(This exercise is based on Siemann, E. and W. E. Rogers. 2001. Genetic differences in growth of an invasive tree species. Ecology Letters 4: 514–518.)
(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)
Invasive plant species often out-compete native species in their new range. One hypothesis proposed to explain the enhanced competitive ability of invasive species is that invasive plants have few natural enemies (such as herbivores) in their introduced range. This hypothesis proposes that as an invasive plant grows, it does not need to allocate as many resources to defenses as do native plants. As a result, an invasive plant can allocate more resources to growth and reproduction, making it a better competitor. Alternatively, the EICA hypothesis (for “evolution of increased competitive ability”) proposes that because they have few enemies in their introduced range, over time, invasive plant populations have evolved to have reduced allocation to defense and increased allocation to growth or reproduction. Note that evolutionary change has occurred in the EICA hypothesis but not in the first hypothesis mentioned.
The EICA hypothesis predicts that genotypes of invasive plants from outside their native range (invasive genotypes) will have a higher competitive ability than genotypes from the native range (native genotypes). The hypothesis also predicts that the invasive genotypes will allocate fewer resources to defenses than will the native genotypes.
The Chinese Tallow Tree, Sapium sebiferum, is native to Asia. It was introduced to Georgia in the late eighteenth century, and to Texas and Louisiana in the early twentieth century. To test the EICA hypothesis with the Chinese Tallow Tree, Siemann and Rogers took measurements on trees collected as seedlings from Asia, Georgia, Louisiana, and Texas that had been grown in a greenhouse in Texas.
Figure 1
Question 1
In Figure 1, the average basal area, a measure of the cross-sectional area of the tree and an indicator of how much the tree has grown, and the proportion of trees that are producing seeds are plotted for each geographic source of the trees. Different lowercase letters (positioned above the bars) indicate means that were significantly different from one another in a statistical test. What differences between the native genotypes (Asia), the genotypes from older introductions (Georgia), and the genotypes from more recent introductions (Louisiana and Texas) are present?
Question 2
Discuss these findings in relation to the EICA hypothesis.
Figure 2 Siemann and Rogers also measured the concentrations of tannins for each genotype of the tree. Tannins are bitter and astringent chemicals that many plants produce in order to deter herbivores. Different lowercase letters (positioned above the bars) indicate means that were significantly different from one another in a statistical test.
Question 3
What differences do you see between the native genotypes, the genotypes from older introductions, and the genotypes from more recent introductions (Louisiana and Texas)?
Question 4
Discuss the tannin findings in relation to the EICA hypothesis.
Question 5
What other experiments would you suggest to further test the EICA hypothesis?
