DIRECCIÓN DE LA COMUNICACIÓN

There is uncertainty surrounding the ability of some species to keep pace with future climate warming, and that their potential range extent will be limited (Thomas et al., 2004; Thuiller et al., 2005). The literature relates largely to trees, and a limited number of other species, which needs to be considered when making reference to species response rates and climate change.

4.4.1 Idiosyncratic Rates of Range Shift

The rates of range shift in response to climate change varies within and between species groups, and this is true for observations more recently (e.g. Holzinger et al., 2008 as discussed later), and for those from the Quaternary; species respond individualistically (Huntley, 1991). This may lead to complete changes in community composition, i.e. non-analog communities (Kullman, 2006), and could lead to the alteration of species’ known niches (Thuiller et al., 2005). Le Roux and McGeoch’s study (2008) found idiosyncratic expansion rates have occurred at Marion Island in South Africa, leading to altered, previously unknown community compositions at intermediate and high altitude rates.

4.4.2 Variation between species

Different species have different patterns and rates of growth, and thus will respond accordingly.

Seed plants are at more of a disadvantage than the likes of ferns whose spores are easily dispersed, while tree dispersal rates may be slower in comparison to that of herbs, as they take longer to reach reproductive maturity (Normand et al., 2011), with consequent longer response lag times. Their capability to keep track of rapid climate change therefore decreases, with the unfortunate likelihood of them being stranded in unfavourable climates. A study by Lenoir et al. (2008) also found that species with certain characteristics have shifted the most over the last century including species with faster life cycles, quicker maturation rates and smaller sizes at maturity, mainly the life forms of herbs, ferns and mosses, in comparison to trees and shrubs who display distribution shifts of a lesser magnitude. There is, however, not always a strong correlation between these two factors.

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Araujo & Pearson (2005) analysed breeding birds, amphibians and reptiles and ca 20% of the European vascular flora. They found that the reptiles and amphibians grouping were the least at equilibrium with their climatic niche, whereas plant assemblages were comparatively closer.

Whether plants will be able to migrate at the rate required depends on their ability for long-distance dispersal (LDD), with there being a notion that plants may be more mobile than initially thought (Nathan et al., 2002). A plants potential for LDD will affect their ability for significant range expansion and thus the extent, if any, of their migrational lag (Normand et al., 2011).

4.4.3. Constraints on Range Expansion

It is assumed that many taxa, during post-glacial migration, were unable to follow their climate space and are currently in disequilibrium with the current climate (Normand et al., 2011). Normand et al.

(2011) assessed the constraints on species range for 1016 European plant species and found that climate was vital for all species, but the magnitude of postglacial colonisation has meant the range of more than 50% of the species is lagging. The constraints on range expansion can be assigned to a number of factors, true for both previous and current day climate change: a specie’s inability to co-exist with previously established vegetation, soil development, geographical barriers, interference of land-use by humans, limited dispersal and reproductive age as is discussed in 4.4.2.

4.4.4 Accessibility to Expand

It was found in the Normand et al. (2011) study that Mediterranean and temperate species in southern Europe were more restricted in their location, with accessibility (or lack of) being more important than climate for between 20-60% of the species. The mountainous landscape and separation of land into many peninsulas in southern Europe is an aspect of the accessibility restriction, impacting long-term dispersal. Northern European species may be more at equilibrium with the climate, as climate is more important than accessibility for these species.

A study by Svenning and Skov (2007) also found that the postglacial expansion of European tree species was limited, linking back to the realistic rate of <100m/yr as estimated by McLachlan et al.

(2005), consequently putting them at disequilibrium with the current climate. Svenning & Skov (2007) state that “geographical accessibility from glacial refugia explains most of the variation in tree diversity in central and northern Europe”. There are locations which would appear to be climatically suitable for some species, but their absence could be a reflection of the migrational lag. Today, anthropogenic barriers and dispersal limitations means that small-range species remain associated with their Last Glacial Maximum refugia and are unable to migrate.

83 4.4.5 Potential Range Filling

The bioclimatic envelope “describes the conditions under which populations of a species persist in the presence of other biota as well as climatic constraints” (Thuiller et al., 2005). The bioclimatic envelope modelling work undertaken by Svenning and Skov (2004) found that the likes of Quercus robur, is one of few trees keeping pace with the climatic changes. In comparison, the likes of Fagus sylvatica and Quercus cerris have failed to keep track due to postglacial migration limitations, as shown in figure 4.1, and consequently potential northern range limits have not been met. Forest herbs are of a similar disposition; they are known to have migration rates at a slow 20m/year or less (Honnay et al., 2002), therefore herbaceous forest flora in Europe may well have been of a different composition today if such species would have been able to fill their potential climatic range, as also demonstrated by bioclimatic envelope modelling (Svenning and Skov, 2004).

Figure 4.1 Current native distribution of three temperate tree species (dots) and their climatic potential range (shading), estimated using bioclimatic envelope modelling from Svenning & Skov

(2004). (Svenning and Skov, 2007)

With many species unable to efficiently expand their range after the last glacial, and thus ‘lagging’

behind climate warming, there is doubt whether species will be able to respond appropriately over the next century to the expected rate of climate change. The ability of a species to adapt, or their life-history traits, will be important in their range expansions (Svenning and Skov, 2007). Species at the northern margin of their range may overcome any restriction of dispersal by undergoing evolutionary processes, with climatic changes being a driver for such processes. Although these studies have been for taxa apart from plants (Pateman, 2012), it nonetheless highlights the adaption processes taxa are developing in response to climate change and dispersal limitation.

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Although a considerable amount of research looking at rates of range shift has been carried out in Europe, the theory relating to species traits could also be applied to species in the UK, as species with higher reproductive rates have the ability to “increase propagule pressure and hence the probability and number of species reaching new areas, facilitating population establishment” (Pateman, 2012).

An assumption, therefore, is that species with these characteristics are more likely to prosper in the UK, taking full advantage of their potential range filling. As the UK is made up of islands, however, there is the possibility that species may run out of future space with resultant likely losses.

4.4.6 Lag times and Extinctions

The composition of a specie’s population across a landscape is known to change over a long time-scale (Pateman, 2012). Those species at the trailing margin of a shifting distribution may lag in comparison to those at the leading edge of the margin as a result of environmental changes (Brook et al., 2009). The individuals in some species which exhibit a long life span, and the seed banks and rhizomes which are able to remain dormant in hostile conditions, explains the natural resistance of many plant species to extinction (Eriksson, 1996). Trees are such an example as they can experience especially long lag times for local extinction, even when their ideal climatic niche moves over time.

Some can survive for lengths of time at their warm range boundary, they may just be unlikely to reproduce, or seedlings be unlikely to establish (Pateman, 2012).

The climatic tolerances of small-leaved lime (Tillia cordata) have for example limited its range, as at its northern range in northern England, the temperature is not warm enough for complete pollen tube formation (Pigott and Huntley, 1981). The tree therefore produces sterile seeds and reproduction is hindered. The tree will, however, have a long lag time for local extinction as it is able to survive, just not able to reproduce.