The
biology of the invasion process as presented in the Invasion Overview section is rational and
experimentally tractable. What is
less apparent is the human component of the selection process that creates
opportunity spaces into which invasive species disperse.
The direct effects of human activity are also more discernable than the
indirect effects. Of critical
importance is the role human perception plays in selection and creation of
opportunity space for invasive species.
anthropology
1: the scientific study of human
beings, their origins, distribution,
2: the study of man, his origins,
physical characteristics, institutions, religious beliefs, social relationships,
etc. (Anonymous, 1979)
1: the skills, arts, etc. of a given
people in a given period (Anonymous,
2: the entire range of customs,
beliefs, social forms, and material traits of a religious, social, or racial
group (Anonymous, 2001)
3: the total of the inherited ideas,
beliefs, values, and knowledge, which constitute the shared bases of social
action
4: the total range of activities and
ideas of a group of people with shared traditions, which are transmitted and
reinforced by members of the group (Anonymous, 1979)
1: centering in man
(Anonymous, 2001)
1: regarding man as the most
important and central factor in the universe
Public
policy can provide a starting point to determine human perceptions of invasive
species, an expression of human values. Of
particular interest is public policy on invasive species promulgated by the U.S.
Federal government in Executive Order 13112 of February 3, 1999 (Anonymous,
1999, 2004b). Research, management
and dissemination of information about invasive species in the U.S. are funded
by government agencies in compliance with this order.
The terminology used in this public policy statement reveals how some
perceive invasion biology. Therein
(Anonymous, 1999, 2004b) they define several terms, below included with
definitions from more conventional sources:
1: an alien species whose
introduction does or is likely to cause economic
2: a species that is non-native (or
alien) to the ecosystem under consideration and whose introduction causes or is
likely to cause economic or environmental harm or harm human health (Anonymous,
2004)
1: with respect to a particular
ecosystem, any species, including seeds,
2: non-native; a species occurring
in an area to which it is not native (Lincoln et al., 1998)
1: with respect to a
particular ecosystem, a species that, other than as a
1: relating to the indigenous
inhabitants of a country or area; a local
2: relating or belonging to a person
or thing by virtue of conditions existing at the time of birth; born in
particular place (Anonymous, 1979)
3: indigenous; living naturally
within a given area; used of a plant species that occurs at least partly in
natural habitats and is consistently associated with certain other species in
these habitats (Lincoln et al., 1998)
1: the doctrine of innate ideas
2: in U.S., the advocacy of the
claim of native as opposed to that of naturalized Americans (Anonymous, 2001)
3: Chiefly
U.S. the policy of favouring the natives of a country over the immigrants
(Anonymous, 1979)
1: of or produced by nature
(Anonymous, 2001)
2:
in accordance with human nature (Anonymous, 1979)
3: not affected by man or
civilization; uncultivated; wild (Anonymous,
1: intentional or unintentional
escape, release, dissemination, or
Several
aspects of invasion biology are revealed in these definitions.
These include the concept of economic, environmental and human harm; the
differentiation 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 importance 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 managed.
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 affected
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. CO2, O3).
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 confounded by the
heterogeneous array of human viewpoints and aesthetic values of what is
desirable in landscapes. This
heterogeneity of opinion is not resolvable but remains at the core of invasion
biology because values guide activity and management.
For better or worse, the actualization of human values creates
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-gatherer and nomadic peoples were
displaced gradually by spatially sedentary agriculturists.
The opportunity space for agriculture was vast.
Humans imposed disturbance 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 (de Wet, 1966; de Wet and Harlan,
1975). Wild progenitor species 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 closely related forms of the same
species-group, an ideal genetic situation for the longevity of the species.
Archetypical examples of these wild-crop-weed complexes are found in Amarathus
(grain amaranth, pigweeds), Setaria
(foxtail millet, green foxtail; Dekker, 2003; 2004b), Brassica
(rapeseed and wild mustards), Helianthus
(sunflowers), Avena (oat), Oryza
(rice), sorghum (crop, johnsongrass), Solanum
(potatoes, nightshades), and Hordeum
(barley, foxtail barley).
The most important current agricultural plant invasion is the
introduction of transgenic crops, often on a vast scale (e.g. glyphosate-resistant
crops). Introduction of any
trans-gene into the crop cultivars of these wild-crop-weed complexes increases
the chances of introgression into its related non-cultivated weedy and wild
phenotypes (e.g. Dekker and Comstock, 1992).
The development of these biotechnologies in wild-crop-weed complexes
fulfill the conjecture provided in the introduction: a critical interaction of
disturbance, dispersal and plant traits adapted for the resultant opportunity
space. The introduction of such
biotechnologies as herbicide-resistant crops provides a mixture of environmental
and economic benefit and harm which makes implementation of public policy as
defined by U.S. Federal policy (Anonymous, 1999, 2004a) somewhat problematic and
highlights the complex interaction of biology and human values.
CONCLUSIONS
"With
the present tremendous population explosion the most common habitat has become
man-made, and it may not be many centuries before this will be the only habitat
available. With the disappearance of
stable habitats, truly wild species will be the first to become extinct.
Wild colonizers may survive as long as habitats remain that are only
sporadically disturbed by man. Eventually
these must also disappear and Homo sapiens,
the ultimate of all weeds, will lord it over the domain he has created for
himself, his companion weeds, his crops and domesticated animals."
-J.M.J.
de Wet, 1966
Human population
size.
Human population size may be the primary cause of invasion biology and
changing community structure. With
expanding populations are consequential changes in land use and spatial
organization, increasing direct and indirect disturbance, increased resource use
and loss, and other changes to habitats.
Global propagule
dispersal.
Human global traffic has increased significantly in recent history.
There exists an increased "transferability" of everything in
human global society: trade goods and services, human travel and transportation,
and ideas. Swept along with this
traffic have been vastly increased opportunities of dispersal of biological
propagules into available opportunity spaces.
World grain traffic alone has moved immense quantities of plant
propagules over historical times, despite our best efforts to control the more
noxious forms.
Human disturbance.
Landscapes and habitats around the world have been influenced by this
byproduct of human activity. Air and
water pollution is ubiquitous and affects almost all spaces on the earth's
surface. Direct and indirect
disturbances by humans have altered most of these spaces and the ecological
relationships in biological communities, leaving vast new opportunity spaces
open and available to species with traits allowing their exploitation.
Human values and
culture.
Human perception of these changes, public policy initiatives defining
environmental and economic harm, and human aesthetic values provide
heterogenous and often conflicting value systems to be compromised in
reaching a consensus on the best solutions.
Contributing to the situation is a recent increase in perceived fear of
alien invasion, and a nativistic reaction to these fears.
Science
of invasion biology. There is
not a meaningful difference between the invasion process and the processes of
ecological community assembly, succession and the evolution of niche
differentiation by speciation. Despite
this, disciplinary barriers are apparent in the differentiation of invasion
biology science in unmanaged and managed habitats: agricultural weed biology and
invasive plant biology are often separated in the scientific academy.
Both these realms are unified by disturbance as a prime motivator of
change in community structure. The
scale of habitats in time and space is continuous; and all communities are
inter-related and inter-dependent. Agriculturalists
often do not completely embrace the invasion process in understanding population
shifts, and ecologists studying unmanaged systems often fail to recognize the
role of indirect disturbance and dependence on adjacent agricultural habitats in
the larger landscape. The science of both will advance when the unifying
principles underlying both types of undesirable species are acknowledged in a
larger view of invasion biology.
Utilizing
and Exploiting Beneficial Plant Invasive Species
weed 1:
a plant whose virtues have not been yet discovered
Imagine
a world in which global warming results in Antarctic ice melting and exposing a
terrestrial habitat. What would the
first higher plant invaders look like? Imagine
that humans colonize Mars. What will
their controlled environment plant communities look like?
In either case, does there now exist plant traits that may provide
answers to these two speculative questions?
Would exploitation of the traits that allow invasives to succeed provide
us with better plant communities, whether Antarctic, Martian, agricultural or
scenic?
As
humans alter the earth's habitats directly and indirectly, some consideration
should be given to preserving and exploiting the germplasm of our best-adapted
invasive and weedy species for such a future.
Preservation of weedy and invasive genotypes is an emerging issue in
science as novel and unique weedy biotypes are lost with the significant changes
biotechnological crop introductions are causing (e.g. Anonymous, 2004a; Marshall
et al., 2003). The traits these
noxious plants possess may provide novel solutions to the problems we create in
expanding human communities. The
only obstacle to utilizing invasive plants as sources of novel, useful traits in
crops for biotechnology is our perception of what is useful, beautiful and
harmful.
Other
ecological services are provided by weeds and invasives.
Purple loosetrife (Lythrum
salicaria) is
a beautiful and pleasing plant species. Zebra
mussels (Dreissena polymorpha)
have cleaned turbidity from many localities in the U.S. great lakes (e.g.
Michigan's Saginaw Bay and Lake Huron), enhancing human health and recreational
value. Weeds are a major food source
for indigenous wildlife bird and animal populations (Martin et al., 1961).
They provide refuge habitat to many other species, and can be alternate
hosts for beneficial insects and microflora in biocontrol tactics.
In addition, their vast phenotypic biodiversity is valuable in its own
right.
(Dekker, 2005)
