Dispersal will limit ability of mammals to track climate change in the Western Hemisphere.
ABSTRACT As they have in response to past climatic changes, many species will shift their distributions in response to modern climate change. However, due to the unprecedented rapidity of projected climatic changes, some species may not be able to move their ranges fast enough to track shifts in suitable climates and associated habitats. Here, we investigate the ability of 493 mammals to keep pace with projected climatic changes in the Western Hemisphere. We modeled the velocities at which species will likely need to move to keep pace with projected changes in suitable climates. We compared these velocities with the velocities at which species are able to move as a function of dispersal distances and dispersal frequencies. Across the Western Hemisphere, on average, 9.2% of mammals at a given location will likely be unable to keep pace with climate change. In some places, up to 39% of mammals may be unable to track shifts in suitable climates. Eighty-seven percent of mammalian species are expected to experience reductions in range size and 20% of these range reductions will likely be due to limited dispersal abilities as opposed to reductions in the area of suitable climate. Because climate change will likely outpace the response capacity of many mammals, mammalian vulnerability to climate change may be more extensive than previously anticipated.
- SourceAvailable from: Christy Marie Mccain[show abstract] [hide abstract]
ABSTRACT: Model predictions of extinction risks from anthropogenic climate change are dire, but still overly simplistic. To reliably predict at-risk species we need to know which species are currently responding, which are not, and what traits are mediating the responses. For mammals, we have yet to identify overarching physiological, behavioral, or biogeographic traits determining species' responses to climate change, but they must exist. To date, 73 mammal species in North America and eight additional species worldwide have been assessed for responses to climate change, including local extirpations, range contractions and shifts, decreased abundance, phenological shifts, morphological or genetic changes. Only 52% of those species have responded as expected, 7% responded opposite to expectations, and the remaining 41% have not responded. Which mammals are and are not responding to climate change is mediated predominantly by body size and activity times (phylogenetic multivariate logistic regressions, P < 0.0001). Large mammals respond more, for example, an elk is 27 times more likely to respond to climate change than a shrew. Obligate diurnal and nocturnal mammals are more than twice as likely to respond as mammals with flexible activity times (P < 0.0001). Among the other traits examined, species with higher latitudinal and elevational ranges were more likely to respond to climate change in some analyses, whereas hibernation, heterothermy, burrowing, nesting, and study location did not influence responses. These results indicate that some mammal species can behaviorally escape climate change whereas others cannot, analogous to paleontology's climate sheltering hypothesis. Including body size and activity flexibility traits into future extinction risk forecasts should substantially improve their predictive utility for conservation and management.Global Change Biology 01/2014; · 6.91 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: Phenotypic plasticity and microevolution are the two primary means by which organisms respond adaptively to local conditions. While these mechanisms are not mutually exclusive, their relative magnitudes will influence both the rate of, and ability to sustain, phenotypic responses to climate change. We review accounts of recent phenotypic changes in wild mammal populations with the purpose of critically evaluating the following: (i) whether climate change has been identified as the causal mechanism producing the observed change; (ii) whether the change is adaptive; and (iii) the relative influences of evolution and/or phenotypic plasticity underlying the change. The available data for mammals are scant. We found twelve studies that report changes in phenology, body weight or litter size. In all cases, the observed response was primarily due to plasticity. Only one study (of advancing parturition dates in American red squirrels) provided convincing evidence of contemporary evolution. Subsequently, however, climate change has been shown to not be the causal mechanism underlying this shift. We also summarize studies that have shown evolutionary potential (i.e. the trait is heritable and/or under selection) in traits with putative associations with climate change and discuss future directions that need to be undertaken before a conclusive demonstration of plastic or evolutionary responses to climate change in wild mammals can be made.Evolutionary Applications 01/2014; 7(1):29-41. · 4.15 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: Characterising patterns of animal movement is a major aim in population ecology, and yet doing so at an appropriate spatial-scale remains a major challenge. Estimating the frequency and distances of movements are of particular importance when species are implicated in the transmission of zoonotic diseases. European badgers (Meles meles) are classically viewed as exhibiting limited dispersal, and yet their movements bring them into conflict with farmers due to their potential to spread bovine tuberculosis in parts of their range. Considerable uncertainty surrounds the movement potential of badgers, and this may be related to the spatial-scale of previous empirical studies. We conducted a large-scale mark-recapture study on badgers (755km(2) ; 2008-2012; 1,935 capture-events; 963 badgers) to investigate movement patterns in badgers, and undertook a comparative meta-analysis using published data from 15 European populations. The dispersal movement (>1km) kernel followed an inverse power-law function, with a substantial 'tail' indicating the occurrence of rare long-distance dispersal attempts during the study period. The mean recorded distance from this distribution was 2.6km., the upper 95%ile was 7.3km and the longest recorded was 22.1km. Dispersal frequency distributions were significantly different between genders; males dispersed more frequently than females but females made proportionally more long-distance dispersal attempts than males. We used a subsampling approach to demonstrate that the appropriate minimum spatial-scale to characterise badger movements in our study population was 80km(2) , substantially larger than many previous badger studies. Furthermore, the meta-analysis indicated a significant association between maximum movement distance and study area size, while controlling for population density. Maximum long-distance movements were often only recorded by chance beyond the boundaries of study areas. These findings suggest that the tail of the badger movement distribution is currently underestimated. The implications of this for understanding the spatial-ecology of badger populations and for the design of disease intervention strategies are potentially significant. This article is protected by copyright. All rights reserved.Journal of Animal Ecology 01/2014; · 4.84 Impact Factor
Dispersal will limit ability of mammals to track climate
change in the Western Hemisphere
Carrie A. Schloss1, Tristan A. Nuñez, and Joshua J. Lawler
School of Environmental and Forest Sciences, University of Washington, Seattle, WA 98195
Edited by Monica G. Turner, University of Wisconsin, Madison, WI, and approved April 13, 2012 (received for review October 12, 2011)
As they have in response to past climatic changes, many species
will shift their distributions in response to modern climate change.
However, due to the unprecedented rapidity of projected climatic
changes, some species may not be able to move their ranges fast
enough to track shifts in suitable climates and associated habitats.
Here, we investigate the ability of 493 mammals to keep pace with
projected climatic changes in the Western Hemisphere. We
modeled the velocities at which species will likely need to move
to keep pace with projected changes in suitable climates. We
compared these velocities with the velocities at which species are
able to move as a function of dispersal distances and dispersal
frequencies. Across the Western Hemisphere, on average, 9.2% of
mammals at a given location will likely be unable to keep pace
with climate change. In some places, up to 39% of mammals may
be unable to track shifts in suitable climates. Eighty-seven percent
of mammalian species are expected to experience reductions in
range size and 20% of these range reductions will likely be due to
limited dispersal abilities as opposed to reductions in the area of
suitable climate. Because climate change will likely outpace the
response capacity of many mammals, mammalian vulnerability to
climate change may be more extensive than previously anticipated.
have been toward the poles or upwards in elevation and have
occurred at an average rate of 6.1 km and 6.1 m per decade,
respectively (3). Given the projected rates of future climatic
changes, rates of range shifts over the coming century are likely
to be even greater (4–6). Several studies have projected potential
shifts in species distributions in response to forecasted climatic
changes. Because these range-shift projections rarely account for
dispersal abilities, they indicate areas that may be climatically
suitable for species in the future, but they do not tell us whether
species will be able to expand their ranges into these newly
In the coming century, the survival of species will in part de-
pend on their abilities to track geographic changes in suitable
climates (7, 8). This ability to keep pace with climate change
depends on both the velocity of the climatic changes a species
will face and species-specific dispersal abilities. Loarie et al. (8)
mapped the velocity of climate change as the ratio of temporal
and spatial gradients of changes in mean temperature, and
separately, in annual precipitation. Temperature and precipita-
tion are expected to change globally at average velocities of 0.42
and 0.22 km/y, respectively, and the velocities of changes in both
of these climatic variables also vary spatially. The velocity of
climate change that a species will face, or the velocity at which
a species will need to move to keep pace with shifts in suitable
climates, therefore, depends on the location of the species and
the velocities of changes in the particular climatic factors that
influence the species’ distribution.
At broad spatial scales, species’ distributions are largely de-
termined by multiple, interacting, climatic factors (9). Bioclimatic
models identify the relationship between species’ distributions and
climate and can be used to project the future locations of suitable
move from a given location to track these shifts in suitable
ecent changes in climate have already caused discernible
shifts in species distributions (1, 2). In general, these shifts
climates can be measured as the distance from that location
within the species’ current range to the closest location projected
to have a suitable climate in the future divided by the time period
of the projected climatic changes. This species- and location-
specific measure of the velocity of climate change will likely vary
for different species at a single location and across different
locations within the range of a single species.
The ability of species to keep pace with climate change also
depends on species-specific dispersal abilities and the degree to
which the landscape facilitates or impedes movement. Dispersal
can be defined as the movement of an individual outside of its
home range without anticipated return (12). Dispersal may occur
before the first mating (natal dispersal) or between breeding
seasons in adults (breeding dispersal). Dispersal velocity, or the
distance that a given species can disperse over a given time pe-
riod, depends on both the distance of individual dispersal events
and the frequency of those events. In addition to dispersal ability,
land use also affects species overall movement. Landscapes that do
reduce the abilities of species to track climatic changes.
We investigated the potential for mammals in the Western
Hemisphere to keep pace with a changing climate by comparing
the velocities of climate change that each of 493 mammals will
likely experience with modeled dispersal velocities for these
species. First, we calculated the velocity of climatic changes
relevant to each species on a cell-by-cell basis across a 0.5° by 0.5°
grid using bioclimatic model projections for the coming century.
Next, we modeled dispersal velocities for each species as a
function of body mass, diet type, the successive time between
generations, and the potential variability in dispersal distances.
We then determined the percentage of species in each grid cell
for which the velocity of climate change will likely exceed the
species’ dispersal velocity, and thus, the percentage of species
that will be unable to keep pace with projected climate changes.
We also investigated the relative degree to which human land
use may further impede movement of species by analyzing land-
use patterns along simple movement routes connecting current
and future suitable climates and calculated the additional dis-
tances that some species may need to travel to avoid movement
through less suitable landscapes.
On average, across the Western Hemisphere and 10 projected
future climates, 9.2% of the mammals in a given grid cell will
likely be unable to keep pace with the changing climate (Fig. 1).
In some areas, up to 39% of the mammals will be unable to track
climate change. Furthermore, projections from three of the cli-
mate models indicate that over 50% of the mammals in given
Author contributions: C.A.S., T.A.N., and J.J.L. designed research; C.A.S. and T.A.N. per-
formed research; C.A.S. analyzed data; and C.A.S., T.A.N., and J.J.L. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
1To whom correspondence should be addressed. E-mail: email@example.com.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
| May 29, 2012
| vol. 109
| no. 22www.pnas.org/cgi/doi/10.1073/pnas.1116791109
locations will be unable to keep pace with climate change (SI
Appendix). Species will be least likely to be able to track climatic
changes in the tropical and subtropical moist broadleaf forest
biome of the western Amazon. In this biome, on average, 14.5%
of species will likely be unable to keep pace with climate change.
The inability of mammals to track climate change will be
driven more by rapid climatic changes in some regions (Fig. 2A)
and more by limited dispersal abilities in others (Fig. 2B). In the
Amazon, the Yucatan Peninsula, the Appalachian Mountains,
and the south–central United States, the subset of species that
will likely be unable to reach suitable climates will experience
high velocities of climate change. For example, in some locations
in the Amazon, the average dispersal velocity for this subset
of species is about 1 km/y, but the average velocity of climate
change experienced by this subset of species can be as high as
8 km/y. In contrast, the subset of species that will likely be unable
to track their climatic niches in western Mexico and the Midwest
and western United States are more limited by dispersal ability.
The average dispersal velocity for these species is only 0.1–0.5
km/y. Thus, despite the relatively low velocities of climate change
projected for these species (about 1 km/y), even lower dispersal
velocities will likely prevent them from tracking climate change.
Our results indicate that primates and eulipotyphla (shrews
and moles) will be the mammalian taxa least able to keep pace
with climate change and carnivora, artiodactyla (even-toed
ungulates), and xenarthra (armadillos, anteaters, and sloths) will
be the most able to keep pace (Fig. 3A). We exclude the order
microbiotheria in the taxonomic evaluation of the results be-
cause the Monito del Monte (Dromiciops gliroides) is the only
species in this order. The primates are more dispersal limited
than the other orders and many primates may need to expand
their ranges over longer distances than species of other orders to
reach suitable climates (Fig. 3B). In contrast, the orders carniv-
ora and artiodactyla are the least-dispersal limited and the dis-
tances they need to disperse are short, relative to the distances
required of species of other orders.
On average, across all 10 climate projections, 87% of mammals
are expected to experience reductions in range size. Twenty per-
cent ofthese reductions aredue todispersallimitationsratherthan
a reduction in the area that is climatically suitable. The average
projected change in range size is a reduction of 37%. Nearly all
primates will experience severe reductions in range size. The
average projected change in range size for species in the order
primates is a reduction of 75%. Conversely, many species ranges in
In many places, land use will likely make it even more difficult
for species to keep pace with climate change (Fig. 5). In the
midwestern and Appalachian regions of the United States,
movement of species, already dispersal limited, may be further
constrained by the extensive human land use in these regions.
Species in the Amazon region are limited by dispersal ability, but
face lower intensities of human land use, except in a few regions
in western Brazil. In contrast, in southeastern Brazil, movement
of mammals may not be dispersal limited, but species will need
to move through landscapes with high human impacts, and so,
their movement may be inhibited or they will have to travel
farther to avoid inhospitable landscapes. To avoid movement
through human-dominated landscapes and large bodies of water,
mammals will need to disperse an average of 0.8 km/y faster to
reach suitable climate (based on a random subset of 38 mammals,
SI Appendix).An additional 2.4% ofspecies’rangeswill beunable
to keep pace with climate change as a result of these alternative
dispersal routes (SI Appendix). The impact of the longer dispersal
greatest for primates and rodents (SI Appendix).
Species vulnerability to climate change depends on exposure,
sensitivity, and the capacity to respond to climatic changes (13).
Although exposure to climate change is projected to be greatest at
higher latitudes (14), our results indicate that most mammalian
species at high latitudes will likely be able to keep pace with these
large changes. The geographic differences between projected
magnitudes of climate change and our results could be due to
higher latitudinal gradients in climate or a greater capacity to re-
spond to large climatic changes, given the greater dispersal abili-
ties of some of the mammals in these regions. Conversely, in the
tropics, many species lack the ability to keep pace with climate
be the result of lower latitudinal gradients in climate in the tropics
and/or higher sensitivity to climatic changes due to narrower cli-
matic niches of tropical species. In tropical regions, species gen-
erally have more restricted physiological tolerances and hence are
expected to be more sensitive to climate change (15, 16).
The orders primates and eulipotyphla are least able to keep
pace with climate change and are the most likely to experience
climate-change–induced range reductions. The primates are also
currently the most threatened taxon in our study with 35% al-
ready classified as critically endangered, endangered, or vulner-
able on the International Union for Conservation of Nature
(IUCN) Red List of Threatened Species (17) and an additional
keep pace with climate change. Results are average percentages across
projections that incorporate the output from 10 climate models run for
a midhigh (SRES A2) emissions scenario.
Percentage of mammalian species that are projected to be unable to
models (A) and average dispersal velocity (B) of all mammals that are pro-
jected to be unable to keep pace with climate change.
Average velocity of climate change as projected by 10 climate
Schloss et al.PNAS
| May 29, 2012
| vol. 109
| no. 22
29% listed as near threatened (SI Appendix). However, the IUCN
Red List lists climate change as a threat for only one of these
species, the black howler monkey (Aluatta pigra). Our results in-
dicate that primates will be further threatened by climate change.
Knowledge of the potential vulnerabilities of this order to climate
change will enable management efforts to focus on species that
are most vulnerable to climate change by minimizing the current
threats to these species and fostering more resilient populations.
Landscape Permeability. We are underestimating the percentages
of species that will be unable to keep pace with climate change
because the percentages are based on dispersal estimates that do
not incorporate landscape permeability. Landscape permeability
is the degree to which the landscape facilitates animal movement
(18). Human-dominated land uses between some species’ ranges
and regions of future suitable climate will increase the percent-
age of species that are unable to keep pace with climate change
by requiring faster dispersal along longer alternative routes. In
regions in which extensive human land use is ubiquitous, in-
hospitable landscapes may not lengthen dispersal routes because
alternative routes through more permeable landscapes may not
be available. In the subsample of 38 species, carnivores will likely
travel through landscapes with extensive human land use, but
those landscape patterns will have relatively little impact on their
movement routes. In these regions and for these species, dis-
persal may be less successful due to the potential for decreased
survival and reproduction in less suitable habitats. Fig. 5 is de-
rived from straight-line movement routes of all 493 mammals
and it highlights the regions from which we estimate land use to
make keeping pace with climate change more difficult either as
a result of increased distances in dispersal routes or decreased
fitness. In eastern Brazil, the eastern United States, southern
Mexico, and Central America, species are likely to reach suitable
climate when the permeability of the landscape along the dis-
persal route is ignored. However, extensive human land use
between these regions and regions of future suitable climate will
likely add additional challenges to the capacity for species to
track their climatic niches. These regions may therefore be im-
portant areas from which to focus connectivity efforts because in
these regions species have the ability to keep pace with climate
change, provided that the landscape does not impede their sur-
vival or movement. In addition to incorporating the human-
dominated land uses into species’ dispersal routes, incorporating
species-specific habitat preferences along with projected changes
in habitat through time would generate more realistic estimates
of the velocities species need to travel to reach suitable climates.
However, the detailed data required for these analyses are not
available for most species and the extensive processing time re-
quired for least-cost modeling makes these analyses infeasible
for large numbers of species.
range from which that species will likely be unable to track shifts in suitable climate. Boxplots incorporate percentages from each of 10 estimates based on the
output from 10 climate models for each mammalian species. Boxes represent the interquartile range. Dashed whiskers extend to the lowest and highest
values within 1.5 times the interquartile range. Values that are more extreme than the dashed whiskers are plotted as circles. (B) Modeled dispersal velocity
plotted against the velocity (averaged across every grid cell in a species range and across projections from 10 climate models), at which each species will need
to move to keep pace with climate change. Species that, on average, will likely be unable to keep pace with a changing climate fall below the solid line,
whereas species that will likely be able to keep pace, on average, with a changing climate are above the solid line. Ten species of carnivora and three species
of xenarthra with dispersal velocities greater than 20 km/y, but with velocities of climate change that were less than 2 km/y are not plotted here (n = 480).
Estimates of the ability of species in each order in the class mammalia to track projected climate change. (A) Percentage of grid cells in each species’
current range sizes. Boxplots incorporate ratios from projections based on
each of 10 climate models individually for each mammalian species. Boxes
represent the interquartile range. Dashed whiskers extend to the lowest and
highest value within 1.5 times the interquartile range. Values that are more
extreme than the dashed whiskers are plotted as circles. Values above the
dashed line (i.e., ratios greater than one) indicate projected expansions in
range size, whereas values below the dashed line (i.e., ratios less than one)
indicate projected contractions in range size.
The ratio of future projected range sizes (as limited by dispersal) to
| www.pnas.org/cgi/doi/10.1073/pnas.1116791109Schloss et al.
Dispersal Assumptions. Some of the assumptions we make in esti-
matingdispersallikely result inunderestimates ofthepercentages
of mammals unable to reach suitable climate. For example, we
assume that successful reproduction occurs at the youngest age
biologically possible andthat offspringin every generationsurvive
to dispersal age and successfully disperse. In reality, dispersal has
high associated mortality (12, 19). We also assume that suitable
habitat and climates for establishment, survival, and reproduction
will exist between regions of current and future climate and that
individuals will disperse directly toward the closest suitable grid
cell. However, smaller-bodied mammals generally have more
tortuous or winding dispersal paths (20). More sinuous dispersal
paths in smaller species that are, in general, already dispersal
limited may further inhibit the ability for small species to keep
pace with climate change. Finally, in our model, dispersal tra-
jectories ignore topography, rivers, roads, and other dispersal
barriers, which may either impede movement or increase travel
distance by requiring animals to take alternate routes. Alterna-
tively, we may overestimate the percentage of mammals unable to
keep pace because reduced habitat suitability may lead to adap-
tation of increased dispersal distances (2, 21).
Although we may overestimate or underestimate dispersal
velocities for particular species, in general, the overall patterns of
our results are relatively insensitive to uncertainties in natal
dispersal velocity. To investigate the impacts of these uncer-
tainties, we applied our models using a range of dispersal velocity
estimates that varied by more than 200 km/y for some species (SI
Appendix). Using the different dispersal estimates resulted in no
change in the relative patterns of geographic and taxonomic
inabilities to keep pace with climate change and little change in
the magnitude of the results (SI Appendix). For example, across
90% of the Western Hemisphere, using the lowest and highest
estimates of dispersal velocity resulted in, at most, a difference of
four mammals that could not keep pace with climate change.
The largest difference was nine mammals and it only occurred in
0.013% of the grid cells of the Western Hemisphere.
Although taxonomic patterns are insensitive to natal dispersal
estimates, the results are sensitive to dispersal frequency. For
a subset of 193 species, a maximum estimate of breeding dis-
persal velocity that ignores the time between generations was
relatively greater for species with delayed reproductive maturity
such as ungulates, carnivores, and primates. These estimates had
negligible impacts on the results for carnivores and ungulates,
but resulted in a 52% average decrease in the percentage of
primates’ ranges that will be unable to keep pace with climate
change (SI Appendix). However, this estimate represents the
upper limit of species’ movement potential rather than a likely
dispersal scenario because natal dispersal is generally more
common than breeding dispersal due to the evolution of a natal
dispersal strategy (22) and is often the single largest long-dis-
tance movement event (12).
Suitable Climate Assumptions. In addition to potential errors in
estimates of dispersal velocities, our estimates of the velocity of
climate change may also be influenced by assumptions and
uncertainties. Most notably, we used bioclimatic model projec-
tions to delineate future suitable climate, and so, we inherently
incorporated some of the assumptions and uncertainties of bio-
climatic modeling. These assumptions have been extensively
reviewed elsewhere (10, 23) and can cause over- or under-
estimates in our results. For example, false assumptions of cli-
matic intolerance due to modeling only the realized niche, and
therefore ignoring species interactions and other nonclimatic
determinants of range boundaries, may increase the projected
distance a species needs to travel to suitable climate and cause
overestimates of the percentages of species unable to track cli-
mate change (e.g., our results in southern Florida). In addition,
correlative models have inherent uncertainties associated with
their inability to project the suitability of novel climates and to
account for evolution. Also, range expansions may occur from
small populations that are already closer to regions of future
suitable climate, but that are absent from our projections due to
the coarse resolution of range maps (7). Conversely, the as-
sumption that the climate throughout a species distribution is
sufficient for persistence of all populations of that species will
lead to underestimates of percentages of mammals unable to
reach suitable climate because current distributions may not exist
in equilibrium with current climate or species may be locally
adapted to specific climates within their distributions. The bio-
climatic model projections in our study also contain uncertainty
because they are based only on climatic variables and derived
from one type of bioclimatic model. Incorporating vegetation or
soil types as predictor variables or using alternate bioclimatic
models would likely result in different projections of species
distributions and possibly in greater or lesser velocities of climate
change (24, 25). Additionally, bioclimatic model projections
contain inherent uncertainty due to their dependence on output
from uncertain climate models (23). However, the patterns pro-
duced by our models are geographically and taxonomically very
similar across the 10 climate-change scenarios used in our models
(SI Appendix). Therefore, our geographic and taxonomic assess-
ment of the ability of mammals to keep pace with climate change
is somewhat insensitive to differences in climate model output.
Impact of Dispersal on Range-Shift Projections. Most previous
range-shift projections have ignored dispersal. Our results in-
dicate that species-specific dispersal is extremely important to
include in range-shift projections. Assumptions of unlimited dis-
persal generally lead to overestimates of potential future range
expansions and underestimates of potential range contractions.
Our models indicate that 58% of range expansions projected
with the assumption of unlimited dispersal will likely be range
contractions when dispersal limitations are taken into account (SI
failure to incorporate dispersal has more of an impact on the
range-shift projections for species that are less able to expand
into newly suitable climate such as primates and eulipotyphla
species attempting to track climate change. Red areas are highly vulnerable
regions due to the inability of mammals to keep pace with climate change,
but have relatively permeable landscapes. Blue areas are areas where
species dispersal velocities and climate change velocities could allow them
to keep pace with climate change; however, high intensities of human land
use may prevent them from doing so. Pink areas highlight regions in which
there are high percentages of species that, given their potential dispersal
velocities and the velocities of climate change, will be unable to keep pace
with climate change and from which dispersers will encounter human-
Degree to which human land use may further impede movement of
Schloss et al.PNAS
| May 29, 2012
| vol. 109
| no. 22
Our study quantifies, over a large geographic area and for a large
number of species, the ability of mammals to keep pace with
climate change. We assessed the combined impact of the expo-
sure and sensitivity of mammals to climate change in the Western
Hemisphere and evaluated one aspect of the capacity of these
mammals to respond. Despite uncertainties, incorporating dis-
persal into range-shift projections should greatly improve pro-
regional vulnerabilities, particularly for dispersal-limited species.
Our results indicate that large percentages of mammals will likely
be unable to keep pace with climate change in the Amazon and
that primates willlikely besome ofthe most vulnerable mammals.
These results are robust to some of the uncertainties in climate
change and in species’ dispersal abilities.
For dispersal-limited species (e.g., primates, shrews, and
moles), reducing nonclimatic stressors will help to foster more
resilient populations (Table 1). Assisted colonization may also be
an option for dispersal-limited species; however, there are many
uncertainties and risks associated with such actions and detailed
assessments are needed if these actions are to be considered
(26). Our study highlights a number of species that may be
candidates for assisted colonization and for which further re-
search into the costs and benefits of these actions may be prac-
tical. Our results also highlight areas in which landscapes could
be made more permeable to allow species to better track climate
change. Conservation planners in these regions can focus on how
land-use practices can facilitate climate-change–related dispersal
and range expansions into regions of newly suitable climate.
Although reducing other threats, intentionally moving species,
and increasing the permeability of the landscape may help some
species track climatic changes, ultimately reducing greenhouse
gas emissions and thus reducing the velocity of climate change is
the only certain method to ensure that many species will be able
to keep pace with climate change.
Velocity of Climate Change. We calculated velocities of climate change (in
kilometers per year) for each grid cell in each species’ current range based on
the distances between those grid cells and the closest grid cell projected to
be climatically suitable for that species over an averaged 30-y period from
2071 to 2100. We divided this distance by 110 y—the difference between the
historical time period (1961–1990) and the future time period on which the
climate projections were based. We determined geographic distributions of
future suitable climates for each species using existing bioclimatic model
projections (6). Bioclimatic models use relationships between current species
distributions and current climate variables in conjunction with projected
future climatic data to forecast the locations where climate might be suit-
able for species in the future (10, 27). The bioclimatic models we used were
based on random forest predictors (28, 29) and were built using current
species distributions derived from digital range maps (30) and 37 different
bioclimatic variables (6). Random forest models—a model-averaging ap-
proach based on classification or regression trees—have been shown to
outperform other modeling approaches for predicting distributions of
mammals in the Western Hemisphere (25). The models we used correctly
predicted at least 80% of the presences and at least 97% of the absences in
semi-independent test datasets that were reserved during the model-
building process. We used projected future distributions of suitable climate
for each mammal from these established relationships and climate projec-
tions from 10 general circulation models (SI Appendix) run for a midhigh
[Special Report on Emissions Scenarios (SRES) A2] (31) greenhouse gas
emissions scenario to calculate the velocity of climate change for each spe-
cies. Although our projections are based on a midhigh emissions scenario,
our results may be conservative because the emissions trajectory of the last
decade has already exceeded the trajectory of this scenario (32).
Velocity of Dispersal. Of the 525 nonvolant mammals for which Lawler et al.
built models (6), we obtained from the literature or estimated body masses,
diet types, and generation lengths for 493 species (SI Appendix). We used
this information to calculate dispersal velocity for 92 carnivores, 233 herbi-
vores, and 168 omnivores. The velocity of dispersal (i.e., the distance that
a species can travel over 110 y converted to velocity by dividing by that
number of years) depends on the distance and frequency of dispersal events.
We modeled the distances for single-natal dispersal events for each of the
493 species using the positive allometric relationships between median dis-
persal distances in kilometers (D) and body masses in kilograms (M) for
carnivores and together for herbivores and omnivores (Eqs. 1 and 2, re-
spectively) and the probability of dispersing shorter or longer distances (d)
relative to this median distance (Eq. 3) (12).
Prðd > DÞ ¼ e−ðD=1:5Þ
Because natal dispersal occurs before the first reproductive event, we esti-
mated the frequency of natal-dispersal events from the successive period
between generations determined by gestation length, age at sexual matu-
rity, and the period until the next breeding season. We determined total
dispersal distances in 110 y by multiplying the distances of single natal-dis-
persal events by the number of potential dispersal events calculated by di-
viding 110 by the period between successive generations.
Table 1.Species- and region-specific examples of management strategies for climate-change adaptation
land useExample order/region
climate change Conservation action
HighLowHighPrimates in Central AmericaVery highIncrease connectivity; assisted migration;
ex situ conservation and reintroduction
Assisted migration; ex situ conservation and
Prevention of habitat loss and fragmentation
to maintain connectivity for range shifts;
potential assisted migration
Increase landscape permeability and
connectivity to facilitate range shifts;
potential assisted migration
Prevention of habitat loss and fragmentation
to maintain connectivity for range shifts;
potential assisted migration
Increase landscape permeability; nonurgent
HighLowLowPrimates in the Amazon Very high
HighHighLowLagomorpha in the western
Low to high
LowLowHighEulipotyphla in the Appalachian
Intermediate to high
LowLowLowEulipotyphla in the western
United States and Canada
Low to high
LowHighHighArtiodactyla and Xenarthra in
Carnivora in eastern Canada
LowHigh LowVery lowContinue current management practices
| www.pnas.org/cgi/doi/10.1073/pnas.1116791109 Schloss et al.
Because rare, long-distance dispersal events can determine the rate of
population spread at the edge of a species’ range (33), we included the
potential for these events in our estimates of dispersal velocity. We used an
exponential distribution to describe the variability about the median dis-
persal distances (12). We multiplied median dispersal distances by the mean
of this distribution, which incorporates both the potential for dispersing less
than the median dispersal distance and the potential for long-distance dis-
persal events (Eq. 3) (12).
Landscape Permeability. To determine the relative degree to which the land-
scape may impede movement in different regions, we calculated the extent of
human-dominated land use that a species would encounter by dispersing from
of human land use (34). We calculated the length-weighted means of human
the difference between these straight-line movement paths and dispersal
compared the velocity species would need to travel to reach suitable climate
and the average degree of human land use that species would encounter
in the current range to regions of future suitable climate (SI Appendix). Least-
cost modeling identifies routes that balance travel through unsuitable
landscapes with the additional travel required to avoid less suitable regions
(35). Weusedthehuman influenceindextoassign relative costs tomovement
through each 1-km grid cell (34).
ACKNOWLEDGMENTS. We thank Denis White for valuable discussions about
the modeling of species distributions and Sandor Toth and Aaron Wirsing
for their helpful edits of the manuscript. We acknowledge the Program
for Climate Model Diagnosis and Intercomparison (PCMDI) and the World
Climate Research Programme (WCRP)’s Working Group on Coupled Model-
ling (WGCM) for their roles in making available the WCRP Coupled Model
Intercomparison Project phase 3 (CMIP3) multimodel dataset. Support of this
dataset is provided by the Department of Energy.
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| vol. 109
| no. 22