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Ecological and Evolutionary Limits to Species Geographic Ranges.
Author(s): Monica A. Geber
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Source: The American Naturalist, Vol. 178, No. S1, Ecological and Evolutionary Limits to
Species Geographic Ranges (October 2011), pp. S1-S5
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vol. 178, supplement
the american naturalist
october 2011
Ecological and Evolutionary Limits to
Species Geographic Ranges*
Monica A. Geber†
Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York 14853
Introduction
The study of geographic ranges and their limits necessarily draws on diverse areas of organismal biology, ecology,
micro- and macroevolution, genetics, and paleobiology
(Gaston 2003, 2009; Holt and Barfield 2011). Understanding range limits requires the integration of theoretical,
analytical, and empirical approaches, and this integration
is equally relevant to paleontological, contemporary, and
future timescales. “Ecology and Evolutionary Limits to
Species Geographic Ranges” was therefore a “natural”
theme for the 2010 Vice Presidential Symposium of the
American Society of Naturalists, clearly meeting the Society’s stated goal of “enhance[ing] the conceptual unification of the biological sciences.”
A fundamental goal of ecology is to understand the
distribution and abundance of organisms (Elton 1930; Andrewartha and Birch 1954). This goal leads naturally to
the study of species geographic range limits. There is no
clearer setting in which to examine the determinants of
distribution—a species is present on one side of the
boundary but absent on other. Explanations of range limits
therefore must consider ecological causes: the diverse abiotic and biotic factors that could limit distribution.
Another fundamental perspective on range limits is historical. The history and geography of species origins explain why certain clades are found in some parts of the
world but not others (Wallace 1876; Darwin 1859; Lomolino et al. 2006). Dispersal limitation and barriers to
movement (e.g., mountain ranges, oceans) are the simplest
explanations for why taxa are absent from seemingly suitable habitat. Such explanations are confirmed by wellknown examples of vicariance events (e.g., Knowlton and
Weigt 1998) and (as if by experiment) by species invasions
over the last several centuries, which were facilitated by
* This issue originated as the 2010 Vice Presidential Symposium presented
at the annual meetings of the American Society of Naturalists.
†
E-mail: [email protected].
Am. Nat. 2011. Vol. 178, pp. S1–S5. 䉷 2011 by The University of Chicago.
0003-0147/2011/178S1-53120$15.00. All rights reserved.
DOI: 10.1086/661899
human-mediated transport of organisms (Williamson
1996).
Explanations for range limits also must consider ecological and evolutionary dynamics operating at various
time scales. Ranges are expected to expand and contract
relatively rapidly as environments change, especially in vagile species (Parmesan 2006). But even over the typical
timescales of climate change (e.g., Pleistocene glaciation
cycles), low dispersal speed may still limit some taxa from
precisely tracking suitable environments (Svenning and
Skov 2004). Ideally, investigations of the causes of geographic range limits should attempt to discern whether
range limits are expanding and have recently done so or
whether range boundaries have been stable over long periods of time (Moeller et al. 2011). This requires good
historical information from the fossil record on species
distributions or studies of population genetics and phylogeography combined with climate reconstruction or, failing such information, experimental transplant studies to
establish whether populations can persist beyond their current range boundaries (see Sexton et al. 2009 for list of
transplant studies). If a species distribution can be shown
to be in equilibrium with its current environment, then
the geographic range roughly corresponds to the organism’s niche, sensu Grinnell (1917) and Hutchinson (1957),
that is, the constellation of environmental conditions, both
abiotic and biotic, where the long-term population growth
rate is, at minimum, 1 (Pulliam 2000). The study of geographic range limits is therefore allied with the study of
limits on niche expansion, that is, niche conservatism
(Wiens et al. 2010). Phrased another way, why does a
species fail to adapt to environmental conditions beyond
its current range boundaries (Antonovics 1976)? We know
that populations can evolve rapidly in response to environmental change (Hendry and Kinnison 1999) and that
invading species are currently adapting to novel conditions
in their new range. What hampers adaptation at the margins of stable range boundaries (Hoffmann and Blows
1994)? This is where the rapidly expanding theory on ecological and evolutionary limits to range expansion becomes
informative (Holt and Barfield 2011), inspiring empirical
S2 The American Naturalist
tests of the role of demography, ecology, dispersal (including gene flow), and patterns of environmental variation in selection in constraining range limits.
Contributions to the symposium consist of a mix of
articles, theoretical and empirical. With demographic, ecological, genetic, or comparative approaches, they address
questions concerning the limits on geographic range expansion, the determinants of geographic range size, and
how limits on expansion might explain differences in regional species diversity. Articles in the symposium place
particular emphasis on the joint effects of ecology, demography, and contemporary or historical evolution on
geographic range limits.
Several excellent reviews of limits on geographic range
expansion have recently been published (Gaston 2003,
2009 (and articles therein); Holt and Keitt 2005 (and articles therein); Bridle and Vines 2007; Kawecki 2008; Sexton et al. 2009; Bridle et al. 2010). Rather than review the
theory in its entirety, Holt and Barfield (2011) present new
models on adaptive evolution at range margins as affected
by the interaction between demography, environment, and
selection in patchy landscapes. Holt has been enormously
influential in the development of theory on range limits,
especially in demonstrating the crucial interplay between
demographic performance (a reflection of both the degree
of local adaptation and persistence through immigration),
gene flow, and landscape patterns of environmental variation that define patterns of selection (see references in
Holt and Barfield 2011). Using individual-based models
incorporating demography and genetics, Holt and Barfield
consider a species, such as the cladoceran Daphnia magna,
which occupies a heterogeneous landscape of patches (water bodies) coupled by dispersal. The authors ask how and
when adaptation evolves to the environments of sink habitats—those where conditions are outside the species original niche. They show that the likelihood of observing
niche evolution (and thus range expansion) into sink habitats depends on the magnitude of initial maladaptation;
the pattern (pulsed vs. continuous, uni- vs. bidirectional),
timing (juvenile vs. adult), and rate of dispersal; the mutation rate (a function of population size); and whether
reproduction is asexual or sexual. Interestingly, the architecture of polygenic adaptation is influenced by the interplay of selection and dispersal. A thorough understanding of the evolutionary limits to range expansion therefore
will need to incorporate fine-scale landscape studies of
environmental variation, demography, and genetics.
Direct tests of whether a species is absent beyond its
current range margin because of limited adaptation or
limited dispersal require transplanting populations into
unoccupied areas. Ideally, transplants should include all
life stages and be followed over multiple years. Because of
the difficulty of such experiments, and perhaps also for
ethical reasons, transplant studies beyond range boundaries have been rare (Sexton et al. 2009). Another approach
is to construct species distribution models (SDMs), also
known as niche models, to assess whether a species is
absent from large areas of apparently suitable habitat, suggesting dispersal limitation, or whether a species appears
to occupy the full geographical extent of suitable habitat,
suggesting limits on adaptation. The most common approach to constructing SDMs uses information on species
presences (and sometimes absences, and occasionally
abundances) along with varying numbers of geo-referenced environmental variables (e.g., precipitation, temperature) to generate predictions of where species ought
to be found (Elith and Leathwick 2009). A second approach is to generate mechanistic or process models of
individual or population performance in relation to environmental variables and to map locations that permit a
threshold level of performance (Morin and Thuiller 2009;
Kearney et al. 2010). Comparison of the actual and predicted distributions can then be used to assess limitation
due to dispersal vs. maladaptation.
In the second article of the symposium, Eckhart et al.
(2011) use a third approach to assess dispersal limitation
versus maladaptation along the eastern range margin of a
native California annual plant, Clarkia xantiana ssp. xantiana. Given that the niche requirements of a species are
defined as the environmental conditions that allow longterm population persistence, Eckhart et al. modify standard SDMs based on presence data and environmental
variables with estimates of stochastic population growth
rate (lS) obtained from multiyear studies of demography
on populations spanning the species geographic range in
the southern Sierra Nevada. Stochastic ls are used to
“weight” the importance to population performance of
changing environmental conditions across the range.
Weighted SDMs, compared to distribution models based
solely on presences, place the eastern range margin closer
to its actual margin and support the hypothesis that the
species border is set by limited adaptation.
Theoretical models of range limits implicate demographic processes, including colonization history, population turnover, and population size changes, that often
take place over timescales too lengthy for the average biologist to study. Absent a good fossil record, insight into
historical population dynamics can be gleaned from the
signatures that they leave on the population structure of
molecular genetic variation. In the third contribution to
this symposium, Moeller et al. (2011) discuss how different
models of range dynamics make different predictions
about demographic history and population genetic structure. Although the predictions of alternative range limits
models are not mutually exclusive, molecular population
genetic analyses can assist in determining whether range
Limits to Geographic Ranges S3
limits are unstable (i.e., contracting versus expanding) or
whether stable range limits are set by demographic (e.g.,
metapopulation dynamics) versus evolutionary phenomena (e.g., maladaptive gene flow). With this background,
Moeller et al. (2011) analyze DNA sequence and microsatellite variation from central and peripheral populations
of C. x. ssp. xantiana, the same species used in Eckhart et
al.’s (2011) study. They show that peripheral populations
contain a subset of the diversity of central populations but
have large effective population sizes and show little evidence for population bottlenecks. Central populations, by
contrast, exhibit a history of population expansion with
bias in gene flow from the range center to periphery, consistent with the abundant center hypothesis (Brown et al.
1995). These results tend to discount strictly demographic
models and more strongly support evolutionary-genetic
models where responses to selection at the range edge are
constrained by a lack of adaptive genetic variation or maladaptive gene flow.
In the fourth contribution to the symposium, Paul et
al. (2011) tackle head-on the oft-cited hypothesis of Ernst
Mayr (1963) that maladaptive gene flow from central to
peripheral populations limits adaptation and thus range
expansion at the periphery, using Mimulus cardinalis, a
perennial plant native to the western United States as a
model. First, for a series of populations from the southern
Sierra Nevada of California to a northern range limit in
southern Oregon, they establish, through growth chamber
studies, the temperature optimum for plant growth, because plant relative growth rate is closely tied to fitness.
They then use environmental data on July maximum temperature from each population’s field location to determine the phenotypic deviation (PD) between the populations’ measured temperature optimum for growth and
July field temperatures during the period of maximum
growth. They hypothesize that PD should be correlated
with the amount of maladaptive gene flow that populations
receive, which should be a function of both the amount
of gene flow and how far away genes are coming from,
since gene flow from distant populations with dissimilar
environments is likely to be more deleterious than an
equivalent amount of gene flow from nearby populations
with similar environments. Furthermore, if the maladaptive gene flow hypothesis on limits to range expansion is
correct, peripheral populations should exhibit greater phenotypic deviation from the field temperature and should
be receiving greater amounts of maladaptive gene flow.
The authors find that, for contemporary gene flow estimates, net immigration into northern populations is from
hotter environments, while net immigration into more
southerly populations is from cooler environments. The
magnitude of phenotypic deviation between observed and
predicted temperature optima is correlated with latitude-
weighted (but not temperature-weighted) estimates of
contemporary gene flow, but there is no evidence that
range limit populations exhibit greater PDs or receive
greater amounts of maladaptive gene flow than central
populations. The approach developed by Paul et al.
(2011)—to connect the degree of mismatch between an
observed phenotype and the optimum phenotype for a
population’s environment (PD) to the amount of maladaptive gene flow a population receives—is a powerful
one that can be applied to many populations and many
landscape contexts, not just range limit contexts.
The last two contributions turn the focus from plants
and population biology to animals and macroecological
and macroevolutionary patterns in geographic range. The
authors of the first use a comparative phylogenetic approach to ask whether trait plasticity affects range size in
Australian Drosophila (Overgaard et al. 2011). Those of
the second ask whether conservatism in climatic niche
limits account for differences in bird diversity between
regions of the Himalayas (Price et al. 2011).
There have been many attempts to identify characteristics that influence variation in range size among species
(Bohning-Gaese et al. 2006; Garcia-Barros and Romo Benito 2010) and to determine whether range size is heritable, with mixed results (Webb and Gaston 2003; Mouillot
and Gaston 2007, 2009). There has been particular interest
in the literature on invasive species on what traits distinguish invasive taxa (with large new ranges) from taxa that
fail to become invasive (e.g., van Kleunen et al. 2010). It
has been suggested that adaptive trait plasticity, the ability
of an individual to adjust its phenotype in response to
environmental variation, might lead to larger geographic
distributions. In an interesting twist on long-term studies
of plant species persistence in the famed Thoreau woods
in Concord, Massachussetts, Willis et al. (2008) have
shown that species that have declined in abundance over
the last century and a half are ones that exhibit low flowering time plasticity in response to warming climate and
that conservatism in plasticity is phylogenetically biased.
In this symposium, Overgaard et al. (2011) use a careful
series of rearing experiments to compare adult thermal
limits to both cold (temperate) and warm (tropical) temperatures across five widespread species and five narrowly
distributed tropical species of Drosophila from eastern Australia. In many ectotherms, geographic distributions are
related to thermal tolerance limits (Stevens 1989; Angilletta
2009), and those that occupy variable thermal environments are expected to evolve physiological or behavioral
plasticity to accommodate variable temperatures (Chown
and Terblanche 2007). The authors assessed thermal limits
following both long-term developmental acclimation as
well as the effect of gradual hardening on upper and lower
thermal limits in order to obtain both innate thermal re-
S4 The American Naturalist
sistance and the ability to modify thermal tolerance during
chronic and gradual exposure to temperature extremes.
Contrary to expectation, while widespread species were
more resistant to colder temperatures, they were not more
plastic in their thermal limits to either cold or warm temperatures, suggesting that range distributions in these Drosophila are more closely tied to innate differences in thermal limits than to differences in plasticity of those limits.
In the final contribution to the symposium, Price et al.
(2011) assess the role of niche conservatism, that is, adaptive evolutionary limits on range (niche) expansion, in
explaining a twofold decline in bird species diversity from
the southeastern to the northwestern Himalayas. Climate
modeling shows the two regions to differ in the amount
and seasonality of precipitation and maximum and range
of temperatures, with the northwest being more seasonal,
cooler, and drier than the southeast and also lacking the
warm and humid tropical broad-leaf forests of the southeast. Regional differences in species diversity arise from
differences in speciation versus extinction rates, colonization rates, or carrying capacities (MacArthur 1969;
Wiens and Donoghue 2004; Jablonski et al. 2006; Mittelbach et al. 2007; Roy and Goldberg 2007). The authors
hypothesize that in vagile taxa, such as birds, where movement per se is not a constraint, the failure to colonize
(establish in) new climate zones (niches) may account for
reductions in diversity. Using a comparative phylogenetic
approach, they find that while there is a phylogenetic signal
to evolutionary changes in elevational and latitudinal extents and in habitat use of southeastern bird species, only
habitat use (forest vs. open) is highly conserved. Furthermore, using climate modeling to predict which southeastern species should extend their ranges northwestward,
they find that more species than expected do extend their
ranges, and they do so by occupying climates zones not
present in the southeast. These results suggest that climate
aspects of niche conservatism do not account for the paucity of species in the northwest. Instead, comparisons of
vegetation cover in the two regions indicate that the quantity and quality of forest, the habitat that accounts for the
greatest discrepancy in species number between the two
regions, are lower in the northwest. Whether or not the
northwestern Himalayas have been fully recolonized by
bird species following Pleistocene climate change (i.e.,
whether the region is at full carrying capacity), their results
suggest a contribution of differences in resource diversity
as a contributing factor to regional variation in species
diversity.
In a single symposium, we have of necessity left out
many relevant and exciting areas of research on species
geographic ranges, including species responses to past and
contemporary environmental change, the promise of invasive species as models for studying range expansion, and
the opportunities provided by genomics to assess the genetic basis of traits and trait combinations that confer
adaptation in limiting environments. Neverthless, the symposium’s contributions to the study of geographic range
limits are helpful in identifying the limits of our current
knowledge. On historical timescales, we are just beginning
to amass data on which “niche traits” are conserved and
which are not, and why. The models of Holt and Barfield
show how complex microevolutionary processes of adaptation can be in heterogeneous landscapes, and they
indicate that the time required to adapt to novel conditions
can often be protracted. While empirical studies are beginning to characterize the abiotic, biotic, demographic,
and genetic factors that impinge on the likelihood of adaptation at range limits, we do not yet know how often
and under what circumstances range limits are in equilibrium with their environment, how abiotic and biotic
factors combine to create limiting environmental conditions, how demography and gene flow interact to accelerate or retard adaptation to novel circumstances, and how
the genetics of traits and trait combinations factor into
limits on range expansion.
Acknowledgments
I thank the American Society of Naturalist for the opportunity to organize this symposium, to showcase new
work on the determinants of species geographic ranges. I
appreciate the hard work of all of the contributors to bringing this volume to a successful conclusion and the comments of V. Eckhart and D. Moeller on a draft of this
introduction. M.A.G. was supported by National Science
Foundation grant DEB-0515428.
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