Many European species could be threatened by future climate change (Fig. 1).
Under the assumption of no migration, more than half of the species we
considered become vulnerable or committed to extinction by 2080. The
impacts of climate change are, naturally, less under the universal
migration because of the possibility for species to move across
landscapes. Under the no-migration assumption and the most severe
climate change scenario (A1-HadCM3), 22% of the species become
critically endangered (>80% range loss), and 2% extinct by 2080.
These numbers decrease for the other scenarios and climate models.
Under the universal migration assumption, the results are, as expected,
less severe. Under tA1-HadCM3, 67% of species would be classified as
low risk, whereas under B1-HadCM3, 76% of the species would be at low
Our results coincide with the direction of predictions made by Thomas et al. (11), although the magnitude of the risks we project is less [and note that we project distributions to 2080, whereas Thomas et al. (11) only projected to 2050].
modeling does not address the proximate causes of species extinction.
Nevertheless, any reduction in the potential geographic range of a
species is likely to lead to an increased risk of local extinction (11). This conclusion is, in fact, the rationale for building IUCN Red Lists (31).
A decrease in range size implies that smaller stochastic events affect
a larger proportion of the species' total population, especially in
fragmented landscapes. If a species becomes restricted to a few sites,
then local catastrophic events (such as droughts or disease outbreaks)
or an increase of land transformation by humans could easily cause the
extinction of that species (32).
Rates of species' loss and turnover show great variation across scenarios (Fig. 2).
In A1-HadCM3, the mean European temperature increases by up to 4.4 K,
leading to a mean species loss of 42% and turnover of 63%. This
scenario provides the widest range of variability across Europe for
both species loss (2.5-86%) and turnover (22-90%). The percentage of
species loss could exceed 80% in some areas, such as northcentral Spain
and the Cevennes and Massif Central in France. B1-HadCM3 gives the
lowest expected mean percentage of species loss (27%), reflecting the
fact that this scenario has the lowest rate of increase in CO2
and temperature by 2080 (mean European temperature increase of 2.7 K).
Other scenarios show intermediate mean rates of species loss (≈30%) and
The relationship between the modeled percentage of species loss and the
anomalies for the two most significantly correlated bioclimatic
variables, growing-degree days (representing accumulated warmth) and a
moisture availability index, was used to uncover the potential causes
of variations in predicted changes in plant diversity across regions
within and across scenarios (Fig. 3).
The strong consistent linear relationship across scenarios indicates
that projected species loss from our models could be estimated from
these two predictors. The Spearman rank-correlation values for the
separate univariate relationships were 0.73 and 0.65, respectively.
Multiple-linear regression by using these two predictors explains 60%
of the variance across scenarios. The temperature of the coldest month,
although being an important predictor of distributions for many species
(6), did not show a strong relationship with species loss overall and was therefore not used in this analysis.
Regional deviations from the inferred relationship (positive and
negative residuals) can be interpreted as indications of particularly
high or low species vulnerability, because of ecological and historical
characteristics of the flora, and/or specific environmental conditions (Fig. 4).
An excess of species loss (red color) is shown for mountain regions
(mid-altitude Alps, mid-altitude Pyrenees, central Spain, French
Cevennes, Balkans, Carpathians). Severe climatic conditions have
occurred in mountains over evolutionary times, promoting highly
specialized species with strong adaptation to the limited opportunities
for growth and survival (33).
The narrow habitat tolerances of the mountain flora, in conjunction
with marginal habitats for many species, are likely to promote higher
rates of species loss for a similar climate anomaly than in any other
part of Europe (34).
By contrast, the southern Mediterranean and part of the Pannonian
regions have a negative residual for species loss (gray color). Both
regions are characterized by hot and dry summers and are occupied by
species that tolerate strong heat and drought. Under the scenarios used
here, these species are likely to continue to be well adapted to future
We finally present mean percentages of species loss and turnover by
environmental zones (M. Metzger, unpublished data) with the A1-HadCM3
scenario of maximum change to best illustrate the spatial patterns (Fig. 5).
The major spatial patterns are similar over all scenarios. The northern
Mediterranean (52%), Lusitanian (60%) and Mediterranean mountain (62%)
regions are the most sensitive regions; the Boreal (29%), northern
Alpine (25%), and Atlantic (31%) regions are consistently less
sensitive. Species turnover shows a somewhat different pattern. The
Boreal region could, in principle, gain many species from further
south, leading to a high species turnover (66%). The Pannonian region
could also theoretically gain eastern Mediterranean species and has a
calculated turnover of 66%. Thus, these regions stand to lose a
substantial part of their plant species diversity, and (in time) to
show a major change in floristic composition. Projected species
turnover peaks at the transition between the Mediterranean and
continental regions (Fig. 5)
with extirpation of Euro-Siberian species and expansion for
Mediterranean or Atlantic species. Southern Fennoscandia is also an
area of high potential turnover with the loss of boreal species and
gain of Euro-Siberian species.
These results cannot be taken as precise forecasts given the
uncertainties in climate change scenarios, the coarse spatial
resolution of the analysis (35), and uncertainties in the modeling techniques used (8, 29).
The relatively coarse grid scale of our study may hide potential
refuges for species and environmental heterogeneity that could enhance
species survival, especially in mountain areas where our estimation of
risks of extinctions could be overestimated. On the other hand,
landscape fragmentation could increase the vulnerability of these
refuges to fire or other disturbances, which in combination with the
lack of propagule flow, could compromise the survival of remnant
populations. There are also major uncertainties due to lags associated
with biotic processes. The recognized time scales for assigning species
IUCN Red List categories are not suited to evaluating the consequences
of slow-acting but persistent threats. We have substituted a time scale
of 80 years (instead of 20) for critically endangered, endangered and
vulnerable, respectively, over which to assess declines. The extent of
species losses may be overestimated, because the plasticity of species
and the survival of species in favorable microhabitats is not
considered. However, even if the numbers are overestimated, patterns
across regions may stand (e.g., the ranking of region in terms of
vulnerability to loss). Species loss does not necessarily imply the
immediate loss of a species from a site, rather it may imply a
potential lack of reproductive success and recruitment that will tend
to extinction on a longer time scale (36).
Migration rates are likely to be species-specific, and resulting biotic
interactions in “no-analogue” assemblages may alter species' realized
Land use and associated habitat fragmentation are likely to generally inhibit migration rates (19).
Further, future species distributions will likely be influenced by
other environmental factors than changing climate. The current
atmospheric CO2 concentration exceeds any experienced during the past 20 million years (12). Plant physiological responses, including growth responses to increased atmospheric CO2 and changes in wateruse efficiency, are expected to ameliorate the response of some plant functional types to climate change (37).
On the other hand, nitrogen deposition, the enhanced potential for
invasion by exotic species, or the promotion of more competitive native
species may change competitive interactions in plant communities,
yielding novel patterns of dominance and ecosystem function (38).
uncertainties, our findings provide illustration of the potential
importance and the likely direction of climate change effects. From a
conservation perspective, a proportion of European plant species could
become vulnerable. The strong positive relationship between projected
species loss and changes in bioclimatic variables implies that action
to reduce greenhouse gas emissions would also mitigate climate-change
effects on plant diversity. However, even under the least severe
scenario considered, the risks to biodiversity appear to be
considerable. Different regions are expected to respond differently to
climate change, with the greatest vulnerability in mountain regions and
the least in the southern Mediterranean and Pannonian regions. Recent
observations (39) and predictions (9)
corroborate our conclusion regarding the climatic sensitivity of
species in European mountain areas. We have also identified a broad
transition zone where the greatest species mixing is likely to occur.
During the Quaternary period, this region acted both as a crossing
point and a refuge zone for Boreo-alpine and Euro-Siberian species (40). This transition zone will be a strategic region for plant-species conservation in a changing climate.