BIODIVERSITY RESEARCH: Current distribution and predicted geographic expansion of the Rufous-backed...

BIODIVERSITYRESEARCH

Current distribution and predictedgeographic expansion of theRufous-backed Robin in Mexico:a fading endemism?

Miguel Angel Martınez-Morales1*, Iriana Zuria2, Leonardo Chapa-Vargas3,

Ian MacGregor-Fors4, Ruben Ortega-Alvarez5, Edgar Romero-Aguila6 and

Pilar Carbo2

1Seccion Hidalgo, Sociedad para el Estudio y

Conservacion de las Aves en Mexico, A.C.

Francisco I. Madero 58, Omitlan, Hidalgo

43560, Mexico, 2Centro de Investigaciones

Biologicas, Universidad Autonoma del Estado

de Hidalgo. Ciudad Universitaria, Carretera

Pachuca-Tulancingo Km 4.5, Colonia

Carboneras, Mineral de la Reforma, Hidalgo

42184, Mexico, 3Division de Ciencias

Ambientales, Instituto Potosino de

Investigacion Cientıfica y Tecnologica, A.C.

Camino a la Presa San Jose 2055, Lomas 4ª

Seccion, San Luis Potosı, San Luis Potosı

78216, Mexico, 4Laboratorio de Ecologıa

Funcional, Centro de Investigaciones en

Ecosistemas, Universidad Nacional Autonoma

de Mexico. Antigua Carretera a Patzcuaro

8701, Colonia Ex-Hacienda de San Jose de La

Huerta, Morelia, Michoacan 58190, Mexico,5Museo de Zoologıa ‘‘Alfonso L. Herrera’’,

Facultad de Ciencias, Universidad Nacional

Autonoma de Mexico. Apartado Postal

70-399, Mexico DF, 04510, Mexico, 6Cuerpo

Academico de Desarrollo Sostenible,

Universidad Intercultural Maya de Quintana

Roo. Calle Primavera s/n, entre Av. Jose

Marıa Morelos y Jacinto Canek, Colonia

Centro, Jose Marıa Morelos, Quintana Roo

77890, Mexico

*Correspondence: Miguel Angel Martınez-

Morales, Seccion Hidalgo, Sociedad para el

Estudio y Conservacion de las Aves en Mexico,

A.C. Francisco I. Madero 58, Omitlan, Hidalgo

43560, Mexico.

E-mail: [email protected]

ABSTRACT

Aim The Rufous-backed Robin Turdus rufopalliatus is a bird endemic to the

Pacific slope of Mexico. The species recently established populations in several

localities in the Mexican Central Highlands. Based on available data, we modelled

the range expansion of the Rufous-backed Robin in Mexico to understand the

pattern, mechanisms and ecological and biogeographic implications of its

expansion.

Location Mexico.

Methods We assessed the species’ presence and habitat requirements at two

spatial scales. At the site level, we evaluated the relationship between land use and

species presence in an urban environment. At the country level, we generated a

niche model. We then produced a dispersion model through the interpolation of

points generated from information derived from the niche model, the location of

records within and outside its native distribution range, the species’ natural

history, habitat requirements and its estimated dispersion rate (4.2 km year)1).

Results The dispersion model predicted that the species will significantly increase

its distribution range in Mexico in the coming decades. Its expansion would occur

by a stepping-stone colonization of suitable habitat in areas of native vegetation

and human settlements. The model predicted that the species should arrive on the

Gulf slope of Mexico before 2025.

Main conclusions Mechanisms that could explain the species’ success in

establishing viable populations outside its native distribution include its

dispersion ability, competitive release, the urban heat island phenomenon and

the trade of wild birds. The geographic range expansion of the Rufous-backed

Robin will probably create new interactions with other species, particularly with

close taxonomic and ecological relatives. The increase in the distribution range of

the Rufous-backed Robin has resulted from direct and indirect human-induced

dispersion; therefore, it cannot be considered a fading endemism. In part of its

expanded range (to date the Mexican Central Highlands), it should be considered

an invasive alien species.

Keywords

Biological invasions, dispersion pathways, niche modelling, range expansion,

Turdus rufopalliatus, urban ecology.

Diversity and Distributions, (Diversity Distrib.) (2010) 16, 786–797

DOI:10.1111/j.1472-4642.2010.00691.x786 www.blackwellpublishing.com/ddi ª 2010 Blackwell Publishing Ltd

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INTRODUCTION

Change in species geographic ranges is a natural phenomenon,

but the rate and magnitude of such change has significantly

increased because of anthropogenic causes. Although in many

cases, human-induced changes in species geographic range

have gone unnoticed because of the lack of knowledge of the

species’ native distribution range, human activities have

proven to be an important factor in shaping present species

distributions (Mooney et al., 2005). Thus, the geographic

range of many species has been reduced because of land cover

changes, habitat fragmentation, presence of alien species and

direct killing (Vitousek et al., 1997; Sala et al., 2000; Chapin

et al., 2001; Mace et al., 2005; Valiela & Martinetto, 2007),

whereas others have expanded their range because of the

creation of new areas of suitable habitat and intentional and

unintentional human transport (Chapin et al., 2001; Marzluff

et al., 2001; Wehtje, 2003; Mace et al., 2005; Munoz & Real,

2006; Alvarez-Romero et al., 2008). In addition, altered species

distributions are expected at some level because of climate

change (Peterson et al., 2001, 2002; Honnay et al., 2002;

Stralberg et al., 2009). In the last few decades, intensive work

has been carried out on modelling species distributions, but

given the pressing needs to apply principles, theories and

analyses to provide management options for biodiversity

conservation, it is paramount to go beyond static species

distribution models to incorporate key dynamic processes

determining species distributions (Richardson & Whittaker,

2010). Such processes can be incorporated in a multimodelling

framework that may include species–environment relation-

ships, landscape dynamics, population dynamics and phylog-

eographic and landscape genetic methods (Franklin, 2010;

Scoble & Lowe, 2010). In addition, ‘hybrid’ models that

incorporate the strengths of a range of different types of

approaches (phenomenological and mechanistic) could pro-

vide the means to make reliable and robust predictions about

species potential distributions, their population dynamics and

the potential outcomes of invasion processes (Gallien et al.,

2010).

In North America, geographic range expansion of native bird

species has been better documented than in other biological

groups. Mechanisms proposed to explain such expansions

include climatic warming, increase of summer rainfall, inten-

tional introductions and anthropogenic land cover changes. In

addition, species features such as ecological flexibility or broad

habitat tolerances, ability to colonize unoccupied habitats and

being a human commensal all increase the possibility of a bird

species expanding its range (Stepney & Power, 1973; Johnson,

1994; Rothstein, 1994; Christensen, 2000; Wehtje, 2003). These

studies have stressed the ecological consequences of range

expansions of native bird species, mainly the emerging

interspecific interactions. However, the endemic nature of a

species might become questionable as the species expands its

geographic range beyond the native distribution that merited

its classification as endemic. This is the case of the Rufous-

backed Robin Turdus rufopalliatus.

The Rufous-backed Robin is a bird endemic to the Pacific

slope of Mexico and the Balsas Basin (Howell & Webb, 1995;

AOU, 1998; Clement, 2000). In Mexico, established popula-

tions outside of the known native distribution range have been

reported in Mexico City since the 1940s (Howell & Webb,

1995; Peterson & Navarro-Siguenza, 2006) and in central

Oaxaca since the 1950s (GBIF, 2009). These populations are

thought to be of captive origin (Wilson & Ceballos-Lascurain,

1986). There are also recent reports of individuals or estab-

lished populations in northern Sonora, Chihuahua, Nuevo

Leon, San Luis Potosı, Aguascalientes, north-eastern Jalisco,

Guanajuato, Queretaro, Hidalgo, northern State of Mexico and

northern Puebla (Contreras et al., 1995; Brooks, 1999; Clem-

ent, 2000; Carbo Ramırez, 2008; Romero-Aguila & Chapa-

Vargas, 2008; GBIF, 2009; Pineda-Lopez & Malagamba Rubio,

2009; authors obs.). In the USA, the species was first recorded

in 1960 in southern Arizona (Harrison, 1962; Johnson &

Simpson, 1971). It is now also reported in California, southern

New Mexico, southern Texas, Oregon, Utah and New York

(AOU, 1998; Clement, 2000; GBIF, 2009). In this study, we

predict the geographic range expansion of the Rufous-backed

Robin in Mexico based on the availability of suitable habitat

(both natural and anthropogenic) and human-induced dis-

persion. We evaluate the possible mechanisms that have

facilitated its dispersal, discuss its dispersal pathways (sensu

Wilson et al., 2009a), dispersal rate and the ecological impli-

cations of the emerging interspecific interactions. Finally, we

examine the endemic condition of this species in the light of its

geographic range expansion.

METHODS

The species

The Rufous-backed Robin is a common resident along the

Mexican Pacific slope from southern Sonora (Rıo Yaqui) and

extreme south-western Chihuahua south to southern Oaxaca

(to about the Isthmus of Tehuantepec). In the interior, it

occurs along the Balsas Basin to Morelos and south-western

Puebla (Fig. 1; Howell & Webb, 1995; AOU, 1998; Clement,

2000). Native habitat of the Rufous-backed Robin includes

tropical deciduous and mixed forests, woodland edge, dense

shrubbery and scrub. The species occurs up to 1500 m in

foothills and the higher valleys, but it is found mostly in

lowland woodlands and frequently at fruiting trees (Howell &

Webb, 1995; Clement, 2000). In anthropogenic environments

it uses plantations, urban and suburban parks, orchards and

large gardens. These areas all have tree cover normally

associated with open areas (lawns, gardens). Trees are used

for nesting, roosting and feeding (fruits), complementing its

feeding (earthworms, insects and other invertebrates) on the

ground (Howell & Webb, 1995; Clement, 2000; Guevara

Tacach, 2006; authors obs.).

Within its native distribution, the Rufous-backed Robin

overlaps, at least seasonally, with other members of the family

Turdidae such as the White-throated Thrush Turdus assimilis,

The Rufous-backed Robin: a fading endemism?

Diversity and Distributions, 16, 786–797, ª 2010 Blackwell Publishing Ltd 787

the American Robin Turdus migratorius, the Orange-billed

Nightingale-Thrush Catharus aurantiirostris, the Swainson’s

Thrush Catharus ustulatus, and the Hermit Thrush Catharus

guttatus (Howell & Webb, 1995; Clement, 2000).

Species records

We gathered information of the presence, habitat requirements

and behaviour of the Rufous-backed Robin in Mexico from

systematic fieldwork carried out in Mexico City, Hidalgo, and

San Luis Potosı. Additionally, we obtained species’ presence

data at several major Mexican urban areas by contacting

ornithologists from each state capital (see Acknowledgements)

and also from extensive opportunistic observations in a

number of localities within and outside of its known native

distribution. We conducted these surveys and observations

from 2003 to 2009. We obtained additional records from

museum collections and databases (see Acknowledgements),

from the Global Biodiversity Information Facility (GBIF) data

portal (http://data.gbif.org/species/), and the Global Network

on Biodiversity Information (REMIB, in Spanish) (http://

www.conabio.gob.mx/remib/doctos/remib_esp.html). We ver-

ified species identity and temporal and spatial precision of

most of the records from these data sources. We gathered a

total of 311 spatially unique records of the species within its

known native distribution and 81 spatially unique records

outside of this distribution range. Dates of the records within

and outside the species’ native distribution spanned from 1925

to 2008 and from 1955 to 2009, respectively.

Scales of analyses for habitat requirements

We used two spatial scales of analyses to understand habitat

requirements of the Rufous-backed Robin: site level (habitat

preferences) and country level (niche modelling). At the site

level, we used a 16-variable database generated to evaluate the

relationship between urban land use and bird communities in

Mexico City (Ortega-Alvarez & MacGregor-Fors, 2009). These

variables described vegetation structure, urban structure, human

activity and abundance of potential predators. Through corre-

lation analyses, we identified all significant moderately to highly

correlated variables (r > 0.5; P < 0.05). When two or more

variables showed such correlation, we only included the variable

with the highest variance in the multiple regression analysis. We

performed a stepwise (backward; P < 0.05) multiple regression

analysis including 11 variables (building maximum height, tree

species richness, tree density, tree diameter at breast height,

shrub species richness, shrub cover, shrub height, herbaceous

plant maximum height, pedestrians min)1, vehicles min)1,

number of dogs and cats observed) to assess which variables

were related to the presence and abundance of the Rufous-

backed Robin in Mexico City.

At the country level, we used 21 environmental variables

(resolution 30 s c. 1 km2) from WorldClim (Hijmans et al.,

2004) and INEGI (1995) for niche modelling: annual mean

temperature, mean diurnal temperature range, isothermality,

temperature seasonality, maximum temperature of warmest

month, minimum temperature of coldest month, temperature

annual range, mean temperature of wettest quarter, mean

Figure 1 Niche model of the Rufous-backed Robin in Mexico based on the ‡75% coincidence area of the 20 best models generated with the

Genetic Algorithm of Rule-Set Prediction. Species habitat suitability is depicted in grey scale. The grey line depicts native distribution sensu

Howell & Webb (1995) (Ridgely et al., 2003). Points represent records within (squares) and outside (circles) its native distribution.

M. A. Martınez-Morales et al.

788 Diversity and Distributions, 16, 786–797, ª 2010 Blackwell Publishing Ltd

temperature of driest quarter, mean temperature of warmest

quarter, mean temperature of coldest quarter, annual precip-

itation, precipitation of wettest month, precipitation of driest

month, precipitation seasonality, precipitation of wettest

quarter, precipitation of driest quarter, precipitation of

warmest quarter, precipitation of coldest quarter, altitude

and slope. We did not include land cover (CONABIO, 1999) as

a variable for niche modelling because of changes occurred

over the time spanned by the species records, but we used this

variable for dispersion modelling.

Niche modelling

We generated a niche model of the Rufous-backed Robin in

Mexico by using the Genetic Algorithm of Rule-Set Prediction

(GARP; Stockwell & Noble, 1992; Stockwell & Peters, 1999).

We carried out GARP modelling on DesktopGARP v. 1.1.6

(Scachetti-Pereira, 2002) software. It has been argued that

GARP performs poorly in predicting species’ distributions

compared with other methods (Elith et al., 2006); however,

Peterson et al. (2007) (but see Phillips, 2008) argued that this

algorithm is appropriate for anticipating most of the species’

distributional potential (the transferability property), which is

particularly valuable in uses such as evaluating the species’

spreading potential. For model construction, we used all

species records within the known native distribution of the

Rufous-backed Robin to take advantage of the information of

all of them (increasing the predictive capacity of GARP), and

also because patterns in intrinsic measures of model perfor-

mance have been found to be consistent with extrinsic ones, i.e.

when data are split in training and testing subsets (Anderson

et al., 2003). Nevertheless, 100 preliminary models were

constructed to allow for the assessment of significance

(departure from random predictions) using a chi-square test

where 60% of records within the known native distribution

were used as training data and 40% as testing data. For model

parameter optimization, we performed 100 runs setting a

convergence limit of 0.01 or 1000 iterations. All rule types

(atomic, range, negated range and logistic regression) were

selected for rule production. To select the optimal models, we

constructed a plot of intrinsic omission error versus intrinsic

commission index; then, we first considered those models with

no omission error and from this subset, we selected the 20

models around the median commission index as the best

model subset. This procedure provides an objective mean of

model evaluation (Anderson et al., 2003). We overlaid and

added these 20 models in a GIS to produce a first niche

distribution map for the Rufous-backed Robin in Mexico.

Since niche modelling can be affected by extension of

modelling area because of the relationship between the value

of environmental variables at the distribution of species

records and variability range of environmental variables in

the modelling area, we refined niche modelling through a

reduction in modelling area. To do this, we redefined

modelling area from the first produced niche distribution

map plus a 50-km buffer area. Within this area, we generated a

definitive niche model of the Rufous-backed Robin as

described previously. We then produced a final niche distri-

bution map for the Rufous-backed Robin by depicting a

gradient of habitat suitability based on the ‡75% coincidence

area of the 20 best models (Fig. 1).

Dispersion modelling

We generated a dispersion model of the Rufous-backed Robin

in Mexico from the information derived from the niche model,

the location of records within and outside its known native

distribution range, its natural history and habitat requirements

in natural and anthropogenic environments. Based on available

empirical evidence, we assumed two means of species disper-

sion: (1) through the establishment of populations in areas of

native vegetation with suitable habitat and (2) through the

establishment of populations in cities and towns larger than

2000 inhabitants. On average, human settlements in Mexico

with more than 2000 inhabitants hold adequate anthropogenic

habitat for the species (e.g. parks and gardens with enough area

and adequate tree cover).

We estimated a species dispersion rate of 4.2 km year)1

(95% CI = 1.3–7.1 km yr)1) outside of its native distribution.

This estimate was calculated from the dispersion rate of 10

subpopulations in five regions of Mexico (Mexico City,

Oaxaca, Hidalgo, northern Jalisco-Guanajuato, and San Luis

Potosı). Calculation of dispersion rates was based on the dates

of first records of established populations in contiguous areas.

These estimates depend on two main assumptions: (1) date of

first record approximates the date of population establishment

and (2) variability in dispersion rate does not vary significantly

among regions in Mexico.

As a first stage for model construction, we generated

concentric 21-km buffers (dispersion distance in 5 years) in a

GIS from the limit of the area of the niche distribution map

where the presence of the Rufous-backed Robin was supported

by records (niche + records area; Fig. 1). Within this area, we

excluded the area of suitable habitat in Chiapas because we

have confirmed the species’ absence from the region (except

for two isolated records of individuals collected at Pueblo

Nuevo Solistahuacan in 1961, now at the Louisiana State

University Museum of Natural Science). Thus, we set the

south-eastern limit of the niche + records area at La Ventosa

region in the Isthmus of Tehuantepec, Oaxaca, where the

southern-most species records from 2008 occur (Fig. 1). We

also generated concentric 21-km buffers for records outside the

species known native distribution range, including those from

the Mexico-USA border. We assumed the starting year for

concentric buffers to be 2008 at the limit of the niche + records

area because this was the year of the most recent records for

niche model construction. In the case of records outside the

species’ native distribution, we considered the year of the

earliest record as the starting year for concentric buffers. We

intersected the concentric buffers with layers representing the

niche model, areas of deciduous, semi-deciduous, and riparian

forests and cities and towns larger than 2000 inhabitants. We



then produced a point map at the intersections, assigning an

estimated year of arrival at each point based on the species rate

of dispersion. We performed adjustments of the year of arrival

at a site whenever a true species absence was confirmed

regarding the estimated year of arrival (e.g., a true absence in

2009 while the estimated year of arrival was 2005). Finally, we

interpolated this point map (744 points) depicting the

estimated year of arrival at sites using the inverse distance

weighting method to produce a dispersion surface model of the

Rufous-backed Robin in Mexico (Fig. 2).

RESULTS

Site-specific relationship with urban variables

The multiple regression model of site-specific urban variables

predicted the abundance of the Rufous-backed Robin in

Mexico City (R2 = 0.55; F5,152 = 13.69; P < 0.001) and re-

vealed that four variables were significantly related to the

species’ abundance. Tree species richness was the only variable

positively related to the abundance of this species. All tree

variables were highly correlated, but only one was used to avoid

multicolinearity. Thus, the tree components, including tree

species richness, tree cover, tree height and tree abundance are

important to determine the presence and abundance of this

species in a particular urban site. On the other hand, elevation,

vehicles min)1 and number of cats and dogs observed were

shown to be negatively related to the presence and abundance

of the Rufous-backed Robin (Table 1). The positive relation-

ship with tree variables suggests that the presence and

abundance of the Rufous-backed Robin is associated with

vegetated areas, such as urban parks, gardens and sports fields

(including courts, gardens and tree areas). On the other hand,

the negative relationship with vehicles and number of cats and

dogs suggests that the species avoids heavy trafficked areas with

high densities of potential bird predators. Furthermore,

although our surveys in Mexico City comprised an elevational

range of only 1040 m (2137–3177 m a.s.l.), this variable

showed a significant negative relationship to the species’

presence and abundance. This indicates that although the

species can inhabit areas with a higher elevation than its native

range (sea level to 1500 m a.s.l.), it remains a limiting factor for

its distribution and abundance in urban environments.

Figure 2 Dispersion model of the Rufous-backed Robin in Mexico. Black lines represent 25-year intervals in species geographic range

expansion. Digital elevation model is depicted in grey scale. The dark grey line depicts its native distribution sensu Howell & Webb (1995)

(Ridgely et al., 2003). Points represent records within (squares) and outside (circles) its native distribution.

Table 1 Multiple regression model of relationship between urban

habitat attributes and the abundance of the Rufous-backed Robin

in Mexico City (general model: R2 = 0.55; F5,152 = 13.69;

P < 0.001).

Parameters Beta SE t P

Intercept 2.887 0.004

Elevation )0.230 0.075 )3.048 0.003

Tree species richness 0.476 0.071 6.717 0.000

Vehicles min)1 )0.159 0.074 )2.159 0.032

No. of dogs and cats )0.141 0.071 )1.984 0.049



Species niche distribution

The niche model for the Rufous-backed Robin (Fig. 1)

provides a good approximation of the species’ known native

distribution. Contiguous to the Pacific slope of Mexico, the

model predicts suitable habitat in the mountains and valleys of

southern Zacatecas and the highlands of Jalisco. The model

also predicts suitable habitat in areas outside its native

distribution, such as the Bajıo region, Mexico City and central

Puebla where the species is present. In addition, the model

predicts suitable habitat in areas where the species does not

currently occur, such as the lowlands of southern Tamaulipas,

the Rıo Verde and Tampaon river basins (San Luis Potosı),

Sierra Gorda (Queretaro and Hidalgo), Barranca de Metztitlan

(Hidalgo), the lowlands of central Veracruz, the eastern

portion of the Tehuacan-Cuicatlan region (Oaxaca and

Puebla), the Chiapas’ Central Depression and an area of the

north-western Yucatan Peninsula.

Current species distribution

Besides its presence within its known native distribution

(Howell & Webb, 1995; Clement, 2000) where it is fairly

common, the Rufous-backed Robin has established popula-

tions outside of its native distribution, mainly in urban areas.

The first of these recently established populations were

recorded in Mexico City and Oaxaca City (Howell & Webb,

1995; Peterson & Navarro-Siguenza, 2006), where the species is

now widespread and fairly common. In the state of Guanaju-

ato, the species was first recorded in Santiago Maravatıo in

1985 (specimen held at the Museo de Zoologıa ‘‘Alfonso L.

Herrera’’, Universidad Nacional Autonoma de Mexico), and

since the early 1990s it has established populations in San

Miguel de Allende and surrounding areas (Brooks, 1999), as

well as in Leon, Celaya and Guanajuato City. It is likely that the

species arrived to Puebla City in the late 1980s through a

population established in Cholula, Puebla, but its presence was

not reported until the late 1990s. Throughout the current

decade, established populations were recorded in Pachuca,

Hidalgo (Carbo Ramırez, 2008), in San Luis Potosı City

(Romero-Aguila & Chapa-Vargas, 2008) and Queretaro City

(Pineda-Lopez & Malagamba Rubio, 2009). The species has

also been recently recorded in western Chihuahua (Madera, in

2002), northern Puebla (Zacapoaxtla, in 2005), northern

Jalisco (Lagos de Moreno, in 2006, and Encarnacion de Dıaz,

in 2007), western Aguascalientes (Calvillo, in 2008) and in

northern Oaxaca (Sierra Norte, in 2008) (Fig. 1). In the USA,

the species was first recorded in southern Arizona, in 1960

(Harrison, 1962; Johnson & Simpson, 1971). However, there

are now additional reports of the species’ presence in several

localities near the Mexico-USA border in California, Arizona,

New Mexico, and Texas (AOU, 1998; Clement, 2000; GBIF,

2009), some of which represent established populations

(Fig. 2). The Rufous-backed Robin was collected in northern

Chiapas in 1961, but apparently did not establish a viable

population because recent surveys did not detected it. In 1965,

it was collected in Sierra Laguna, Baja California Sur (specimen

held at the Coleccion Nacional de Aves, Universidad Nacional

Autonoma de Mexico), but the fate of this population, if ever

established, is unknown.

Dispersion pattern

Given the estimated dispersion rate, the dispersion model

predicts that the Rufous-backed Robin will significantly

increase its distribution range in Mexico in the following

decades (Fig. 2). As an endemic to the Pacific slope of

Mexico, the Rufous-backed Robin has expanded its distribu-

tion to sites in the Mexican Central Highlands, and it is

expected to arrive on the Gulf slope of Mexico before 2025.

Based on the dispersion model, there are two main possible

routes where the species could reach the Gulf lowlands by a

stepping-stone colonization of suitable habitat in areas of

native vegetation and human settlements: (1) through

successive colonization of cities and towns from the estab-

lished population at Comaltepec in Zacapoaxtla, Puebla

(Fig. 3, route a), passing through Papantla and then to

Tecolutla, in costal Veracruz, or passing through Teziutlan,

Puebla, and then to Nautla, in costal Veracruz. (2) Alterna-

tively, the species could reach the Gulf slope from the

population established in Pachuca, Hidalgo (Fig. 3, route b),

through the Barranca de Metztitlan (Hidalgo), though it

would need to overcome first the elevational barrier of the

Sierra de Pachuca and the Sierra de las Navajas (with

elevations above 3000 m). From the Barranca de Metztitlan,

the species could move down through the Panuco river basin,

through the Huasteca region (Hidalgo, San Luis Potosı and

Veracruz), to the lowlands of Veracruz and Tamaulipas.

Additionally, from the Barranca de Metztitlan it could

colonize the Sierra Gorda (Hidalgo and Queretaro) and then

move through San Luis Potosı, where suitable habitat exists

at the Tampaon, Tamuin and Panuco river basins reaching

the lowlands of Veracruz and Tamaulipas. The species could

also expand its distribution range in northern Mexico from

the populations established in the USA, near the Mexican

border. The species could expand into the Baja California

Peninsula through successive establishment of populations in

cities and towns, both in the north from the population

established at Imperial Valley (California), and from the

south if a population exists at Sierra Laguna (Baja California

Sur). It would take longer for the Rufous-backed Robin to

colonize the Yucatan Peninsula from the south-eastern

populations at the Isthmus of Tehuantepec in Oaxaca

(Fig. 2).

One inconsistency was detected in the model for Baja

California Sur. Based on the model, the species would have

reached the city of La Paz by 1978 from the population at

Sierra Laguna, but to date the species has not been detected in

the city (R. Erickson, pers. com.). This fact leads to doubt as to

whether the species was ever established at Sierra Laguna. If

this is the case, the colonization of the Baja California

Peninsula would take much longer.



DISCUSSION

Can the niche model predict colonization areas?

As shown in Fig. 1, the niche model failed to predict areas

already colonized by the Rufous-backed Robin, most corre-

sponding to human settlements. Broennimann et al. (2007)

have demonstrated that during biological invasions, species

may experience a niche shift between native and non-native

ranges. At this point, it is difficult to infer whether or not

the Rufous-backed Robin is experiencing a niche shift as a

result of its range expansion. The species can be a human

commensal both within and outside its native range.

Although it has colonized human settlements well above

its elevational range, the urban heat island phenomenon

(Ferguson & Woodbury, 2007) may be enabling the species

to occur outside its elevational range, but within its

temperature range. The annual mean minimum temperature

of cities where the species occurs inside and outside its

native range does not show a significant difference. Addi-

tionally, we have not found evidence that the species occurs

in areas surrounding the human settlements it has colonized

beyond its native range. Its colonization of human settle-

ments above its elevational range may represent non-

analogue environmental conditions that the niche model

cannot resolve (Thuiller et al., 2004; Fitzpatrick & Hargrove,

2009). However, the finer scale of analysis of its habitat

requirements allowed us to infer those areas prone to be

colonized that the broader scale of the niche model could

not identify.

Why is the Rufous-backed Robin dispersing?

Based on our field observations, two non-exclusive means of

dispersion for the Rufous-backed Robin may be inferred: (1)

through the establishment of populations in areas of suitable

habitat of native vegetation and (2) through the colonization

of human settlements. In addition to the species’ intrinsic

dispersion ability, the mechanisms that would explain its

success in establishing viable populations outside of its native

range could be the absence of competitors (competitive

release), the growth of human settlements, the warmer local

temperatures in cities and towns and the wild bird trade in

Mexico. In the case of competitive release, no resident

populations of other Turdus or Catharus species occur in the

Mexican Central Highlands or in the Baja California Peninsula

(Howell & Webb, 1995; Clement, 2000), either in native

vegetation or in most of the anthropogenic environments.

Here, we assume that niche overlap would be more important

between the Rufous-backed Robin and related species than

with other bird species. On the other hand, the growth in size

of human settlements (villages becoming towns, and towns

becoming cities) may represent the appearance of new urban

habitat available for the species’ expansion. In addition, the

warmer local temperatures and drier environments in cities

and towns, because of the urban heat island phenomenon

Figure 3 Possible routes for the Rufous-backed Robin to reach the lowlands of the Gulf of Mexico from the populations at

Comaltepec, Puebla (a) and Pachuca, Hidalgo (b). Points represent records within (squares) and outside (circles) its native distribution.

The grey line depicts the native distribution of the Rufous-backed Robin, and the grey dashed line depicts the native distribution

of the Clay-coloured Thrush sensu Howell & Webb (1995) (Ridgely et al., 2003). Digital elevation model is depicted in grey scale.



(Ferguson & Woodbury, 2007), may enable the species to

expand its elevational range to higher sites which would

otherwise be inhospitable (MacGregor-Fors et al., 2008), such

as Queretaro City (1800 m a.s.l.), San Luis Potosı City (1950 m

a.s.l.), Puebla City (2100 m a.s.l.), Mexico City (2200 m a.s.l.)

and Pachuca in Hidalgo (2400 m a.s.l.) among others. Finally,

trade involving some native wild bird species is permitted by

Mexican legislation (INE, 1996), including the harvest of wild

or captive-bred populations. Based on this legislation, the

Rufous-backed Robin can be harvested in most of its current

distribution and can be traded anywhere in Mexico. Usually,

the trade of the Rufous-backed Robin in Mexico is carried out

at a local level, but the species may also be traded far from the

collection site. This trade raises the possibility of bird escapes

or releases.

Dispersal pathways

The Rufous-backed Robin seems to be expanding its range

through several types of human-induced dispersal pathways

sensu Wilson et al. (2009a): leading-edge dispersal, jump

dispersal and extreme long-distance dispersal. These dispersal

pathways and the differences between the species’ native and

non-native ranges have ecological and evolutionary conse-

quences (Wilson et al., 2009a,b). Leading-edge dispersal is

likely to occur according to the normal dispersal distance of

the species at the limit of its native distribution range, that is at

the boundary between the Pacific slope of Mexico and the

Mexican Central Highlands. Examples of this could be the

populations established in southern Guanajuato, central Pue-

bla and central Oaxaca. Jump dispersal, which is a form of

long-distance dispersal where there is still a possibility of gene

flow between the new and the previous species’ range over

ecological time-scales, seems to be operating in all other sites

that the species has colonized in Mexico and in the Mexico–

USA border. Finally, extreme long-distance dispersal is the

pathway that should explain the species occurrences in all

other sites in the USA because this represents colonization of

areas far beyond its dispersal capacity in ecological time-scales.

In the leading-edge dispersal pathway, both direct and indirect

human intervention could be involved, whereas in jump and

extreme long-distance dispersal pathways a direct human

intervention should be the main driver. During post-coloni-

zation or post-introduction spread, leading-edge dispersal and

jump dispersal could be the main dispersal pathways for the

species. It is very likely that, in most cases, colonization of a

particular site by the Rufous-backed Robin is the consequence

of several colonization or introduction events, which also

increases the likelihood of successful establishments. Human-

induced dispersal pathways tend to introduce larger propor-

tions of genetic variation from more diverse sources over

shorter periods of time than natural pathways (Wilson et al.,

2009a). The assessment of the proportion of the species’

genetic diversity at a site compared total genetic diversity of the

species is in fact, an interesting area for research because of the

biogeographic, ecological and management implications.

Expansion rate

The dispersion model generated in this study was based on the

assumption of a linear species’ dispersion rate outside its

native distribution. However, some factors could accelerate or

retard the species’ dispersion depicted in the model. On the

one hand, the wild bird trade in Mexico may hasten

dispersion, since this species is broadly traded in Mexico

(although it is not very common in most markets). On the

other hand, competitive exclusion by other taxonomically

related species or functionally similar species could restrict its

expansion.

The trade of the Rufous-backed Robin in Mexico raises the

possibility of bird escapes or releases potentially increasing its

dispersion rate. Not all bird escapes and releases end up

establishing viable populations, but some do (Munoz & Real,

2006; Strubbe & Matthysen, 2009). This could explain the

presence of distant and isolated established populations of the

Rufous-backed Robin outside its native distribution, such as in

San Luis Potosı City, Pachuca in Hidalgo, and Comaltepec in

Puebla. There is no doubt that all species records and

established populations in the USA are the result of escapees

from captive populations rather than natural expansion. The

USA was the world largest single importer of live wild bird

species at least before the Wild Bird Conservation Act of 1992

came into effect (Thomsen et al., 1992; Beissinger, 2001).

On the other hand, if competitive exclusion effectively

occurs between the Rufous-backed Robin and the Clay-

coloured Thrush Turdus grayi, the presence of the latter could

slow or even stop the dispersion of the former. This could

prevent the arrival of the Rufous-backed Robin to the Gulf

slope of Mexico, Chiapas and the Yucatan Peninsula. An

example of competitive exclusion is suggested by the replace-

ment of the American Robin by the Clay-coloured Thrush over

much of the range of the latter species from southern Mexico

and Central America (Clement, 2000). Alternatively, the

Rufous-backed Robin could competitively exclude local pop-

ulations of the Clay-coloured Thrush. Under this scenario, the

geographic expansion of the Rufous-backed Robin at the

expense of the Clay-coloured Thrush could represent a threat

to the long-term viability of some thrush populations. Finally,

another possibility could be that both species coexist if there

are mechanisms that prevent potential niche overlap, such as

differential use of resources (food, nesting sites) in space or

time between the two species. This possibility seems likely,

given the known coexistence of populations of both species in

areas colonized by the Rufous-backed Robin in Oaxaca

(Forcey, 2002) and San Luis Potosı (Romero-Aguila &

Chapa-Vargas, 2008). However, more information is needed

to discriminate among these alternative hypotheses.

Emerging interspecific interactions

The geographic range expansion of the Rufous-backed Robin

will probably originate new interactions with other species,

particularly with those closely related taxonomically and



ecologically. New interspecific interactions could emerge with

the Clay-coloured Thrush and seasonally with the Veery

Catharus fuscescens, the Gray-cheeked Thrush Catharus min-

imus, and the Wood Thrush Hylocichla mustelina during the

non-breeding season. There is no available information on the

ecology of interactions between the Rufous-backed Robin and

other Turdus or Catharus species which would allow us to infer

potential emerging interspecific interactions as the species

expands its geographic range; in fact, this is an area that merits

further research.

The Clay-coloured Thrush has similar habits and habitat

requirements to the Rufous-backed Robin (Howell & Webb,

1995; Clement, 2000). If the Rufous-backed Robin reaches the

Gulf slope of Mexico, it will likely interact with populations of

the Clay-coloured Thrush in the overlapping areas, mainly in

cities and towns (Fig. 3). This coexistence has already been

reported in Oaxaca City (Howell, 1990; Forcey, 2002) and in

San Luis Potosı City (Romero-Aguila & Chapa-Vargas, 2008),

but no interspecific interactions have been reported so far.

These areas, however, are outside of the native geographic

range of both species. The inability of the Rufous-backed

Robin to expand into Chiapas is intriguing and may be

evidence of competitive exclusion because of the presence by

the Clay-coloured Thrush.

A fading endemism?

Evidence gathered so far on the geographic expansion of the

Rufous-backed Robin has identified both direct and indirect

roles of humans as driving factors. The direct role has occurred

through intentional or unintentional species releases at sites

outside its native distribution. However, an indirect role is

implicated by the establishment of populations in human

settlements that are becoming suitable habitat for the species.

Based on these facts, we conclude that the Rufous-backed

Robin remains an endemic bird species of the Pacific slope of

Mexico and the Balsas Basin, but it should be regarded as an

invasive alien species (sensu Pysek et al., 2004) in the Mexican

Central Highlands because of direct and indirect human-

induced dispersion.

van Kleunen & Richardson (2007) have suggested that

many endemic species might have evolved traits that allow

them to persist in small populations and prevent them from

going extinct. Such traits could explain the rapid spread from

small founder populations when introduced to new areas.

However, range expansion or invasion of endemic species

seems to be rather uncommon in nature. Endemic species

usually have restricted ranges either because they have limited

dispersal abilities, or because they have highly specific habitat

requirements, or both. Although 27% of all birds are

considered restricted-range species (Stattersfield et al., 1998),

we have no knowledge of other endemic birds expanding

their distribution range or invading new areas. More

empirical studies are urgently needed to document this

process to understand its patterns, mechanisms and conse-

quences.

Research and management recommendations

The dispersion model produced in this study should be

regarded as a hypothesis to stimulate research and monitoring

in those areas where the Rufous-backed Robin is expected to

arrive, and also in those areas where it has already established.

This would be particularly feasible in urban and suburban

environments where the probability of detecting the species is

greater, but areas covered with native vegetation should also be

surveyed because here ecological impacts, if any, would be of

more concern. Additionally, evidence of niche shifts should be

sought, considering their evolutionary and ecological implica-

tions, and also given its potential application in providing

empirical evidence in managed relocation strategies in a

scenario of climate change (e.g., Richardson et al., 2009).

Governments also need to be more cautious when defining

and implementing wildlife trade policies. In Mexico, current

wildlife policies (including trade) should be carefully reviewed

to reduce the risk of intentional or unintentional dispersal of

species outside their native ranges. In the case of the Rufous-

backed Robin, trade should be limited to sites within its native

distribution, at least until more information is available on the

ecological consequences of such range expansion.

ACKNOWLEDGEMENTS

We thank the following ornithologists for communicating the

presence-absence of the Rufous-backed Robin in several major

cities in Mexico: S. Arriaga-Weiss, E. Blancas-Calva, J. Chable-

Santos, D. Colon, J. Correa Sandoval, R. Erickson, H. Garza-

Torres, M. Grosselet, A. Lafon-Terrazas, L.F. Lozano Roman,

E. Mellink, R. Pineda-Lopez, Y. Rubio, I. Ruvalcaba, D.

Valenzuela, J. Vargas Soriano, and J. Vega. We also thank the

following institutions and data bases for access to their data:

California Academy of Sciences, Cornell University Museum of

Vertebrates, Delaware Museum of Natural History, Louisiana

State University Museum of Natural Science, Macaulay

Library, Michigan State University Museum, National Mu-

seum of Natural History, Santa Barbara Museum of Natural

History, Universidad Nacional Autonoma de Mexico (Cole-

ccion Nacional de Aves and Museo de Zoologıa ‘‘Alfonso L.

Herrera’’), University of Kansas Biodiversity Research Center,

University of Michigan Museum of Zoology, University of

Minnesota Bell Museum of Natural History, University of

Washington Burke Museum, Yale University Peabody Mu-

seum, and eBird, Great Backyard Bird Count, and Project

FeederWatch. We thank K. Renton, J.L. Rangel, B. Patterson,

D.M. Richardson, and two anonymous reviewers whose

comments helped to improve the manuscript.

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BIOSKETCH

Miguel Angel Martınez-Morales Landscape ecologist and

conservation biologist interested in niche modelling, priority

settings for conservation, alien species ecology and population

ecology of endangered species.

Author contributions: M.A.M.M., I.Z., and L.C.V. conceived

the ideas. M.A.M.M., I.Z., L.C.V., I.M.F., R.O.A., E.R.A., and

P.C. collected the data. M.A.M.M., I.M.F., and R.O.A. analysed

the data. M.A.M.M. led the writing.

Editor: Bruce Patterson



BIODIVERSITY RESEARCH: Current distribution and predicted geographic expansion of the Rufous-backed...

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