Biological Journal of the Linnean Society, 2008, 94, 869–878. With 4 figures
Phylogenetic perspective on ecological niche evolution
in american blackbirds (Family Icteridae)
MUIR D. EATON*, JORGE SOBERÓN and A. TOWNSEND PETERSON
Natural History Museum and Biodiversity Research Center, The University of Kansas, Lawrence,
KS 66045, USA
Received 11 September 2007; accepted for publication 1 November 2007
Analysis of ecological characters on phylogenetic frameworks has only recently appeared in the literature, with
several studies addressing patterns of niche evolution, generally over relatively recent time frames. In the present
study, we examined patterns of niche evolution for a broad radiation of American blackbird species (Family
Icteridae), exploring more deeply into phylogenetic history. Within each of three major blackbird lineages, overlap
of ecological niches in principal components analysis transformed environmental space varied from high to none.
Comparative phylogenetic analyses of ecological niche characteristics showed a general pattern of niche conservatism over evolutionary time, with differing degrees of innovation among lineages. Although blackbird niches were
evolutionarily plastic over differing periods of time, they diverged within a limited set of ecological possibilities,
resulting in examples of niche convergence among extant blackbird species. Hence, an understanding of the
patterns of ecological niche evolution on broad phylogenetic scales sets the stage for framing questions of
evolutionary causation, historical biogeography, and ancestral ecological characteristics more appropriately.
© 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94, 869–878.
ADDITIONAL KEYWORDS: character evolution – comparative method – convergent evolution – GARP –
niche modelling.
INTRODUCTION
Over the past decade, historical information has
increasingly been incorporated into ecological studies
to illuminate evolutionary processes behind ecological
patterns (Freckleton, Harvey & Pagel, 2002). Systematists are now beginning to show similar interest in
incorporating ecological information into systematic
and biogeographic studies (Wiens & Graham, 2005).
The idea is that all biogeographic processes occur in
ecological contexts, so ecological information, particularly in paleoecological contexts, can inform an understanding of biogeography, distribution, and evolution
(Wiens, 2004a, b). Still, in spite of the recent wave of
interest, studies linking ecological and phylogenetic
analyses are only beginning to appear in the
literature (Rice, Martínez-Meyer & Peterson, 2003;
Graham et al., 2004).
*Corresponding author. Current address: Drake University,
Biology Department, 2507 University Avenue, Des Moines,
IA 50311, USA. E-mail: [email protected]
The New World blackbirds (Family Icteridae)
provide an ideal opportunity to explore ideas of
ecological niche conservatism and niche evolution
in a phylogenetic framework. The approximately 100
blackbird species assort into several major monophyletic lineages (Lanyon & Omland, 1999), and
robust, well-supported species-level phylogenies
based on molecular sequence data have been developed for nearly all lineages (Johnson & Lanyon,
1999; Omland & Lanyon, 2000; Price & Lanyon,
2004). What is more, icterids in general occupy
diverse habitats and ecological niches throughout
the New World (Jaramillo & Burke, 1999): oropendolas and caciques are tropical forest dwellers;
meadowlarks are open-country birds; orioles generally inhabit more open riverine forests; and the
grackles and allies are most diverse in terms of ecological associations. This broad range of coarse-scale
ecological attributes, combined with the availability
of detailed phylogenetic frameworks (Lanyon, 1993),
make icterids a prime choice for exploring and
understanding niche evolution.
© 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94, 869–878
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M. D. EATON ET AL.
Previous studies of niche evolution have generally
focused on relatively recent time frames (Peterson,
Soberón & Sánchez-Cordero, 1999; Graham et al.,
2004), such as most recent speciation events, with
relatively few studies extending back to deeper portions of phylogenetic time (Andreas et al., 2001;
Yesson & Culham, 2006a, b). In the present study, we
explore more deeply into phylogenetic history, viewing
back over a broad radiation of the blackbirds.
Comparisons not just of closely-related taxa, but also
distantly-related groups, open the possibility of
detecting convergent evolution of niche characteristics, in which areas presenting similar conditions may
be invaded independently by distantly-related taxa.
More generally, we assess ecological niche evolution
in a phylogenetic framework across broad time scales.
MATERIAL AND METHODS
INPUT DATA
Primary occurrence data (i.e. unique geographic coordinates of known occurrences for each species) were
assembled for all species of New World blackbirds
from the following sources: North American Breeding
Bird Survey (Sauer, Hines & Fallon, 2001), the Atlas
of Mexican Bird Distributions (Navarro-Sigüenza,
Peterson & Gordillo-Martinez, 2002, 2003), and
several North American natural history museum
collections (Academy of Natural Sciences, American
Museum of Natural History, University of Washington Burke Museum, Natural History Museum of Los
Angeles County, Peabody Museum of Natural History,
Western Foundation of Vertebrate Zoology, University
of Kansas Natural History Museum); the latter
were queried in part via the ORNIS portal (http://
ornisnet.org). Occurrence data were restricted to
breeding season dates for migratory species (e.g.
Icterus galbula, Euphagus carolinus). Geographic
coordinates for all localities were drawn from the
GeoNet Names Server (http://gnswww.nga.mil/
geonames/GNS/index.jsp).
Phylogenies for three major clades of New World
blackbirds were drawn from the literature: oropendolas and caciques (Price & Lanyon, 2004), orioles
(Omland & Lanyon, 2000), and grackles and allies
(Johnson & Lanyon, 1999; Eaton, 2006). Because
the resolution of the basal relationships among
the primary icterid clades remains weak (Lanyon &
Omland, 1999), subsequent comparative analyses
were restricted to comparisons within these three
major lineages. The meadowlarks (genus Sturnella)
were excluded from the study due to the lack of a
robust, well-supported, species-level phylogenetic
hypothesis. In addition, four species were excluded for
lack of sufficient input occurrence data or because
comprehensive niche models could not be developed
given the very small sample sizes of occurrence data
or severely limited island distributions: Nesopsar
nigerrimus, Icterus laudabilis, Icterus oberi, and
Icterus bonana.
ECOLOGICAL
NICHE MODELING
To characterize ecological landscapes over the broad
range of this group, we used nine of the ‘bioclimatic’
variables from the WorldClim data set (Hijmans,
Cameron & Parra, 2005): annual mean temperature,
maximum temperature of warmest month, minimum
temperature of coldest month, annual temperature
range, temperature seasonality, annual precipitation,
precipitation of wettest and driest months, and precipitation seasonality. Given that the occurrence data
were of rather low precision (approximately 10 km),
we used the coarsest-resolution data set in WorldClim
(pixel resolution 0.17°). Although our niche analyses
(see below) focused on climatic dimensions, we
included four dimensions of topography (elevation,
slope, aspect, compound topographic index) from the
Hydro-1K data set (USGS, 2001) in model development because these variables can modify how a
species experiences climates (e.g. north-facing slopes
versus south-facing slopes), and are known to contribute positively to model quality (Peterson & Cohoon,
1999). Topographic variables were resampled to 0.17°
to match the resolution of the WorldClim data. Thus,
for each cell in the geographic grid, we had 13 values
of environmental variables.
The next challenge was to model ecological niches
for each species. We emphasize that the general ideas
and approaches explored herein do not depend on
any particular ecological niche modeling approach, as
many options exist (Carpenter, Gillison & Winter,
1993; Huntley et al., 1995; Guisan & Zimmermann,
2000; Elith & Burgman, 2002; Phillips, Dudik &
Schapire, 2004; Segurado & Araujo, 2004; Elith et al.,
2006). In the present study, we modeled ecological
niches using the Genetic Algorithm for Rule-set
Prediction (GARP; Stockwell & Peters, 1999, 2002b).
GARP is an evolutionary-computing approach to
discovery of nonrandom associations between occurrences and raster GIS data layers that describe
potentially relevant aspects of ecological landscapes.
As GARP has been applied quite widely to questions
of New World bird distributions, invariably resulting
in statistically significant predictions of species’
distributions (Peterson et al., 1999; Peterson, 2001,
2005; Anderson, Gomez-Laverde & Peterson, 2002;
Stockwell & Peterson, 2002a; Anderson, Lew &
Peterson, 2003), we do not present detailed descriptions of the niche modeling methodology; also, given
the quite-variable sample sizes and space limitations,
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ECOLOGICAL NICHE EVOLUTION IN BLACKBIRDS
we do not present statistical tests of the model predictions used.
In general, all analyses were run on default settings, and the best-subsets procedure (Anderson
et al., 2003) was used to choose a subset of models for
further consideration, which were then summed to
produce a single grid summarizing levels of model
agreement in predicting presence versus absence.
This grid was converted to a binary prediction of
presence versus absence by choosing the lowest
threshold at which the species was known to occur
(Rice et al., 2003; Pearson et al., 2006). The result was
a set of binary grids summarizing the geographic
extents of the environmental niches calculated by
GARP for all species. Therefore, corresponding to the
geographic extent of the niche of a species is the set
of all environmental combinations (points in the
13-dimensional space of raw environmental variables)
occurring in that geographic space. The niche of the
species can in fact be defined as this set of environmental combinations.
NICHE
CHARACTERIZATION AND VISUALIZATION
Because variables of different units and variances
were used in building models, the first step in characterizing modeled niches was to standardize each
variable by centering them to their mean and dividing
them by their standard deviations (Hirzel et al., 2002;
Peterson, 2007). This step solves the problem of
sensitivity to units of measure, but creates another:
effectively assigning equal weights to variables of
unequal variance. No solution to this dilemma is
available that retains the interpretability of the original niche axes; one simply has to choose which
problem is more important to the purposes of the
study. We transformed the database for the entire
Western Hemisphere (156 933 cells in geographic
space) to standard normal variables.
To address problems with inter-correlations among
environmental variables and consequent nonindependence of axes, a principal components analysis (PCA) was used to reduce dimensionality. The
use of PCA-transformed variables to estimate niche
breadth is common (Rotenberry & Wiens, 1980;
Carnes & Slade, 1982; Litvak & Hansell, 1990). We
retained the first five principal components, which
together accounted for more than 99% of the total
variance (loadings of raw environmental variables on
principal component axes available from authors
upon request). For each geographic cell, a vector of
five variables corresponding to its PCA-transformed
environment was thus created. Figure 1 visualizes
the ecological space of the Americas in two sets of
axes: raw environmental space and the reduced PCA
space. In both visualizations, the irregular features of
871
environmental space are apparent: two elongated
pincer-like arms corresponding to the extremely wet
regions of the Choco in Colombia and the temperate
rain forests of southern Chile. Hence, the set of grid
cells in geographic space corresponding to the niche of
a species defines a set of PCA-transformed variables
that can be used to depict that niche in the transformed space. Notice that, if repetitions of environmental combinations are not allowed (i.e. if all pixels
in the environmental space are unique), the number
of elements in a niche is strictly equal to the number
of elements in the corresponding geographic space.
We further characterized each species in terms of
niche breadth and niche volume, following computations derived and detailed elsewhere. Niche breadth is
summarized as the multivariate variance in the standardized PCA space (Carnes & Slade, 1982). Niche
volume is calculated as the total number of elements in
the set of niche variables for a given species; this latter
number equals the spatial extent of the geographic
projection of the suitable conditions for each species.
We calculated ecological distances between niches for
all pairs of species within each of three major lineages
as the average of all pairwise Euclidean distances
between all environmental combinations within the
niche of one species to all environmental combinations
of another species. In addition, geographic distances
between centroids of distributional areas for all pairs
of species within each of the three lineages were
calculated using ArcGIS, version 9.0.
To visualize evolution of ecological niches within
each lineage, we used a branch-length fitting
approach. Pairwise ecological niche distances among
species were fit to the topology of the respective
phylogenies using the Fitch optimization option in
PHYLIP (Felsenstein, 1989). This method allows
visualization of total amount of ecological niche
change (phylogenetic distances) along each branch in
the phylogeny, essentially constraining patterns of
ecological similarity or difference to reflect the independent phylogenetic hypotheses, and allowing us
to distinguish ecological convergence from ecological
conservatism.
RESULTS
For each species, distributional predictions derived
from niche models were consistent with known distributions (Jaramillo & Burke, 1999); as discussed in
the Material and Methods, detailed tests of model
predictions were not developed in the present study.
As in previous studies, raw model predictions also
showed disjunct areas of potential distribution
(Guisan & Thuiller, 2005; Soberón, 2007). This phenomenon is illustrated for Icterus auratus, which is
endemic to the Yucatan Peninsula in southeastern
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M. D. EATON ET AL.
Figure 1. Visualization of the ecological niche of Icterus auratus in (A) geographic space, (B) a two-dimensional view of
raw environmental variable space, and (C) a two-dimensional view of ecological principal components space, as an
example.
Mexico, but for which disjunct potential distributional
areas are also predicted in the Chaco of southern
South America and the Caribbean (Fig. 1A). Niche
dimensions can be understood in raw environmental
space (Fig. 1B) and, perhaps most usefully, in principal components space (Fig. 1C). In general, each
species was restricted to a subsector of the overall
ecological diversity of the Americas.
Within each major clade, pairwise distances
between ecological niches varied from high (i.e. nonoverlapping niches) to almost nil. For example,
the niches of Cacicus melanicterus and Cacicus
chrysopterus overlapped considerably, whereas no
overlap was observed between the niches of either of
these species and that of Cacicus sclateri (Fig. 2).
Furthermore, these three taxa illustrate the dynamics of niche evolution: the sister species pair C. sclateri and C. chrysopterus has diverged ecologically,
whereas distantly-related taxa in distinct regions can
converge on the same niche (e.g. C. chrysopterus
and C. melanicterus) in southern South America
and western Mexico, respectively. More commonly,
though, sister species pairs show similar niche characteristics (see Supplementary material).
Ecological, geographic, genetic, and phylogenetic
distances across all taxa in each major lineage show
© 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94, 869–878
ECOLOGICAL NICHE EVOLUTION IN BLACKBIRDS
4
C. sclateri
C. chrysopterus
3
Principal Component 2
C. melanicterus
2
1
0
-1
-2
-3
-4
-5
0
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2
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4
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Figure 2. Visualization of overlapping niches (Cacicus
chrysopterus, Cacicus melanicterus) and non-overlapping
niches (Cacicus sclateri versus C. chrysopterus and C.
melanicterus) in two-dimensional ecological principal components space.
intriguing interrelationships (Fig. 3). Genetic and
geographic distances are positively related, but
species pairs exist that are distant genetically but
close geographically (e.g. Agelaius phoeniceus and
Quiscalus quiscula in eastern North America); fewer
species pairs are distant geographically, but genetically close (e.g. I. galbula and Icterus abeillei in
eastern North America and central Mexico, respectively). Similarly, ecological distances are positively
related to genetic and geographic distances, but many
examples exist of species pairs that are similar ecologically but distant genetically or geographically.
Finally, and most interestingly, plots of pairwise
phylogenetic ecological distances versus raw ecological distances (Fig. 3, bottom) illustrate evolutionary
convergence in ecological traits. By definition of the
phylogenetic distances, they always equal or exceed
raw ecological niche distances. Points in the extreme
lower right sector of these graphs indicate ecological
niche convergence; these species pairs are ecologically
quite similar, but when that similarity is constrained
to fit to the phylogenetic topology, considerable overall
ecological change must have occurred. Convergence
for these extreme examples was confirmed by visual
inspection of PCA score reconstructions on the respective phylogenies (see Discussion). It should also be
noted that the great majority of the points in these
graphs are clustered close to the origin; these points
reflect general conservatism of niches over much of
the evolutionary history of these species.
Visualizing these optimizations on the phylogenies
of each major clade shows that the phylogenetic signal
of niche evolution concentrates mainly in the terminal
branches (Fig. 4). The oropendola and cacique tree
indicates little overall niche differentiation, but
873
cacique niches have diverged somewhat from oropendola niches. Large amounts of niche differentiation are
seen only in C. melanicterus, C. sclateri, Psarocolius
atrovirens, and Ocyalus latirostris + Psarocolius oseryi.
Among orioles, again, overall change is slight
(although more than in the oropendolas and caciques)
and the signal is focused in the terminal branches;
only a few species account for substantial niche
divergences, namely Icterus chrysocephalus, Icterus
spurius, Icterus parisorum, Icterus leucopteryx, Icterus
bullockii, I. galbula, and Icterus cucullatus. The grackles and allies show more substantial niche divergence
among clades: in particular, large niche divergence
has occurred in the Gymnomystax + Lampropsar
+ Hypopyrrhus, Euphagus, and Macroagelaius clades.
In addition, some niche divergence is detectable
between the two main clades (mostly South American
species versus mostly North American species) within
the larger grackle and allies phylogeny.
DISCUSSION
Previous studies of the present type have produced a
variety of results regarding the idea of conservatism
of ecological niches over evolutionary time, including
studies developed in an explicit phylogenetic context
(Andreas et al., 2001; Rice et al., 2003; Graham et al.,
2004; Knouft et al., 2006; Jakob, Ihlow & Blattner,
2007; Waltari et al., 2007). A general pattern emerging from such studies is that of only minor niche
differentiation, with the exceptions being large niche
divergences in terminal branches of the tree (Yesson
& Culham, 2006b). The present study is different in
that it considers a much deeper clade that covers a
broader geographic region and more diverse species
than most previous studies, although a few exceptions
exist (Andreas et al., 2001). Overall, patterns of niche
evolution in the icterids support the idea of phylogenetic inertia in niche characteristics, with most
closely-related species showing closely similar ecological characteristics. However, in some cases, niche
divergence was detectable among terminal branches
of the phylogenies, and some deeper phylogenetic
signal in ecological characters was detectable in the
grackles and allies (Fig. 4).
The present study is one of an emerging suite of
explorations of phylogenetic dimensions of coarse-scale
ecological features of evolving clades. The broader goal
is to reconstruct ancestral ecological characteristics
and the distributional patterns that would be linked to
them, which would be a powerful tool in understanding
historical biogeography. However, as is clear from the
results presented herein, the understanding of how
ecological traits evolve over the history of major clades
is only beginning to be shaped, and so such bigger goals
remain as future challenges. What is more, beyond the
© 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94, 869–878
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M. D. EATON ET AL.
GRACKLES AND ALLIES CLADE
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ORIOLE CLADE
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OROPENDOLAS AND CACIQUES CLADE
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Figure 3. Plots of all pairwise species comparisons within each of three New World blackbird clades (vertical columns,
respectively) of (A) genetic distance (uncorrected sequence divergence) versus geographic distance; (B) genetic distance
versus ecological niche distance; (C) geographic distance versus ecological niche distance; (D) phylogenetic distance
(Fig. 4) versus ecological niche distance.
challenge of dealing with ecological characters, many
challenges remain simply in the realm of reconstructing continuous characters on phylogenies (Oakley &
Cunningham, 2000).
One point that we emphasize herein, however, is
that comparisons of patristic and raw ecological distances reveal widespread niche convergence (Fig. 3).
Here, distantly-related taxa converge on similar niche
characteristics. Visual inspection of the principal
component axes summarizing ecological niche space
reconstructed on the phylogeny for each of the three
clades confirmed that these species pairs are, indeed,
converging on similar values of niche variables (see
Supplementary material). In general, the contrast
between raw and tree-constrained ecological distances makes these convergent cases detectable.
The nature of these convergence events then
becomes of considerable interest. Does convergence
result from reinvasion of the same geographic region,
or can it result from invasion of similar niche conditions in different geographic areas? The former situation can be observed in O. latirostris and C. sclateri
© 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94, 869–878
ECOLOGICAL NICHE EVOLUTION IN BLACKBIRDS
875
Cacicus melanicterus
Cacicus solitarius
A
Psarocolius wagleri
Psarocolius angustifrons
Psarocolius atrovirens
Psarocolius decumanus
Psarocolius viridis
Dives warszewiczi
Psarocolius montezuma
Dives atroviolacea
C
Dives dives
Psarocolius yuracares
Agelaius tricolor
Psarocolius bifasciatus
Agelaius phoeniceus
Agelaius assimilis
Cacicus chrysonotus
Agelaius xanthomus
Agelaius humeralis
Cacicus haemorrhous
Molothrus rufoaxillaris
Psarocolius oseryi
Scaphidura (Molothrus) oryzivora
Molothrus aeneus
Ocyalus latirostris
Molothrus ater
Cacicus cela
Molothrus bonariensis
Euphagus carolinus
Cacicus uropygialis
Euphagus cyanocephalus
Quiscalus quiscula
Cacicus microrhynchus
Quiscalus lugubris
Cacicus chrysopterus
Quiscalus niger
Quiscalus mexicanus
Cacicus sclateri
1
Quiscalus major
Macroagelaius imthurni
Macroagelaius subularis
I. maculialatus
Gnorimopsar chopi
B
I. wagleri
Agelaius thilius
I. cucullatus
Agelaius xanthophthalmus
I. chrysocephalus
Agelaius cyanopus
Agelaius ruficapillus
I. cayanensis
Agelaius icterocephalus
I. spurius
Xanthopsar (Agelaius) flavus
I. dominicensis
Pseudoleistes virescens
I. mesomelas
Pseudoleistes guirahuro
I. icterus
Molothrus badius
I. croconotus
Oreopsar bolivianus
I. graceannae
Amblyramphus holosericeus
I. pectoralis
Curaeus curaeus
I. parisorum
Curaeus forbesi
Gymnomystax mexicanus
I. graduacauda
Lampropsar tanagrinus
I. chrysater
Hypopyrrhus pyrohypogaster
I. leucopteryx
1
I. auratus
I. nigrogularis
I. gularis
I. bullockii
I. pustulatus
I. galbula
I. abeillei
1
Figure 4. Evolution of ecological niches in each of three New World blackbird clades: (A) oropendolas and caciques;
(B) orioles; and (C) grackles and allies. Branch lengths represent the amount of ecological niche divergence between
nodes and between nodes and terminal taxa.
© 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94, 869–878
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M. D. EATON ET AL.
(Fig. 4A), which share higher values of PC1, lower
values of PC2, and higher values of PC5 (see Supplemental material, Fig. S1A), and are endemic to the
same restricted area of northeastern Peru. The latter
situation, however, is also observable; for example,
C. melanicterus and C. chrysopterus (Fig. 4A) occur
in western Mexico and east-central South America,
respectively, but converge on extreme lower values of
PC1, extreme higher values of PC2, and intermediate
values of PC5. Other such convergent, allopatric
examples include I. galbula and I. bullockii (Fig. 4B)
in eastern and western North America, and E. carolinus and Curaeus curaeus (Fig. 4C) in northern
North America and southern South America. These
examples serve to illustrate the repeated patterns of
convergence on coarse-scale ecological niche types
within the different icterid lineages, and offer a potential general explanation for observations of niche
conservatism and convergence on broad evolutionary
scales, consistent with evolutionary constraints on
niches over longer periods of time (Peterson et al.,
1999; Yesson & Culham, 2006b).
Of the emerging swell of studies of niche evolution
versus conservatism, most have concluded conservatism of niche characteristics (Peterson et al., 1999;
Martínez-Meyer, Peterson & Hargrove, 2004; Knouft
et al., 2006; Martínez-Meyer & Peterson, 2006), but
several have found evidence of more rapid change
(Rice et al., 2003; Graham et al., 2004). A notable
difference in the present study is the recognition
that some lineages tend to innovate ecologically
whereas others do not, with a general constraint of
niche conservatism among closely-related taxa. For
example, the grackles and allies show greater overall
evolutionary innovation of niches (Fig. 4C) compared
to the oropendolas and caciques; grackles’ and allies’
niches are more broadly distributed throughout principal component environmental niche space (compare
range of PC values in Supplemental material,
Fig. S1A–C). This spread reflects the ecological diversity realized by these species as a group, inhabiting
marshes (e.g. Agelaius spp.), open grasslands (e.g.
some Quiscalus and Molothrus spp.), dry scrub (e.g.
Oreopsar bolivianus), and temperate and tropical
forests (e.g. C. curaeus and Lampropsar tanagrinus)
throughout the Americas, with some distributions
ranging into the high latitudes of both hemispheres
(Jaramillo & Burke, 1999). By contrast, the oropendolas and caciques exhibit much more restricted distributions of niches across species (Fig. 4A), occurring
exclusively in tropical latitudes, and inhabiting
lowland tropical and subtropical forest, although
some species prefer drier forest (Jaramillo & Burke,
1999). As a clade, their niches are relatively narrowly
defined along the PC1 axis, with only three species
showing notable innovation beyond the general ‘oro-
pendola and cacique’ niche: O. latirostris, C. sclateri,
and C. melanicterus show extreme PC2 values (see
Supplementary material) indicating invasion into
wetter (the former two species) and drier (the latter)
conditions. Orioles appear intermediate in degree
of niche innovation (Fig. 4B), inhabiting more open
woodlands, gallery forests, and savannas, with many
species preferring more xeric habitats (Jaramillo &
Burke, 1999). Although oriole niches, as a clade, are
defined by relatively higher values along the PC2 axis
(i.e. drier conditions), a subclade of six species represents the principal ecological novelty among orioles: I.
galbula, I. bullockii, I. spurius, I. cucullatus, I. abeillei, and I. parisorum all exhibit much lower values of
PC 1 (see Supplementary material, Fig. S1B), reflecting their more high northern latitudinal distributions, contrasting with the mainly northern tropical
latitude distributions of the rest of the group. Thus,
as clades, the niches of the three icterid lineages
concentrate in different portions of the available environmental principal components space, and, as discussed above, invasions into more extreme niche
space have been achieved independently by relatively
distantly-related taxa (i.e. niche convergence).
In conclusion, the evolutionary history of ecological
niches is certainly likely to be complex, given the
myriad evolutionary forces that shape them (Rice
et al., 2003; Wiens & Graham, 2005). Icterids show a
general pattern of niche conservatism (i.e. evolutionary constraint) on a broad phylogenetic scale, with
differing degrees of innovation within each major
clade. Although niches can certainly be evolutionarily
plastic, as indicated by examples of divergence among
relatively closely-related taxa (Rice et al., 2003),
icterid niches diverge across a relatively limited set of
possibilities, and, as such, relatively distantly-related
taxa may converge on similar ecological characteristics, as has been observed in other studies (Knouft
et al., 2006). The principal goal of the present study
was to understand patterns of niche evolution on
broad phylogenetic scales; by integrating the ecological niche characterizations with a phylogenetic framework, we have succeeded in distinguishing between
overall niche conservatism and convergent evolution,
both of which are widespread in the blackbirds. It is
a very different question to ask why niches are conserved versus why they have converged on the same
niche independently; thus, our results set the stage
for proper framing of questions about evolutionary
causation of niche change and historical biogeography, both in icterids and among other organisms.
ACKNOWLEDGEMENTS
We thank the curators and collection managers at
the aforementioned museums for providing requested
© 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94, 869–878
ECOLOGICAL NICHE EVOLUTION IN BLACKBIRDS
museum records. We thank S. Lanyon for initiating
discussion of niche evolution in blackbirds and M.
Papes for training with georeferencing. We thank two
anonymous reviewers for their helpful comments on
drafts of the manuscript. This work was funded in
part by the IRACDA program at the University of
Kansas.
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SUPPLEMENTARY MATERIAL
The following material is available for this article online:
Figure S1. Continuous squared change parsimony optimization of centroid values for each of the five principal
component (PC) axes defining niche breadth (see Methods), reconstructed on the molecular phylogenies of (A)
oropendolas and caciques, (B) orioles, and (C) grackles and allies, respectively, using Mesquite (version 1.06).
This material is available as part of the online article from:
http://www.blackwell-synergy.com/doi/abs/10.1111/j.1095-8312.2008.01040.x
(This link will take you to the article abstract).
Please note: Blackwell Publishing are not responsible for the content or functionality of any supplementary
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© 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94, 869–878