Competitive Exclusion in Japanese Dandelion

環境学の授業で課題が出ました。

教科書に載っている論文を一本通しで読んで軽いまとめを書いてくること、という課題。折角日本人なので西洋タンポポと日本タンポポの競争についての論文を読ませていただいてまとめました。所要時間二時間くらい。結構かかりました。シンガポール大学のオンラインの蔵書数は本当に物凄いなあと改めて感じました。Google Scholarに載っている論文殆ど無料で読めるんですからね。大学パワーで殴りに行ってる感じですね。論文のリンクは一番下に貼ってあります。面白かったので他にも同系列の論文読んでみても良いかもしれません。

A) Background

This study is aimed to investigate the impact of invasive alien species in the ecosystem. It focuses on competition for pollination, taking Japanese native dandelion (Taraxacum spp) and invasive dandelion (T. officinale).

B) Question

How do native dandelion species compete with invasive species over pollination?

C) Assumption

Native dandelion and Invasive dandelion share the same niche, meaning they use the same means of pollen-transfer.

D) Hypothesis

Native species may suffer from the reduction of seed set upon the invasion of alien species because invasive species deprive native species of pollinators.

E) Study Set and Results

1) which species attract the larger number of pollinators? 

Researchers chose mixed vegetation of two species, randomly selected the same numbers of both species for treatment. They observed a total number of visitation of pollinators for both species for 3- minutes period. The right graph shows that for each treatment, invasive species attract more pollinators than native species, indicating that invasive species are more competitive than native species.

2) when two species are mixed, do invasive dandelion species deprive native species of pollinators?

To investigate the pollinator visitation to each species, researchers prepared both species with the same number of capitula. There are three treatments, 1) 30 of Native dandelion, 2) 15 of native and 15 of invasive dandelion, and 3) 30 of Native and Invasive dandelion. (1 and 2 have the same number of total dandelions, 1 and 3 have the same number of native dandelion). Then count the pollinator visitation for a 13-17-day period at two locations a and b. The graph shows the pollinator visitation to native species. They observed a significant reduction in visitation, suggesting that the derivation of native species is real.

3) Do pollinators move around in mixed vegetation?

The third phase of the experiment is structured to focus on whether there is any interbreeding of natives and invasive species due to pollinators in mixed plots. Researchers who observed the movements of pollinators in mixed plot found that the pollinators prefer invasive species to native species, which was suggested in experiment 2 as well, hence they tend to move from native to invasive rather than from native to native. This may cause pollen contamination and a low rate of successful pollination.

4) does the presence of invasive species reduce the seed set in native species?

To investigate the seed production of native species with the presence of invasive species, the researchers placed mixed plots to see the influence of invasive species. After natural pollinators freely pollinate the flowers, researchers looked into the proportion of seed set of native species. The graph indicates a significant reduction in seed production in native dandelion species.

5) does the same phenomenon happen in the natural environment?

To show the same pattern as experiment 4 in the natural environment, they chose seven sites and investigated the seed production of native species. At all seven sites, the proportion of seed set is always lower in the mixed population. We can conclude that the presence of invasive species does decrease the rate of successful pollination of native dandelion species.

F) Conclusion

The experiments reveal that the seed set of the native species is reduced by the presence of the invasive species. This is caused by the invasive species depriving native species of pollinator visits because pollinators prefer invasive species. The mixed environment also plays a role as the interspecific movement of pollinators contaminates the pollen.

The result of the study suggests that the hypothesis about the interspecific competition between native dandelion species and invasive species are happening. In other words, invasive species are competitively excluding the native species (competitive exclusion)

G) Bibliography

Kandori, Ikuo, et al. "An invasive dandelion unilaterally reduces the reproduction of a native congener through competition for pollination." Oecologia 159.3 (2009): 559-569.