All plants have their own means of dispersal, whether it is being released into the air and by a strong breeze, or passed along by animals. Humans have contributed largely to the spread of many plant species all over the world, creating an environment of non-native species rather than natives for human use or simply by accident. The introduction of these foreign species has devastated natural ecosystems and eradicated many native species living in those areas. Invasive species are largely to blame for this. Invasive species hold many self-benefiting characteristics that are extremely harmful to other species. These characteristics include rapid growth and maturity, competitive ability, copious seed production, low maintenance care, and adaptability (Groves, 2009). These attributes allow invasive species to flourish while stripping ecosystems of all nutrients leading to the creation of dense new forests inhabited primarily by foreign plants.
A dominance of invasive species has been seen all over the world. These species have been introduced purposefully by humans intended to aid people in one form or another. For example, Lonicera japonica, also known as the Japanese honeysuckle, is an invasive species mainly seen on the east coast of the United States. It was introduced from Eastern Asia to Long Island, NY in 1806 for ornamental use and erosion control. This plant is known for being aggressive; creating large groups of vines that smother out other competing species in its path (Grebenstein, 2016). Since its introduction to the U.S., the plant has spread many hundreds of miles along the east coast pushing out other native plants. This plant is only one example of many that are found throughout the world.
This study observes the impact of an invasive species located on an isolated group of islands that is well- known for the prevention and confining of invasive species to specific areas to prevent those species from dominating the islands and destroying the natural habitat: the Galápagos Islands. The two islands observed were the island of San Cristóbal and Española. San Cristóbal is an inhabited island containing about 6,000 people while Española Island remains uninhabited. The three invasive species studied included the Rubus niveus (blackberry), Psidium guajava (guava), and Lantana camara (flower bush). Rubus niveus was introduced in the 1970s to the islands and is considered one of the worst alien plants introduced (Groves, 2009) Psidium guajava was introduced in 1869, and L. camara was introduced in 1938 and is also known to be very invasive to the Hawaiian Islands.
We hypothesized that greater numbers of these three plants would be found on San Cristóbal Island rather than Española Island due to San Cristóbal Island being inhabited and containing numerous non-native animals. We also theorized that more invasive plant species would be found closer to heavier human populations. Humans have brought many non-native organisms to the island, and some travel between the mainland and the island chain. Heavier human traffic can help the spread of invasive seeds for wider dispersal.
For observing and measuring invasive species on the island of San Cristóbal, we specified the invasive species that we would be recording as the Rubus niveus (blackberry), Psidium guajava (guava), and Lantana camara (flower bush). These species are widely known to be extremely invasive, common on the island, and easily identified by observation.
To measure the number of invasive species in selected areas, we performed multiple transects of 100 feet each (Fig. 1). Four transects were taken total; three performed on the island of San Cristóbal and one on the island of Española (Fig. 2). Two researchers measured the 100 feet distance using a measuring tape. We would then walk along the designated area defined by the measuring tape and count the number of invasive species seen within a 10-foot range of the measuring tape. After, we took observations of factors that could contribute to the numbers recorded and marked the geographical located of where the transects were taken for calculation of the distance from heavy human civilization. This process was repeated exactly the same for each of the four transacts taken.
On the island of Española, no invasives were found, so we recorded variance in natives present. The native species studied include a Poaceae sp., Trianthema portulacastrum, and Heliotropium sp. This transact was performed in a similar way to the previous. Transacts consisted of 100 feet with observations taken within a 10-foot range of the tape measure. Instead of counting individual plants, percentages of the amounts observed were taken, because high plant density prohibited counts.
The results of each of these transects were then inputted into Microsoft Excel for data manipulation and calculations. Graphical comparisons explored variation in invasive plant density at Finca Guadalupe, an abundance of invasive species between the islands San Cristóbal and Española, and variations in native species on Española Island.
Abundance and variance in native and invasive species were calculated from transects taken on the islands Española and San Cristóbal. We found that invasive plants were in greater abundance on the island of San Cristóbal (Fig. 3). Out of the three invasive species studied, we found that L. camara was most abundant with 1.1 individuals per 1.0 meters (Fig. 4). Psidium guajava was the second most abundant with an average of 0.65 individuals per meter (Fig. 4). The least abundant invasive plant was the blackberry which was seen an average of 0.02 individuals per meter (Fig. 4). On the island of Española, no invasive species were found (Fig. 3). This led to the measurement of variance in native species on the island. In the first transect, we found the plant of greatest abundance was Poaceae sp. at 60% coverage (Fig. 5). However, in the second transect, the percentage dropped significantly to around 20% coverage (Fig 5). Trianthema portulacastrum was the second most abundant in the first transect with a percentage of 25% and the most abundant in the second transect with a percentage of 50% (Fig 5). The species Heliotropium sp. was found in the lowest abundance overall with a coverage percentage of 10% in the first transect and 20% in the second transect (Fig 5). We also noticed that when comparing the variance in native species to the variance in invasive species, the native species showed a greater variance between the different species.
In support of our hypothesis, the island of Española had fewer instances of invasive species compared to Santa Cruz, an island settled by humans. Suggesting that the local methods of conservation, performed by professional conservationists and the local population, are effective.
The results for the blackberry were unexpected. The blackberry is a highly competitive invasive species, and we predicted that it would be more abundant. Indeed, the farmer removed blackberry in order to plant elephant grass for adult tortoises hosted on his farm, in collaboration with local conservationists. He did this by using a time-consuming method of cutting down the plants by hand, to ensure its complete removal. Guava plants are deliberately cultivated for their fruit. Despite the classification as an ‘invasive species’ farmers have learned how to best keep them from taking over the environment. By trimming them down when they get too big, clearing out competitors, and ensuring that their dietary needs are met, farmers keep them from trying to spread out beyond control. In contrast, the lantana was found wild, growing in clusters. This allows it to grow uninhibited by other plant species. Thus, limiting potential interactions with rival plants.
The island of Española had no invasive plant species. This was expected, given the geographical distance the island has from human settlement, as well as the native populace’s conversation efforts; the most common being to ensure no seeds are transferred by humans, along with liberal use of soap. We tested whether the variation in native species would be greater than those found in invasive species; we predicted this under the assumption that native plants, living in an ideal resource-rich environment, wouldn’t stifle one another in competition. We found Poaceae sp., Trianthema portulacastrum, and Heliotropium sp. along the rocky footpath leading further into the island.
The most widespread was the Heliotropium sp., with the Poaceae spp. thinly distributed among it. Trianthema portulacastrum was far away from both other plants and spread out amongst themselves. Even though the Heliotropium sp. is by far the most abundant, it coexists with the other two species.
One thing to look out for in the future is whether or not the blackberry returns. The farmer has not completely eradicated it from his property, just in one area, and even then, there were still two plants left. To say nothing to the fact that there is still much of Española we were unable to explore. Our transects were done in an area that sees heavy foot traffic from visitors; it would help to explore deeper into the island that receive little to no human interaction, to observe whether or not greater plant growth would spur competition.
MORE TO EXPLORE
- Blackburn, Tim M., et al. “On the Island Biogeography of Aliens: a Global Analysis of the Richness of Plant and Bird Species on Oceanic Islands.” Global Ecology and Biogeography, vol. 25, no. 7, 2015, pp. 859–868. Wiley Online Library, doi:10.1111/geb.12339.
- Whittaker, Robert J., et al. “A General Dynamic Theory of Oceanic Island Biogeography.” Journal of Biogeography, vol. 35, no. 6, 2008, pp. 977–994. Wiley Online, doi:10.1111/j.1365-2699.2008.01892.x.
- Rentería, Jorge Luis, et al. “Possible Impacts of the Invasive Plant Rubus Niveus on the Native Vegetation of the Scalesia Forest in the Galápagos Islands.” PLoS ONE, NCBI, 31 Oct. 2012, www.ncbi.nlm.nih.gov/pmc/articles/PMC3485296/.
- Groves, R. H. Weed Risk Assessment. SBS, 2009.
- Grebenstein, Emily. “Escape of the Invasives: Top Six Invasive Plant Species in the United States.” Smithsonian Insider, 16 Mar. 2016, insider.si.edu/2013/04/top-six-invasive-plant-species-in-the-united-states/.
THE AUTHORS & NOTES FROM THE FIELD
Alyssa Feldewert is a student at Webster University. She studies Biological Sciences with an emphasis in Health & Medicine. Alyssa plans to attend nursing school after completing her degree.
The most amazing thing I witnessed during this research was the dedicationand passion in the local people about conservation. They rely so much on the environment to supply them with all their needs such as energy, water, and food and put every ounce of effort they could give to preserve resources. This also goes along with their efforts to prevent further contamination of invasive species to other islands or specific areas. Seeing this really changed my view on the world and how important it is to work with the environment rather just using it.
The lack of any invasive plants on Española was surprising, even if it logically made sense. I mean, you’d think that there would be one slip-up, one thing that slipped through the cracks of the conservationist’s efforts to keep the island clean, but there wasn’t. To be honest, it was really impressive, and made it sink how much the locals care for their home. At the same time, it was also impressive how little work it took to make sure that the islands were clean. A little soap here, brushing there, pick up a little trash there, and you’ve made a difference.
Our locations, a farm and the footpath on Española, have low instances of invasive species. When traveling to Española the precautions taken to prevent the introduction of new species were even as thorough as making sure that there were no seeds on our clothes and wiping our shoes down. Española is a conservation site, so although it is not an island inhabited by humans, it still has the influence and efforts to prevent and eradicate invasive species. We did not observe any non-native species on Española, indicating that conservation efforts in populated areas or areas with a lot of interference from people are effective. Further conservation efforts in areas of human interest will continue to be effective if expanded and regulated.