ORIGINALARTICLE

Areography of the genus Dendroctonus(Coleoptera: Curculionidae: Scolytinae)in Mexico

Yolanda Salinas-Moreno1, Ma. Guadalupe Mendoza1, Miguel A. Barrios2,

Ramon Cisneros1, Jorge Macıas-Samano3 and Gerardo Zuniga1*

1Laboratorio de Variacion Biologica y

Evolucion, Departamento de Zoologıa. Escuela

Nacional de Ciencias Biologicas-IPN. Prol. de

Carpio y Plan de Ayala s/n, Del. Miguel

Hidalgo, 2Laboratorio de Fanerogamas,

Departamento de Botanica. Escuela Nacional

de Ciencias Biologicas-IPN. Prol. de Carpio y

Plan de Ayala s/n, Del. Miguel Hidalgo and 3El

Colegio de la Frontera Sur. Carretera Antiguo

Aeropuerto km 2.5 Tapachula, Chiapas,

Mexico

*Correspondence: Gerardo Zuniga, Laboratorio

de Variacion Biologica y Evolucion,

Departamento de Zoologıa, Escuela Nacional de

Ciencias Biologicas-IPN, Prolongacion de

Carpio y Plan de Ayala s/n, Col. Sto. Tomas.

C. P. 11340, Del. Miguel Hidalgo, Mexico D.F.

E-mail: gzuniga@bios.encb.ipn.mx (or)

gzuniga_1960@yahoo.com

ABSTRACT

Objective To analyse whether the geographical ranges of Dendroctonus species

are (1) associated with factors such as host species or elevation, and (2) in

agreement with Halffter’s Nearctic distribution pattern. (3) To identify and

discuss the factors that are likely to act as barriers to the genus’ geographical

distribution. (4) To explore whether there is an association between the size of the

geographical ranges of Dendroctonus species and the number of Pinus host species

used by each of them, and (5) to assess if these host species are most common at

the elevations preferred by the individual Dendroctonus species.

Site Mexico.

Methods Records of 12 species of Dendroctonus were gathered from

entomological collections in Mexico. Distribution ranges were defined by using

the propinquity method (Rapoport, 1975a). Analysed parameters were: (1)

geographical distribution of single species, (2) overlapping of species ranges, (3)

disjunction patterns and barriers by means of isoprobabilistic lines, based on the

morphotectonic subdivision of Mexico (Ferrusquıa-Villafranca, 1998), (4) spatial

variation in species richness with respect to latitude and altitude, (5) size of

geographical ranges, and (6) host species for each Dendroctonus species. A

correlation was determined between area size and number of pine host species.

Results The species ranges varied in shape and size. Geographical ranges tend to

be discontinuous in shape. Composite patterns showed that disjunctions among

ranges do not closely follow Mexico’s morphotectonic subdivision. There are

repeated discontinuities among individual distributions, which define five areas:

(1) Baja California Peninsula, (2) Sierra Madre Occidental (SMOC), (3) northern

Sierra Madre Oriental (SMOR), (4) Sierra Madre de Chiapas, and (5)

SMOR + Faja Volcanica Transmexicana (FVT) + Sierra Madre del Sur. The

isoprobabilistic lines confirm that the inner part of SMOC provides an optimal

environment for the genus, and the FVT province constitutes the broader

corridor for it in the country. Richness does not directly decrease or increase with

latitude. Richness behaviour of the insect is not associated with that of its host.

Elevation distributions showed that most Dendroctonus species move within

broad margins of tolerance and species richness is concentrated in the montane

interval. Dendroctonus attack 24 of the 47 Pinus species distributed in Mexico.

Preferred pine species belong predominantly to Leiophyllae, Ponderosae and

Oocarpae subsections. The Spearman rank correlation between area size and

number of pine host species was not significant. Dendroctonus clearly belongs to a

Nearctic distribution pattern (sensu Halffter, 1987).

Main conclusions Dendroctonus is present in all montane systems of Mexico

and its species coexist within a high geographical sympatry. Overlapping of

species distribution appears to be the result of two elements – generalized

Journal of Biogeography (J. Biogeogr.) (2004) 31, 1163–1177

ª 2004 Blackwell Publishing Ltd www.blackwellpublishing.com/jbi 1163

INTRODUCTION

Mexico is a transitional zone between the Nearctic and

Neotropical regions, whose biota are a composition that

originate from two North American tracks (one from the east

and another from the west) and a Gondwanic track related to

Central and South America (Contreras-Medina & Eliosa-Leon,

2001; Morrone & Marquez, 2001). Several biogeographical

studies of the Mexican montane entomofauna indicate that in

this region there is a convergence of different elements of

Nearctic, Paleoamerican and Mesoamerican origin (Ball, 1968;

Mateu, 1974; Halffter, 1976, 1987; Noguera-Martınez &

Atkinson, 1990; Liebherr, 1991, 1994; Llorente-Bousquets &

Escalante-Pliego, 1992; Thomas, 1993). The distribution range

of these taxa resulted from dispersal events, and differentiation

in situ and vicariant processes occurred in a complex

morphotectonical scenario (Thomas, 1993; Ferrusquıa-

Villafranca, 1998).

At the end of the Cretaceous (65 million years ago (Ma))

the Laramidian orogeny started and determined the main

physiographic features of the mountains in Mexico and

northern Central America, with the exception of the Faja

Volcanica Transmexicana (FVT) whose origin was in the

Oligocene (c. 30 Ma). However, the actual confirmation of the

FVT was not finished until the Holocene. The penetration of

Pinus into Mexico occurred in two stages from the Rocky

Mountains (Farjon & Styles, 1997). The first stage was at the

Oligocene in the Sierra Madre Occidental (Mirov, 1967; Miller,

1977), and the second occurred in the Pliocene (5 Ma) at the

edge of the Gulf of Mexico along the Sierra Madre Oriental

(Martin & Harrell, 1957). Climate changes at the Early

Quaternary promoted the diversification of the genus in

Mexico (Eguiluz Piedra, 1985; Styles, 1993; Farjon & Styles,

1997).

In this scenario, big barriers and corridors would be present

for the expansion of the Pinus genus and the associated

phytophagous insects. Subsequently, the climate and geologic

stability in this region, together with the emergence of the

Isthmus of Tehuantepec after the Miocene (24 Ma), favoured

the penetration of the cold-temperate biota towards south of

Mexico, and the invasion of Central America.

Biogeographical patterns of Mexican entomofauna has been

affected by flora movement, environmental heterogeny, and

geographical barriers. Particularly, geographical distribution of

phytophagous insects have been associated with those of its

host. In this context, the morphotectonic conformation of

Mexico was the frame to Pinus diversification, however, the

time of association between this genus and Dendroctonus is

unknown.

The genus Dendroctonus Erichson is a Holarctic taxon

composed of 19 species, distributed from Alaska to Nicaragua

in North and Central America and across the boreal region

of Europe and Asia (Wood, 1982). At present, no global

biogeographical studies of this genus exist. The only infor-

mation available can be found in a taxonomic monograph

for North and Central American species, which includes

geographical distribution maps (Wood, 1982). Regional

studies on the geographical ranges of these species are

common (e.g. Hendrichs, 1977; Perusquıa, 1978; Gudino,

1985; Zuniga et al., 1999), but they do not include systematic

analyses of size, location, deformation and overlapping areas

of Dendroctonus species, which are important for character-

izing local distribution patterns (Udvardy, 1969; Rapoport,

1975a,b, 1979; Letcher & Harvey, 1994; Brown & Lomolino,

1998). Studies of this kind allow identification of habitats

(host), barriers, corridors or ecological variables that

determine their distribution patterns (Rapoport, 1975a;

Kohlmann & Sanchez, 1984; Antunez & Marquez, 1992;

Ruggiero et al., 1998).

The aim of this paper was to analyse several aspects of the

geographical distribution of 12 Dendroctonus species located in

Mexico, which differ in their host specificity from the rest of

the North American species, because they parasite mainly

Pinus Linnnaeus species and rarely have been reported on

other host species (Cibrian-Tovar et al., 1995; Zuniga et al.,

1999). Specifically, we were interested in determining whether:

1. The geographical ranges of Dendroctonus species are associ-

ated with factors such as host or elevation, which ultimately

limit species distributions. For this, we analysed the geograph-

ical distribution of single species (hereafter called individual

distribution) and the overlapping of species geographical ranges

(hereafter called composite pattern), and the disjunction

patterns by means of isoprobabilistic lines. These analysis

allowed us to identify and discuss the factors that are likely to

act as barriers to the geographical distribution (sensu Rapoport,

1975a,b, 1979), and the patterns of resemblance among

geographical ranges. These analyses can indicate the occurrence

of significant environmental transitions (e.g. biogeographical

ecotones), as well as suggest the effect of common historical

and/or ecological constraints on distribution patterns.

polyphagy inside Pinus and a wide elevation tolerance within mountainous

environments. This behaviour, linked to a high vagility, has allowed the genus

Dendroctonus to expand its distribution across Mexico and to employ

mountainous systems as corridors separated by barriers that exert a low

selective filter effect.

Keywords

Dendroctonus, biogeography, Mexico, bark beetle, Pinus.

Y. Salinas-Moreno et al.

1164 Journal of Biogeography 31, 1163–1177, ª 2004 Blackwell Publishing Ltd

2. The geographical ranges of Dendroctonus species agree with

Halffter’s Nearctic distribution pattern (Halffter, 1976, 1987;

Halffter et al., 1995). This pattern is followed by Holarctic and

Nearctic genera with relatively recent penetration into the

Mexican Transition Zone. This pattern predicts a species

richness peak for the mountainous entomofauna above

1700 m, and a decrease in richness as the latitude decreases.

Hence, we test whether the spatial variation in the richness of

Dendroctonus species with respect to latitude and elevation

agrees with these predictions.

3. There is an association between the size of the geographical

ranges of Dendroctonus species and the number of Pinus host

species used by each of them. The size, location and

deformation of the species ranges provide useful information

regarding preferential habitats and ecological requirements of

species. The widespread species are more likely to be generalist

with respect to resource used than species with restricted

geographical ranges, which tend to be specialists (Brown, 1984;

Letcher & Harvey, 1994). We estimated the sizes of the

geographical ranges and the host number of each Dendroctonus

species to determine whether there is a correlation between

them.

4. Finally, we tried to elucidate whether Dendroctonus species

preferentially parasite a specific host and if this host species is

the most common in the preferred elevation of the insect. To

achieve this, frequency tables were developed to show how

many pine species were parasitized by each beetle species,

within the preferred elevation interval.

MATERIALS AND METHODS

Data base

Collection records (museum specimens) of Dendroctonus were

gathered from 10 entomological collections in Mexico

(Table 1), which are well distributed in all the mountain

systems. Geographical distribution analysis assumes that the

identification of each species is correct. Data considered for

each collection record were selected on the basis of specific

criteria (determinator, curatorial work and experience)

increasing the reliability of the geographical distribution

records for each species (Brown et al., 1996). All specimens

were checked personally by authors following the latter criteria,

and the external morphology and seminal rod were used to

determine the species of Dendroctonus. In addition, each

collection was considered as an independent observation. The

data base includes species, determinator, locality, elevation and

host species. A total of 854 records were obtained for the

following species: Dendroctonus adjunctus Blandford, D.

approximatus Dietz, D. brevicomis LeConte, D. frontalis

Zimmermann, D. jeffreyi Hopkins, D. mexicanus Hopkins, D.

parallelocollis Chapuis, D. ponderosae Hopkins, D. pseudotsugae

Hopkins, D. rhizophagus Thomas and Bright, D. valens

LeConte, and D. vitei Wood. These collection data can be

consulted in the following web page http://www.cona-

for.gob.mx.

Geographical distributions

Based on these data, we made geographical distribution maps

(scale ¼ 1 : 5,000,000) for each Dendroctonus species. Distri-

bution boundaries were defined by using the propinquity

method (Rapoport, 1975a), which consists of joining the

nearest locations or nodes by means of lines or arcs. This

procedure is displayed on distribution maps depicting a

maximum propinquity tree with all nodes interconnected. All

arcs were measured and arithmetic means were calculated for

all data sets. With a compass and a radius equal to the mean,

contours were traced around each node and on both sides of

the arcs. Finally, a general contour was marked, resulting in

isolated areas separated by distances larger than two arith-

metic means. Distribution areas of each species were projec-

ted on a standardized grid map of Mexico (quadrants of one

geographical degree on a side). The composite pattern (sensu

Udvardy, 1969) was obtained by overlapping individual

distributions.

Disjunctions

The individual distributions and composite patterns were

described and analysed upon the map of morphotectonic

provinces of Mexico proposed by Ferrusquıa-Villafranca

(1998) (Fig. 1). The morphotectonic scenario was used in this

study only as reference frame to describe the geographical

Table 1 Entomological collections used

Colegio de Posgraduados, Montecillos, Mex. (CP)

Centro de Investigaciones Biologicas, Universidad Autonoma del Estado de Morelos, Cuernavaca, Mor. (CIB-UAEM)

Division de Bosques, Universidad Autonoma de Chapingo, Chapingo, Mex. (DB-UACH)

Escuela Nacional de Ciencias Biologicas, Instituto Politecnico Nacional, Mexico, D.F. (ENCB)

Instituto de Biologıa, Universidad Nacional Autonoma de Mexico, Mexico, D.F. (IB-UNAM)

Instituto Nacional de Investigaciones Forestales Agrıcolas y Pecuarias, Secretarıa de Agricultura, Ganaderıa, Desarrollo Rural,

Pesca y Alimentacion, Mexico, D.F. (INIFAP-SAGARPA)

Instituto de Silvicultura, Universidad Autonoma de Nuevo Leon, Linares, Nvo. Leon. (IS-UANL)

Museo Historia Natural, Mexico, D.F. (MHN)

Sanidad Forestal, Secretarıa del Medio Ambiente y Recursos Naturales, Mexico, D.F. (SF-SEMARNAT)

Sanidad Vegetal, Secretaria de Agricultura, Ganaderıa, Desarrollo Rural, Pesca y Alimentacion, Mexico, D.F. (SV-SAGARPA)

Journal of Biogeography 31, 1163–1177, ª 2004 Blackwell Publishing Ltd 1165

Areography of Dendroctonus in Mexico

distribution from insects. Disjunctions were analysed to

determine whether it is related to the physiographic or

ecologic nature of the habitats within each province.

Isoprobabilistic maps

Isoprobabilistic lines were traced around an arbitrary point

where largest specific richness is present. The quadrants with

the largest specific richness are located at the Faja Volcanica

Transmexicana (FVT), Sierra Madre de Chiapas (SMCH),

Sierra Madre Occidental (SMOC) and Sierra Madre Oriental

(SMOR). From these points, the ratio of number of shared

species to number of species present was calculated for the rest

of the quadrants of the composite pattern. Cells with the same

value were connected by lines, obtaining a map with contour

lines of the same probability that defined those zones where

barriers had been more effective, due to abrupt probability

changes on a short geographical distance.

Resemblance

Based on the fragmentation that described the Dendroctonus

composite pattern, common areas of sympatry or disjunction

areas were established, defining areographic units in which the

general pattern was divided. Then, similarity among these areas

of sympatry was estimated through the Simpson Index (Sanchez

& Lopez, 1988) and data were organized in a similarity matrix.

Species richness patterns

The behaviour of distributions was analysed in an elevation

scenario in relation to those abiotic factors that limit species

distribution. Maximum and minimum limits were determined

for each species, as well as the mean value and standard

deviation. These data were used to represent in box plots the

tendency of the genus. Although we did not have enough data

to compare all 12 species, an attempt was made to use data

from eight species to find out whether each species had a

preferred elevation, that is, the elevation most common to

where a species was collected. The preferred elevation interval

was established for each species using frequency histograms.

These were built using the records found within 500 m

intervals between 1500 and 3500 m elevation range.

To test for the existence of a latitudinal pattern, a species

density map was developed with the total number of species

per cell in the grid map. In addition, mean species density

(med) was calculated for all quadrants in two geographical

degrees, starting at 32� N and ending at 14� N. Data were

displayed on a graph of mean species density vs. latitude.

Locations of maximum richness values were analysed in

relation to historic and ecological characteristics of the

Mexican morphotectonic provinces and in relation to the

Pinus species richness pattern (Farjon & Styles, 1997).

Shape, size and perimeter of geographical ranges

The size and perimeter for each range were determined by

digitizing individual distributions using AUTOCAD (vers. 2000).

The range deformation was defined by the axis that best

described preferred direction of the composite pattern. The

perimeter to �area ratio was calculated for each species:

deviations from a circle (p/�a � 3.5) suggest areas with

barriers to expansion (Rapoport, 1975a).

Range size vs. host

To determine the probable causes for range size, the number

of Pinus host species for each Dendroctonus species, the

degree of occurrence of Dendroctonus species on each host

species (incidence percentage), and the most frequently used

host species per beetle species were determined. A Spearman

rank correlation was conducted between the size of the

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PBC

PSN

SMOC

MCCHyC

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MC

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SMS

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PY

PCG

PCG

Figure 1 Morphotectonic provinces of

Mexico sensu Ferrusquıa-Villafranca (1998).

PBC, Baja California Peninsula; PSN, Plani-

cies and Sierras del Noroeste; SMOC, Sierra

Madre Occidental; MCCHyC, Mesetas and

Cordilleras de Chihuahua y Coahuila; SMOR,

Sierra Madre Oriental; PCG, Planicie Costera

del Golfo; MC, Meseta Central; FVT, Faja

Volcanica Transmexicana; SMS, Sierra Madre

del Sur; SMCH, Sierra Madre de Chiapas; PY,

Plataforma de Yucatan.

1166 Journal of Biogeography 31, 1163–1177, ª 2004 Blackwell Publishing Ltd

Y. Salinas-Moreno et al.

geographical ranges of Dendroctonus and the number of Pinus

host species. To assess a potential association between range

shape and Pinus distribution, a comparison of Dendroctonus

composite pattern and the geographical distribution pub-

lished for the genus Pinus (Farjon & Styles, 1997) was

established.

Finally, to determine how many hosts were parasitized by

Dendroctonus species within their preferred elevation, we

elaborated association tables among host, altitude and Dendr-

octonus species.

RESULTS

Geographical range patterns

The 12 individual distributions varied in shape and size. The

tendency of the shape is discontinuous for geographical

ranges, with the exception of D. ponderosae, D. jeffreyi and

D. vitei. The patterns of distribution indicated four groups of

species: (1) D. approximatus, D. mexicanus and D. valens are

the species with broadest distribution because they are located

in all mountainous systems of the country. These three species

showed disjunct patterns in the mountain ranges of the

morphotectonic provinces of Peninsula de Baja California

(PBC), SMOR, SMOC, Sierra Madre del Sur (SMS) and

SMCH. The exception to this occurred at the contact zone

between FVT and SMOR, and FVT and SMS, where a wide

and continuous pattern occurred (Fig. 2a–c). (2) D. adjunctus,

D. frontalis and D. parallelocollis were absent from PBC;

however, they show a disjunct pattern similar to D. mexicanus

and D. valens in the rest of the territory (Figs 2d, 3a,b).

3) D. brevicomis and D. pseudotsugae are located only in

the northern portions of SMOC and SMOR (Fig. 3c,d).

4) D. ponderosae and D. jeffreyi distribution patterns are

restricted to PBC (Fig. 3c). Dendroctonus vitei and D. rhizoph-

agus were each unique. Dendroctonus vitei was located

exclusively within SMCH (Fig. 3c) and D. rhizophagus showed

a disjunction within SMOC and PBC (Fig. 3d).

The composite pattern showed that disjunctions among

ranges do not follow closely Mexico’s morphotectonic subdi-

vision (Fig. 4). Taken all together, there is a marginal junction

between FVT and SMOC, and FVT and SMOR, and an evident

continuity between FVT and SMS, such that only PBC and

SMCH are isolated provinces. Several discontinuities or

disjunctions are repeated among individual distributions,

defining five regions where the ranges of multiple species

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Figure 2 Geographical distribution in Mexico of: (a) Dendroctonus approximatus Dietz, (b) D. mexicanus Hopkins, (c) D. valens

LeConte and (d) D. adjunctus Blandford.

Journal of Biogeography 31, 1163–1177, ª 2004 Blackwell Publishing Ltd 1167

Areography of Dendroctonus in Mexico

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Figure 3 Geographical distribution in Mexico of species: (a) Dendroctonus frontalis Zimmerman, (b) D. parallelocollis Chapuis,

(c) —— D. brevicomis LeConte, D. jeffreyi Hopkins, w D. ponderosae Hopkins, D. vitei Wood, (d) —— D. rhizophagus Thomas and

Bright, Æ - Æ - Æ - Æ D. pseudotsugae Hopkins.

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D. brevicomis

D. jeffreyi

D. ponderosae

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D. mexicanus

D. frontalis

D. parallelocollis

D. pseudotsugaeD. rhizophagusD.valens

SMOC

N-SMOR

CM (FVT+S-SMOR+SMS)

SMCH

PBC

Figure 4 Composite pattern of the 12

Dendroctonus species present in Mexico. PBC,

Baja California Peninsula; SMOC, Sierra

Madre Occidental; N-SMOR, North of the

Sierra Madre Oriental; CM, Central Mexico;

SMCH, Sierra Madre de Chiapas.

1168 Journal of Biogeography 31, 1163–1177, ª 2004 Blackwell Publishing Ltd

Y. Salinas-Moreno et al.

Figure 5 Geographical distribution of Pinus

in Mexico sensu Farjon and Styles (1997).

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Figure 6 Isoprobabilistic lines from four main density species centres. (a) Sierra Madre Occidental, (b) Sierra Madre Oriental, (c) Faja

Volcanica Transmexicana and (d) Sierra Madre de Chiapas.

Journal of Biogeography 31, 1163–1177, ª 2004 Blackwell Publishing Ltd 1169

Areography of Dendroctonus in Mexico

overlap extensively in concentrated areas of sympatry that

correspond to: (1) PBC, (2) SMOC, (3) northern SMOR

(N-SMOR), (4) SMCH, and (5) the area integrated by the

southern SMOR (S-SMOR) + FVT + SMS (here called Cen-

tral Mexico, CM) (Fig. 4).

Comparisons between distributions of the genus Pinus

(Fig. 5) and Dendroctonus better reflect the beetle’s com-

posite pattern, especially at the intersection of FVT, where

SMOR joins, and SMS. A more accurate picture of the

beetle disjunction pattern is depicted by the isoprobabilistic

curves (Fig. 6). In fact, all maps confirmed that PBC and

SMCH provinces are separated from the rest. Additionally,

these maps suggest that SMOC is independent of FVT and

that the inside of SMOC provides optimal environment for

the distribution of the genus. However, lines of equal

richness support the relationships among SMOR, FVT and

SMS.

In terms of environmental restrictions, FVT showed the

most favourable conditions for Dendroctonus. It is the broadest

corridor in the country, with an east–west orientation and

marginally extending, in its eastern part, towards the Meseta

Central Province.

Resemblance

There was high similarity (0.5–1.0) among all five centres of

sympatry (Table 2). Minimum values are between PBC

sympatric area and those areas from SMOC, N-SMOR,

CM and SMCH. SMOC and N-SMOR had perfect similar-

ities.

Species richness patterns

Dendroctonus exhibited a heterogeneous species density in

Mexico (Fig. 7). Nine geographical quadrants had a species

density ‡ 50% (six to eight species), one quadrant in each of

the following provinces: PBC, SMOR, SMS, SMCH and FVT,

and four in SMOC.

This suggests that SMOC represents a favourable environ-

mental mosaic for the genus. Species density was the least

heterogenous within FVT, with five species within each of

seven quadrants.

Mean species density does not directly decrease or increase

with latitude (Fig. 8). Its behaviour is homogeneous through

the latitude gradient, except in the ending portions of SMOC

and SMOR, and the ending of the SMCH. However, the

analysis of total richness pattern in each quadrant showed a

low richness pattern at high latitude (PBC), attaining an

increment (eight species) towards 26�–24� N (SMOC and

SMOR), and ending in a lower diversity (six species) at low

latitudes (SMCH). One would expect that such geographical

behaviour of richness would show some association with that

of their host. However, that is not the case, because pine

reaches a maximum richness at the western side of FVT

(Farjon & Styles, 1997), whereas Dendroctonus depicts a higher

richness above FVT.

Dendroctonus was distributed from 800 m to 3929 m. The

distributions analysis suggests that each species have a

preferred elevation range (Table 3); however, confirmation of

this is needed. The mean elevation per species shows a more

Table 2 Pairwise similarity matrix among areas of sympatry of

Dendroctonus estimated by Simpson’s index

PBC SMOC N-SMOR CM SMCH

PBC

SMOC 0.66

N-SMOR 0.5 1

CM 0.5 1 1

SMCH 0.5 0.85 0.85 1

PBC, Baja California Peninsula; SMOC, Sierra Madre Occidental;

N-SMOR, North of the Sierra Madre Oriental; CM, Central Mexico;

SMCH, Sierra Madre de Chiapas.

0 200

4

1

37

1 3 3

37

2

8

22

2 2284

11 6

32 1

111

113

4 4

22 2

45

555555

2 4 6 3 1 14

2 2 631 2 3 6

1

1

41

16

UNITED STATES OF AMERICA

GULF OF MEXICO

PACIFIC OCEAN

114 110 106 102112 108 104 98100 9496 9092 88 86

12

14

16

18

20

22

24

26

28

30

32

Figure 7 Density of Dendroctonus species in

Mexico. Each quadrant shows species num-

bers present inside each area.

1170 Journal of Biogeography 31, 1163–1177, ª 2004 Blackwell Publishing Ltd

Y. Salinas-Moreno et al.

regular condition (Fig. 9), because 90% mean elevation falls

between 1700 and 2300 m. This richness is concentrated in this

montane interval, an exception is D. adjunctus, which can live

in this interval, but more frequently is found in locations above

3000 m.

Shape, size and perimeter of the geographical

ranges

Because information is scarce for D. jeffreyi, D. ponderosae and

D. vitei, it was not possible to apply the propinquity method to

define the boundaries of their ranges. The median and mean

area size for the remaining nine species were 364,730.9

and 390,640.2 km2, respectively (Table 4). Areas were

mostly similar, three were larger than the estimated average

and six were smaller.

Range deformation analysis to the composite pattern shows

six main axes (Fig. 10). They describe a NW–SE orientation,

the exceptions are the axes located in the middle part of the

country (FVT and SMS). All ranges follow the orientation of

the main mountain systems.

Ranges deformation described by perimeter to �area ratio

diverges from the expected values for a circular expansion

(Table 4). Dendroctonus mexicanus and D. valens exhibit the

two highest values (15.45 and 15.05, respectively), whereas

D. pseudotsugae and D. brevicomis exhibit the two lowest values

(5.43 and 6.01, respectively).

Area size vs. host

Species of Dendroctonus attack 24 of the 47 Pinus species

distributed in Mexico (Farjon & Styles, 1997). Dendroctonus

pseudotsugae is the only species that has been found on

Pseudotsuga menziesii (¼P. flahaulti). The number of host

species attacked by these bark beetles varies widely. Whereas

D. mexicanus and D. valens infest 20 or more species, D. jeffreyi

and D. vitei infest only one species (Table 5). Among the

Dendroctonus species that can be found on several host

species, a preference for one conifer is observed. For example,

D. mexicanus, D. parallelocollis and D. valens are most

frequently found on Pinus leiophylla Schiede ex Schlechtendal

& Chamisso (Table 5). Preferred pine species are predomin-

antly Leiophyllae, Ponderosae and Oocarpae, subsections of

Pinus.

Species with the maximal area extension do not necessarily

parasitize a larger number of hosts (rs ¼ )0.216, P ¼ 0.515).

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

32–30 30–28 28–26 26–24 24–22 22–20 20–18 18–16 16–14

Spe

cies

mea

n di

vers

ity

Latitude (North degrees)

Figure 8 Mean diversity of species vs. latitudinal distribution

of Dendroctonus species.

Table 3 Elevation range and preferred elevation range of each

Dendroctonus species in Mexico

Species

Elevation range

(m a.s.l.) Preferred interval

Dendroctonus adjunctus 1600–3929 3100–3500

D. approximatus 1900–2800 2100–2500

D. brevicomis 1680–2835 Data unavailable

D. frontalis 960–2835 1100–2000

D. mexicanus 800–3400 2100–2500

D. parallelocollis 800–3100 2100–2500

D. rhizophagus 1200–2600 2100–2500

D. valens 1100–3760 2100–2500

D.

brev

icom

is

D.

fron

talis

D.

mex

ican

us

D. p

aral

lelo

colli

s

D.

rhiz

opha

gus

D. v

alen

s

Species

Ele

vatio

n (m

)

D.

adju

nctu

s

D.

appr

oxim

atus

4000

3000

2000

1000

Figure 9 Distribution of Dendroctonus species by elevation above

sea level. The box plot was constructed with mean elevation, one

standard error, and maximum and minimum elevation values.

Table 4 Size of geographical ranges (km2) and perimeter to �area

ratio for Dendroctonus species in Mexico

Species

Geographical

area (km2)

Perimeter/

�area

Dendroctonus approximatus 553,852 10.9

D. parallelocollis 531,908 9.93

D. pseudotsugae 466,707 5.43

D. brevicomis 386,581 6.01

D. frontalis 376,645 10.97

D. valens 352,473 15.05

D. mexicanus 336,158 15.45

D. adjunctus 335,824 12.12

D. rhizophagus 175,608 8.41

Average 390,640

Journal of Biogeography 31, 1163–1177, ª 2004 Blackwell Publishing Ltd 1171

Areography of Dendroctonus in Mexico

Finally, the preferred elevation–preferred host comparison

showed no strict association between these elements, given

that Dendroctonus species attacked more than one Pinus

species within the preferred elevation range of the insect

(Table 6).

DISCUSSION

Contrary to other coleopteran genera occupying montane

habitats, Dendroctonus in Mexico underwent few events of

diversification, resulting in only a single endemic species and

Table 5 Host species and percentages of Dendroctonus species incidence on each one of them

Pinus species

(Farjon & Styles,

1997)

Dendroctonus species

D. adjunctus D. approximatus D. brevicomis D. frontalis D. jeffreyi D. mexicanus D. parallelocollis D. rhizophagus D. vitei D. valens

P. arizonica 2.7 15 1.8 2.5 2.1 24.3 7.6

P. ayacahuite 0.4 0.7

P. cembroides 0.4 1.4

P. devoniana 22.2 3.6 9.4 12.8 4.3 100 9.0

P. douglasiana 3.6 0.8

P. durangensis 5.3 3.7 40 3.6 1.2 6.4 14.3 3.5

P. engelmannii 7.4 15 1.2 44.3 4.9

P. gregii 1.2 2.8

P. hartwegii 80 14.8 1.8 1.2 4.3 3.5

P. herrerae 1.3 0.4 0.7

P. jeffreyi 3.7 100 0.4 4.3 1.4

P. lawsonii 1.8 0.8

P. leiophylla 2.7 14.8 15 3.6 35.6 27.7 20.1

P. lumholtzii 0.4 4.3 1.4

P. maximinoi 1.3 1.2

P. montezumae 1.3 7.4 1.8 7.4 21.3 10.4

P. oocarpa 3.7 1.5 37.5 5.7 4.3 9.0

P. patula 2.7 4.9 2.0

P. pinceana 1.3

P. ponderosae 4.3 1.4

P. pringlei 19.7 1.6 2.1 2.0

P. psudostrobus 3.7 5.4 9.0 4.3 6.9

P. quadriflora 0.7

P. teocote 1.3 18.5 16 13.9 14.9 9.7

Total host 10 10 5 12 1 21 10 7 1 20

Numbers in bold represent higher percentages of incidence.

114 889092949698100102104106108110112

0 300 Km

14

16

18

20

22

24

26

28

30

32

UNITED STATES OF AMERICA

GULF OF MEXICO

PACIFIC OCEAN

Figure 10 Preferred directions of the

Dendroctonus composite pattern.

1172 Journal of Biogeography 31, 1163–1177, ª 2004 Blackwell Publishing Ltd

Y. Salinas-Moreno et al.

another that shares endemism with a contiguous region in

Guatemala. Despite complex morphotectonic scenario in the

mountainous environment and the heterogeneity of pine

species distribution (Farjon & Styles, 1997), interspecific

geographical disjunction is not the main characteristic of

Dendroctonus distribution patterns. It can be said that Mexico

represents an area of overlapping of species with wide

distribution and of species with restricted ones. All of them

nested in five areas of sympatry. The causes for this

disjunction are probably biotic, climatic and geological

barriers present at this time or the result of conditions

imposed by the geological, paleogeological or paleoclimatic

history of Mexico.

If we analyse the sympatry location areas with respect to the

physiographic and geological scenery of Mexico, the disjunc-

tions where the genus is distributed do not correspond

specifically to the morphotectonic provinces, with the excep-

tion of PBC, SMOC and SMCH. In other words, the

disjunction patterns do not directly reflect the origin and

formation of Mexico’s tectonic units. However, they are more

similar with patterns for other mountainous taxa vicariants

and speciation centres (Ball, 1968; Noonan, 1988; Liebherr,

1991).

An example is the vicariation hypothesis pattern revealed by

the general area cladogram for Carabidae (Liebherr, 1991).

This predicts that the relationship among these areas can be

used to analyse the behaviour of other taxa that are ecologically

restricted to mountainous forests and that have sister groups

inhabiting North America. SMOR appears to be subdivided

into two, with the northern portion having a close affinity to

SMOC, a situation that might be the result from an old

relationship during the Pleistocene, when conditions were

more mesic at the Chihuahua desert. In Dendroctonus, such

north–south division, in addition to the relation of the north

portion with SMOC, is confirmed with isoprobabilistic lines,

the similarity values between both zones, and by the exclusively

shared presence of D. pseudotsugae and D. brevicomis. How-

ever, the southern region of SMOR seems more related to FVT

and SMS than to the northern areas. Such a relationship would

in Dendroctonus help explain the eastern location of the area of

sympatry that we called CM, which is clearly shown in the

isoprobabilistic lines.

This close connection between FVT and SMS observed in

Carabidae (Liebherr, 1991), Dendroctonus and other species is

explained through the continuous mountainous habitat

offered by the Mountains of Oaxaca, which in turn favours

biotic exchange. Nevertheless, loss of continuity with the west

of SMS is due to the Balsas Basin, which has acted as an

important biogeographic barrier for many groups of organ-

isms, among them insects and pines.

The other areas of sympatry – PBC and SMCH – can be

explained by significant barriers, such as the Sonora desert and

the Tehuantepec isthmus, respectively, for taxa with Nearctic

affinities.

Analysing the average similarity among the areas of

sympatry, it becomes evident that at least two-thirds of the

present species are shared by several of them, which might

imply that fragmentation of Dendroctonus ranges are rather

the result of ecological restrictions than due to historical

events. It seems that dispersal activities of Dendroctonus

individuals in conjunction with their high vagility could be

the causes of this distribution pattern (Noonan, 1988;

Turchin & Thoeny, 1993; Byers, 2000).

The only exception to this condition is PBC, because it

exhibits the greatest differentiation with respect to the rest of

the areas. These values result from the presence of D. jeffreyi

and D. ponderosae, exclusive to this zone, and the absence of

D. frontalis, D. adjunctus and D. parallelocollis, species broadly

distributed in Mexico. However, PBC is a province with a flora

and fauna known for its singularity and for its greater affinity

with the Californian province to the north than with the rest of

the provinces in Mexico (Alvarez & de Lachica, 1974;

Rzedowski, 1981; Morrone et al., 2002).

The fact that disjunction patterns among most species are

congruent, suggests that individuals of those species are

closely linked to common habitats and, therefore, limited by

similar factors. The topographic diversity of the continent,

position and orientation of mountain ranges might be very

important in determining the location and the shape of

geographical ranges (Brown & Maurer, 1989). Nevertheless,

climate must set the absolute limits of a species distribution

and habitat is important in the discontinuities in species

assemblages (Letcher & Harvey, 1994). Disjunctions common

to all Dendroctonus species are directly explained by the

absence of pines, if we consider most pine species in Mexico

as confined to mountains or high plateaus. Isolation of PBC

is due to the loss of elevation above sea level, and the

mountains of Baja are surrounded by desert zones with

xerophytic vegetation. Disruption between SMOC and FVT,

despite their short distance, can be explained because pine

forest continuity is interrupted by depressions like the Valley

of the Mezquital River and by the Santiago River watershed,

both of which are covered by deciduous tropical forest.

Between the northern and southern parts of SMOR,

physiographic and ecological conditions impose pine char-

acteristic patterns and consequently affect Dendroctonus too.

It is a mountainous system with low elevation zones

(< 1000 m) between 21� N and 23� N, with ample plateaus

and low elevation covered with xerophytes, shrubs and

deciduous tropical forests.

Table 6 Incidence percentages of Dendroctonus species on their

preferred host within their preferred elevation range

Bark beetle species Preferred host

Preferred

elevation % Incidence

Dendroctonus adjunctus Pinus hartwegii 3100–3500 83.3

D. approximatus P. devoniana 2100–2500 38.4

D. frontalis P. oocarpa 1100–2000 42.3

D. mexicanus P. leiophylla 2100-2500 34.9

D. parallelocollis P. leiophylla 2100-2500 23.8

D. valens P. leiophylla 2100-2500 33.3

Journal of Biogeography 31, 1163–1177, ª 2004 Blackwell Publishing Ltd 1173

Areography of Dendroctonus in Mexico

The disjunction between CM and SMCH is given by the

Tehuantepec isthmus. This barrier represents a depression below

1000 m, covered with evergreen and deciduous tropical forests.

However, the isoprobabilistic analysis shows the existence of

zones within SMOC and CM, where faunistic similarity

decrease rapidly. The reason for this, in the case of SMOC, is

the high discontinuity of Dendroctonus species ranges.

Although it is a system where 65% of its surface is between

2000 m and 3000 m, it is formed by numerous isolated

mountains that represent an elevation and climatic mosaic for

many Dendroctonus species. This tendency of elevation

heterogeneity is enhanced towards the south. In this zone,

the conifer forest is interrupted by deciduous tropical forests

giving rise to isolated forested areas, observed in the composite

pattern as centres of major sympatry.

The CM is a distinctive case, which may be perceived by

Dendroctonus as a homogeneous area despite being a hetero-

geneous physiographic and elevation system. The wide separ-

ation of the isoprobabilistic lines at the eastern portion seems

to be explained by its predominant elevation between 1500 m

and 2500 m, while the western half, with some exceptions, lies

below 1500 m. Thus elevation and climatic conditions are

more favourable in the east portion.

The isoprobabilistic lines showed that the main corridors

were associated with mountain ranges, thus the geographical

barriers could be related to elevation and presumably to

climatic factors, such as temperature and humidity.

The relationship among elevation, Dendroctonus richness

tendency and host species in Mexico indicates a preference of

these beetles for montane systems, which are rich in Pinus

species (Farjon & Styles, 1997). High host richness promotes

greater habitat availability and permits high species richness of

beetles.

Some facts suggest that distribution of the genus Dendroc-

tonus is determined by cold temperate climate conditions

(Williams & Liebhold, 2002). First, the distribution limit of

some Dendroctonus species is northern rather than southern in

Mexico. Secondly, the highest species richness is found in the

north and their values increase at the FVT latitudinal belt,

where Mexico has its highest peaks. Thirdly, the lack of a

match between geographical location of species richness

centres of Dendroctonus and Pinus. Several studies seem to

confirm that scolytid distribution is influenced by environ-

mental factors such as temperature during winter and summer

(Swaine, 1925; Lekander et al., 1977; Heliovara et al., 1991;

Ungerer et al., 1999).

If we accept the hypothesis for the Nearctic origin of the

genus Dendroctonus (Zuniga et al., 2002a,b) and its association

with the dispersal of its main host, the genus Pinus, into

Mexico (Mirov, 1967; Farjon & Styles, 1997; Millar, 1998), it is

possible to explain the northern sites with the highest density

of species, located in PBC, SMOR and SMOC, not only as a

result of elevation, latitude, climate and host diversification,

but also of dispersal.

These richness centres are characterized by three groups of

species. The first composed of boreal species, such as

D. brevicomis, D. jeffreyi, D. ponderosae and D. pseudotsugae,

which are typical of the western region of the USA and south-

western Canada and whose geographical distribution in

Mexico is narrow. The second group is formed by widely

distributed species in North America, such as D. adjunctus,

D. approximatus, D. frontalis and D. valens. The third group

includes species that are indigenous to Mexico such as

D. mexicanus, D. parallelocollis and D. rhizophagus.

The species richness centres located at FVT, SMS and SMCH

are compose of widely distributed species such as D. adjunctus,

D. approximatus, D. frontalis and D. valens, as well as by

Mesoamerican native elements (sensu Maldonado-Koerdell,

1964) such as D. mexicanus, D. parallelocollis and D. vitei. The

absence of boreal elements and the presence of indigenous

ones make the composition of species in this region different

from that of the north.

From the 17 Dendroctonus species of North and Central

America, 62% occurr in Mexico. Of these, 50% reach

distribution limits in northern Mexico, and most have used

SMOC, SMOR, FVT and SMS as dispersion corridors towards

Mesoamerica. Following this line of thought, with the excep-

tion of PBC, there is a real latitudinal decrease in Dendroctonus

species richness within the continent, despite its increase in

FVT. Finally, because of its presence in conifer forests, and

their major specific richness located above 1700 m, we are led

to consider that Dendroctonus belongs to the Nearctic distri-

bution pattern (sensu Halffter, 1987).

Although area deformation suggests that Dendroctonus track

mountain systems, the perimeter to �area ratio reveal that

their distribution is not homogeneous (within the altitudinal

range that characterizes the Nearctic pattern) and that there

are some degrees of environmental resistance.

For phytophagous insects, the area size is important in order

to determine habitat (Udvardy, 1969; Hengeveld, 1990;

Gaston, 1991, 1996) and host availability (Anderson, 1984;

Gaston, 1990, 1996). Therefore, it would be expected that taxa

having a wide area would have better ecological opportunities

than those with narrow areas.

Trends in range size of Dendroctonus species in Mexico agree

with the general assumption that most taxa occupy small ranges

(Rapoport, 1975a; Gaston, 1990). However, our results suggest

that ranges inhabited by Dendroctonus in Mexico do not depend

on how widespread these species are in different mountain

systems (e.g. D. mexicanus occupies a small size area), nor in the

number of host species (i.e. correlations were not significant).

When distribution of herbivore species are independent of the

host species distribution, as is the case for Dendroctonus and

Pinus which exhibit richness centres at different latitudes, it

suggests that others factors (tolerance, dispersal ability, beha-

viour) could be involved (Kennedy & Southwood, 1984). This

could explain why species with narrow distributions such as

D. brevicomis and D. rhizophagus, and others with wide

distributions, including D. adjunctus, D. mexicanus, D. frontalis

and D. valens, have area sizes below the average.

The data show that the majority of Dendroctonus species in

Mexico have a low specificity towards its host (Pinus), the host

1174 Journal of Biogeography 31, 1163–1177, ª 2004 Blackwell Publishing Ltd

Y. Salinas-Moreno et al.

species ranges are wider than those of the insects that feed on

them, and the number of pine species is four times larger than

that of Dendroctonus. Of 24 species that could potentially be

infested, only P. arizonica Engelmann, P. hartwegii Lindley,

P. montezumae Lambert, P. durangensis Martınez (subsec.

Ponderosae), P. oocarpa Schiede ex Schlechtendal, P. teocote

Schiede ex Schlechtendal & Chamisso (subsec. Oocarpae) and

P. leiophylla (subsec. Leiophyllae) are used by at least 50% of

the Dendroctonus species (Zuniga et al., 1999). Only 30% of

Dendroctonus species show an exclusive relationship, including

D. adjunctus that shows an 80% incidence on P. hartwegii. The

rest of the species have just one preferred host but their

incidences are not higher than 44%. Although 75% of the

records show that D. mexicanus has a preference for P. devoni-

ana Lindley, P. leiophylla, P. montezumae, P. pseudostrobus

Lindley and P. teocote, this species infests 21 host species along

its distribution range in Mexico (Zuniga et al., 1999). All of

this suggests that Dendroctonus behaves more as a polyphagous

generalist inside the genus Pinus, with a potential for frequent

host switching. The correlation observed among the Dendr-

octonus species and host species belonging to certain Pinus

subsections (Kelley & Farrell, 1998) seems to be only the result

of ecological opportunities exploited by the insect rather than a

strict evolutionary association between both taxa, for example,

D. adjunctus, D. brevicomis, D. frontalis, D. ponderosae and

D. valens, which attack host species of the Ponderosae,

Oocarpae and Leiophyllae subsections (Zuniga et al., 1999),

have been found on other pine species beyond Mexico (Wood,

1982). In the same sense is the fact that P. leiophylla is the

preferred host of D. parallelocollis, D. mexicanus and D. valens,

but the distributions of these species are found outside the

range of P. leiophylla.

It has been proposed that insect species richness is a function

of the range size of hosts; thus hosts with wide distribution

present an increase in the numbers of phytophagous species

(Strong, 1979). In this context, the hosts preferentially attacked

by Dendroctonus, P. devoniana, P. durangensis, P. engelmannii

Carriere, P. hartwegii, P. leiophylla and P. oocarpa are distri-

buted only in three of the six Mexican morphotectonic

provinces where these insects are located.

Species specialization is a complex process involving

ecological and historical factors (Thompson, 1994). The scarce

specialization observed in phytophagous insects can be the

result of either a low diversity in secondary chemistry of the

host (Tahvanainen & Niemela, 1987), or insect-plant associ-

ations that are casual, haphazard or labile (Strong, 1979). In

Dendroctonus, the comparison of their molecular phylogeny

with their host (Pinus, Picea, Pseudotsuga and Larix) suggests a

poor association among them, despite sibling species attacking

similar groups of pine species (Kelley & Farell, 1998).

In summary, Dendroctonus habits montane systems of

Mexico and their species show a high geographical sympatry.

The coexistence of species appears to be the result of the

polyphagy inside the genus Pinus and wide elevation tolerance.

This behaviour corresponds to ecological opportunities

rather than to a direct host association. Despite wide host

distribution, some species limit their ranges to the northern

forest of Mexico. The high vagility of species has allowed the

genus Dendroctonus to expand its distribution across Mexico

and to perceive mountainous systems as corridors.

ACKNOWLEDGMENTS

We thank A. Ruggiero, J.J. Morrone, J.L. Hayes, A. Cognato,

S.T. Kelley, J. Moser, O.J. Polaco, S. Sanchez Colon and

R. Galvan for review and constructive comments on the

manuscript. This work was supported by a CONAFOR

research grant (CO1-5829).

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BIOSKETCHES

Yolanda Salinas-Moreno is a professor of biogeography at

ENCB-IPN. Her main research interest is the biogeography of

the genus Dendroctonus in North and Central America.

Miguel A. Barrios is a professor at ENCB-IPN and a botanist

interested in the taxonomy and biogeography of the Mexican

flora.

Ramon Cisneros is a biology graduate of the Universidad

Nacional Autonoma de Mexico, with interests in several

aspects of the life history of the genus Dendroctonus, such as

the biogeography, phylogeography and phylogenetic analysis

of American populations of this genus.

Jorge Macıas-Samano is a senior researcher at ECOSUR

whose main interest is the study of interactions between

insects and trees, using trees and scolytids and cerambycids as

models of interactions. He is the Head of Graduate Studies of

ECOSUR.

Gerardo Zuniga is a professor of evolution and population

genetics at ENCB-IPN, and a senior researcher whose main

interests are the population genetics and phylogeography of

Dendroctonus species.

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Areography of Dendroctonus in Mexico