Competition between the invasive macrophyte Caulerpa taxifolia and the seagrass Posidonia oceanica: contrasting strategies

Abstract

Background: Plant defense strategy is usually a result of trade-offs between growth and differentiation (i.e. Optimal Defense Theory--ODT, Growth Differentiation Balance hypothesis--GDB, Plant Apparency Theory--PAT). Interaction between the introduced green alga Caulerpa taxifolia and the endemic seagrass Posidonia oceanica in the Mediterranean Sea offers the opportunity to investigate the plausibility of these theories. We have accordingly investigated defense metabolite content and growth year-round, on the basis of an interaction gradient.

Results: When in competition with P. oceanica, C. taxifolia exhibits increased frond length and decreased Caulerpenyne--CYN content (major terpene compound). In contrast, the length of P. oceanica leaves decreases when in competition with C. taxifolia. However, the turnover is faster, resulting in a reduction of leaf longevity and an increase on the number of leaves produced per year. The primary production is therefore enhanced by the presence of C. taxifolia. While the overall concentration of phenolic compounds does not decline, there is an increase in some phenolic compounds (including ferulic acid and a methyl 12-acetoxyricinoleate) and the density of tannin cells.

Conclusion: Interference between these two species determines the reaction of both, confirming that they compete for space and/or resources. C. taxifolia invests in growth rather than in chemical defense, more or less matching the assumptions of the ODT and/or PAT theories. In contrast, P. oceanica apparently invests in defense rather than growth, as predicted by the GDB hypothesis. However, on the basis of closer scrutiny of our results, the possibility that P. oceanica is successful in finding a compromise between more growth and more defense cannot be ruled out.

Figure 1

Leaf number. Mean number of intermediate and adult leaves of Posidonia oceanica according to the period and the level of interaction with Caulerpa taxifolia. Bars: Confidence level 95%, n = 30 shoots.

Figure 2

Leaf length. Mean length of intermediate leaves of Posidonia oceanica according to the period and the level of interaction with Caulerpa taxifolia. Bars: Confidence level 95%, n = 30 shoots.

Figure 3

Frond length. Mean length of Caulerpa taxifolia fronds according to the period and the level of interaction with Posidonia oceanica. Bars: Confidence level 95%, n = 40 fronds.

Figure 4

Leaf production. Mean number of Posidonia oceanica leaves produced per shoot during the year of study according to the level of interaction with Caulerpa taxifolia. Bars: Confidence level 95%, n = 30 shoots.

Figure 5

Leaf life-span. Mean life-span of Posidonia oceanica leaves, according to the level of interaction with Caulerpa taxifolia. Bars: Confidence level 95%, n = 5 (L0) n = 7 (L1), n = 10 (L2).

Figure 6

Posidonia primary productionr. Net primary production (in g dry weight shoot^-1 yr^-1) of Posidonia oceanica dedicated to rhizomes, sheaths and blades, according to the level of interaction with Caulerpa taxifolia. The method of estimation (see [21] Pergent and Pergent-Martini, 1991) does not allow calculation of a confidence interval.

Figure 7

Tanin cells. Change in the mean density of tannin cells along the length of Posidonia oceanica adult leaves (sheaths and blades) as a function of the level of interaction with Caulerpa taxifolia. Distance along the leaf was measured below and above the limit between sheath and blade. For each adult leaf of each shoot several replicates were performed; the total number of replicates was n = 78 (L0) n = 89 (L1), n = 83 (L2). Bars: Confidence level 95%,.

Figure 8

Phenolic compounds. Mean annual content in P1 (a mixture with ferulic acid) and P2 (methyl 12-acetoxyricinoleate) phenolic compounds in adult leaves of Posidonia oceanica, as a function of the level of interaction with Caulerpa taxifolia. Bars: Confidence level 95%, n = 17 (L0) n = 18 (L1), n = 18 (L2).

Figure 9

Caulerpenyne content. Caulerpenyne (CYN) content (in mg CYN g wet weight^-1) in Caulerpa taxifolia fronds as a function of both season and level of interaction with Posidonia oceanica. Bars: Confidence level 95%, n = 5.

Figure 10

Energy allocation and strategy. Different energy allocations and competition strategy between Posidonia oceanica (high left) and Caulerpa taxifolia (down right). (E⇗): Increase in energy needs; (E⇘): Decrease in energy needs.