numbers of alien species being introduced (Figure SPM.4). 
Some of these have become invasive (well established) {2.1, 
3.2.3}. Even without the introduction of new species, given 
the opportunity, many already-established alien species in a 
region may continue to expand their geographic ranges and 
spread into new countries and regions (well established)
{2.6.1}, including into remote environments such as 
mountain, polar (i.e., Antarctica and the Arctic) and desert 
ecosystems (well established) {2.5.2.8, 2.5.2.7, Box 2.7, 
Box 3.11}. Under a "business-as-usual" scenario, which 
assumes the continuation of past trends in drivers, the total 
number of alien species is projected to further increase 
globally, and by 2050 is expected to be approximately 
36 per cent higher than in 2005 (established but incomplete)
{2.6.1}. As trends in major drivers are predicted to 
accelerate in the future (well established) {3.1.1}, the 
number of alien species worldwide is expected to increase 
faster than predicted under the "business-as-usual" scenario 
(established but incomplete) {2.6.1}. There is a lack of 
quantified projections for invasive alien species under 
different scenarios (Table SPM.A1), which impedes a 
comparison of trends for alternative futures (well established)
{2.6.5}. Projections of long-term trends for invasive alien 
species numbers are not available but they are expected to 
be similar to those for established alien species (established 
but incomplete) {2.2.1}. The documented global economic 
cost of biological invasions has increased fourfold every 
10 years since 1970 (Figure SPM.3) and is anticipated to 
continue rising (established but incomplete) {Box 4.13}.
SUMMARY FOR POLICYMAKERS
19SUMMARY FOR POLICYMAKERS
BACKGROUND
A GLOBAL DISTRIBUTION OF ESTABLISHED ALIEN SPECIES 