Biology and anthropology of plant invasions - Invasive Plants: Ecological and Agricultural Aspects - page 218

Several aspects of invasion biology are revealed in these definitions. These
include the concept of economic, environmental and human harm; the differ-
entiation between alien and native species; the existence of natural conditions;
and the purposeful introduction of a plant species to a locality.
The purpose of this paper is only to highlight the explicit statements of
human goals and values that may influence invasion biology. Of specific impor-
tance to public policy is the value placed on nativism, natural conditions and the
different categories of harm. How public policy is implemented with these
guiding, often subjective, concepts is at the heart of how these species are man-
aged. The management elicited by public policy is the selection pressure these
invasive species will respond and adapt to in their subsequent evolution.
Human values
The historical expansion of human populations, and their activities, has affect-
ed almost every habitat on earth to some extent, either directly or indirectly.
Air and water pollution alone have affected much of the surface biology of
earth (e.g., CO 2 ,O 3 ). Human perception of what is natural and indigenous,
what is disturbed and artificial, is therefore compromised to some degree. In
one form or another, willingly or not, the earth is the garden of humanity. The
equivocal nature of what harm is caused by invasive species is therefore con-
founded by the heterogeneous array of human viewpoints and aesthetic values
of what is desirable in landscapes. This heterogeneity of opinion is not resolv-
able but remains at the core of invasion biology because values guide activity
and management. For better or worse, the actualization of human values cre-
ates opportunity space for new species to invade: they are a direct reflection of
human activity.
The best expression of human-mediated invasion biology can be found in
agriculture. With the advent of agriculture some 10,000 years ago, hunter-gath-
erer and nomadic peoples were displaced gradually by spatially sedentary agri-
culturists. The opportunity space for agriculture was vast. Humans imposed dis-
turbance regimes on those spaces (e.g., soil tillage) and favored plant species
with desirable phenotypic traits to cultivate and harvest. Evolutionary changes
in those cultivated species led to somewhat ironic consequences: the formation
of stable, long-lived wild-crop-weed complexes [11, 12]. Wild progenitor spe-
cies were domesticated. Crop phenotypes escaped cultivation and developed
weedy habits ideal for infestation with their crop relative, and both shared space
with the original wild relatives. Gene flow was continuous between these close-
ly related forms of the same species-group, an ideal genetic situation for the
longevity of the species. Archetypical examples of these wild-crop-weed com-
plexes are found in Amarathus (grain amaranth, pigweeds), Setaria (foxtail mil-
let, green foxtail [13, 14]), Brassica (rapeseed and wild mustards), Helianthus
(sunflowers), Avena (oat), Oryza (rice), sorghum (crop, johnsongrass), Solanum
(potatoes, nightshades), and Hordeum (barley, foxtail barley).