Domestication, Invasive Characteristics in Plants and Crop Improvement.
1. There is no doubt in my mind that transfer from one ecosystem to another can
result in a plant species causing major change. However, I think some people
who research invasive plants can overreach in their claims or in the projected
scope of their subject. Read the paper by Anderson, Galatowitsch and Gomez
(2006). In that paper there are two claims. The first sentence states “Crop
domestication is widely understood to be one of the main pathways increasing
the rate and geographic scope of biological invasions (Reichard & White,
2001).” Further on the authors state “ While other agriculture and horticulture
sectors are also involved in new crop development, the floriculture industry
can be used as an example to illustrate the possible creation and flow of
invasive crops through the commercial trade and how plant breeders may aid
in preventing their creation and spread during the domestication phase.”
2. Domestication is the process of cropping and selection that is considered to
have taken place in making wild plants more productive and amenable to
repeated use in agriculture. The paper by Burger, Chapman, and Burke (2008)
gives a useful introduction. Note that the essential trait in domestication of a
grain plant results in prevention of seed dispersal. In rice, for example, there
have apparently been two mutations that reduce or stop “shattering” i.e., the
separation of the seed from the plant when it becomes ripe. These mutations
interfere with the abscission layer that normally forms at the base of the seed
and similar mutations have occurred in other cereals. Volbrect and Sigmon
(2005) discuss the particular case the evolution of the maize ear—which I
recommend you read. So a very important characteristic of cereal crops is that
they do not spread readily (or in the case of maize do not spread at all) unless
humans harvest and plant them. The floriculture industry may be important to
study but it is not a good general example for problems associated with
domestication.
3. But what about the possible effects of current plant breeding? May that make
crop plants more likely to become invasive? An interesting thing about plant
breeders is that they use group selection and most often in a completely
empirical way, i.e., they do not take much account of any design criteria but
just look at results of repeated crosses and yields from crosses to judge the
parents. (I do not count genetically engineered organisms here partly because
most plant breeding does not use that technique.) In a number of cases,
notably for cereals of the “Green Revolution” but also in European wheat
varieties, this has lead to dwarfing—material that would previously have gone
into the stem goes into the ear. These varieties are likely to be less competitive
with other species (sometimes called weeds) —should they ever come into
contact with them! Read the paper by Duvick, Smith and Cooper (2004)
which gives an account of improvement in maize over about six decades.
Notice the characters that have changed.
4. I hope you noticed that one of the characters that showed a marked change in
progression of maize hybrids was foliage angle and that it became markedly
more upright over the 1970s decade and onward. Please read Ford, Cocke.
Horton, Fellner and Van Volkenburgh (2008). Well, there are some sums!
There is a companion paper (still on the stocks) that shows the upright leaf
variety to be more productive and to let more light reach the ground. I doubt it
will be more competitive with other species. Farmers have shown resistance
to sowing upright leaf varieties because of those dratted weeds! If planting
density is not increased then sufficient light reaches the ground early in the life
of the crop for weeds to become of major importance.
Generally, a weed is a plant in an undesired place. (But I love them!)