Ecology Letters, (2012)
LETTER
Brian Tilston Smith,1* Robert W.
Bryson Jr.,2† Derek D. Houston3 and
John Klicka2†
doi: 10.1111/j.1461-0248.2012.01855.x
An asymmetry in niche conservatism contributes to the
latitudinal species diversity gradient in New World vertebrates
Abstract
The Tropical Niche Conservatism hypothesis is a leading explanation for why biodiversity increases
towards the equator. The model suggests that most lineages have tropical origins, with few dispersing into
temperate regions. However, biotas are comprised of lineages with differing geographical origins, thus it is
unclear whether lineages that originated on different continents will exhibit similar patterns of niche conservatism. Here, we summarised biogeographical patterns of New World vertebrates and compared species
diversity patterns between families that originated in North and South America. Overall, families with
southern origins exhibit niche conservatism with many lineages restricted to the Neotropics, whereas many
northern-origin families are distributed across the Neotropics and the Nearctic. Consequently, northern lineages have contributed to high tropical biodiversity, but southern lineages have contributed relatively little
to temperate biodiversity in North America. The asymmetry in niche conservatism between northern and
southern lineages is an important contributor to the biodiversity gradient.
Keywords
Biodiversity, biogeography, nearctic region, neotropical region, niche evolution, tropical niche conservatism.
Ecology Letters (2012)
INTRODUCTION
One of the most ubiquitous ecological patterns in nature is the
increase in species diversity towards the equator, generally referred to
as the latitudinal species diversity gradient (LDG hereafter; Mittelbach
et al. 2007). The Tropical Niche Conservatism hypothesis (TNC) has
become a leading model used to explain why biodiversity is higher in
tropical regions (Wiens & Donoghue 2004). This model suggests that:
(1) most clades evolved in the tropics when tropical environments
extended into high latitudes, (2) dispersal into temperate regions has
been limited because adaptation to colder temperatures appears to be
uncommon and (3) the deeper history of lineages adapted to conditions in the tropics has allowed these clades more time for speciation
to occur (Wiens & Donoghue 2004). Since the conception of the
TNC, an abundance of new phylogenetic data have emerged allowing
researchers to fully test predictions of the model. Evidence of TNC
has been demonstrated in a variety of organisms, including invertebrates (Hawkins & DeVries 2009; Condamine et al. 2012), birds
(Hawkins et al. 2006), amphibians (Wiens et al. 2006, 2009) and bats
(Stevens 2011). The TNC predicts that historical processes such as
post-Eocene climatic cooling have played an important role in the formation of the LDG (Wiens & Donoghue 2004). Other lesser studied
Earth history processes may have also impacted the LDG.
The formation of mountain chains, land bridges, riverine systems
and other landscape features has strongly influenced the diversification of biotas (Feduccia 1999; Sanmartı́n et al. 2001; Hoorn et al.
2010). The large-scale biogeography of some clades (Mao et al. 2012)
developed as early as the Mesozoic (251–65 million years ago, or Ma)
1
Museum of Natural Science, Louisiana State University, 119 Foster Hall,
and is attributed to continental drift. In the Cenozoic (65 Ma to present), the landmasses moved into their current positions and the continents of North and South America were isolated from one another
for most of this time period. Although the GAARlandia land bridge
that linked South America and the Greater Antilles during the midCenozoic may have also provided a corridor to North America (Iturralde-Vinent & MacPhee 1999), South America probably did not
become connected to North America until 12 Ma at the earliest
(Montes et al. 2012). Throughout most of the Cenozoic, South America was isolated from all other landmasses and was in effect a vast
island continent. In contrast, North America, which extends from
Alaska through Panama, has maintained a long history of intermittent
connectivity with eastern Eurasia over the last 50 million years by way
of the recurring Bering land bridge, and with Europe via an older
trans-Atlantic connection that was present 190 Ma (Sanmartı́n et al.
2001). The differences in the geological histories of North and South
America have important implications for understanding the evolution
and current-day distributions of each continent’s biota.
The mammalian fossil record has revealed evidence that North and
South American lineages may have been differentially influenced by
geological history (Simpson 1980). The fossil record suggests that c.
3 Ma, there was an abrupt change in the mammalian diversity of
North and South America (Simpson 1980) as lineages unique to each
continent were exchanged in an event now known as the ‘Great
American Biotic Interchange’ (Marshall et al. 1982). The sudden
appearance of new taxa on each land mass was linked to the completion of the Panamanian Isthmus, a land bridge that served as a corridor connecting the once isolated continents (Marshall et al. 1982).
†
Current address: Department of Biology and Burke Museum, Box 351800,
Baton Rouge, LA, 70803, USA
University of Washington, Seattle, Washington, 98195, USA
2
*Correspondence: E-mail: [email protected]
Marjorie Barrick Museum of Natural History, University of Nevada Las Vegas,
4505 S Maryland Parkway, Las Vegas, NV, 89154, USA
3
Department of Biology, Brigham Young University, 401 WIDB, Provo, UT,
84602, USA
© 2012 Blackwell Publishing Ltd/CNRS
2 B. T. Smith et al.
Recent studies using phylogenetic data have examined whether a similar exchange occurred in other taxa (animals and plants, Cody et al.
2010; birds, Smith & Klicka 2010; fishes, Chakrabarty & Albert 2011).
This research indicated that some lineages, such as land animals, largely depended on the closure of the Panamanian land bridge to disperse between continents (Cody et al. 2010; Smith & Klicka 2010).
Other lineages, such as plants and freshwater fishes, dispersed across
the oceanic barrier well before the continents were connected (Cody
et al. 2010; Chakrabarty & Albert 2011).
One of the more remarkable outcomes from the Great American
Biotic Interchange was the asymmetric success of North and South
American mammals after crossing the land bridge. Northern mammals
radiated across the South American continent and replaced a significant portion of the endemic fauna (Webb & Marshall 1982). Today,
more than 50% of modern South American mammalian genera have
North American origins (Webb & Marshall 1982). This pattern contrasts strongly with the relatively poor success of southern mammals
that dispersed northward (Webb & Marshall 1982). Relatively few of
the mammalian lineages that evolved in the tropics of South America
had success in colonising the temperate regions of North America as
only 10% of extant genera have southern origins (Webb & Marshall
1982). A similar pattern of asymmetry in colonisation success between
North and South American faunas has been demonstrated in some lineages of birds and was postulated to be a possible contributor to the
LDG (Smith & Klicka 2010). The geographical distributions of some
groups of birds and mammals indicate that there may be asymmetric
niche conservatism between widely distributed lineages that originated
in North America and lineages with a South American origin that are
restricted to the Neotropics. The importance and generality of asymmetric niche conservatism between North and South American faunas
and the impact of this asymmetry on the formation of the New World
LDG has not been fully evaluated.
Here, we focused on broad-scale distributions of New World vertebrates to examine patterns of niche conservatism between taxonomic families that had ancestral origins in either North or South
America. We evaluated two alternative explanations for the broadscale latitudinal ranges of New World vertebrates: (1) assemblages
with North and South American origins both exhibit similar patterns of being mostly restricted to the Neotropics as predicted by
the TNC or (2) northern faunas more commonly occur in both the
Neotropics and Nearctic, whereas southern faunas are restricted to
the Neotropics. We compiled data on the proportion of vertebrate
species distributed in the Neotropical and Nearctic regions and calculated the latitudinal range limits in a large percentage of New
World families from major vertebrate clades (ray-finned fishes,
amphibians, turtles, squamates, mammals and birds). We then tested
whether families with North American origins have a greater proportion of species in the Neotropics than families with South
American origins have in the Nearctic. Understanding species distribution patterns in faunas that originated in different regions will
provide insight into how lineages with differing biogeographical histories may have helped shape the LDG across the New World.
MATERIALS AND METHODS
New World vertebrate families
For our study, we assessed the degree of niche conservatism at relatively deep phylogenetic scales. To accomplish this objective, we
© 2012 Blackwell Publishing Ltd/CNRS
Letter
examined patterns at the family level that allowed us to determine
whether or not related taxa maintained broad-scale climatic niches
over time. We compiled species lists, geographical distributions and
family-level latitudinal ranges for six major New World vertebrate
clades from several sources: amphibians (Frost 2011; Accessed July
2011), birds (American Ornithologist’s Union Check-lists of North
and South American birds; www.aou.org; Accessed July 2011),
mammals (Wilson & Reeder 2005; www.bucknell.edu/msw3/), rayfinned fishes (hereafter fishes; Nelson 2006), testudines (hereafter
turtles) and squamates (lizards and snakes) (The Reptile Database;
http://www.reptile-database.org/; Accessed July 2011). In some
cases, we used Google Earth (http://earth.google.com) and the
IUCN Red List of Threatened Species (www.iucnredlist.org) to validate the latitudinal ranges of families.
Defining biogeographical regions
To define the Nearctic and Neotropical regions, we used biogeographical areas delimited by the Sclater–Wallace classification (Lomolino et al. 2010) and the transition zone proposed by Morrone &
Márquez (2001). The Nearctic region extends from Alaska, USA
south to the Mexican highlands (the Sierra Madre del Sur, Sierra
Madre Occidental, Sierra Madre Oriental, and Trans-Mexican Volcanic Belt). The Neotropical region encompasses the tropical coastal
lowlands adjacent to the Mexican highlands south through Central
America and the entire continent of South America. We omitted all
Caribbean, oceanic island and marine species, and considered only
those taxa distributed on mainland North and South America. For
each family, we counted the number of species distributed in the
Nearctic and Neotropical regions to describe whether a family was
constrained to a biogeographical region or if it was distributed in
both regions. We assigned species that were distributed in both the
Nearctic and Neotropical regions to both regions.
Ancestral geographical areas
To incorporate the ancestral geographical origins of families, we summarised published studies that inferred the ancestral geographical origin of lineages (see Supplementary data for citation list). These studies
used phylogenetic data and/or the fossil record to indicate whether a
family first occurred in North America (defined as the landmass
extending from present-day Alaska south through Panama) or South
America. We chose to identify the geographical origin of lineages
(North or South America) instead of biogeographical origin (Neotropical or Nearctic region) for two reasons: (1) biogeographical
regions describe the geographical distribution of extant biotas and
may not be applicable to the time periods in which lineages originated
and (2) by identifying the geographical origin of lineages we were able
to distinguish between Neotropical lineages that originated in South
America from those that originated west of the Isthmus of Panama.
Published studies based on phylogenetic data used various approaches
to infer the ancestral geographical origin of a clade, including ancestral
state reconstruction (Pagel 1999), dispersal-vicariance analysis (Ronquist 1997), the dispersal-extinction-cladogenesis model (Ree & Smith
2008), and inferences made from the order of branching events in a
phylogeny where more basal divergences represent area of origin and
subsequent divergences represent the direction of range evolution. In
studies that inferred areas of origin from fossils, ancestral areas were
determined from the oldest described fossil for a lineage and/or using
Letter
cladistics to infer the phylogenetic placement of extinct taxa. In many
instances, studies used both the fossil record and phylogenetic data.
As lineages may have evolved outside of the New World and dispersed into North or South America, our designations of North
American (hereafter termed ‘northern’) and South American (‘southern’) faunas ultimately refer to geographical origination of ancestral
lineages in the New World. Families for which the ancestral geographical origins could not be inferred were omitted from subsequent
analyses.
Statistical analyses
For each of the six major vertebrate clades (ray-finned fishes, amphibians, turtles, squamates, mammals and birds), we identified the maximum southern range limit of species within each family, and then
averaged these southernmost limits across families within each vertebrate clade. Similarly, we obtained the northernmost range limit for
each family and the average northernmost range limit for each vertebrate clade. We also calculated the mean latitudinal range for all families within each major vertebrate clade using the maximum northern
limit and maximum southern limit for any species within the respective family. In addition, for each family, we estimated the total number of species that occur in the Neotropical and Nearctic regions (see
Supplementary data). The Neotropical–Nearctic transition represents
the environmental turnover from relatively constant warm temperatures throughout the year in the tropics to the high temperature seasonality in the temperate zone (Lomolino et al. 2010). Geographical
distributions at this biogeographical scale can be used as proxies for
assessing niche conservatism or niche evolution. Lineages restricted
to the Neotropical region are consistent with tropical niche conservatism and lineages distributed in both the Neotropical and Nearctic
regions are consistent with niche evolution. To examine niche conservatism among continental biotas, we assessed the broad-scale biogeography of lineages at the expense of accounting for potential niche
evolution within tropical and temperate lineages in the Neotropics.
Since southern temperate taxa represent a small subset of Neotropical
diversity and their distributions and diversity are consistent with the
predictions of the TNC, we assumed that niche conservatism patterns
within the Neotropics would not influence our interpretations.
To test whether or not there was a significant difference between
the proportions of northern species in each family distributed in the
Neotropical region relative to the proportions of southern species in
each family distributed in the Nearctic region, we used a generalised
linear model with a quasi-binomial error term to account for overdispersion. Generalised linear modelling was independently performed on fishes, amphibians, squamates, mammals and birds (we
did not include turtles because of the small number of families). For
the response variable in each model, we combined a list containing
the number of species distributed in the Nearctic and Neotropical
region in each northern family with an analogous list of the biogeographical distributions of the number of species in each southern
family. We specified the geographical origin of a family as the predictor variable in each model. We performed all analyses using native
libraries in the R programming language.
RESULTS
After filtering out families with ambiguous ancestral geographical
origins, we compiled a list of 188 New World vertebrate families
Latitudinal asymmetry in niche conservatism for vertebrates 3
comprised of 12 101 species (see Supplementary data for family
and species totals). Of all species currently distributed on mainland
North and South America, our analysis included 57% of fishes,
87% of amphibians, 100% of turtles, 61% of squamates, 78% of
mammals and 75% of birds. The low sampling of fishes in our
study was largely a result of poorly studied families that are
restricted to South America, particularly in the Amazon Basin. The
lower sampling in squamates was driven by the exclusion of the
speciose snake family Colubridae that is distributed across all continents except Antarctica. Of the clades sampled, 95 families were of
northern ancestral origin and 93 families were of southern origin
(see Supplementary data): fishes (northern families: 13; southern
families: 22), amphibians (northern families: 11; southern families:
18), turtles (northern families: 7; southern families: 3), squamates
(northern families: 12; southern families: 9), mammals (northern
families: 23; southern families: 15) and birds (northern families: 29;
southern families: 26).
Each of the southern vertebrate clades had mean northern latitudes that did not extend into the Nearctic region (Fig. 1;
Table 1). In contrast, the northern vertebrate clades showed a
strikingly different pattern with squamate, turtle, mammalian and
bird families that extended their southern limits south of the
equator (Fig. 1; Table 1). The distributions of northern amphibian
and fish families indicated that these clades were largely restricted
to the temperate region (Fig. 1; Table 1). The mean northernmost latitude of northern clades was similar across vertebrate
clades and extended well into the temperate region of North
America (Table 1). A similar pattern was shown in the southern
clades; their mean southernmost latitude extended into the temperate zone of South America (Table 1). Mammalian and bird
families with ancestral northern origins had larger latitudinal
amplitudes than their southern counterparts (Table 2). Southern
amphibian, squamate and fish families had wider latitudinal amplitudes than northern-origin lineages (Table 2).
Amphibian, squamate, mammal and bird families with northern
origins had a significantly greater proportion of species in the Neotropical region than families with southern origins in the Nearctic
(Table 3). In contrast, fish species from both northern and southern
families were largely restricted to Nearctic and Neotropical regions
respectively (Table 3).
DISCUSSION
Niche conservatism and the latitudinal species diversity gradient
Our results suggest that the ‘out of the tropics’ model (Wiens &
Donoghue 2004; Jablonski et al. 2006) presents an incomplete picture of the historical assembly of the LDG in the New World.
When the ancestral geographical origins of taxonomic lineages are
explicitly taken into account, a striking pattern is apparent. Families
that evolved in South America exhibit strong tropical niche conservatism with many lineages seemingly unable to expand into temperate North America (Fig. 1). Conversely, many terrestrial families
with northern geographical origins show a remarkable ability to
inhabit both the Nearctic and Neotropical regions (Fig. 1). This
finding is significant for understanding the origins of biodiversity
patterns in the New World because northern lineages have contributed to the high species diversity of the tropics of North and South
America, but southern faunas have contributed very little to the
© 2012 Blackwell Publishing Ltd/CNRS
4 B. T. Smith et al.
Letter
Northernmost latitude (°)
N
90
80
70
60
50
40
30
20
10
0
S
N
–10
–20
–30
–40
Fishes
Amphibians
Turtles
Mammals
Birds
90
80
70
60
50
40
30
20
10
0
S
–10
–20
Squamates
–30
–40
–60–50–40–30–20–10 0 10 20 30 40 50 60 70
S
–60–50–40–30–20–10 0 10 20 30 40 50 60 70
N
S
N
–60–50–40–30–20–10 0 10 20 30 40 50 60 70
S
N
Southernmost latitude (°)
Figure 1 Pairwise plots showing latitudinal range (latitudinal maximum and minimum) of New World vertebrate families with inferred ancestral origins. Families that
originated in North America and South America are shown using light-blue squares and orange circles respectively. Lines cross at the approximate northern limits of the
Neotropical–Nearctic transition (27° N). The geographical demarcation between North and South America occurs at approximately 9.3° N. Points in the upper right box
are distributions of lineages restricted to the Nearctic region, points in the lower left box represent lineages that are restricted to Neotropical region, and points in the
upper left box denote lineages that occur in both the Nearctic and Neotropical regions. The x-axis shows southernmost latitude and the y-axis shows northernmost latitude.
Table 1 Latitudinal range patterns of New World vertebrates
Clade
Mean Northern
Latitude
(Northern
Families)
95% CI
Fishes
Amphibians
Turtles
Squamates
Mammals
Birds
50.8°
48.5°
41.0°
37.0°
61.8°
60.6°
(41.1°
(40.8°
(29.0°
(31.1°
(52.3°
(54.2°
–
–
–
–
–
–
60.6°)
56.2°)
52.9°)
42.8°)
71.3°)
67.1°)
Mean Northern
Latitude
(Southern
Families)
13.1°
16.6°
11.7°
22.7°
5.9°
19.9°
Mean Southern
Latitude
(Southern
Families)
95% CI
Mean Southern
Latitude
(Northern
Families)
95% CI
(10.5° – 15.6°)
(4.5° – 28.7°)
(9.7° – 13.7°)
(13.2° – 32.3°)
(1.4° – 30.3°)
(13.0° – 26.7°)
23.9°
17.8°
1.0°
3.1°
36.9°
13.6°
(19.6° – 28.3°)
(5.2° – 30.5°)
( 21.4° – 23.3°)
( 23.1° – 16.9°)
( 47.5° – 26.3°)
( 26.5 – 0.8°)
95% CI
( 35.6° –
( 34.4° –
N/A
( 42.6° –
( 43.9° –
( 42.4° –
31.5°
27.4°
30.8°
29.8°
37.6°
36.2°
27.5°)
20.4°)
16.9°)
31.4°)
30.1°)
Shown are mean latitudes and 95% confidence intervals of the average range limits across families within each vertebrate clade. Average range limits for each vertebrate
clade come from the northernmost and southernmost range limits from species distributions in each family.
Table 2 Latitudinal ranges of New World vertebrates
Clade
Mean
Latitudinal
Amplitude
(Northern
Families)
Fishes
Amphibians
Turtles
Squamates
Mammals
Birds
26.9°
30.7°
40.0°
40.1°
98.3°
74.3°
Table 3 Proportion of species in each biogeographical region
95% CI
Mean
Latitudinal
Amplitude
(Southern
Families)
95% CI
(17.7°–36.1°)
(14.0°–47.4°)
(11.0°–69.1°)
(15.7°–64.5°)
(78.8°–117.8°)
(59.8°–88.8°)
44.6°
43.6°
42.5°
52.5°
53.9°
56.1°
(39.2°–50.0°)
(27.3°–59.9°)
(4.7°–80.3°)
(35.°5–69.5°)
(38.5°–69.2°)
(45.4°–66.8°)
Means and 95% confidence intervals of latitudinal ranges for northern and southern vertebrate families. Latitudinal ranges were estimated by averaging across the
maximum and minimum latitude for each family within a vertebrate clade.
© 2012 Blackwell Publishing Ltd/CNRS
Clade
Proportion of
species distributed
in the Neotropics
from northern
families
Proportion of
species distributed
in the Nearctic
from southern
families
t -statistic
Fishes
Amphibians
Squamates
Mammals
Birds
0.02
0.38
0.53
0.60
0.72
0.00
0.07
0.06
0.01
0.04
t
t
t
t
t
>
>
>
>
>
1.86
5.48
3.26
3.62
10.94
P
0.07
< 0.0001
0.004
0.0009
< 0.0001
Shown are the overall proportion of species from northern vertebrate families
distributed in the Neotropics and the proportion of species from southern vertebrate families in the Nearctic. Reported are the t-statistics and P-values from the
generalised linear models.
Letter
diversity of temperate North America. Potential asymmetry in niche
conservatism between continental faunas has not been previously
factored into the TNC (Wiens & Donoghue 2004), nor has it been
incorporated into any other LDG model (Mittelbach et al. 2007).
While the TNC hypothesis provides a causal linkage between historical processes and present-day patterns, we suggest that the model
should now be extended to incorporate the mosaic of biogeographical histories of different lineages (see Condamine et al. 2012; Davies
& Buckley 2012; Hawkins et al. 2012).
We found that vertebrate clades show strong tropical niche conservatism within families of southern geographic origin, but the
degree of niche evolution displayed in northern clades varies
(Fig. 2). The pattern of niche evolution was most prevalent in turtles, squamates, mammals and birds, which have mean southern latitudes near or below the equator and latitudinal ranges that extend
into both the Nearctic and Neotropical regions (Fig. 1; Table 1).
Although we found significant statistical support for asymmetric
niche conservatism in amphibians, as evident in the northern geographical origin of plethodontid salamanders (Zhang & Wake 2009)
and ranid frogs (Bossuyt et al. 2006; Wiens et al. 2009) that have
invaded the Neotropical region, the pattern is likely influenced by
the relatively low diversity of northern amphibians. Among families
with northern origins, niche divergence was least apparent within
fishes. The comparatively little mixing that has occurred between
northern and southern fish lineages may be because freshwater
Latitudinal asymmetry in niche conservatism for vertebrates 5
fishes face the greatest biogeographical barriers in attempting to disperse across the landscape. Nevertheless, these biogeographical patterns in fishes reinforce the historic isolation of the North and
South American biotas.
We found that vertebrate clades with northern ancestral origins
have more species in the Neotropics than clades with southern
ancestral origins have in the Nearctic (Fig. 2; Table 3). From our
data, it is not directly apparent whether our results represent a general process that has contributed to the formation of species richness patterns or if the asymmetric pattern is attributable to
historical contingencies restricted to New World biotas. The palaeogeography of North and South America set the stage for a unique
historical scenario that shaped the evolution of New World faunas
(Simpson 1980), but this pattern may not be as pronounced across
the Old World because northern and southern Old World land
masses were more connected during the Cenozoic. However, higher
diversification rates in the tropics due to differences in rates of speciation, extinction or both could generate a similar asymmetric pattern (Mittelbach et al. 2007; Wiens 2007), especially if speciation
rates increased in northern lineages after their colonisation of the
tropics. Nonetheless, there appears to be evidence of inherent differences in niche conservatism among temperate and tropical taxa.
A study that assessed niche conservatism among New World temperate and tropical montane species found that tropical species
exhibited greater niche conservatism (Cadena et al. 2012). Future
Figure 2 Biogeographical patterns of species diversity in families that have a North American ancestral origin (Northern clades; top) or a South American ancestral origin
(Southern clades; bottom). Light-blue vertical columns show the proportion of species in each family distributed in the Nearctic region, whereas orange vertical columns
show the proportion distributed in the Neotropical region. Taxonomic families have been separated into the major vertebrate clades, which are denoted by coloured bars
above and below the graph as follows: fishes (green), amphibians (brown), turtles (light purple), squamates (rouge), mammals (dark blue) and birds (yellow).
© 2012 Blackwell Publishing Ltd/CNRS
6 B. T. Smith et al.
studies of the TNC across the Old World should consider potential
biogeographical range differences between northern and southern
faunas to assess whether asymmetric niche conservatism is a general
pattern.
Time and niche conservatism
While asymmetry in niche conservatism between northern and
southern clades of New World vertebrates examined in our study is
clear, more detailed and finer scale biogeographical analyses will be
required to understand the processes that resulted in this
asymmetry. Molecular dating and the fossil record indicate that
many groups that evolved in South America, such as parrots
(Wright et al. 2008), hylid frogs (Wiens et al. 2011) and caviomorph
rodents (Antoine et al. 2011), have origins that date back to the
early Cenozoic and evolved on the continent during its period of
isolation from other land masses. Many of the southern lineages
that have dispersed into Central America but not into temperate
North America appear to have only dispersed across the Isthmus of
Panama in the last few million years (birds, Smith et al. 2012; mammals, Simpson 1980). In addition, temperate southern lineages were
not a prominent component of the faunas that dispersed northward
(Simpson 1980; Smith & Klicka 2010). The southern lineages that
have expanded into the Nearctic such as hummingbirds (Smith &
Klicka 2010) and eleutherodactyline frogs (Heinicke et al. 2007)
likely colonised North America prior to the formation of the Panamanian land bridge.
The biogeographical history of the North American fauna is
dynamic. Origination of some North American vertebrate clades
date to the Cretaceous (e.g., pleurodont lizards, Townsend et al.
2011) whereas others are more recent Miocene arrivals (e.g., nineprimaried oscine songbirds, Barker et al. 2004). Clades such as
viperid snakes (Wüster et al. 2008), numerous mammalian lineages
(Simpson 1980; Weinstock et al. 2005) and ranid frogs (Bossuyt
et al. 2006), evolved in the Old World during the Eocene and presumably colonised North America via a Beringian connection
(Sanmartı́n et al. 2001). The timing and degree of niche divergence
in these northern groups will require further analyses, but in some
cases, such as in oscine songbirds (Barker et al. 2004) and squirrels
(Mercer & Roth 2003), niche evolution appears to have occurred
relatively rapidly. The degree to which niches have evolved will
additionally need to be assessed to understand whether the niches
of northern lineages have actually evolved or whether they had historically broad niches.
Potential causes of asymmetric niche divergence
There are several plausible explanations for the observed differences
in biogeographical distributions of northern and southern faunas
detected in our study. In mammals, it has been suggested that
northern lineages are better migrators, better survivors and speciators and better competitors (reviewed in Lomolino et al. 2010), or
have ecologies more suitable for Late Cenozoic environments (Vrba
1992). There are multiple examples of extraordinary radiations of
northern lineages in South America such as the sigmodontine
rodents (Simpson 1980) and thraupid tanagers (Feduccia 1999). In
contrast, there are no comparable radiations by southern vertebrate
lineages in North America. In addition, it has been proposed that
because northern biotas are part of a more cosmopolitan biota that
© 2012 Blackwell Publishing Ltd/CNRS
Letter
evolved on a ‘world continent’ exposed to greater competition,
more diseases, and more parasites, they were better able than their
southern counterparts to take advantage of niches that opened in
novel geographical areas (Wilson 1992). The asymmetry was also
likely influenced by the differences in abiotic conditions that taxa
were exposed to over time. Northern taxa may have experienced
greater environmental heterogeneity because North America occurs
across a large latitudinal and longitudinal gradient in comparison
with South America that is more centred on the equator and has a
much smaller temperate zone.
Differences in metabolic capacity between tropical and temperate
lineages represent another possible explanation for asymmetric niche
conservatism. Temperate species are predicted to have undergone
stronger selection for higher thermogenic capacity and cold tolerance than tropical species because tropical species experience less
variability in temperature seasonality (Wiersma et al. 2007). Under
such a scenario, tropical species would require cold tolerance to disperse into the temperate region, whereas temperate species would
not require any physiological adaptations to disperse into the tropics. Studies in birds (Wiersma et al. 2007) and mammals (Rezende
et al. 2004) have shown that temperate species have higher metabolic rates than tropical species, which is consistent with the finding
that tropical environments have not selected for high thermogenic
capacity. Complimentary findings have been used to explain broadscale biogeographical distributions in the northern origin oscine passerine birds that have higher metabolic rates than southern-origin
suboscine passerine birds that are largely restricted to the tropics
(Swanson & Bozinovic 2011). The causes of asymmetric niche conservatism between northern and southern lineages are not directly
apparent. Accordingly, the asymmetric niche conservatism identified
in our study highlights the need for future research that investigates
potential ecological and evolutionary characteristics of lineages that
may explain the mechanisms responsible for the pattern.
CONCLUSION
The latitudinal gradient in species diversity has been generated by the
synergy of speciation, extinction and dispersal, yet our understanding
of these interactions across time and space has been challenging
(Wiens & Donoghue 2004). In this study, we evaluated species distribution patterns of New World vertebrates to determine the degree of
niche conservatism in lineages that had North or South American
ancestral origins. The isolation of South America from other landmasses for most of the Cenozoic and the protracted history of intermittent connectivity of North America with the Old World has led to
evolution of two biotas that currently exhibit asymmetric niche conservatism with respect to broad-scale biogeographical distributions.
Overall, we found that families with South American origins exhibit
strong niche conservatism with many lineages geographically
restricted to the Neotropics. In contrast, many families with North
American origins occur widely across both the Nearctic and Neotropical regions. Thus, northern lineages have contributed to the high
tropical biodiversity, but southern lineages have contributed relatively
little to temperate biodiversity of North America. Based on the findings of our study and recently published works (Wiens et al. 2011;
Condamine et al. 2012; Davies & Buckley 2012; Hawkins et al. 2012),
biogeographical history can influence the degree of niche conservatism observed in lineages. Therefore, accounting for the relationship
between geographical origin and niche conservatism in comparative
Letter
analyses may help explain some of the variance in temporal and species richness patterns observed across lineages and should provide
greater explanatory power among correlations in studies examining
the latitudinal diversity gradient.
ACKNOWLEDGEMENTS
We thank R. Brumfield, P. Chakrabarty, A. Cuervo, D. Fletcher,
M. Harvey, R. Terrill, G. Seeholzer and E. Rittmeyer for their comments, feedback and assistance in preparing this manuscript. We thank
J.J. Wiens, F. Condamine and an anonymous referee for providing
comments and suggestions that greatly improved this manuscript.
AUTHOR CONTRIBUTIONS
B.T.S. and J.K. designed the study. B.T.S., R.W.B. and D.D.H collected the data. B.T.S analysed the data. All authors contributed to
the writing of the paper.
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