Biological control impacts and the Cartesian mirage

As humans spread over the globe,they carried with themselves plants and animals that they considered useful at their new residences.Sometimes they did this because they depended on them.Typically,domesticated plants are not the most common or widespread today where they were domesticated—they have been spread over vast areas,and also outside their original area of distribution(Piperno,2017).Sometimes people just wanted to be surrounded by familiar species—thus the"naturalisation societies"of the European colonies that set out to make their surroundings"more familiar"by introducing various plant and animal species from their old homeland to the new ones(Wilson,2004).It would be illogical to expect that these species would forever remain without consumers,be those herbivores or pathogens,and indeed they had not been.Often the consumers travelled with the host from the original area of distribution,otherwise there was a delay—but once established,they often caused grave problems in the new environments.These"following"organisms were often termed invasive species(while their host species,even if extensively planted,have never been declared such),and one persistent idea why they could reach damaging densities is the"enemy release hypothesis".This hypothesis claims that these consumers have left their natural enemies behind,and now have little to check their intrinsic rate of increase(Williamson&Fitter,1996).This can quickly create problems;it is enough to remember Darwin′s(1859)famous example to realise that virtually any species can become"a pest"—i.e.is capable to reach high densities that will create problems for humans.