Saturday, November 21, 2009

Wetland Plant of the Week #33

Hypericum fasciculatum “Peelbark St. Johnswort”

Peelbark St. Johnswort, also known as ‘marsh St. Johnswort,’ is a Florida native and widely distributed member of the Hypericaceae Family that can commonly be found in swamps, marshes and just about any locality having sufficient water to satisfy its Obligate lifestyle. The multi-branched growth pattern of this upright shrub gives it a very bushy appearance, and provides ample structure for numerous arthropods species to nest and hide (including ticks, take caution).

Though typically about four feet tall, the woody stems of Hypericum fasciculatum can push the plant upwards to reach heights of over two meters (6ft). In an effort to increase available surface area for oxygen absorption, the reddish-bark of the stems is exfoliated giving it a soft and crumbly look and feel.

The needle-like leaves of the plant grow in bundles and average about 2.6cm in length with slightly longer leaves at the top of the stem. The flowers, although not pictured here, display five petals arranged in a whorled pattern and can be found spring-through-summer at the terminal ends of the branches.

The abundance of Hypericum fasciculatum, when combined with its multi-branched physiognomy and its habitat preference for plentiful water, make the plant an integral component of aquatic ecosystems here in Florida. As mentioned previously, the structure provided by the plant’s branches, branchlets and leaves attract a myriad of arthropod species. Once attracted by the ‘peelbark,’ these same arthropods will, in turn, move to occupy niches in proximity to the plant. There they’ll take on roles as pollinators, predators and prey for other organisms. Through such species interactions, the trophic effects of seemingly unconnected organisms become intertwined and bound.

As an example of how complex life histories can become tangled, consider for a moment a hypothetical swamp in which a hypothetical fish is swimming around the exfoliated base of a hypothetical Hypericum fasciculatum …

As with many fish, the hypothetical one feeds on aquatic insects like water beetles, mayflies and larval dragonflies. Examining dragonflies in particular, the loss of dragonfly larvae via fish predation ultimately results in the emergence of fewer adult dragonflies than would be predicted in the absence of the fish. Compounding the process further, the presence of fewer adult dragonflies in nearby ecological communities translates to less aerial predation of flying insects. Flying insects, in addition to being food-stuffs for dragonflies, also pollinate plants; from this relationship it can be inferred that with fewer dragonflies, more insect pollinators find the nectar-rich flowers they seek… The end result of this hypothetical situation is that the Hypericum’s proximity to the fish allows the plant to host a greater number of pollinators, and thus to experience a greater level of reproductive success itself.

Image from Cited Article, interaction web showing the pathway by which fish facilitate plant reproduction. Solid arrows ndicate direct interactions; dashed arrows denote indirect interactions. The sign refers to the expected direction of the direct or indirect effect.

As is characteristic of most ecosystem dynamics, the above scenario can also be run in reverse to show that the presence of the common Hypericum fasciculatum could lead to increased fitness in the hypothetical fish species.

Knight, T., McCoy, M., Chase, J., McCoy, K., & Holt, R. (2005). Trophic cascades across ecosystems Nature, 437 (7060), 880-883 DOI: 10.1038/nature03962

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