Global Change Biology, FS 2017
Ewa Czyż
How fast will plants migrate, and do we need to consider assisted migration?
Date: 24.04.2017
Supervisor: Harald Bugmann
Introduction
Global climate change is causing shifts of climatic zones. Temperature rise is shaping new abiotic conditions
for many plants, which together with human activities like land cover change (Higgins et al., 2003) are
forming new compositions and abundances of species at different spatial scales.
Every organism has specific tolerances for environmental conditions (i.e., ecological tolerance). The organisms of narrow ecological tolerance are more susceptible to environmental changes than organisms of wide
ecological tolerance. If due to environmental change the ecological tolerance of an organism is not overlapping with the new conditions, the organism can: (1) adopt to new conditions, (2) migrate to an environment
featuring suitable conditions, or (3) stay without changing. The latter would cause organism, and at the
larger scales, species extinction (Aitken et al., 2008). It has been shown that instead of adaptation to new
environmental conditions, plants tend to migrate to areas where conditions are suitable (Bradshaw, 1991).
The rate of plant migration involves two functions: one connected with organism biology, and one connected
with organism dispersal (movements of offspring; Weinberger, 1982). The migration rate of a population
can be modulated or estimated from empirical data. In the empirical approach the migration rate of a population is equal to dispersal range divided by the duration of one plant generation. Diverse ways of seed
dispersal and rare events of long-distance seed transport are the cause of big difficulties for evaluating the
speed of plant migration in theoretical approaches. The prediction of the future speed of migration in the
context of climatic change scenarios is even more difficult.
In the global change context, it is important to know the speed of plant migration, and to estimate if plants
would be able to track suitable abiotic conditions. Active actions like assisted migration are sometimes
considered, where the species is translocated by humans to the areas where environmental conditions are
suitable and the risk of extinction is smaller (Ste-Marie et al., 2011). Such a human-aided improvement of
the migration rate could be a method to reduce biodiversity loss under global change, but it is unlikely to be
a universal solution. Local adaptation of plants to conditions other than climate (soil, biotic environment)
(Bucharova, 2016) and the invasion risk of relocated plants (Mueller & Hellmann, 2008) should also be
considered.
Question
How fast will plants migrate? Do we need to consider assisted migration?
Results
The estimation of (future) plant migration speed has to consider both species-specific migration speed and
environmental changes.
Empirical estimates for the dispersal distances of plants in Tropical East Asia vary between nearly 0 and
100 km, depending on plant biology and the vector that is transporting the seeds. Distances <10 m are mostly
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Global Change Biology, FS 2017
Ewa Czyż
achieved by mechanical processes and ant transport. Distances >100 km are achieved by wind, water and
animals as vectors (Corlett, 2009). Plant maturity is reached in 1-30 yr, and a mean dispersal distance of
0.05 – 1.5 km implies a plant migration speed of 0.0017 – 1.5 km/yr (Corlett & Westcott, 2013). However,
there is a large but unknown contribution of long-distance dispersal (Higgins et al., 1999). Furthermore,
intraspecific competition and other biotic interactions are difficult to incorporate in migration models.
The mean global change velocity, defined as the velocity at which something must move over the surface of
the Earth to maintain constant climatic conditions, is 0.42 – 0.22 km/yr (Loarie, 2009). After incorporation
of surface topography, the value of global change velocity is estimated as 10 km/yr for lowlands to 10 m/yr
on steep slopes in the equatorial region (Corlett & Westcott, 2013). General Circulation Models do not
incorporate all of the climatic factors and stochastic events that could be of importance for determining
migration speeds.
Due to many uncertainties in climate change and incomplete knowledge of species biology and biotic interactions, it is impossible to estimate the exact speed of plant migration under global changes. However,
relative migration speed can be induced. The minimum speed of migration to track climate conditions after
Loarie (2013) is 0.42 km/yr, provided that maximum speed of migration, without long-distance dispersal
events, is 1,5 km/yr. This shows that the migration rate of plants is not large enough to track climatic change.
Based on empirical migration rates (Corlett, 2009), only in dry, geomorphologically undifferentiated lowlands areas, where the minimum speed of migration is 10 km/yr (Corlett & Westcott, 2013), the tracking of
climatic change by plants could be achieved. Thus, plants migrate too slowly to track suitable climatic conditions. For example, the modeled range of Fagus sylvatica suggests a strong range shrinking under global
changes (Saltre et al., 2015).
Furthermore, land fragmentation is slowing down plant migration speed. For example, for Leucodendron
rubrum the mean theoretical dispersal distance is 63 m, but the realized dispersal distance in an environment
with barriers is merely 28 m. The seeds of L. rubrum plants are transported by wind, but since seed release
is induced by fire, the secondary transport over the soil surface has a strong influence on dispersal distance.
After combining the theoretical dispersal distance with plant biological attributes, the realized migration
rate was found to be 83 m/yr; without barriers it would be 216 m/yr (Higgins et al., 2003). However, the
influence of other components of the changed habitat, such as longer migration distance of animals being
vectors, could improve the dispersal distance of the plants under land cover changes (Cote et al., 2017).
Furthermore, land fragmentation and patching the habitats contributes to genetic fragmentation of a population. The reduction of genetic diversity, due to inbreeding in isolated areas, could cause smaller overall
phenotypic plasticity, and hence bigger susceptibility to environmental changes and a reduction in dispersal
ability (Cote et al., 2017).
Thus, it is worth considering assisted migration, where plants are transported by humans to areas with suitable conditions. The concept of assisted migration was introduced in 1997 in a commercial paper and 2007
in the scientific community. Since then, the idea of translocation of species in the context of climatic change
is growing rapidly (Ste-Marie et al., 2011). Other, similar concepts applied at different spatial and genetic
scales are: assisted colonization, where the species is translocated to totally new environment; and assisted
population migration, where a representative from a different population is translocated. Active action of
biodiversity protection where the assisted migration concept was used are the reintroduction of Cypripedium
arietinum and Lespedeza leptostachya, where the former was successful but the latter not (Vitt et al., 2016).
The example of unsuccessful (re-)introductions shows that assisted migration cannot be a universal solution
for biodiversity maintenance, among others because of necessary local adaptations of plants to conditions
other than climate (e.g., soil, biotic environment) and the impossibility of achieving fully reliable experimental conditions for the evaluation of the fitness of a plant in a new environment (Bucharova, 2016).
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Global Change Biology, FS 2017
Ewa Czyż
Furthermore, incomplete knowledge about species interactions could give rise to a high invasion risk of
relocated plants (Mueller & Hellmann, 2008).
Conclusion
-
It is impossible to estimate the exact migration speed of plants in the future because of incomplete
knowledge about plant biology, the complexity of climatic change, and stochastic events.
The average worldwide migration estimations show that the speed of plant migration is typically too low
to track climatic change.
More topographically diverse environments are better suited for tracking the climatic changes by plants.
Land fragmentation is generally slowing down plant migration.
Assisted migration could be a solution for the maintenance of biodiversity threatened by global change.
It is not a universal concept, because of local plant adaptations and experiment restrictions.
Incomplete knowledge of species interactions may cause invasion risk of translocated species.
References
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