Rubus Nivieus - (Hill raspberry) - Found on San Christobal and Isabella. This weed is by far the least of a problem of the four. This plant interferes with the highland ecosystem but the good news is that farmers hate it because it also interferes with agriculture. The plant is spread by animals and birds that injest the fruit but has no known form of pollination. It is thought to come originally from Indian to southeastern Asia and is usually controlled by chemical means because it has thorns. In Hawaii Rubus Argustus has been controlled biologically using Croesia zimmermani (blackberry moth).
Latana Camara - (Multicolored Latana) - Major problem on Floreana since it escaped from cultivation and also occurs on San Christobal and Santa Cruz. The bad news is it is extremely adaptable and also is propagated mainly by dispersal by birds and other animals. In other words it can grow anywhere. Present methods of removal in the Galapagos are manual removal and use of herbacide. Why it needs to be controlled: It is a direct threat to many of the critically endangered plants of the Galapagos and grows mainly in the arid lowlands and moist upper lands. It completely changes the ecology of the areas it is introduced to. Again Hawaii has tried biocontrol including the defoliating caterpillar Hypena strigata; the seed-destroying fly Ophiomyia lantanae, and the lace bug Teleonemia scrupulosa that have been the most effective methods.
Psidium guajava L. - (Common Guava) - Major threat to the native vegetation of the islands. The plant is dispersed by cattle and tortoises. This species invades disturbed areas and forms dense thickets obviously changing the ecological nature of these areas. On the Galapagos it grows in the moist uplands making it a particular problem. It is thought to have come from Central America originally. It has formed dense stands on Isabela and San Christobal out competing the native species. Guava is controlled by physical and chemical methods (or goats which we surely don't want any more of).
Cinchona succirubra - (Quinine tree) - This plant spreads at an alarming rate due to wind- bore seeds and crowds out the herb and shrub layers creating trees where there weren't any before. It is very hard to destroy (it will repair itself even if it's bark is removed from up to 1 meter of its stem). It also is pretty much resistant to herbicides so the herbicides that need to be used to kill it are pretty toxic. It grows fast and produces enormous amounts of seeds.
Weeds are often a good indicator of land degradation. The monitoring of these species can tell us a lot about the health of the ecosystem. For instance, if the amount and distribution of invasives changes drastically it is a good indication that something is changing in the terrestrial environment. One study suggests a five-point scale ranging from "insignificant" to "dominated by invasive species." This assigns a quantitative amount to a qualitative statement. Plotting is usually done with grids. Areas are visited by random selection. Data is then mapped onto the grid and quantitative amounts are assigned to each of the numbers to determine the health of the ecosystem.
Sources:
McMullen, Conley K. Flowering Plants of the Galapagos. New York: Cornell University Press, 1999.
"Potential value of indicator species for conservation and management of New Zealand terrestrial communities." Science for Conservation. 19 Nov.
ISSG Database. 19 Nov. 2004