Edited by Charles Perrings, Mark Williamson and Silvana Dalmazzone
12. Conclusions Charles Perrings, Mark Williamson and Silvana Dalmazzone 1 WHY ECONOMICS MATTERS TO THE CONTROL OF INVASIVE SPECIES Invasive species have been found in most parts of the world, but the pattern differs from biome to biome. Within terrestrial systems, xeric (deserts, semi-deserts, tropical dry forests and woodlands) and northern arctic systems are typically least affected, while in island systems invasive species have been directly responsible for a number of documented extinctions. Lake, river and near-shore marine systems are similarly more affected than pelagic marine environments (Heywood, 1995). This pattern probably reﬂects differences in the susceptibility of the underlying ecosystems to invasions. The difﬁculties in drawing conclusions from the raw data were stressed by Lonsdale (1999) who concluded that species properties, ecosystem properties and propagule pressure (Williamson, 1996) needed more study. The pattern also undoubtedly reﬂects differences in human behaviour, land use, demographic, market and institutional structures, the regulatory framework and the control strategies adopted, all of which will incidentally affect propagule pressure. That is, the susceptibility of ecosystems to biological invasions seems to us to depend in large part on how those ecosystems are used, and on the measures taken to protect them against the effects of pests and pathogens. In the existing literature, the relation between human behaviour and invasives is reﬂected in the focus on disturbance of ecosystems. Habitat fragmentation and clearance are both argued to have increased the susceptibility of remaining fragments to invasion (Neiring, 1990) as has agricultural disturbance. But...
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