
72
3.60 m
39
1.95 rn
TABLE
6. Estimates (in millions) of the number of arthropod species
in the canopy of tropical rain forests based on the number of beetles
species and the proportion beetles comprise of the canopy fauna
Estimates of beetle spp. in the canopy
Beetles as [;lo
of the canopy fauna
40%
20':.o
-.
8.15
1.95
20.36
40.72
4.88
9.75
total than for the other guilds (Table4). Even with a figure of 5% for the
herbivores and 17; for the other guilds, the number of beetle species specific to
L. seernannii is 39.
Erwin's suggestion that the beetles represent 40% of the canopy arthropod
species is almost certainly a considerable over-estimate. I n British and S. African
trees the beetles represented only 7.2% and 15.6%, respectively (Southwood et al.,
1982b) and in Bornean trees only 22.9% (Stork, unpublished observations). In
this respect a figure of 20% is perhaps a moderate guess (Table 6). Similarly,
Erwin's evaluation of the relative species composition of the canopy and the
ground fauna is probably biased towards the canopy (Table 7). It is more likely
that there are even more species in the ground fauna than the canopy
(Hammond, personal communication).
Taking all these factors into account a range of possible figures can be
calculated. At the more liberal end of the scale estimates of over 80 million can be
N. E. STORK
336
TABLE
7. Estimates (in millions) of numbers of a r t h r o p o d species i n
tropical r a i n forests based on estimates of t h e c a n o p y f a u n a (from
T a b l e 6) i n relation to t h e ground f a u n a
Canopy species
Ratio of ground
to canopy species
40.7
20.4
9.8
4.9
1:2
1:l
61.1
81.4
30.5
40.7
14.7
29.4
7.3
9.8
achieved and at the lower, more conservative end, a figure of 7 to 10 million is
possible. T o arrive at real global estimates several other groups need to be
included: non-specific species on trees, freshwater species and temperate species.
Addition of these provides added weight to the validity of a lower estimate of 10
million. Such a figure provides fuel for the worries of conservationists but also
provides real problems for humble taxonomists and curators. If it has taken 230
years since Linnaeus to describe one million species of insects what chance d o we
have of describing the other 9 (or 79!) million, even with the help of desktop
computers? Given that the British Museum (Natural History) fills a six-floor
building with representatives of about 0.5 million species of insects, where would
we house representatives of the rest, plus the associated literature? With these
kinds of problems taxonomists and curators have to take their heads out of the
sand and adjust their priorities for future collection building and research
programmes.
These figures are of even greater significance to those concerned with habitat
and species conservation. Recent studies indicate that 7.4 million hectares
(equivalent to the area of Scotland or the State of Maine) of tropical forest is
being converted into other types of vegetation every year (Holdgate, 1987). How
many species of arthropods are also lost every year? T h e methods of estimating
global numbers of insect species discussed above may still be ‘hand waving’ but
perhaps more effort and greater resources should be funnelled towards answering
this question.
ACKNOWLEDGEMENTS
I am grateful to C. H. C. Lyal for comments on this paper and K . M. Shaw for
statistical advice. I thank M . J. D. Brendell for fieldwork and technical assistance.
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