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
The Rufous-backed Robin: a fading endemism?
Diversity and Distributions, 16, 786–797, ª 2010 Blackwell Publishing Ltd 789
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
M. A. Martınez-Morales et al.
790 Diversity and Distributions, 16, 786–797, ª 2010 Blackwell Publishing Ltd
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.
The Rufous-backed Robin: a fading endemism?
Diversity and Distributions, 16, 786–797, ª 2010 Blackwell Publishing Ltd 791
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.
M. A. Martınez-Morales et al.
792 Diversity and Distributions, 16, 786–797, ª 2010 Blackwell Publishing Ltd
(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
The Rufous-backed Robin: a fading endemism?
Diversity and Distributions, 16, 786–797, ª 2010 Blackwell Publishing Ltd 793
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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Wilson, J.R.U., Dormontt, E.E., Prentis, P.J., Lowe, A.J. &
Richardson, D.M. (2009a) Something in the way you move:
dispersal pathways affect invasion success. Trends in Ecology
and Evolution, 24, 136–144.
Wilson, J.R.U., Dormontt, E.E., Prentis, P.J., Lowe, A.J. &
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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
The Rufous-backed Robin: a fading endemism?
Diversity and Distributions, 16, 786–797, ª 2010 Blackwell Publishing Ltd 797
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