A major way humans are changing the environment is by moving plants around to new habitats. We are taking plants out of the habitats in which they evolved and are putting them in new ones, which has a huge effect on plant diversity—some plants become invasive, and invasive plants threaten native plant diversity by outcompeting the plants native to an area. Invasive plants reproduce very quickly and spread to new areas. These plants don't only decrease native diversity, but can cost a lot of money in damage control. Invasive species are thought to be the second greatest threat to biodiversity, after habitat loss (Wilcove et al. 1998).
When plants are moved into an environment in which they did not evolve, sometimes they just don't survive or reproduce there, and they die. However, some plants are good at dealing with a wide range of conditions. These plants are the ones that might be problematic.
Some famous invasive plants are kudzu in the southeastern United States, purple loosestrife in most of the United States, and prickly pear cactus in Australia. In the United States, over $100 million a year is spent controlling invasive plants in waterways. That doesn't include control efforts in forests, farms, or national parks. So how do these plants get so out of control?
Kudzu is native to Japan and China but was introduced to the United States in the late 1800s. It has spread across much of the eastern US, and forms dense thickets, covering other plants in its path.
(Public domain image from US Fish and Wildlife Service, available at http://commons.wikimedia.org/wiki/File:Kudzu.jpg)
There are lots of different theories about how invasive plants succeed in new habitats, but two in particular have to do with evolution:
• Enemy Release Hypothesis (ERH)
• Evolution of Increased Competitive Ability Hypothesis (EICA)
The idea behind Enemy Release is that in their native habitat, plants have animals that like to eat them or pathogens (fungi or bacteria) that infect them. These "natural enemies" probably co-evolved with the plant. This means some plants die from the enemy attacks, but a big battle of natural selection is going on. The plants that survive longer and produce more offspring than other plants (that get eaten or infected sooner) may have some sort of resistance to the enemy. Maybe they are less tasty, or hairier, or more resistant to disease. They will pass these traits on to their offspring, and the next generation will be slightly more resistant to enemy attack. However, natural selection is happening on the enemy's side too. Let's say the plants have chemicals in their leaves that are toxic to bugs. Most bugs die when they eat the toxic leaves. Yet some bugs, for some reason, are resistant to the toxin and can eat the leaves and survive to reproduce. They might pass this trait onto their offspring, and all of a sudden there are lots of bugs that can eat the leaves without dying.
The way natural selection works, plants that produce more potent toxins will survive in greater numbers than plants producing less toxic leaves. However, the bugs' natural resistance can keep growing from generation to generation. This whole scenario is an example of what's called an evolutionary arms race—each side keeps building up their defense strategy but each side catches up with the other.
Now let's say we take the plant out of its natural environment (e.g. Florida) and take it to a new one (e.g. Hawaii). The plant still has its toxic leaves, but the little bug that was developing resistance to it is not in the new environment. In the new environment, there is nothing that can eat the leaves of the plant because they are so toxic. The plant has escaped its natural enemies and can survive and reproduce quite well because nothing has evolved the ability to eat it.
The Evolution of Increased Competitive Ability (EICA) Hypothesis builds on the co-evolution idea proposed in the Enemy Release hypothesis, but involves the plant evolving in its new habitat. A fundamental difference is that EICA assumes that the plants are not as successful when they first arrive in the new habitat as they are in later generations.
The creators of EICA, Bernd Blossey and Rolf Notzold, realized that there is often a lag between the time a plant is introduced somewhere and the time it starts spreading rapidly around the land. They proposed that because the plant was taken out of its natural habitat and no longer has to produce energetically costly defenses against its natural enemies, it could put those resources into growing bigger and stronger (or bigger and more prolific in its seed production). In other words, the plant evolves to be a better competitor in its new environment.
Blossey and Notzold tested the EICA Hypothesis by comparing plant growth of purple loosestrife from European seeds (native range) and American seeds (introduced range) in the same conditions. They found that the American seeds grew into bigger plants even though they were growing in the same soil and under the same conditions as the European plants. But hey, you could just say the plants were following the American Dream. The American plants were also less resistant to a beetle that attacks purple loosestrife in its native range.
Not all invasive species become bigger in their introduced ranges, so EICA does not hold for all invasives. Either the ERH or EICA could apply to different invasive plants, and some invasive species are successful for other reasons. This is an active area of research, so scientists don't have all the answers yet.