The federal government defines an invasive species as follows:
An "invasive species" is defined as a species that is 1) non-native (or alien) to the ecosystem under consideration and 2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health. ( Executive Order 13112 ).
Invasive species can be plants, animals, and other organisms (e.g., microbes). Human actions are the primary means of invasive species introductions.
Human activities have resulted in the establishment of alien species of plants and animals throughout the world. Some of these species have become invasive with many negative effects. The danger of introducing alien species, whether accidentally or on purpose, is that it is impossible to predict with any certainty which ones will become invasive. Some things most invasive species share is the lack of natural predators in their new location and the ability to reproduce at a high rate. Plants are capable of reproducing either sexually or vegetatively. Some invasive plants reproduce only vegetatively but they can still be quite successful, particularly in aquatic environments where they can be spread by water movement.
Invasive species often displace native species, resulting in cascading effects through ecosystems.
Almost half of the invasive plants in the U.S. were introduced for horticultural purposes. There have been thousands of plants brought in for this reason and only a very small percentage of them have become problems.
Some of the worst and most aggressive ones have been aquatics. They are often capable of reproducing vegetatively and can spread through waterways very rapidly. They clog waterways, reduce oxygen levels in water when they decompose, and displace native plants that are important to animals. An infestation can be started by the simple dumping of an aquarium into or near a river or lake and may end up costing $millions to clean up.
One of the most destructive invasive species in Texas and other parts of the Southwest is Tamarix (Salt Cedar). It is a rather attractive shrub/small tree that can grow in very dry conditions and as many as eight different species were introduced at various times. The most common and invasive Tamarix is probably a hybrid between two different species. This hybrid has not been found in its countries of origin because the two species do not occur together. However, they came together in the U.S. and produced a hybrid that has created some very serious problems. Salt Cedar puts down very deep roots and removes large amounts of water from the soil. It also secretes salt in its leaves, which builds up in the soil. It outcompetes all the other vegetation by removing available water and increasing the salinity of the soil to a level that most plants can't tolerate.
So far, effective control of Salt Cedar requires the use of herbicides. One approach is to cut the plants off near the ground and coat the cut with herbicide. This works well in small areas but it is labor intensive. Another approach that has been used in Texas for large areas is aerial spraying. This appears to be effective but the long range effects are not known yet.
Manual methods and the use of herbicides are widely used to control invasive plants but they have their limitations and are not often effective. Another relatively new approach to dealing with the invasive species problem is the use of biocontrol agents. In the case of plants, this often involves going to the invasive plant's country of origin and collecting insects that attack it. These insects are brought back but they can't be released without a series of test to determine if they are going to be a problem themselves. There were cases early in the use of biocontrols where the biocontrol agent turned out to be a worse problem than the organism it was supposed to control. After it is determined that the biocontrol agent actually can control the invasive plant and will not cause harm to useful plants, it is selectively released for further testing.
The advantage of biocontrol agents is that they essentially work for free. They typically do not immediately control the invasive plant and they will probably never completely eradicate it on their own.
The complete eradication of most invasive species is going to be very difficult and expensive and will probably not be accomplished for the majority of invasives. The best we can do is limit the damage they cause.
A good nonplant example of invasive species control is the imported fire ant. Huge sums of money have been spent on their eradication and control, with little more than local control in small areas to show for it. There are some biocontrol agents now being tested and released that show promise but you can bet that there will always be imported fire ants present in the warmer areas of the U.S. that get sufficient amounts of rainfall to support the ants. They are still expanding their range, which will eventually include the entire West Coast.
LOCAL INVASIVE PLANTS
A list of the invasive plants that occur just in this area is too long to discuss even briefly. There are also new ones showing up all the time. A recent example is a parasitic plant called Orobanche ramosa (Branched Broomrape). A yellow form of this plant has been known to be in a small area in Carnes County (near San Antonio) for over ten years. Extensive eradication and control efforts have prevented it from spreading to the agricultural regions of the Rio Grande Valley, where it would possibly prevent the exportation of vegetable crops from this area. This would have a devastating effect on the Valley and Texas agriculture in general.
Recently, probably around 1996 or 1997, another infestation of Branched Broomrape occurred, possibly in association with the planting of Crimson Clover contaminated with Branched Broomrape seeds along the medians of I-45. This plant is a different form from the one in Carnes County and has blue flowers. It appeared in great numbers in a very short period of time and no organized efforts were undertaken to control it. The seeds of this plant are tiny, almost as small as dust particles and they attach to anything going through the plants. It is likely that the seeds were spread by mowers and other equipment that went through the roadsides and ditches. It is possible to just walk through the plants when they are in fruit and get large numbers of seeds attached to your clothes and shoes by the attraction of static electricity.
The new infestation first came to our attention by students in Botn 301 who made collections of the plants. In 1997, it was probably present in only one or two counties but it is now found in 30+ counties. It is too late to eradicate it and all that we can do now is prevent it from getting into the areas of the state where it can pose a major problem for agriculture.
Some countries and states have laws that force landowners who have infestations of invasive species to control them, usually with assistance from the government. That is not the case with Texas.
We recently met with a scientist from Australia where Branched Broomrape is also a problem. They have as many as 60 people assigned fulltime to the monitoring and control of this plant at the time of the year it is growing. In spite of this effort, it is still spreading although they appear to be on the verge of halting the spread. In Texas, there is nothing even close to this level of effort devoted to the control of any invasive plant. The state of Texas employs only a few individuals whose prime duty is to survey and control invasive plants. The work on plants is often done by entomologists or anyone else who spends time in the field, usually working on other projects. The general approach is to wait until the problem is so large that it can no longer be ignored, at which point eradication is impossible and even partial control very difficult and expensive.