Nordic Society Oikos
The Relationship between Population Density and Body Size: The Role of Extinction and
Mobility
Author(s): Bo Ebenman, Anders Hendenstrom, Uno Wennergren, Borje Ekstam, Jan Landin,
Tommy Tyrberg
Source: Oikos, Vol. 73, No. 2 (Jun., 1995), pp. 225-230
Published by: Blackwell Publishing on behalf of Nordic Society Oikos
Stable URL: http://www.jstor.org/stable/3545912
Accessed: 07/01/2010 03:26
Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at
http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless
you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you
may use content in the JSTOR archive only for your personal, non-commercial use.
Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at
http://www.jstor.org/action/showPublisher?publisherCode=black.
Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed
page of such transmission.
JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of
content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms
of scholarship. For more information about JSTOR, please contact [email protected].
Blackwell Publishing and Nordic Society Oikos are collaborating with JSTOR to digitize, preserve and extend
access to Oikos.
http://www.jstor.org
OIKOS 73: 225-230. Copenhagen1995
The relationship between population density and body size:
the role of extinction and mobility
Bo Ebenman, Anders Hedenstrom, Uno Wennergren, Borje Ekstam, Jan Landin and Tommy Tyrberg
Ebenman,B., Hedenstrom,A., Wennergren,
U., Ekstam,B., Landin,J. andTyrberg,T.
1995.Therelationshipbetweenpopulationdensityandbodysize:theroleof extinction
and mobility.- Oikos 73: 225-230.
Therelationshipbetweenabundanceandbodysize is the subjectof considerabledebate
in ecology. Herewe presentnew dataon the relationshipbetweenpopulationdensity
andbody massfor flightlessbirds.Unlikeflyingbirds,flightlessbirds(andmammals)
show a strong negative relationshipbetween populationdensity and body mass.
Densitydecreasesas the -1.40 powerof bodymass,whichis significantlysteeperthan
in flyingbirds.Thisdifferenceis mainlydueto a realabsenceof smallspecieswithlow
populationdensitiesin flightlessbirds.Smallpopulationsof small-bodiedspeciesmay
notbe ableto persistunlesssustainedby immigration.
Thehighmobilityof flyingbirds
may allow even small local populationsof small-bodiedspecies to persistthrough
supplyof immigrants,andthuscan explainthe weakrelationshipbetweenabundance
and body size for this groupcomparedto thatof flightlessbirds(andmammals).
B. Ebenman, U. Wennergren, B. Ekstam and J. Landin, Dept of Biology, Univ. of
Linkoping, S-581 83 Linkoping, Sweden. - A. Hedenstr6m, Dept of Ecology, Theoretical Ecology, Lund Univ., Ecology Building, S-22362 Lund, Sweden. - T. Tyrberg,
Kimstadv. 37, S-61020 Kimstad, Sweden.
A number of qualitative patterns have been documented,
linking body size, abundance and distribution of species
(Gaston and Lawton 1988a,b, May 1988, Brown and
Maurer 1989). Especially, the relationship between population density and body mass has received considerable
attention (Damuth 1981, 1987, Peters and Wassenberg
1983, Peters and Raelson 1984, Brown and Maurer 1986,
1987, Juanes 1986, Robinson and Redford 1986, Lawton
1989, 1990, Blackburn et al. 1990, Marquet et al. 1990,
Carrascal and Telleria 1991, Nee et al. 1991, Cotgreave
and Harvey 1992, Griffiths 1992, Blackburn et al.
1993a,b, Cotgreave 1993, Currie 1993, Blackburn and
Lawton 1994). In spite of a growing literature,it has been
difficult to reach a concensus about the patterns and
causes of size-abundance scaling. Damuth (1987) proposed that population density scales to body mass with a
slope of -0.75 (log-scale), implying that all species use
the same amount of energy, since metabolic rate scales to
body mass with a slope of 0.75. Peters (1983) suggested
that population density scales to body mass with a slope
of -1, implying that all species attain the same biomass
(but see Damuth 1994).
However, it has been argued that these relationships
between body mass and population density may only
define the upper bound of a distribution, with many
species falling below (Brown and Maurer 1986, 1987).
Explanations for a negative relationship between abundance and body mass based on arguments about per
capita use of resources (Peters 1983, Damuth 1987) apply
to maximum densities only, while the overall relationship
will depend on the entire distribution;both on maximum
densities (upper bound) and minimum densities (lower
bound). The lower bound needs not to be parallel to the
upper bound. Although it may not be unreasonable to
expect that the upper bound is set by energetic constraints
(Blackburn et al. 1992, but see Blackburn et al. 1993b),
Accepted20 December1994
Copyright? OIKOS1995
ISSN 0030-1299
Printedin Denmark- all rightsreserved
15
OIKOS 73:2 (1995)
225
there is no consensus about the causes behind the lower
bound, although the problem has been discussed (Peters
'I)
and Raelson 1984, Brown and Maurer 1987, Morse et al.
0)
1988, Lawton 1989, 1990, Cotgreave 1993).
7C)
a,
As has been pointed out by Lawton (1990) there also
0
exists a dichotomy in patterns of size and abundance.
0)
0
Thus, local ecological assemblages from one habitat of0
C)
':i:i:::!:
ten show a weak negative relationship between size and
Lo
body::size
0)
ig-b:::
abundance while compilations of data from many taxa
and different geographical locations often show a strong
Log body size
negative relationship (see also Currie 1993).
In global compilations of data of mammals both the
C
upper and lower bound of the distribution are negative
105
and the overall relationship is highly significant (Damuth
1981, 1987, Peters and Wassenberg 1983, Peters and
.
Raelson 1984, Robinson and Redford 1986, Arita et al.
104
1990, Silva and Downing 1994) (Fig. 1A). Moreover, the
pattern found for the combined data of all mammals
o0
0
seems to reflect that found within habitat types (Damuth
CO 3
11981). A note of caution is in place here. It has been
suggested (Brown and Maurer 1986, see also Lawton
E
1989) that there may be a bias in the mammal data sets, in
- * 102.
2
that
small and rare species may be underrepresented,
i
0
the lower bound to be negative (but see Currie
.
causing
.t:o
S
1993, Silva and Downing 1994). On the other hand, for
flying birds even pooled data from large areas show a
.5
weak relationship between population density and body
size (Brown and Maurer 1986, 1987, Juanes 1986, Cot0
Q..A, 1U
greave and Harvey 1992, Blackburn et al. 1993a). The
*
0
upper bound of the distribution declines with increasing
body mass, whereas the lower bound is approximately
horizontal (Fig. iB). An exception to this pattern are
10-1birds of prey which show a strong negative relationship
102
100
101
10o10-2
between abundance and body size (Newton 1979).
Body mass (kg, log-scale)
There is one evident difference between birds and
mammals - the extraordinarymobility of birds stemming
Fig. 1. (A) The principalshapeof a log-log plot of population from their ability to fly - that we expect will influence the
densityvs body size for mammals.(B) The same as in (A) but
for birds.The positiveslope of the upperboundfor very small relationship between population density and body mass,
speciesis uncertainandmaybe a samplingphenomenon.Notea especially the slope of the lower bound (minimum densimore or less horizontallower bound in (B) comparedto a ties).
parallelupperandlowerboundin (A). (C) Populationdensityvs
data(slope
bodymassforflightlessbirdson log-logtransformed
>1
B
A
:s. I
= -1.40; slope s.e. = 0.20; intercept = 5.70; r2 = 0.76; n= 17;
P<0.001). Note the absenceof smallspecieswith low densities
in flightlessbirdscomparedto flying birds.The statisticalerror
variancein body massfor individualspeciesis smallrelativeto Methods
the rangeof body mass studied,thusjustifyingstandardlinear
regression.The species in decreasingorderof body mass and Given the possible bias in the mammal data (but see
Currie 1993, Silva and Downing 1994), we have comsources are: Struthio camelus (Brown et al. 1982); Casuarius
casuarius johnsonii (Crome 1976); Dromaius novaehollaniae
piled data on abundances and body masses for 17 species
(Campbelland Lack 1985, Marchantand Higgins 1990); Pte- of flightless birds (sources are given in the legend to Fig.
rocmenia tarapecensis (Blake 1977, Fjeldsa pers. comm.); Pte1) in order to investigate the effects of mobility. The
rocmenia pennata garleppi (Blake 1977, Cajal 1988); Apteryx
haasti (Marchant and Higgins 1990); Notornis mantelli (Reid
pooled data of flightless birds includes even the rarest
and Stack 1974, Ripley 1977); Apteryx australis scotti (Colspecies. Actually, many of the flightless species, both
boure and Kleinpaste 1983, 1984); Strigops habroptilus (Best
small and large ones, that have been studied are endemic
and Powlesland 1985); Apteryx ovenii (Marchant and Higgins
and rare. There is no reason to believe that the species
1990); Gallinula mortieri (Ripley 1977); Gallirallus australis
included have been sampled in a way that would bias the
(Brothers and Skira 1984, Beauchamp 1987); Rollandia micropterum (Livezey 1989, Fjeldsa pers. comm.); Anas aucklandica
estimated slope of the size-abundance relation. Thus,
(Johnsgard 1978, Weller 1975); Gallirallus sylvestris (Recher
even if low-density populations in marginal or suboptiand Clark 1974, Ripley 1977); Dryolimnas cuvieri (Penny and
mal habitats were under-represented, such under-repreDiamond 1971); Atlantisia rogersi (Ripley 1977).
226
OIKOS 73:2 (1995)
Table 1. Regressionanalysesof log populationdensity(ind./km2)on log body mass (g) for birds.Slopes are given with (standard
P is the level of significanceof the slope (ns = not significant);n is samplesize; hyphen
error);r2is the coefficientof determination;
= values unknown.
Group
Flying birds
Flying birds
Slope
Intercept
r2
Range(g)
P
n
Sourcee
-0.19 (0.14)
-0.089 (0.03)
1.36
-
0.03
0.019
10-4000
3-10470
ns
0.01
60
380
1
2
-
2
2
0.001
0.001
0.001
ns
ns
0.001
0.001
564
437
147
77
70
206
47
3
4
5
5
5
5
6
0.001
0.001
0.001
112
97
17
467
-0.66a
Ob
Flying birds
Flying birds
Flying birdsc
Passerinesc
Non-pass.c
Swedishbirds
Passerines
-0.49 (0.04)
-0.60
-0.75 (0.16)
-0.14 (0.36)
-0.19 (0.27)
-0.771(0.12)
-0.50 (0.13)
Farmlandbirdsd
Woodlandbirdsd
Flightlessbirds
Mammals
-0.475 (0.12)
-0.489(0.12)
-1.40 (0.20)
-0.78 (0.03)
0b
-
-
1.96
5.64
5.04
3.98
1.42
0.18
0.15
0.14
0.002
0.007
0.179
0.26
5.70
4.06
0.125
0.149
0.76
0.64
-
-
3-4536
3-10000
5-10000
5-1000
20-10000
6-160
_
-0.001
37-107000
5-3160000
6
7
7
8
9
aestimated slope of the upperbound(maximumpopulationdensities)of the distribution.
b estimated
slope of the lower boundof the distribution.
c regionaldensities
(all otherdataare basedon ecologicaldensities).
counts.
edterritory
1, PetersandWassenberg1983;2, BrownandMaurer1987;3, Juanes1986;4, CotgreaveandHarvey1992;5, Nee et al. 1991;
6, Carrascaland Telleria1991;7, Blackburnet al. 1993a;8, this study;9, Damuth1987.
sentation would be expected to be independent of body
size. (A systematic under-representationof populations in
marginal/suboptimal habitats independent of body size
will bias the estimated intercept but not the slope). We
have not included colony breeding species (e.g. penguins) because of difficulties in estimating ecological
densities for such species.
As our measure of abundance we used ecological densities, which is the densities in areas actually occupied by
the species (Damuth 1987). For two of the large species
where density estimates were available from more than
one population we used the arithmetic mean. Averaging
reduces the representation of the lowest densities. However, averaging has only been done for two large species
which anyhow have low densities. Body masses for
males and females are often different and therefore the
average masses of the sexes were used in cases of significant sexual dimorphism. The body masses of the
species span over a range of approximately four orders of
magnitude, comparable to that of flying birds.
Results
The overall slopes documented for flying birds vary between -0.60 and -0.09 and the regressions are characterized by low coefficients of determination (r2 values)
(Table 1). (Analyses based on regional densities - densities in a specified geographical area, e.g. Great Britain tend to give steeper slopes for flying birds. For instance,
Nee et al. (1991) found that for British birds abundance
15* OIKOS 73:2 (1995)
across all species declines with the -0.75 power of body
mass, conforming to the slope shown by mammals. However, the significantly negative relationship between size
and abundance in British birds arises because of a difference between passerines and non-passerines. Neither
within the passerines nor within the non-passerines alone,
is there any evidence of any association between abundance and body mass (see also Blackburn et al. 1994).
Moreover, if ecological densities (species distributions
from Sharrock 1976) are used, abundance of British birds
declines with a -0.58 power of body mass (our calculations), which approach the slopes reported for other
flying birds).
Compared to flying birds, flightless birds have a narrower distribution in the size-abundance space with a
size-dependent lower bound (no small species with low
densities, Fig. 1C). Density decreases as the -1.40 power
of body mass (linear regression on log-transformed data;
r2=0.76, 95% confidence interval of the slope = [-1.83,
-0.97], p < 0.001). The slope is significantly steeper than
those found in flying birds (Peters and Wassenberg 1983,
Juanes 1986, Brown and Maurer 1987, Blackburn et al.
1993a) (non-overlapping 95% confidence limits). In fact,
the patternfound in flightless birds is more similar to that
found in mammals than to that of flying birds.
It is unlikely that the differences between flying and
flightless birds is due to biased sampling (see above). The
strong negative relationship between size and abundance
in flightless birds is most likely explained by a real
absence of small-bodied species with low population
densities.
It has been shown that phylogeny may influence the
227
relationship between body mass and abundance (Nee et
al. 1991, Cotgreave and Harvey 1992, 1994, Blackburn et
al. 1994). Thus, treating species as independent observations may cause problems with the interpretationof comparative data. For instance, the relation within individual
taxa may be different from the overall pattern across all
species. Within taxa the relationship may even be positive. Moreover, it has been shown that taxonomically
distinct tribes of birds (low degree of genetic relatedness
to other tribes) and tribes which radiated from a common
ancestor a long time ago (high degree of genetic divergence within the tribe) are more likely to show a
positive relation (Nee et al. 1991, Blackburn et al. 1994,
Cotgreave and Harvey 1994). The number of species of
flightless birds in our data set is too low for making
regressions for individual taxa. However, based on information in Sibley and Ahlquist (1990) and Cooper et al.
(1992), we have corrected for phylogeny in the whole
data set of flightless birds (treating Dromaius/Kasuaris,
Struthio, Pterocmenia, Apteryx, Rallidae, Rollandia,
Anas and Strigops as individual independent data points,
i.e. 8 data points). This correction for phylogeny does not
change the general pattern. There is still a strong and
significantly negative relationship between population
density and body size (slope = -1.44, r2= 0.70, p< 0.01,
df= 6, linear regression on the log-transformed means of
the constituent taxa).
The strong relationship shown by flightless birds is
neither an artefact of small species living on isolated
islands and large ones living on continents, i.e. an island/
mainland effect. We have performed separate regression
analyses on island and mainland species. Both show a
strong and significantly negative relationship between
population density and body mass (slopes: -1.58 and
-1.09, respectively) with relatively high r2 values (0.57
and 0.79, respectively). (Differences in slopes and intercepts were not significant [ANCOVA; note, however,
that the sample size is small]).
Discussion
There are basically two arguments for a negative slope of
the lower bound, both based on the concept of minimum
viable population size: geographical range and hence
total population size increases with body size (Brown and
Maurer 1987, Arita et al. 1990) and/or population size
variability decreases with increasing body size (Gaston
and Lawton 1988a,b, Lawton 1989, 1990). Both causes
will reduce the vulnerability to extinction for populations
of a given density/size (Leigh 1981, Karr 1982, Goodman
1987, Schoener and Spiller 1987, Lande 1993), which
suggests that the lower bound should have a negative
slope. The second argument hinges on the assumption
that low population density implies low population size
for small-bodied species (Lawton 1990). However, numerical analyses of a stochastic population growth model
228
by Belovsky (1987) suggest that the risk of extinction is
more sensitive to population variability than to population size. Thus, in mammals a 10-g species has a shorter
expected persistence time than a 106-g species even if its
population size is nearly 100 times larger than that of the
large-bodied species.
Several studies are consistent with the hypothesis that,
for a given population size, small-bodied species are
often more vulnerable to extinction than large-bodied
ones (Peters and Raelson 1984, Belovsky 1987, Soule et
al. 1988, Pimm et al. 1988, Gotelli and Graves 1990,
Tracy and George 1992) although other factors may also
be important (see Tracy and George 1992).
Another factor that may lead to a negative slope of the
lower bound (minimum densities) are the difficulties for
individuals of small-bodied species to find mates when
population densities are low, especially in species with
low mobility (Morse et al. 1988, Lawton 1989).
On balance, theoretical considerations suggest negative slopes for both the upper and the lower bound of the
size-abundance distribution. The patterns shown by
flightless birds and mammals are consistent with this
prediction. Why, then, have flying birds a size-independent, horizontal lower bound? For a given mass a
flying bird can disperse much longer per unit of time and
at a lesser cost per unit of distance than a running animal
(Peters 1983: chapter 6, Calder 1984: chapter 7). Moreover, flight velocities are less sensitive to body mass than
are running speeds (Peters 1983, Calder 1984). The high
mobility of flying birds may allow even small local populations of small-bodied species to persist through supply
of immigrants (the rescue effect, see also Lawton 1989),
leading to the observed constancy of minimum population densities over the entire range of body sizes, which
in turn can explain the weak overall relationship between
abundance and body size for this group. On the other
hand, if dispersal abilities are poor, like in flightless birds
or mammals, small-bodied species that have low population densities will not be able to persist, because the low
rate of immigration, caused by low mobility, cannot keep
up with the rate of local extinction.
Dispersal between local populations may also have a
stabilizing effect on the dynamics of the local populations, which will tend to decrease the extinction risks of
the local populations further (McCallum 1992, Hastings
1993, Stone 1993).
It can be added that both flightlessness and isolation
(e.g. living on isolated "islands") should affect dispersal
and hence the persistence of small populations in the
same negative way. Therefore, terrestrial flying birds
living on isolated "islands" should show a similar sizeabundance patternas flightless birds, i.e. no small-bodied
species with low population densities. Some studies indicate that this might be the case (Faaborg 1982, Soule et
al. 1988).
- We thankT. Alerstamfor helpful comAcknowledogements
ments,S. Akessonfor help with transforming
regionaldensities
OIKOS 73:2 (1995)
into ecological densities (Great Britain birds) and S. Douwes for
drawing the figure. We are also grateful to P. Cotgreave for
information on two of the flightless species and to J. Lawton for
constructive criticism of an earlier version of the manuscript.
Research funded by Swedish Natural Science Research Council
grant to B.E.
References
Arita, H.T., Robinson, J. G. and Redford, K. H. 1990. Rarity in
Neotropical forest mammals and its ecological correlates. Conserv. Biol. 4: 181-192.
Beauchamp, A.J. 1987. The social structure of the Weka (Gallirallus australis) at Double Cove, Marlboruogh Sounds. Notornis 34: 317-325.
Belovsky, G.E. 1987. Extinction models and mammalian persistence. - In: Soul6, M. E. (ed.), Viable populations for conservation. Cambridge Univ. Press, Cambridge, pp. 35-57.
Best, H. and Powlesland, R. 1985. Kakapo. - New Zealand
Wildlife Service, Dunedin.
Blackburn, T. M. and Lawton, J. H. 1994. Population abundance
and body size in animal assemblages. - Philos. Trans. R.
Soc. Lond. B 343: 33-39.
, Harvey, P.H. and Pagel, M.D. 1990. Species number,
population density and body size relationships in natural
communities. - J. Anim. Ecol. 59: 335-345.
- , Lawton, J. H. and Perry, J. N. 1992. A method of estimating
the slope of upper bounds of plots of body size and abundance in natural animal assemblages. - Oikos 65: 107-112.
- , Brown, V.K., Doube, B.M., Greenwood, J.J.D., Lawton,
J. H. and Stork, N. E. 1993a. The relationship between abundance and body size in natural animal assemblages. - J.
Anim. Ecol. 62: 519-528.
-, Lawton, J.H. and Pimm, S.L. 1993b. Non-metabolic explanations for the relationship between body size and animal
abundance. - J. Anim. Ecol. 62: 694-702.
, Gates, S., Lawton, J.H. and Greenwood, J.J.D. 1994.
Relations between body size, abundance and taxonomy of
birds wintering in Britain and Ireland. - Philos. Trans. R.
Soc. Lond. B 343: 135-144.
Blake, E.R. 1977. Manual of Neotropical birds. Vol. 1. - Univ.
of Chicago Press, Chicago.
Brothers, N.P. and Skira, I.J. 1984. The Weka on Macquarie
Island. - Notornis 31: 145-154.
Brown, J.H. and Maurer, B.A. 1986. Body size, ecological
dominance and Cope's rule. - Nature 324: 248-250.
- and Maurer, B.A. 1987. Evolution of species assemblages:
effects of energetic constraints and species dynamics on the
diversification of North American avifauna. - Am. Nat. 130:
1-17.
- and Maurer, B.A. 1989. Macroecology: The division of
food and space among species on continents. - Science 243:
1145-1150.
Brown, L. H., Urban, E. K. and Newman, K. 1982. The birds of
Africa. Vol. 1. - Academic Press, London.
Cajal, J. L. 1988. The lesser rhea in the Argentine Puna Region:
Present situation. - Biol. Conserv. 45: 81-91.
Calder, W.A. 1984. Size, function, and life history. - Harvard
Univ. Press, Cambridge, MA.
Campbell, B. and Lack, E. (eds) 1985. A dictionary of birds. T & AD Poyser, Calton.
Carrascal, L. M. and Tellerfa, J. L. 1991. Bird size and density: a
regional approach. - Am. Nat. 138: 777-784.
Colbourne, R. and Kleinpaste, R. 1983. A banding study of
North Island brown kiwis in an exotic forest. - Notornis 30:
109-124.
- and Kleinpaste, R. 1984. North Island brown kiwi vocalisations and their use in censusing populations. - Notornis 31:
191-201.
Cooper, A., Mourer-Chauvir6,C., Chambers, G. K., von HaeseOIKOS 73:2 (1995)
ler, A., Wilson, A.C. and Paabo, S. 1992. Independent
origins of New Zealand moas and kiwis. - Proc. Natl. Acad.
Sci. USA 89: 8741-8744.
Cotgreave, P. 1993. The relationship between body size and
population abundance in animals. - Trends Ecol. Evol. 8:
244-248.
- and Harvey, P.H. 1992. Relationships between body size,
abundance and phylogeny in bird communities. - Funct.
Ecol. 6: 248-256.
- and Harvey, P.H. 1994. Phylogeny and the relationship
between body size and abundance in bird communities. Funct. Ecol. 8: 219-228.
Crome, F.H.J. 1976. Some observations on the biology of the
cassowary in Northern Qeensland. - Emu 76: 8-14.
Currie, D.J. 1993. What shape is the relationship between body
size and population density? - Oikos 66: 353-358.
Damuth, J. 1981. Population density and body size in mammals.
- Nature 290: 699-700.
- 1987. Interspecific allometry of population density in mammals and other animals: the independence of body mass and
population energy-use. - Biol. J. Linn. Soc. 31: 193-246.
- 1994. No conflict among abundance rules. - Trends Ecol.
Evol. 9: 487.
Faaborg, J. 1982. Trophic and size structureof West Indian bird
communities. - Proc. Nat. Acad. Sci. USA 79: 1563-1567.
Gaston, K.J. and Lawton, J.H. 1988a. Patterns in the distribution and abundance of insect populations. - Nature 331:
709-712.
- and Lawton, J.H. 1988b. Patterns in body size, population
dynamics, and regional distribution of bracken herbivores. Am. Nat. 132: 662-680.
Goodman, D. 1987. The demography of chance extinction. - In:
Soule, M.E. (ed.), Viable populations for conservation.
Cambridge Univ. Press, Cambridge, pp. 11-34.
Gotelli, N.J. and Graves, G.R. 1990. Body size and the occurrence of avian species on land-bridge islands. - J. Biogeogr.
17: 315-325.
Griffiths, D. 1992. Size, abundance, and energy use in communities. - J. Anim. Ecol. 61: 307-315.
Hastings, A. 1993. Complex interactions between dispersal and
dynamics: Lessons from coupled logistic equations. Ecology 74: 1362-1372.
Johnsgard, P.A. 1978. Ducks, geese and swans of the world. Lincoln.
Juanes, F. 1986. Population density and body size in birds. Am. Nat. 128: 921-929.
Karr, J.R. 1982. Population variability and extinction in the
avifauna of a tropical land bridge island. - Ecology 63:
1975-1978.
Lande, R. 1993. Risk of population extinction from demographic and environmental stochasticity and random catastrophes. - Am. Nat. 142: 911-927.
Lawton, J. H. 1989. What is the relationship between population
density and body size in animals? - Oikos 55: 429-434.
- 1990. Species richness and population dynamics of animal
assemblages. Patterns in body size: abundance space.Philos. Trans. R. Soc. Lond. B 330: 283-291.
Leigh, E.G. 1981. The average lifetime of a population in a
varying environment. - J. Theor. Biol. 90: 213-239.
Livezey, B.C. 1989. Flightlessness in grebes (Aves, Podicipedidae): its independent evolution in three genera. - Evolution
43: 29-54.
Marchant, S. and Higgins, P. (eds) 1990. Handbook of Australian, New Zealand and Antarctic birds. Vol. 1. - Oxford
Univ. Press, Oxford.
Marquet, P. A., Navarrete, S. A. and Castilla, J. C. 1990. Scaling
population density to body size in rocky intertidalcommunities. - Science 250: 1125-1127.
May, R.M. 1988. How many species are there on Earth? Science 241: 1441-1449.
McCallum, H. I. 1992. Effects of immigration on chaotic population dynamics. - J. Theor. Biol. 154: 277-284.
229
Morse, D.R., Stork, K.E. and Lawton, J.H. 1988. Species
number, species abundance and body length relationships of
arborealbeetles in Bornean lowland rain forest trees. - Ecol.
Entomol. 13: 25-37.
Nee, S., Read, A. F., Greenwood, J. J. D. and Harvey, P. H. 1991.
The relationship between abundance and body size in British
birds. - Nature 351: 312-313.
Newton, I. 1979. Population ecology of raptors.- Buteo Books,
Vermillion, SD.
Penny, M.J. and Diamond, A.W. 1971. The white-throated rail
Dryolimnas cuvieri in Aldabra. - Philos. Trans. R. Soc.
Lond. B 260: 529-548.
Peters, R.H. 1983. The ecological implications of body size. Cambridge Univ. Press, Cambridge.
- and Wassenberg, K. 1983. The effect of body size on animal
abundance. - Oecologia 60: 89-96.
- and Raelson, J.V. 1984. Relations between individual size
and mammalian population density. - Am. Nat. 124: 498517.
Pimm, S.L., Jones, H.L. and Diamond, J. 1988. On the risk of
extinction. - Am. Nat. 132: 757-785.
Recher, H. F. and Clark, S.S. 1974. A biological survey of Lord
Howe Island with recommendations for the conservation of
the island's wildlife. - Biol. Conserv. 6: 263-273.
Reid, B. and Stack, D.J. 1974. An assessment of the numbers of
Takahe in the 'Special area' of the Murchison mountains
during the years 1963-1967. - Notornis 21: 296-305.
230
Ripley, S.D. 1977. Rails of the world. - David R. Godin,
Boston.
Robinson, J.G. and Redford, K.H. 1986. Body size, diet, and
population density of Neotropical forest mammals. - Am.
Nat. 128: 665-680.
Schoener, T.W. and Spiller, D.A. 1987. High population persistence in a system with high turnover. - Nature 330:
474-477.
Sharrock, J.T.R. 1976. The atlas of breeding birds in Britain
and Ireland. - BTO, Tring.
Sibley, C. G. and Ahlquist, J. E. 1990. Phylogeny and classification of birds: a study in molecular evolution. - Yale Univ.
Press, New Haven, CT.
Silva, M. and Downing, J.A. 1994. Allometric scaling of minimal mammal densities. - Conserv. Biol. 8: 732-743.
Soul6, M. E., Bolger, D. T., Alberts, A. C., Wright, J., Sorice, M.
and Hill, S. 1988. Reconstructed dynamics of rapid extinctions of chaparral-requiringbirds in urban habitat islands. Conserv. Biol. 2: 75-92.
Stone, L. 1993. Period-doubling reversals and chaos in simple
ecological models. - Nature 365: 617-620.
Tracy, C.R. and George, T.L. 1992. On the determinants of
extinction. - Am. Nat. 139: 102-122.
Weller, M.W. 1975. Ecological studies of the Auckland island
flightless teal. - Auk 92: 280-297.
OIKOS 73:2 (1995)