META- One-dimensional analyses of Rapoport’s rule … · the Rapoport effect through the...

© 2007 The Authors DOI: 10.1111/j.1466-8238.2006.00303.xJournal compilation © 2007 Blackwell Publishing Ltd www.blackwellpublishing.com/geb

401

Global Ecology and Biogeography, (Global Ecol. Biogeogr.)

(2007)

16

, 401–414

META-ANALYSIS

Blackwell Publishing Ltd

One-dimensional analyses of Rapoport’s rule reviewed through meta-analysis

Adriana Ruggiero

1

* and Victoria Werenkraut

ABSTRACT

Aim

To analyse quantitatively the extent to which several methodological,geographical and taxonomic variables affect the magnitude of the tendency for thelatitudinal ranges of species to increase with latitude (the Rapoport effect).

Location

Global.

Methods

A meta-analysis of 49 published studies was used to evaluate the effect ofseveral methodological and biological moderator variables on the magnitude of thepattern.

Results

The method used to depict the latitudinal variation in range sizes is astrong moderator variable that accounts for differences in the magnitude of thepattern. In contrast, the extent of the study or the use of areal or linear estimations ofrange sizes does not affect the magnitude of the pattern. The effect of geography ismore consistent than the effect of taxonomy in accounting for differences in themagnitude of the pattern. The Rapoport effect is indeed strong in Eurasia and NorthAmerica. Weaker or non-significant latitudinal trends are found at the global scale,and in Australia, South America and the New World. There are no significantdifferences in the magnitude of the pattern between different habitats, however, theoverall pattern is weaker in oceans than in terrestrial regions of the world.

Main conclusions

The Rapoport effect is indeed strong in continental land-masses of the Northern Hemisphere. The magnitude of the effect is primarilyaffected by methodological and biogeographical factors. Ecological and spatial scaleeffects seem to be less important. We suggest that not all methodological approachesmay be equally useful for analysing the pattern.

Keywords

Ecogeographical rule, geographical range, latitudinal gradient, macroecology,

meta-analysis, Rapoport effect, range size, species ranges.

*Correspondence: Adriana Ruggiero, Laboratorio Ecotono, Centro Regional Universitario Bariloche, Universidad Nacional del Comahue/CONICET, Quintral 1250 (8400) Bariloche, Rio Negro, Argentina.E-mail: [email protected]

Laboratorio Ecotono, Centro Regional

Universitario Bariloche, Universidad Nacional

del Comahue/CONICET, Quintral

1

250 (8400)

Bariloche, Rio Negro, Argentina

INTRODUCTION

In contrast to the latitudinal diversity gradient, which is a universal

feature of the spatial variation in biodiversity (Hillebrand,

2004), the tendency for the geographical ranges of species to

increase with latitude (known as Rapoport’s rule; Stevens, 1989)

is nowadays considered to be only a regional or local pheno-

menon restricted to high latitudes in the Northern Hemisphere

(Rohde, 1996, 1999; Gaston

et al

., 1998; but see Cardillo, 2002).

Studies that focused on different regions and taxa, or that applied

different methods of analysis, have come up with different

perceptions of the pattern (Lyons & Willig, 1997; Gaston

et al.

, 1998;

Rohde, 1999; Ashton, 2001; Gaston, 2003). The existence of

equivocal evidence in support of a Rapoport effect (i.e. 15 out of

24 studies reported in Gaston

et al

., 1998, support the pattern;

see also Cowlishaw & Hacker, 1998; but results from simulation

models also contradict the pattern; see Stauffer & Rohde, 2006;

Ribas & Schoereder, 2006) has led to the dismissal of the pattern

as a universal ecogeographical rule. Nonetheless, the analysis of

the Rapoport effect has continued until today. Thus, it is timely

to elaborate a new, quantitative, revision of the literature for re-

assessment of the overall magnitude of the pattern. Given that the

number of significant outcomes may have little relationship to

the magnitude of an effect (Hedges & Olkin, 1985; Rosenberg

A. Ruggiero and V. Werenkraut

© 2007 The Authors

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Global Ecology and Biogeography

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, 401–414, Journal compilation © 2007 Blackwell Publishing Ltd

et al

., 2000), the extent to which some consistency might be

found in the magnitude of the Rapoport effect across different

taxa or regions of the world still remains debatable.

Most of the tests of the Rapoport pattern have been based on a

statistical one-dimensional approach of analysis that uses scatter

plots to show the latitudinal variation in range size over latitudinal

bands, across individual species, or have analysed changes in

the latitude:range size association with time through phylo-

genetic comparative analysis. More rarely, studies have adopted a

truly geographical perspective to map directly the mean varia-

tion of range size over continents (Australian mammals, Smith

et al

., 1994; African primates, Eeley & Foley, 1999; carabid beetles

in the Northern Hemisphere, Noonan, 1999; South American

birds, Graves & Rahbek, 2005; birds at a global scale, Orme

et al

.,

2006; New World birds, Hawkins & Diniz-Filho, 2006). Thus,

most of the analyses of range size patterns that tested the

Rapoport pattern are impregnated by the imprint of thinking in

one ‘latitudinal’ dimension (see Hawkins & Diniz-Filho, 2004) and

lack a truly geographical approach (see Ruggiero & Hawkins,

2006). Within this context, several methods of analysis have been

developed, which may influence further our perception of the

pattern (see e.g. Lyons & Willig, 1997; Blackburn & Gaston,

2006). Blackburn & Gaston (2006) have suggested that the

different approaches applied so far to the analysis of the

Rapoport effect have been equally useful for understanding the

pattern as they provide complementary information on species

ranges (cf. Ruggiero & Hawkins, 2006). The present study will

be useful for evaluating this premise in more detail. We will

show the extent to which the several methodological approaches

have influenced the magnitude of the Rapoport effect reported in

the literature.

In the present study, we analyse one-dimensional studies of

the Rapoport effect through the application of meta-analytical

techniques to quantify the overall strength of the latitudinal

gradient in range size globally, and to examine several key issues

related to the analysis of the Rapoport effect. Meta-analysis

allows the estimation of an overall magnitude of the Rapoport

effect by taking into account not merely the number of signific-

ant and non-significant outcomes obtained by different studies,

but also the strength of the latitude:range size association, as

expressed by a general measurement of ‘effect size’. A weighted

overall effect size is estimated by taking into account that the

non-systematic variance of estimates of effect size is inversely

proportional to the sample size of the studies on which estimates

are based (Hedges & Olkin, 1985; Rosenberg

et al

., 2000). Then,

such estimations of effect sizes, insensitive to the effects of

sample size, are useful for evaluating consistency among studies

and the kind of biological and non-biological moderator variables

that might be responsible for heterogeneity in the results obtained

by different authors (see Hedges & Olkin, 1985; Rosenberg

et al.

,

2000). More specifically, in the present study we evaluate

whether the strength of the latitude:range size association is

significantly correlated with the extent and focus of the study

(Rohde, 1996; Gaston

et al

., 1998; Gaston & Chown, 1999).

Although it has been suggested that the pattern is largely con-

fined to high latitudes in the Northern Hemisphere (Rohde,

1996; Gaston

et al

., 1998), the degree to which the scale and/or

the focus of studies affects the perception of the Rapoport effect

is unknown. Here, we evaluate the effect of the geographical

and taxonomic focus of a study by analysing whether the pattern

differs in strength among different taxa, habitats and/or regions

of the world.

METHODS

Choice of data

We conducted a systematic search for papers that contain the

words ‘Rapoport’ and ‘Rule’ in two literature data bases, which

retrieved 100 (JSTOR) and 79 (ISI Web of Science) papers that

matched this query. Out of the total number of papers found,

we selected a working list of 49 papers (see reference list) that

quantified the relationship between latitude and range size, thus

complementing the lists of papers previously reported by Gaston

et al

. (1998) and Gaston (2003). We distinguished among studies

that used four broad families of methods to quantify the latitudi-

nal range size variation.

(1)

Band methods

average species ranges over latitudinal bands

to quantify the latitudinal range size patterns, with three main

variants: (i) Stevens’ (1989) method shows the latitudinal

variation in the range sizes of all species recorded at each

band; examples include France (1992), Letcher & Harvey (1994),

Macpherson & Duarte (1994), Roy

et al

. (1994), Ruggiero

(1994), Hughes

et al

. (1996), Kalin-Arroyo

et al

. (1996), Price

et al

. (1997), Stevens & Enquist (1998), Santelices & Marquet

(1998), Eeley & Foley (1999), Sax (2001), Husak & Husak (2003),

Fortes & Absalao (2004) and Arita

et al

. (2005); (ii) Rohde

et al

.’s

(1993) method shows the latitudinal variation in the range sizes

of all species whose midpoints lie on each band; examples

include Rohde & Heap (1996), Lyons & Willig (1997), Mourelle

& Ezcurra (1997), Roy

et al

. (1998), Ruggiero & Lawton (1998),

Hecnar (1999) and Koleff & Gaston (2001); and (iii) Pagel

et al

.’s

(1991) method shows the latitudinal variation in the range sizes

of all species whose most distal points lie on each band; examples

include Lyons & Willig (1997) and Ruggiero (1999).

(2) The

individual species as independent points methods

use

the geographical range and location of each species as independent

data points for the analysis of the latitudinal range size variation.

Examples include Juliano (1983), Smith

et al

. (1994), Taylor &

Gotelli (1994), Blackburn & Gaston (1996), Meladiou &

Troumbis (1997), Price

et al

. (1997), Murray

et al

. (1998),

Noonan (1999), Harcourt (2000), Gillespie (2002), Ramos de

Sant’Ana

et al

. (2002), Garcillán

et al

. (2003), Macpherson

(2003), Reed (2003), Valgas

et al

. (2003), Pfenninger (2004) and

Folgarait

et al

. (2005).

(3)

Phylogenetic comparative methods

(hereafter referred to as

PC methods) use different techniques to control for phylogenetic

effects while estimating the magnitude of the latitude:range size

association. Individual species are not statistically independent

sample points and phylogenetic effects on range size have been

reported by a number of authors (e.g. Jablonski, 1987; Ricklefs &

Latham, 1992; Brown, 1995). Phylogeny has been taken into

Meta-analysis of the Rapoport effect

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403

account in several analyses of the Rapoport effect to evaluate

whether there has been a positive range size:latitude association

through evolutionary time. Examples include Blackburn &

Gaston (1996), Edwards & Westoby (1996), Cowlishaw & Hacker

(1997), Price

et al

. (1997), Blackburn

et al

. (1998), Johnson

(1998), Ruggiero & Lawton (1998), Harcourt (2000), Diniz-

Filho & Tôrres (2002), Cardillo (2002), Olifiers

et al

. (2004),

Pfenninger (2004) and Cruz

et al

. (2005).

(4)

Across cell analyses

: studies that map directly the mean

variation of range size over continents (see references in the

Introduction) usually correlate the values of mean range size and

latitude for each cell in the map to provide a statistical estimation

of the magnitude of the Rapoport effect.

Estimation of effect sizes

In the present study we used the correlation coefficient (

r

) and

the number of observations (

n

) reported in studies to estimate a

common measure of effect size (Fisher’s

Z

-transform,

Zr

; for a

formal definition see Hedges & Olkin, 1985) using MetaWin v.2

(Rosenberg

et al

., 2000). If correlation coefficients were not avail-

able, we square-root transformed

R

2

and converted

t

-values to

r

(Rosenberg

et al

., 2000; Hillebrand, 2004).

Ideally, each independent study should be included only once

in a single meta-analysis to avoid pseudoreplication (Rosenberg

et al

., 2000). However, in the present case, the implementation of

this decision was complicated because most often studies provide

several estimations of effect size. It is a common practice in most

macroecological papers to evaluate the consistency of the latitu-

dinal range size patterns estimated in several ways. For instance,

within any single study, the latitude:range size association may be

estimated first by using species as independent points, then by

averaging ranges sizes over latitudinal bands and finally by the

calculation of independent contrasts. It is also common to

provide estimations of the latitude:range size association for

different subsets or regrouping of data, i.e. at the global scale and

then dividing the same data into Northern and Southern

Hemispheres, or considering division of all the data into different

subsets (see examples in Table 1 of Gaston

et al

., 1998). As a

consequence, we extracted a total of 246 estimations of effect

sizes from a total of 49 studies analysed. The estimation of an

overall cumulative effect size based on all these data taken

together is affected by the high level of pseudoreplication present

(246/49 or

c.

5). Thus, we conducted preliminary analyses based

on the whole data set to examine whether differences in the mag-

nitude of the pattern were associated with different methods of

analysis. Then, we conducted separate and specific meta-analyses

on the outcomes of studies that measure the latitudinal variation

in species range sizes in different ways.

We reasoned that the unit of analysis of those studies that test

the Rapoport effect through averaging range sizes over latitudinal

bands (i.e. those applying Stevens’, Rohde’s and Pagel’s methods)

are not strictly comparable to the unit of analyses of studies that

use species as independent sample points (i.e. individual ranges)

or examine correlated patterns of changes in both variables

through time using phylogenetic comparative methods (i.e.

calculating phylogenetically independent contrasts). On the

other hand, given the problem of spatial autocorrelation inherent

in Stevens’ method (see Rohde

et al

., 1993, for discussion) a great

number of studies based on this method do not provide a

quantitative estimation of an effect magnitude but rely only on visual

interpretation of trends. In this case, we calculated effect sizes

from values within the graphs. These estimations of effect size are

not based on the exact values obtained by the original researcher

but on the relative positions of data points obtained after digiti-

zation. Thus, to evaluate the effect of several biological and

methodological moderators on the magnitude of the latitude:

range size association, it seemed reasonable to conduct a separate

meta-analysis on the outcomes of studies that showed the latitu-

dinal range size variation based on different methods.

We also repeated the analyses on a restricted subset of data to

evaluate the robustness of trends after further reduction of the

level of pseudoreplication. The inclusion of papers within this

reduced subset involved the application of several criteria. If a

paper provided several estimations of effect sizes based on the

same data base, we took into account: (1) the geographical scale

— those estimations based on global-scale analyses were included

over those based on single continents or biogeographical regions;

those based on the analysis of both hemispheres were included

over those based on the Northern or the Southern Hemispheres;

(2) taxonomic inclusiveness — those estimations of effect sizes

based on a high-order taxon (e.g. mammals) were included over

those based on subordinate taxa (e.g. primates, carnivores); and

(3) range size estimations — effect sizes based on the use of linear

estimations of range sizes (i.e. latitudinal extent) were included

over those based on areal estimations. The level of pseudoreplica-

tion after the application of these inclusion criteria decreased

considerably, from

c.

5 (246/49: all effects/all studies) to

c.

2

(38/20: Stevens’ method),

c.

1.75 (21/12: Rohde’s method),

c.

1.75 (42/24: individual species as independent points) and

c.

1.4 (14/10: phylogenetic comparative methods). The list of

papers with estimations of effect sizes used and the results

obtained from this reduced subset of data are provided in

Appendix S1 in Supplementary Material.

Choice of moderator variables

Each estimation of effect size was classified with respect to the

following eight variables characterizing aspects of methodology,

taxonomy and biogeography that we considered might poten-

tially affect the magnitude of the Rapoport effect. (1) Extent of

study. This is a continuous quantitative variable. The latitudinal

range of each study was estimated as the difference between the

northernmost and southernmost latitudinal limits of the study.

The approximate latitudinal limits of the regions/continents

studied were used for those studies that did not report the exact

latitudinal limits. (2) Unit of analysis — a categorical variable

with three levels: bands, contrasts, species as independent points.

(3) Type of band method — a categorical variable with three

levels, namely Stevens’, Pagel’s and Rohde’s methods. (4) High

order taxon — a categorical variable with seven levels: mammals,

birds, invertebrates, vascular plants, fishes, reptiles, amphibians.


© 2007 The Authors

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(5) Type of habitat — a categorical variable with five levels:

terrestrial, marine, freshwater, freshwater and coastal, freshwater

and terrestrial. (6) Terrestrial regions — a categorical variable

with 10 levels: Eurasia (in this level were included studies focused

on Eurasia, Europa, Asia, Palaearctic, Southeast Asia), Africa,

Madagascar, Holarctic, North America (included studies that

focused on North America, North and Central America), Central

America, South America (

idem

South America, Neotropics,

Andes), New World, Australia, Global. (7) Marine regions — a

categorical variable with three levels: Atlantic, Pacific (included

studies that focused on Pacific, Indo-Pacific), Pacific/Atlantic.

(8) Hemisphere — a categorical variable with three levels:

Northern, Southern, both.

Analyses of data

We conducted a weighted regression analysis in

metawin

2.0

(Rosenberg

et al

., 2000) to test for the effect of continuous

moderator variables on the magnitude of the Rapoport effect.

This allows us to estimate whether the total amount of hetero-

geneity explained by the continuous moderator variable (

Q

M

)

is statistically significant.

metawin

also allows us to test

whether categorical variables explained a significant amount of

heterogeneity in the effect sizes using a random effect model,

through comparing the overall heterogeneity in the effect sizes

explained by the categorical variable (

Q

B

) with the residual

heterogeneity (Rosenberg

et al

., 2000). The significance levels

(

P

) for the analyses of heterogeneity are obtained by 999

randomizations. Non-overlapping 95% confidence intervals

(95% CI) indicate a significant difference between levels within

categorical variables.

One disadvantage of the meta-analytic approach is that it does

not allow the interactions between explanatory variables to be

quantified. It is not possible to formally test the extent to which

the effect of the method of analysis interacts with the effect of

taxa and/or biogeographical regions to account for differences in

the magnitude of the Rapoport effect. Note, for instance, that the

distribution of taxa and biogeographical regions within different

methods of analysis is not homogeneous (results from a

G

-test:

G

taxa

= 101,

χ

2

(0.05; 28) = 41.34,

P

= 0.003;

G

biog reg

= 156.3,

χ

2

(0.05; 48) = 65.17,

P

< 0.001; see also below); neither is

the distribution of taxa within biogeographical regions (

G

=

362.5,

χ

2

(0.05; 96) = 119.87). In part, we try to partially over-

come this problem by conducting a separate meta-analysis on

effect sizes obtained through different methods but we must

caution that this only partially reduces the problem.

RESULTS

A preliminary cumulative meta-analysis showed that there is

a significant, although low, Rapoport effect across all data

(

n

= 246):

Zr

= 0.3109 (

r

= 0.30, 95% CI = 0.296, 0.326). However,

the high level of heterogeneity

Q

total

= 4426.884 (

P

< 0.00001)

suggests the existence of significant moderator variables that may

account for differences in the magnitude of the effect within this

data set.

The effect of different methods of analysis on the test of the latitudinal Rapoport effect

When the 246 estimations of effect sizes are taken together, the

method of analysis applied to examine the latitude:range size

association has a significant influence on the magnitude of the

Rapoport effect (

Q

B

= 15.17,

P

= 0.074). Although all methods

suggest an overall positive latitudinal trend in the range size of

species, a stronger latitude:range size association is revealed

through the application of band methods (

Zr

= 0.50, 95% CI =

0.40, 0.61,

n

= 118) and independent contrasts (

Zr

= 0.50, 95%

CI = 0.30, 0.70,

n

= 31) rather than through the use of individual

species as independent points (

Zr

= 0.22, 95% CI = 0.11, 0.33,

n

= 31). The number of studies that quantify the variation

of mean range size across cells in a grid map is too small (

n

= 3)

to reveal significance, however, these kind of analyses still

suggest a positive overall latitudinal pattern (

Zr

= 0.40, 95%

CI =

−

0.87, 1.68).

When we examine in more detail the outcomes of studies that

applied different band methods, it is clear that the magnitude

of the Rapoport effect is affected by the way studies average

the range size of species over latitudinal bands. Studies that

applied Stevens’ method show a significant and strong positive

latitudinal pattern in the mean range size of species (

Zr

= 0.88,

95% CI = 0.62, 1.14,

n

= 60); the same positive — although non-

significant — trend is suggested by studies that applied Pagel’s

method (

Zr

= 0.67, 95% CI =

−

0.17, 1.51,

n

= 8), but those that

followed Rohde’s do not reveal latitudinal trends (

Zr

= 0.03, 95%

CI =

−

0.26, 0.32,

n

= 50).

After diminishing the level of pseudoreplication in the

reduced subset of data there are still differences in the strength of

the pattern associated with different methods of analysis (see

Table S1 in Supplementary Material). Studies that applied

Stevens’ method show a significant and strong positive latitudinal

pattern in the mean range size of species. A positive, though

weaker, latitude:range size association is suggested by studies that

use individual species as independent points. In contrast, no

significant latitudinal trends, though overall still positive, emerge

from studies that applied Rohde’s and phylogenetic comparative

methods (Table S1). Fail-safe tests indicate that the significance

of results obtained from studies that applied Stevens’ method or use

individual species as independent points is not seriously affected

by the file-drawer problem (i.e. the deficit of non-significant

published studies; Arnqvist & Wooster, 1995; Rosenberg

et al

.,

2000). The year of publication also does not account for dif-

ferences in the effect sizes reported by different studies. This con-

firms that the method chosen to analyse the latitudinal variation

in range sizes can be considered a significant moderator variable

that accounts for differences in the magnitude of the Rapoport

effect reported in the published literature. In contrast, other

methodological decisions, such as the use of linear (i.e. latitudinal

extents) or areal estimations of range do not affect the outcomes

of the analyses (

Q

B

= 0.38,

P

= 0.704,

n

= 246; Stevens’ method:

Q

B

= 0.153,

P

= 0.743; Rohde’s method:

Q

B

= 0.119,

P

= 0.733;

independent contrasts:

Q

B

= 1.499,

P = 0.375; individual species

as independent points: QB = 4.792, P = 0.054).


© 2007 The Authors Global Ecology and Biogeography, 16, 401–414, Journal compilation © 2007 Blackwell Publishing Ltd 405

The effect of geographical scale and location of the latitudinal limits of studies on the test of the latitudinal Rapoport effect

The latitudinal extent of the study does not have an influence on

the magnitude of the Rapoport effect when the 246 estimations

of effect sizes are analysed together (slope: s = −0.0006, P =

0.106). The division of the data into different methods (Fig. 1 &

Table S2 in Supplementary Material) shows that Stevens’

band method suggests a significant tendency for the strength of

the pattern to decrease with the extent of the study, although the

proportion of variance in the magnitude of the Rapoport effect

explained by the extent of the study is low (Fig. 1 & Table S2).

The analysis of the effect of the most extreme latitudinal limits

of studies offers another way to test for the effect of geographical

location on the magnitude of the Rapoport effect. The 246 esti-

mations of effect sizes analysed together show no evidence of a

significant effect of the northernmost latitudinal limit of studies

on the magnitude of the Rapoport effect (s = −0.0006, d.f. = 243,

P < 0.098; QM = 0.61, P = 0.434). A tendency for the Rapoport

effect to be stronger as the northern latitudinal limits of studies

extend towards the north is found only within studies that used

species as independent points (Fig. 2 & Table S2). Rather there

is a more consistent effect of the location of the southernmost

limits of studies on the magnitude of the Rapoport effect

(s = 0.0049, d.f. = 243, P = 0.001; QM = 21.04, P < 0.00001). The

outcomes of analyses applying Stevens’, Pagel’s and PC methods

suggest that the latitudinal Rapoport effect is less detectable as

the latitudinal limits of studies extend towards the south (Fig. 3

& Table S2).

Figure 1 Test of the effect of the extent of the study on the magnitude of the Rapoport effect: S, slope obtained from weighted regression analysis; P, probability values; QM, amount of heterogeneity in the data explained by the latitudinal extent of studies; Zr, Fisher’s Z-transformation of correlation coefficients.


© 2007 The Authors406 Global Ecology and Biogeography, 16, 401–414, Journal compilation © 2007 Blackwell Publishing Ltd

The effect of focus of the study on the test of the latitudinal Rapoport effect

The 246 estimations of effect sizes analysed together show signi-

ficant differences in the magnitude of the Rapoport effect across

taxa (QB = 64.12, P = 0.001). Nonetheless, after dividing the data

into different methodological approaches, studies that applied

band methods do not reveal differences in the magnitude of the

Rapoport effect across taxa in all analyses performed (Fig. 4 &

Table S3 in Supplementary Material). Taxonomic effects are per-

ceived only in studies that use individual species as independent

points or examine the latitude:range size association through

time by the estimation of independent contrasts (Fig. 4). Studies

that used individual species as independent points show that

birds conform to a negative — though not significant — overall

pattern, which differs from the overall positive trends shown in

other taxa. Studies that applied PC methods confirm an overall

tendency for the range size of species to increase with latitude

through time only for mammals (Fig. 5). These taxonomic

effects are not very robust to the extent that they are not con-

firmed in the reduced subset of data (Table S3).

The present study shows that there is no significant overall

effect of the type of environment on the magnitude of the

Rapoport effect (QB = 10.94, P = 0.316, n = 246). Although there are

no significant differences in the magnitude of the pattern across

habitats, there is a tendency for the Rapoport pattern to be

stronger in freshwater (Zr = 0.94, 95% CI = 0.20, 1.69, n = 5) and

terrestrial environments (Zr = 0.43, 95% CI = 0.43, 0.52, n = 156)

compared to marine ones (Zr = 0.26, 95% CI = 0.11, 0.40, n = 58).

In addition, dividing the data into different methods of analysis

does not reveal any habitat effect (Stevens’ method: QB = 9.507,

P = 0.095; Rohde’s method: QB = 4.703, P = 0.232; Pagel’s

method: QB = 0.033; P = 0.824; individual species as independent

points: QB = 4.442, P = 0.202; see also Table S3).

There is a tendency for the latitude:range size association to be

stronger in the Northern Hemisphere (QB = 25.55, P = 0.006,

Figure 2 Test of the effect of the location of the northern latitudinal limit of the study on the magnitude of the Rapoport effect: S, slope obtained from weighted regression analysis; P, probability value; QM, amount of heterogeneity in the data explained by the location of the northern latitudinal limits of studies; Zr, Fisher’s Z-transformation of correlation coefficients.



n = 246), however, the significant effect of hemisphere is

confirmed only within studies that applied Stevens’ method

(QB = 10.840, P = 0.016). No significant differences in the mag-

nitude of the Rapoport effect between hemispheres is observed

within studies concerned with other methods of analysis (Rohde’s

method: QB = 5.645, P = 0.078; Pagel’s method: QB = 3.404,

P = 0.088; individual species: QB = 3.176, P = 0.300; PC methods:

QB = 1.757, P = 0.656); neither is the effect of hemisphere

confirmed in the reduced data set (Table S3).

When the total estimations of effect sizes are considered as

a whole, there are significant differences in the magnitude of

the Rapoport effect across different terrestrial biogeographical

regions (QB = 113.28, P = 0.001, n = 188). The strong positive

patterns found in the northern landmasses (North America:

Zr = 1.11, 95% CI = 0.90, 1.32, n = 30, Eurasia: Zr = 0.76, 95%

CI = 0.53, 1.00) contrast with the weak trends found at the global

scale (Zr = 0.15, 95% CI = −0.18, −0.48, n = 13) and in South America

(Zr = 0.29, 95% CI = 0.12, 0.45, n = 45) or the negative patterns

reported in Australia (Zr = −0.20, 95% CI = −0.67, 0.27, n = 9)

and Central America (Zr = −0.50, 95% CI = −0.82, −0.18, n = 13).

Dividing the data into different methods of analysis basically

suggests the same trends. Effect sizes derived from the application

of Stevens’ band method suggest that the strongest Rapoport

pattern is found in North America. The positive latitude:

range size association there differs significantly from patterns

shown in South America, in the New World or in Australia (Fig. 5).

Studies that used individual species as independent points

also suggest a stronger Rapoport effect for analyses restricted to

North America and Eurasia (Palaearctic) regions. In general,

North America differs significantly from the positive — though

weaker — latitudinal trends found in the New World, South

America, Australia and the whole world. Eurasia differs signi-

ficantly from the trends found in South America and the whole

world. Also, the negative latitudinal pattern found in Central

Figure 3 Test of the effect of the location of the southern latitudinal limit of the study on the magnitude of the Rapoport effect: S, slope obtained from weighted regression analysis; P, probability value; QM, amount of heterogeneity in the data explained by the location of the southern latitudinal limits of studies; Zr, Fisher’s Z-transformation of correlation coefficients.



America is quite distinct from overall trends found in other parts

of the world (Fig. 6 & Table S3). Studies focused on marine environ-

ments differ from those focused on terrestrial ones given that

they do not show a significant difference in the magnitude of the

Rapoport effect across different oceans (all estimations of effect

size taken together: QB = 3.60, d.f. = 57, P = 0.517; Stevens’

method: QB = 0.151, P = 0.95; Rohde’s method: QB = 1.331,

P = 0.549; see also Table S3).

DISCUSSION

The generality of Rapoport’s rule revisited: is the pattern confined to some taxa, environments or regions of the world?

Although, at present, it is widely accepted that Rapoport’s rule is

a local phenomenon restricted to high latitudes in the Northern

Hemisphere (Rohde, 1996; Gaston et al., 1998; Gaston & Chown,

1999), this idea was recently questioned by Cardillo (2002) who

suggested that the pattern could be more general than is

currently recognized. He found that the latitudinal range sizes

of birds follow a latitudinal Rapoport effect at a global scale,

although the pattern is not as strong in the Southern Hemisphere

compared with the Northern Hemisphere (but see also Orme

et al., 2006). The present meta-analysis shows that both views are

partially supported by evidence. On the one hand, we found that

there is a significant latitudinal increase in the range size of

species across all data considered, although the magnitude of the

pattern is low (r = c. 0.30) and shows systematic variation that

can be associated with both methodological and biological

moderator variables. On the other hand, specific meta-analysis

after the separation of the data into different methodological

approaches suggested that trends at a global scale are weak,

although there is a strong increase in the range sizes of species

with latitude in the northern landmasses (Eurasia and North

America) as compared with South America and Australia. Thus,

Figure 4 Test of the effect of taxonomy on the magnitude of the Rapoport effect: QB, amount of heterogeneity in the data explained by taxonomic effects; P, probability level; Zr, Fisher’s Z-transformation of correlation coefficients. Significant differences between levels of categorical variables are indicated by non-overlapping 95% confidence intervals.



we confirm that, in contrast to the ubiquitous latitudinal gradi-

ent in species diversity (Hillebrand, 2004) or the species–area

relationship (Drakare et al., 2006), the Rapoport effect is a ‘local

phenomenon’ (sensu Rohde, 1996).

The lack of generality of the Rapoport effect at the global scale has

often been understood as reflecting differences in land availability

and climatic conditions between the Northern and Southern

Hemispheres (e.g. Gaston & Chown, 1999; Cardillo, 2002).

Nonetheless, the present meta-analysis shows that there is no

significant effect of hemisphere on the magnitude of the pattern

for most methods of analysis (all but Stevens’). Neither are there

differences in the magnitude of the pattern that can be associated

with differences in habitat type, although a weak tendency for the

pattern to be stronger in mainland rather than in marine habitats

is suggested. This opens the possibility that differences in the

magnitude of the Rapoport pattern across northern and southern

continents are probably mediated through the role of biogeo-

graphical factors rather than being the consequence of climatic

or ecological differences between the Northern and Southern

Hemispheres.

Although we think that there is considerable added value in

the application of a meta-analysis to study the Rapoport effect,

we are aware of the kind of potential problems associated with

this kind of quantitative revision. The application of a meta-

analysis, by definition, obscures the details of particular cases,

which represents considerable loss of biological information (see

e.g. Simberloff, 2006, for a related discussion on this specific

topic). Such kinds of detail are evident only in those primary

works that applied a truly geographical perspective (e.g. Hawkins

& Diniz-Filho, 2006; Orme et al., 2006). On the other hand,

although doing a specific meta-analysis for subsets of data,

as performed in the present study, allows the evaluation of the

effect of specific taxonomic or geographical variables on the

magnitude of the Rapoport effect, there are still some other issues

that complicate the interpretation. Specifically, the uneven

distribution of studies across taxa or regions (Fig. 6) produces an

interaction between the imprints of geographical and taxonomic

effects on the magnitude of the pattern that potentially may

influence conclusions derived from the present study. Thus, the

significant Rapoport effect in Africa could be because the majority

of studies conducted there are focused on mammals rather than

due to pure geographical effects. The extreme negative value of

Central America can be due to the influence of most of the studies

there being based on birds (Fig. 6). Given the paucity of studies

for Africa and Central America we are, at present, unable to

disentangle the relative role of both factors (taxonomic and

biogeographical) on the magnitude of the pattern found in these

regions. In contrast, we can affirm that the negative trend found

in Australia is truly geographical, given the sufficient number of

studies focused on different taxa there that suggest the same neg-

ative pattern. We can have confidence in the effect of taxonomy

on effect sizes derived from studies that applied the independent

contrast method, which do not reveal a significant effect of bio-

geographical region. Similarly, we can be confident that there is

an effect of geography affecting the magnitude of the Rapoport

effect derived from studies that used Stevens’ method which, in

Figure 5 Test of the effect of geographical region on the magnitude of the Rapoport effect: QB, amount of heterogeneity in the data explained by geography. Zr, Fisher’s Z-transformation of correlation coefficients. Significant differences between levels of categorical variables are indicated by non-overlapping 95% confidence intervals.



turn, do not reveal a significant effect of taxonomy on the

magnitude of the Rapoport effect. The interaction between

taxonomic and geographical effects remains an issue that deserves

further study.

Are all methodological approaches equally useful for understanding the pattern?

We showed that indeed the method of analysis used to show the

latitudinal variation in range size is a strong moderator variable

of the magnitude of the Rapoport pattern. Actually, it is one of

the reasons for the existence of equivocal evidence in support of

the pattern. In general, studies that average range sizes over

latitudinal bands or examine the relationship between range size

and latitude through the estimation of phylogenetically in-

dependent contrasts show more consistent overall positive latitudinal

trends in range sizes than studies that use individual species as

independent points or examine the variation in the mean range

size within cells. The same tendency for phylogenetic methods to

provide significant evidence in support of the Rapoport effect (in

contrast to non-significant latitudinal variation found by cross-

species analysis) was suggested in an independent analysis of

global range size patterns in birds (Cardillo, 2002). Differences in

the outcomes of phylogenetic and non-phylogenetic approaches

applied to the analysis of patterns in species ranges may be rather

expected given that only phylogenetic methods are able to reveal

the evolutionary history underlying range size patterns (Black-

burn & Gaston, 2006). However, the present study emphasizes

that studies that take into account the effects of phylogeny could

indeed be more appropriate than band methods to test the

Rapoport pattern across different taxa.

In our opinion, the very contrasting results that can be

obtained from the application of different methods cannot be

understood as merely representing complementary information

on species range patterns (see also Lyons & Willig, 1997). The

methods applied to show the latitudinal variation in range sizes

represent very different ways of reducing the original informa-

tion to basic data susceptible to analysis by the statistical method.

This is an important issue to take into account, particularly when

statistics is taken as the ultimate indication of the latitude:range

size association. For instance, as shown by the present analysis,

using mean range sizes averaged over bands as basic units for

statistical analysis will tend to reduce the number of observations,

and hence the power of statistical tests. Trends will eventually

vanish when information is reduced to the extreme so as to con-

form to the statistical assumptions of independence through the

application of Rohde’s method. As shown by the present analysis,

Rohde’s method tends to reveal only a weak overall positive lati-

tudinal trend in range size throughout, which is also insensitive

to the effects of most moderator variables considered. On the

other hand, although using individual species as independent

points may greatly increase the number of observations, it intro-

duces a great amount of dispersion into the data.

Rohde (pers. comm.; see also Rohde et al., 1993; Rohde, 1999;

Stauffer & Rohde, 2006) emphasizes that his midpoint method

makes better biological sense because it is likely that species have

originated somewhere near the midpoint of their distributions,

from which they have spread into adjacent latitudes, rather than

at the margins of their distributions. This implies, for instance,

an assumption that widespread species that cross tropical lati-

tudes are likely to be of tropical origin and, thus, should be

excluded from the analyses at high latitudes. The assumption

about the tropical origin of widespread species is rather related to

the idea that latitudinal gradients in species diversity may have

arisen as the result of faster evolutionary rates at high tempera-

tures (Rohde, 1992; Jablonski, 1993; Wright et al., 2006), which

implies that many new species may have arisen predominantly in

Figure 6 Relative distribution of effect sizes estimated on (a) geographical region within each method, (b) taxa within each method, and (c) taxa within each geographical region. Abbrevia-tions: Afr, Africa; Atl, Atlantic Ocean; Aus, Australia; CAm, Central America; Eur, Eurasia; G, global; Hol, Holarctic; Mad, Madagascar; NAm, North America; NWo, New World; P/A, Pacific and Atlantic Oceans; Pac, Pacific Ocean; SAm, South America.



the tropics from where they spread into higher latitudes (e.g.

Rohde & Stauffer, 2005; out of the tropics model, Jablonski et al.,

2006). However, we think that caution is needed when making

generalizations about the origin of species based exclusively on

the location of midpoints of species’ present distributions (see

also Stauffer & Rohde, 2006). Historical shifts in distributions

complicate the identification of the area of origin of species (see

Jablonski et al., 2006, for discussion). Provided that phylogenetic

relationships among species are taken into account, such historical

shifts can be traced back to make a hypothesis of the likely

location of a species’ ancestral area (e.g. Nelson, 1969; Bremer,

1992). Thus, even when Rohde’s method is correct from the

statistical point of view, we think it is worth asking whether it

is equally valid from a biological perspective.

Studies that incorporate a geographical perspective into the

test of the Rapoport effect often use data taken from cells in a

grid map as the basis for the statistical analysis. However, after

the extraction of data from the cells, the geographical structure is

lost (unless specific spatial statistics is incorporated into the ana-

lysis) and the use of individual cells as units of analysis introduces

a great amount of dispersion into the data, which may result in

weaker and more variable trends. Within a geographical context,

the existence of the Rapoport effect could be more easily deducible

directly from visual examination of maps on a simple qualitative

basis. In contrast, if the analysis is on an ‘individual species

focus’ (sensu Ruggiero & Hawkins, 2006) the application of the

phylogenetic comparative method would be recommended to

reduce the original information according to a phylogenetic

design that, at the same time, produces statistically independent

basic data.

The present meta-analysis suggests that the decision of using

areal or linear estimations of range sizes has no significant effect

on the magnitude of the Rapoport effect reported in the literature.

This is likely reflecting that the magnitudes of the latitudinal and

longitudinal axes of species distributions tend to be positively

correlated (Gaston et al., 1998). The scale of analysis, as estimated

by the extent and geographical location of the latitudinal limits

of studies, also has a minimal effect on the magnitude of the

Rapoport effect. The confinement of the Rapoport effect to some

continental regions of the world might partially explain the lack

of influence of the spatial scale of analysis on the magnitude of

the effect. This contrasts with the significant effects of spatial

scale known to influence large-scale patterns in species diversity

(e.g. Hillebrand, 2004; Rahbek, 2005; Drakare et al., 2006).

We think it is timely to begin methodological agreements to

ease comparison between the outcomes of different studies.

Although so far the most common method of studying Rap-

oport’s rule is to plot a measure of range size versus latitude, this

should not represent a conceptual constraint for critical assessment

of current methods and the development of new approaches.

After more than a decade of intense study of this pattern, it is

difficult to disentangle the role of competing hypotheses invoked

to explain the spatial variation in the range size of species at

the continental scale (e.g. Brown, 1995; Gaston et al., 1998).

The present study shows that the different methodological

approaches applied so far to test the Rapoport effect have had

a strong impact in our perception of the pattern and have com-

plicated the interpretation. Even when a statistical approach is

adopted to disentangle the role of competing hypotheses, the way

the basic information on species distribution is compiled and

reduced for statistical analysis depends strongly on the way in

which the relationship between latitude and range is shown. As

also suggested by Lomolino et al. (2006), coordinated efforts are

needed to move conceptually from the primary objective of test-

ing the pattern to a more integrative approach that allows the

elaboration and testing of a specific hypothesis that may account

for differences in the magnitude of the pattern between different

taxa and regions of the world.

ACKNOWLEDGEMENTS

H. Hillebrand, P. Rodríguez Moreno, K. Rohde, J. Lockwood and

an anonymous referee gave valuable comments and suggestions

to improve the manuscript. F. Cruz, J.A.F. Diniz-Filho, P.P. Gar-

cillán, S.A. Juliano and M. Pfenninger contributed with papers,

original data and information that otherwise would have been

unavailable to us. A.R. and V.W. receive financial support from

CONICET (PIP2005-5113), Universidad Nacional del Comahue

and Agencia-FONCYT (PICT2002/01-11826).

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Editor: Julie Lockwood

SUPPLEMENTARY MATERIAL

The following supplementary material is available for this article:

Appendix S1 List of papers and estimations of effect sizes and

variance estimated from the data set with the lower level of pseudo-

replication (reduced data set)

Table S1 Results from cumulative meta-analysis of studies that

test the Rapoport effect using different methods, based on the

restricted data set with the lower level of pseudoreplication

Table S2 Test of the effect of extent of the study on the test of the

Rapoport effect, based on the restricted data set with the lower

level of pseudoreplication

Table S3 Test of the effect of taxonomic, ecological and geo-

graphical focus of the study on the magnitude of the Rapoport

effect, based on the restricted data set with the lower level of

pseudoreplication

This material is available as part of the online article from:

http://www.blackwell-synergy.com/doi/abs/10.1111/j.1466-

8238.2006.00303.x

(This link will take you to the article abstract).

Please note: Blackwell Publishing is not responsible for the con-

tent or functionality of any supplementary materials supplied by

the authors. Any queries (other than missing material) should be

directed to the corresponding author for the article.

BIOSKETCHES

Adriana Ruggiero is a researcher interested in

biogeography and macroecology.

Victoria Werenkraut is a graduate student at the

Universidad Nacional de Buenos Aires (Argentina).

She is doing her doctoral research at Laboratorio Ecotono,

at the Universidad Nacional del Comahue. She is studying

altitudinal patterns in species richness, geographical range

and body size in insects in north-west Patagonia,

Argentina.

http://www.blackwell-synergy.com/doi/abs/10.1111/j.1466-

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