IMPACTS OF SPECIES INFESTATIONS ON YIELD
Invasive alien species (IAS) are now thought to be the that climate change is altering the distribution, incidence and in-
second gravest threat to global biodiversity and ecosys- tensity of animal and plant pests and diseases such as Bluetongue,
tems, after habitat destruction and degradation (Mooney a sheep disease that is moving north into more temperate zones of
et al., 2000; CBD, 2001; Kenis et al., 2009). The steady Europe (van Wuijckhuise et al., 2006; FAO, 2008). According to
rise in the number of invasive alien species is predicted FAO (2008), climate scenarios with more winter rain in the Sahel
to continue under many future global biodiversity sce- may provide better breeding conditions for migratory plant pests
narios (Sala et al., 2000; Gaston et al, 2003; MA, 2005), such as desert locust (Schistocerca gregaria) that are totally depen-
although environmental change could also cause non- dent on rain, temperature and vegetation, with catastrophic im-
alien species to become invasive. Environmental change pacts on crop and livestock production.
(e.g., rising atmospheric CO
2
, increased nitrogen depo-
sition, habitat fragmentation and climate change) could People relying most directly on ecosystem services, such as small and
promote further invasions (Macdonald, 1994; Malcolm et subsistence farmers, the rural poor and traditional societies, face the
al., 2002; Le Maitre et al., 2004; Vilà et al., 2006; Song most serious and immediate risks from IAS. These people depend
et al., 2008). As invasive or alien species comprise over
70% of all weeds in agriculture (estimated in the US) (Pi-
mentel et al., 2005), increases in invasive species pose a
Worldwide 67,000 pest species attack crops: 9,000 insects and
major threat to food production (Mack et al., 2000; MA,
mites, 50,000 pathogens and 8,000 weeds. Up to 70% of them are
2005; Pimentel et al., 2005; Chenje and Katerere, 2006;
introduced, with major impacts on global food production.
van Wilgen et al., 2007).
Across Africa, IAS of the genus Striga have a direct impact on local
In Australia, the varroa mite, a serious pest in honeybee
livelihoods: it affects more than 100 million people and as much as
hives, may result in the loss of $30 million a year in free
40% of arable land in the savannahs. These invasive species stunt
pollination services from feral bees (CSIRO, 2008). The
maize plant growth by attacking the roots and sucking nutrients
varroa mite has recently invaded New Zealand and is ex-
and water, and thus in addition to the direct financial costs, have
implications for food security (Chenje and Katerere, 2006).
pected to have an economic cost of US$267–US$602
million, forcing beekeepers to alter the way they manage
Invasive alien species such as pests and diseases have been esti-
their hives (GISP, 2008). Invasive alien species such as
mated to cause an annual loss of US$12.8 billion in yield of eight of
pests and diseases also impose major constraints on world
Africa’s principal crops (Oerke et al., 1994).
crop and livestock production (Oerke et al., 1994). Pests
and pathogens have had particularly severe effects on crop
In West Africa the larger grain borer (Prostephanus truncates), is
yields in the world’s poorest and most food insecure region
responsible for cassava losses of approximately US$ 800 million
of Sub-Saharan Africa. They have been estimated to cause
per year thereby jeopardizing food security (Farrell and Schulten,
an annual loss of US$12.8 billion in yield of eight of Afri-
2002).
ca’s principal crops, and may reduce yields in developing
countries overall by around 50% (Oerke et al., 1994). In Tanzania the larger grain borer (Prostephanus truncates) causes
some US$ 91 million in maize losses per year (GISP, 2008).
Importantly, increased climate extremes may promote
the spread of invasive species, plant diseases and pest
Pimentel et al. (2001) estimated that crop losses due to introduced
outbreaks (Alig et al., 2004; Anderson et al., 2004; Gan,
arthropods in South Africa amount to US$ 1.25 billion per year.
2004; FAO, 2008). For instance, there is clear evidence
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