Threats facing CWR

1.6.1 The biodiversity crisis

CWR are a component of biodiversity and differ from wild plants only in that they are related to one or more crop species. The CBD describes biodiversity as ‘the variability among living organisms from all sources...and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems’ (CBD, 1992). To the present day this remains the favoured definition (Mace et al., 2012) as it highlights the three distinctive levels of diversity: genetics, species and ecosystems. Biodiversity is threatened at all three of these levels. The Millennium Ecosystem Assessment (MEA, 2004) found that the impact of humans on the environment has meant that the vast majority of ecosystems worldwide have been significantly changed requiring species adaptation and migration to ensure survival. Species diversity was shown to be in decline, with species extinction rates having increased by up to one thousand times the expected background rate (MEA, 2004). Considering only species of well-studied higher taxa, between 12% and 52% are threatened

according to the International Union for Conservation of Nature (IUCN) red list criteria (MEA, 2004; IUCN, 2012). Genetic diversity has also declined across the globe, particularly within domesticated species (MEA, 2004). An assessment of the factors driving these losses of biodiversity, such as habitat destruction, over-exploitation, invasive species and climate change, concluded that in the majority of cases the impact of these factors was either ongoing or increasing (MEA, 2004). These results are supported in a study by Butchart et al. (2010), which demonstrated that there has been no reduction in the rate of global biodiversity loss over the last four decades and that the CBD 2010 target ‘to achieve by 2010 a significant

reduction of the current rate of biodiversity loss’ was missed (CBD, 2002). This is likely to be

due to the increasing pressures being exerted on the environment due to the unsustainable rate of global human population growth (Loreau et al., 2006). Though the profile of biodiversity and its ongoing decline was raised following the United Nations Earth Summit conference in Rio de Janeiro in 1992, when the CBD was ratified, many years later biodiversity as a whole remains undervalued (Loreau et al., 2006). It is imperative that more is done to maintain biodiversity and improve conservation efforts. It is clear that although legislation for conservation exists, it is insufficient without action, crucially this must stem from improved communication between research and policy makers (Loreau et al., 2006; Butchart et al., 2010).

1.6.2 Focus on genetic diversity

Genetic diversity has been studied to a lesser extent than species and ecosystem diversity (Hargreaves, 2011) but it is this level of biodiversity that is widely accepted as imperative for populations to persist (Frankel and Soulé, 1981; Gilpin and Soulé, 1986), and in the case of CWR it has the added value of providing traits to expand the limited genetic diversity in modern crop varieties. Natural selection is a non-random process whereby populations adapt

to cope with environmental change, resulting in a change in population allele frequencies and higher numbers of individuals able to tolerate the environmental change i.e. they have an increased evolutionary fitness (Bijlsma and Loeschcke, 2012). The capacity of a population to adapt is positively correlated with the amount of extant genetic variation and it follows that with limited genetic variation, natural selection will also be limited (Reed and Frankham, 2003; Blows and Hoffmann, 2005).

The processes of inbreeding and genetic drift (together known as ‘genetic erosion’) both act to reduce the genetic diversity of populations, the former due to breeding between related individuals and the latter due to the random loss of alleles in populations where not all individuals contribute equally to the next generation, both processes result in increased homozygosity (Charlesworth and Charlesworth, 1987; Bijlsma and Loeschcke, 2012). With low genetic diversity, individual and population fitness is reduced because there is less capacity for adaptation to changing environments, leading to a higher risk of extinction (Höglund, 2009). This is particularly concerning in light of the predicted effects of climate change (section 1.2).

Habitat destruction and land use change are exacerbating this issue as both lead to increasingly fragmented and isolated habitats and populations, between which gene flow is limited (Bijlsma and Loeschcke, 2012). This results in small populations that are particularly susceptible to genetic erosion and therefore, extinction. The effects of genetic drift have been shown to have a larger effect as population sizes become smaller, resulting in a higher rate of allele loss, particularly of rare alleles (Bijlsma and Loeschcke, 2012). As stress resistant/tolerant genes often occur at low frequencies (McNair, 1997) this will greatly affect the capacity of populations to persist under changing environments and is a concern for

probability of extinction of a small population significantly increases due to a higher rate of inbreeding and is further increased when exposed to environmental stress. This suggests that environmental change, particularly the combined effects of climate change and habitat destruction, will have a strong and detrimental impact on the adaptive capacity of populations; the impact is also likely to be greater for outbreeding species (Bijlsma et al., 2000). As a result, active conservation of CWR to maintain sufficient population sizes and genetic variation is crucial to ensure their long-term persistence, their ongoing adaptation and their availability for use for crop improvement. An additional factor that may result in a higher susceptibility of CWR genetic diversity is the association of some populations with disturbed, early-successional plant communities (Grime, 1977; Maxted and Kell, 2009). These habitats are likely to be more susceptible to the impacts of anthropogenic change, experiencing higher levels of stress than perennial, mid to late-successional plant communities as they are not usually considered for inclusion in conservation designations (JNCC, 2013).