Predictions of Mammal Diversity on Four Land Masses
RODRIGO A. MEDELLÍN* AND JORGE SOBERÓN†
*Instituto de Ecología, UNAM. Ap. Postal 70-275, Mexico, D.F., 04510, Mexico,
email [email protected]
†Comisión Nacional para el Conocimiento y Uso de la Biodiversidad, Fernández Leal 43, col. Barrio de la Concepción,
Coyoacán, 04020, Mexico, D.F., Mexico
Abstract: Knowledge about the number of living species is fundamental to the practice of conservation biology, but we are far from knowing the number of species even for relatively well-known groups such as mammals. We analyzed a database that contained information on the year of description, distribution, size class,
and taxonomic order of each known mammalian species. Our goal was to predict the size class, order, and
number of mammalian species on each of four land masses—America, Eurasia, Africa, Oceania. We constructed cumulative functions of species described versus year of description, and we fitted a logarithmic
model to calculate instantaneous rates of species description in 1992 and 2032 and to estimate the number of
species to be described in each group in the 40-year interval. Africa and Eurasia had the highest absolute
number of species today and the greatest number of species expected to be described in species-rich orders (except for Rodentia, in which the highest number of known and expected species was in the American land
mass). Per unit area, Oceania had the highest number of known species and the greatest number of species
expected to be described. Our estimates, based on the historical patterns of species accumulation, place the total number of mammalian species in the year 2032 at 4875, which is 247 more than in 1992. The majority of
the new species will be small (,100 g) and will be in the orders Insectivora, Chiroptera, and Rodentia. Our
analyses will help identify particular groups and land masses for which field surveys, conservation efforts,
and taxonomic study deserve particular attention.
Predicciones de Diversidad de Mamíferos en Cuatro Masas Continentales
Resumen: El número de especies vivas es una pieza de información clave para la biología de la conservación y la toma de decisiones. Aún estamos lejos de conocer el número exacto de especies vivientes incluso
para un grupo relativamente bien conocido como los mamíferos. Analizamos la base de datos formada por
el año de descripción, la distribución regional, el tamaño corporal y el orden taxonómico de todas las especies conocidas de mamíferos. Nuestro objetivo fue predecir el número total de especies de mamíferos en el
mundo, y para cada región del mundo, clases de tamaño y orden taxonómico. Construímos funciones acumulativas de descripción de especies contra año de descripción y les ajustamos un modelo logarítmico para
calcular las tasas instantáneas de descripción de especies en 1992 y en 2032 y el número estimado de especies
que serán descritas en cada grupo en el lapso de 40 años hacia el futuro. Nuestras estimaciones, siguiendo los
patrones históricos de acumulación de especies, colocan el total de especies de mamíferos en el año 2032 en
4875, 247 más que en 1992. La mayoría de las especies nuevas serán pequeñas (,100 g) y pertenecerán a
los órdenes Insectivora, Chiroptera y Rodentia. Africa y Eurasia presentaron los números absolutos más altos
de especies conocidas y los mayores números de especies esperadas en los órdenes grandes (excepto Rodentia;
el más alto número de especies conocidas y esperadas fue en América), pero por unidad de área Oceanía arrojó los mayores números de especies conocidas y esperadas. Nuestros análisis contribuyen a enfocar estudios
de reconocimiento, esfuerzos de conservación y a poner especial atención a trabajar en la taxonomía de grupos particulares en regiones determinadas.
Paper submitted August 6, 1997; revised manuscript accepted March 31, 1998.
143
Conservation Biology, Pages 143–149
Volume 13, No. 1, February 1999
144
Predictions of Mammal Diversity
Introduction
Knowledge about the number of extant species is fundamental for scientific purposes and for decision-making
processes related to conservation and sustainable development. Questions about numbers of species and factors
determining species richness have been asked for decades (Hershkovitz 1958; Hutchinson 1959; Hagmeier
1966; Paine 1966; Whittaker 1972; Cody 1975; Schall &
Pianka 1978; May 1990). Estimates of the number of living
species vary widely, from 5 to 50 million or more (U.S.
Congress 1987; Wilson 1988; McNeely et al. 1990; Hawksworth et al. 1995), with the uncertainty concentrated in
groups such as fungi, insects, and nematodes. Even for
groups considered well known, the number of species
they contain and their geographic distributions remain incomplete. Mammals represent such a group, despite the
fact that they are relatively large animals, they are charismatic species that people find easy to relate to, and they
have ecological, scientific, cultural, and practical importance. Furthermore, mammals were among the first
groups of animals to be studied by early naturalists; therefore, the number of species should be well known. Nonetheless, we are far from knowing the real number of mammalian species that exist (Morell 1996).
Although we recognize 4628 species (Wilson & Reeder
1993), some nonquantitative estimates of species to be described after revision alone—not counting description of
morphologically distinct species (e.g., Morell 1996)—place
the total expected number at 8000. Of the 459 species
added in the 10 years between the two editions of Mammal Species of the World, only 37% were new morphotypes unknown to science; the remainder were sibling spe-
Medellín & Soberón
cies separated thanks to technological advances. Over half
of the known species are small, with a body mass of under
100 g. These are much less conspicuous than larger species, so a straightforward prediction is that most unknown
mammal species will weigh under 100 g. Small mammals,
however, belong to at least three mammalian orders distributed on at least three of the four land masses we considered
(America, Africa, Eurasia, and Oceania): insectivores live in
America, Africa, and Eurasia, and rodents and bats live in all
four. Due to their differential evolutionary origin and radiation, we expect that these orders will differ in the number
of species predicted to be described in each land mass.
At a regional scale, the tropics harbor many more species than temperate areas of the same size, and countries
exceptionally rich in species (“megadiversity” countries)
are almost all included within the tropics and contain
over one-half of the world’s species (Wilson 1988). Thus,
temperate countries tend to contain far fewer species
than tropical ones, even though tropical areas are much
smaller. For example, Costa Rica (area: 322,460 km2) contains at least 203 species of mammals, 796 species of
birds, and 218 species of reptiles, whereas Canada (area:
9,221,000 km2 ) has 94 species of mammals, 434 species
of birds, and 32 species of reptiles (U.S. Congress 1987;
McNeely et al. 1990). Similarly, insularity affects number
of species through endemism, and regions containing numerous islands have more species than a continuous area
equivalent in size, climate, and topography.
Our goal was to generate numerical estimates of the
expected number of mammal species to be added to
current inventories through the year 2032. We calculated this number for different groups (taxonomic and
size class) and land masses.
Methods
Figure 1. Cumulative curve of species description and
fitted model for all land mammal species of the world.
The curve and model start in the year 1890 (year 0 in
the plot). The number of species in that year is 2364
(species 0 in the plot). Inset is the complete cumulative
curve starting in 1758. Because of the shape of the
curve, the model cannot be fitted to the complete
curve.
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Volume 13, No. 1, February 1999
We used Wilson and Reeder’s (1993) comprehensive
listing of mammal species, taxonomic arrangement, type
localities, and year of description to generate the initial
database. We assigned each type locality to one of four
land masses: America, Eurasia, Africa, and Oceania. We
did not separate North from South America or Europe
from Asia, and we lumped Oceania (including Australia)
to avoid confusion in calculating the number of species
inhabiting each land mass. With this arrangement we
could assume that, with only a few exceptions, each
species was endemic to one of those four land masses.
The few species occurring in more than one land mass
were assumed to be endemic to the land mass of their
type locality and absent in the others. We also assigned
each mammal species to one of five size categories: 1,
,100 g; 2, .100 and ,1000 g; 3, .1000 and ,10,000
g; 4, .10,000 and ,100,000 g; and 5, .100,000 g.
To simplify analyses, we pooled the four orders of
Oceanian marsupials and the three orders of American
Medellín & Soberón
Predictions of Mammal Diversity
marsupials into a single “order” because these orders are
more closely related than others, and because until recently they were in fact considered a single order. We
did not include Cetacea, Otariidae, Odobenidae, or Pho-
145
cidae because we cannot assume that marine species are
endemic to one region. To estimate species richness and
the degree of completion of inventories, and to provide
an assessment of the rate of description of new species,
Table 1. Land mass north-south latitudinal span, surface area, mammal species groupings used, equation parameters,a annual rates of species
descriptiona in years 1992 and 2032, known number of species in 1992, and predicted number of species in year 2032 of each mammal group.
Land mass
(latitude span
in degrees)
Surface area
(km2)
America (136)
42,081,000
Eurasia (90)
Africa (72)
Oceania (72)
54,430,000
30,271,000
8,503,195
Mammal species
grouping
zb
ac
Annual
description
rate in 1992
Annual
description
rate in 2032
Number of
known species
(1992)
Species
predicted
in 40 years
4875
1608
953
390
214
38
13
67
27
80
290
84
75
28
922
Entire world
Total
size 1
size 2
size 3
size 4
size 5
Didelphimorphia
Lagomorpha
Insectivora
Chiroptera
Primates
Carnivora
Artiodactyla
Rodentia
0.001
0.00375
0.0055
0.0175
0.0371
88.38
48.758
36.29
11.25
2.191
8.53
2.48
1.70
0.53
0.24
6.29
1.83
1.25
0.39
0.18
0.0743
0.651
0.0647
0.015
0.066
0.209
0.411
0.00609
1.434
7.914
3.239
3.987
0.301
0.622
0.113
39.966
0.12
0.02
0.14
0.56
0.10
0.04
0.02
1.55
0.09
0.01
0.11
0.43
0.08
0.03
0.02
1.14
4628
1538
905
375
207
38
13
63
26
74
280
84
73
27
868
Total
size 1
size 2
size 3
size 4
size 5
Insectivora
Chiroptera
Primates
Carnivora
Artiodactyla
Rodentia
Lagomorpha
0.00299
0.00384
0.0146
0.17
0.22
0.26
0.015
0.0126
0.282
0.743
0.098
0.0056
0.117
20.305
12.41
6.026
3.19
0.89
0.09
2.41
4.339
1.4
1.126
0.24
10.96
0.324
2.82
2.12
0.60
0.06
0.51
0.66
0.03
0.01
0.07
1.51
0.07
2.39
1.86
0.49
0.04
0.03
0.03
0.48
0.57
0.03
0.01
0.07
1.28
0.06
1463
818
322
171
104
48
166
337
62
96
99
621
44
1543
881
341
171
104
48
186
356
63
96
99
663
44
Total
size 1
size 2
size 3
size 4
size 5
Insectivora
Chiroptera
Primates
Carnivora
Artiodactyla
Rodentia
Lagomorpha
0.0044
0.00486
0.047
0.107
0.462
21.97
13.88
5.34
3.029
5.18
2.02
1.76
0.20
0.09
0.02
1.51
1.33
0.15
0.07
0.01
0.0115
0.022
0.232
0.22
0.33
0.009
3.077
4.04
2.91
1.38
7.36
10.39
0.67
0.40
0.04
0.04
0.03
0.99
0.52
0.30
0.03
0.03
0.02
0.74
1069
645
183
123
79
39
188
195
87
67
94
394
10
1129
693
193
125
81
39
201
209
87
69
94
425
10
Total
size 1
size 2
size 3
size 4
Chiroptera
Marsupials
Rodentia
0.0056
0.0056
0.026
0.065
3.97
1.67
1.36
1.09
1.21
0.85
0.30
0.13
0.97
0.72
0.23
0.10
0.014
0.0199
0.014
0.78
1.286
1.93
0.37
0.36
0.51
0.31
0.28
0.40
451
208
126
97
20
113
203
132
474
208
132
99
20
122
204
146
a
Parameters and rates are given only in those cases in which there was an increase in number of species.
The z is related to the slowing down of description rate.
c
Rate of description of new species near the origin.
b
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Volume 13, No. 1, February 1999
146
Predictions of Mammal Diversity
Medellín & Soberón
Figure 2. Cumulative curves of species description and fitted models of four of the five size categories of mammals
analyzed on four land masses. The curves start in the year 1890 (year 0), when the number of species was treated
as zero. Real numbers of species in that year in each size category and land mass were, respectively, body size 1, 2,
3, 4, 5: 342, 201, 146, 34, 13, America; 350, 164, 146, 87, 44, Eurasia; 210, 116, 90, 68, 36, Africa; 77, 63, 62, 19, 0,
Oceania. Numbers for size 4 were too small to attempt adjusting a continuous model.
we used cumulative curves as a tool to model the process of adding new species (Soberón & Llorente 1993;
Colwell & Coddington 1995). We generated cumulative
curves of species as a function of time elapsed since the
beginning of modern scientific explorations for each
land mass and within each land mass for each of the size
categories and each mammalian order with more than
60 species. Most such curves have a pronounced S
shape because from 1758 to about 1850 the number of
species accumulated slowly; subsequently, an explosive
period of new descriptions lasted from about 1875 to
1930. Later, the addition of species slowed significantly
as the number of species approached a hypothetical
“true” number in a given land mass and category (Fig. 1).
To fit the accumulation equations to those data, after
1890 we ignored early data and used only points in the
convex portion (Fig. 1).
Conservation Biology
Volume 13, No. 1, February 1999
The fitted equations were then used to extrapolate to
a standard and arbitrary 40 more years of collecting effort, thus fixing predictions of species numbers to 2032.
Instantaneous rates of species addition were also obtained from curves. Curves were fitted to only those data
sets with enough species to be modeled by a continuous
model. The equation used the logarithmic model because, as discussed by Soberón and Llorente (1993), in a
large area the probability of adding a new species to a
collection decreases without becoming zero for a long
period of time. This is well modeled by the following
equation:
S = ( 1 ⁄ z )log ( 1 + zaT ),
(1)
where a is the rate of species description near the origin, z is another parameter related to the slowing of that
rate, and T is the time in years.
Medellín & Soberón
Predictions of Mammal Diversity
Table 2. Number of species of land mammals for each of the five
size categories in each land mass, corrected for number of species
per 100,000 km2, and number of species expected to be described in
the next 40 years.
Land massa
Size
Number
category of species America Africa Eurasia Oceania Total
1
2
3
4
5
Total
Absolute
Corrected
Expected
Corrected
Absolute
Corrected
Expected
Corrected
Absolute
Corrected
Expected
Corrected
Absolute
Corrected
Expected
Corrected
Absolute
Corrected
Expected
Corrected
Absolute
Corrected
Expected
Corrected
905b
2.15
48
0.11
375b
0.89
15
0.04
207b
0.49
7b
0.02b
38
0.09
0
0.00
13
0.03
0
0.00
1538b
3.65
68
0.16
645
2.13
49
0.16
183
0.60
10
0.03
123
0.41
2
0.01
79
0.26b
2b
0.01b
39
0.13b
0
0.00
1069
3.53
59
0.19
818
208
2576
1.50 2.45b
61b
20
178
0.11 0.24b
322
126
1006
0.59 1.48b
19b
6
50
0.03 0.07b
171
97
598
0.31 1.14b
0
2
11
0.00 0.02b
104b
20
241
0.19 0.24
0
0
2
0.00 0.00
48b
0
100
0.09 0.00
0
0
0
0.00 0.00
1463
451
4521
2.69 5.30b
78b
22
227
0.14 0.26b
a
Land mass areas per 100,000 km2: America, 420.81; Africa,
302.70; Eurasia, 544.30; Oceania, 85.03; total, 1352.84.
b
Highest value in each row.
Results and Discussion
The analysis covered an interval of 234 years, from the
establishment of the Linnean system of nomenclature
(1758) through 1992. When the total time period is examined, the cumulative curve for all mammal species
(Fig. 1) is not near the asymptote; in fact, the section of
the curve depicting species descriptions in the last 70
years alone is linear and has a steep slope. The nonlinear
fitting, however, reveals a significant curvature of the accumulation process. Over the last 15 years the process
accelerated again because of the advent of newer taxonomic tools, such as karyology and molecular genetics
(Baker 1984; Dipenaar & Rautenbach 1986; Baker et al.
1988), although it is also true that many species with distinct morphotypes are still being described (Flannery
1990; Reyes et al. 1991).
Based on our analysis, we are still far from knowing
how many species of mammals there are, but we may be
able to predict where and in what groups new species
are most likely to be found. Our analysis by size class indicates that small mammals are much less well known
than any other size group, as expected. For example, in
Africa there are now 645 mammal species of size class 1,
and in 40 years we expect there will be approximately
147
693, an increase of 7.4% (Table 1; Fig. 2a). There are,
however, 183 mammals of size class 2, and in 40 years
we predict there will be 193, a 5.5% increase (Fig. 2b).
There are 123 species of size class 3, and we predict
only two species to be described in the next 40 years
(Fig 2c). There are 79 species of size class 4, and two
will be added; none will be added beyond the 39 known
species of size class 5 (Table 1; Fig. 2d).
We expect 228 species of small mammals (178 size 1
and 50 size 2) to be described in the next 40 years on all
land masses (Table 2; Fig. 2). It is striking that the trend
of number of species in sizes 1 and 2 is maintained on all
land masses except Africa, which has a large drop in
number of species from size 1 to size 2. Similarly, from
size 2 to size 3 (Fig. 2c), Eurasia has a large drop in number
of species, leaving only America with relatively high values.
A greater number of species will be described from
Eurasia in the next 40 years. If we correct for land mass
area, however, Oceania has a higher value of expected
new species per unit area than in any other land mass
(Table 2). This is probably related to Oceania’s high degree of insularity, which correlates with species density
because of high endemism and small distribution ranges.
Insectivores occur in three of the four land masses and
are more species-rich in Africa; when area is corrected
for, they are twice as rich in Africa as in Eurasia and over
3.5 times richer in Africa than in America (Table 3). Surprisingly, a greater number of species is expected to be
described in Eurasia in the next 40 years, although Africa
Table 3. Number of species for each of the 10 most species-rich
orders of land mammals in each land mass corrected for number of
species per 100,000 km2, and number of species expected to be
described in the next 40 years.
Order
Land mass
Number
of species America Africa Eurasia Oceania Total
Insectivora
Absolute
74
Corrected
0.18
Expected
6
Corrected
0.01
Chiroptera Absolute 280
Corrected
0.67
Expected
9
Corrected
0.02
Primates
Absolute
84
Corrected
0.20
Expected
0
Corrected
0.00
Carnivora
Absolute
73
Corrected
0.17
Expected
2*
Corrected
0.00
Artiodactyla Absolute
27
Corrected
0.06
Expected
1*
Corrected
0.00
Rodentia
Absolute 868*
Corrected
2.06*
Expected 51*
Corrected
0.12
188*
166
0.62*
0.30
13
19*
0.04*
0.03
195
337*
0.64
0.62
13
18*
0.04
0.03
87*
62
0.29*
0.11
0
1*
0.00
0.00
67
96*
0.22*
0.18
2*
0
0.01*
0.00
94
99*
0.31*
0.18
0
0
0.00
0.00
394
621
1.30
1.14
32
42
0.11
0.08
428
38
113
1.33*
9
0.11*
925
49
233
1
236
4
220
1
132
1.55
14
0.16*
2015
139
* Highest value in each row.
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Volume 13, No. 1, February 1999
148
Predictions of Mammal Diversity
still has a greater predicted species description rate per
unit area (Fig. 3a).
Eurasia is the land mass richest in bat species, although America, Africa, and Eurasia have a similar bat-
Medellín & Soberón
species richness once area is corrected for. Bat species
richness in Oceania is about half that of the other three
land masses. Eurasia will have the greatest number of
new species, although, per unit area, Oceania again has
the highest number of species to be described. This is
probably also related to Oceania’s high degree of insularity (Fig. 3b).
The absolute species richness of rodents is highest in
America, where by far the highest species density and
the largest expected number of species to be described
also occurs. Although for methodological restrictions
we pooled North and South America, intuitively South
America will be the main source of new species. As with
Chiroptera, Oceania has the highest number of species
expected to be described per unit area (Table 1; Fig. 3c).
Primates and carnivores show similar values of species
richness and species packing on all four land masses.
Eurasia is relatively poor in primates, and Africa has the
greatest values per unit area. There remain few species
to be described in the next 40 years. Artiodactyls show
far fewer species in America, probably a result of Pleistocene extinctions.
The emerging pattern shows that, as expected, small
mammals will compose the bulk of the species to be
described in the next half century. If past description
patterns continue, bats and insectivores in Eurasia will
contribute an important percentage of those species. Although we pooled North and South America, the North
American mammal fauna is clearly much better known
than that of South America. Similarly, Asia and Europe
were combined in our analyses, but most of the new
species will surely come from Asia. Africa and Oceania
will yield a greater number of species per unit area (Table 1). Unless there are radical shifts with discoveries of
unknown hotspots of undescribed species or a change
in taxonomic philosophy, our predictions should be robust enough to be useful as a tool to plan prospective
and exploratory work and as a guide to focus conservation and research efforts.
Acknowledgments
Figure 3. Cumulative curves of species description
and fitted model of the orders Insectivora, Chiroptera,
and Rodentia on four land masses. The curves start in
the year 1890 (year 0), when the number of species
was treated as zero. Real numbers of species in that
year in each order and land mass are Insectivora, Chiroptera, Rodentia, respectively: 27, 142, 339, America;
64, 185, 263, Eurasia; 47, 93, 147, Africa; 0, 56, 35,
Oceania.
Conservation Biology
Volume 13, No. 1, February 1999
We thank H. Gómez de Silva, D. Wilson, G. Meffe, E.
Main, and two anonymous reviewers for critically reading an earlier version of this manuscript. H. Arita, G.
Ceballos, M. Equihua, and C. Equihua provided fruitful
discussion of these ideas.
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Conservation Biology
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