Since there seems to be so much confusion in this area, a few thoughts on conservation seem in order. Here are a few key points as I see them.
Habitat preservation is essential. Send the Nature Conservancy some money. They simply buy up critical habitat, while most of the other conservation organizations do not put the money where it does the most good.
Organisms always have and will continue to colonize new habitats, adapting to them over time. Some are successful while others go extinct, but in general things adapt to new circumstances; it's the way the world works.
New colonizers--be they plant or animal--generally build to high populations in a new environment, but will eventually settle into normal levels. This occurs as the system adapts; it's the way the world works.
Organisms reproduce in large numbers and the excess is utilized by other members of the system. This excess capacity can be harvested for human use safely and forever as long as you pay attention to population levels, birth rates, death rates etc. Here in Michigan, hunters annually harvest large numbers of deer and cars get a bunch more, yet still their population is at near record levels. Removing animals for human use has not harmed the population. Annual increase more than makes up for what is removed; it's the way the world works.
Collecting small amounts of seed from stable populations will not harm that population. Birds and insects and rodents do this all the time. Think about it-- gardeners plant the seed not eat it, which actually results in an increase of the base population; it's the way the world works.
Collecting reasonable numbers of plants from a stable population will not cause harm either. Although more care needs to be taken not to cut deeply into its reproductive base, there is always attrition from grazing and natural losses due to fire and other natural causes. Things do re-appear from seed stored in the soils natural seed bank, however a selective harvest approach is much more ecologically sound than wholesale clear cutting. Digging a plant or two and moving it to your garden from a thriving non-endangered population harms nothing; it's the way the world works.
Propagation of plants or animals under controlled conditions with the exclusion of predation can produce far more offspring than would result under natural circumstances; it's the way the world works.
It is better to work with organisms before they become endangered when there is still good genetic diversity to incorporate into the breeding program. Waiting until you have a handful of live California Condors left is a mistake. However successful you are, there is still a genetic bottleneck; it's the way the world works.
If you want to save stuff in third world countries establish backup populations elsewhere. Most of these countries will slash, burn, and develop at the expense of biodiversity. (Loss of biodiversity represents a threat orders of magnitude greater then a few weeds.) We did the same to the United States; it's the way the world works.
C.I.T.E.S. and the Rio craziness does not help matters; it makes things worse. Encourage legislation that promotes captive propagation. Put more faith in the public sector than governments. Zoos and Botanical Gardens can and should be part of the solution but their arrogant attitude that only they are qualified to do these sorts of things should stop. Look at tropical fish and reptiles. I daresay there are a great many more animals produced by private individuals than by Zoos and Public Aquaria. So what if they are driven by money. This is a good thing; it's the way the world works.
High populations of introduced species (the weed problem) will work itself out over time. This happens either by things adapting to eat the newfound resource or by the process of succession. Most weeds are early succession colonizers that will be crowded out naturally and they serve a very important purpose. They prevent soil erosion and limit the spread of deserts due to human disturbance. Climax vegetation weeds are rather rare and do represent a fundamental change but not necessarily for the worse. Human planners need to adopt a view that encompasses more of a geological time scale and not overreact due to short-term thinking. Look at the fuss people raise about introductions to volcanic islands. If they would stop and think for a moment they would realize that everything there rafted from somewhere else and most of them went through a weed phase before reaching stable population levels. New introductions will do exactly the same thing; it's the way the world works.