April 12, 2010

History of land use determines threat and rarity in mangrove tree species

This post was chosen as an Editor's Selection for ResearchBlogging.orgA new study from PLoS ONE was published last week assessing the threat to mangrove tree species around the world based on IUCN Red List data. At first glance the paper might seem to be just another bleak walk through the anthropogenic dismantling of a fragile biome, but there are some excellent issues presented regarding our relationship between the land and its inhabitants and the interconnectedness of rarity and threat level.

The major transition of land use to land management (with a cons bio or ecological base) is a shift in public perception driven by the shift in the perceived, publicized and tangible wants and needs of Western culture molded and implemented by government officials, politicians, philosophers and activists. When you juxtapose historical procedure and law regarding resource acquisition with our modern standards, the inescapable constant is Western prerogative, which definitely gives environmentalists a steep rhetorical hill to climb when trying to rationalize proposed protections, especially those that would effectively rope off or reign in particular resources from public access in foreign countries. One of the largest factors in the decline of mangroves worldwide is the proliferation of aquaculture, which is established by local (or not so local) business people to feed the Western-inspired globalized desire for seafood of particular types. It must be delightfully contradictory for locals to simultaneously receive pleas for the environment and orders for product from the same countries.

Portugal found value in the mangroves going as far back as the early 1700’s, when a law was established in Brazil making it illegal to fell a tree without also using the bark. This wasn’t an indicator of some kind of European protoenvironmentalism, however; it protected the tanneries’ interests in the trees, essentially granting exclusive rights to the tanneries for logging. Tannin was big business until more recently, evidenced by chemical evaluations like this:


That passage comes from the second volume on “the tannins”, preceded by historical data on the English interest in mangrove tannin in the early 19th century, so the commercial interest in these areas has been constant even if the primary interests have changed.

There are 70 species of tree that can be classified as “true” mangrove species, though not all of them are closely related. Mangrove trees have two main environmental stressors: an overabundance of salt from the water and a deficiency of oxygen from the soil. These plants have developed root structures like pneumatophores or above-ground, “aerial” roots to absorb oxygen , poking through the largely hypoxic mud. In some mangrove trees, the roots contain high levels of waxy suberin to mitigate the level of salt entering cells; in others, like the white or grey mangrove, the organism is able to secrete excess salts.

But perhaps the most unique adaptation to the high level of salts in the water and soil is the way some mangrove trees nurture and disperse their seeds. Unlike most plants, mangrove trees such as Aegialitis or Rhizophora are viviparous – the seeds germinate while still attached to the tree, forming a buoyant propagule, a protective vessel highly resistant to the desiccating waters encompassing the forest. Blair Niles, Mary Blair Beebe and William Beebe describe these structures in their 1910 book Our Search for Wilderness:

Far out on the tip of a lofty branch a mangrove seed will germinate before it falls assuming the appearance of a loaded club from eight to fifteen inches in length One day it lets go and drops like a plummet into the soft mud where it sticks upright Soon the tide rises and if there is too strong a current the young plant is swept away to perish far out at sea but if it can maintain its hold roots soon spring out and the ideal of the mangrove is realized the purpose for which all this interesting phenomena is intended the forest has gained a few yards and mud and leaves will soon choke out the intervening water.

This mangrove forest in eastern Venezuela, the Orinoco delta, is one of the areas of least concern for this biome. The forests are relatively protected in the area, and many of the species are replicated in other areas of the world, as far away as Africa. This is not the case, however, in other places of the world.

Mangrove Distribution

Just north, the mangrove forests along the Pacific and Atlantic narrows of Central America contain the highest proportional number of threatened mangrove tree species in the world, about 25 to 40 percent depending on the area, according to the authors of the new PLoSOne paper I mentioned, Polidoro et al. There are approximately 10 species of trees in the area, a stark contrast to the Indo Malay Philippine Archipelago, which harbors 36 – 46 species out of the 70 known of which less than 15 percent are threatened.

Percent of Mangroves threatened per area

That number can be deceiving however; the habitat has been reduced by 30 percent in the past 30 years due mainly to the establishment of fish and shrimp farms, and the protections on paper are not always translating into enforced policies. Two species in particular are of chief concern due to an 80 percent reduction in their already patchy habitats of late, Sonneratia griffithii and Bruguiera lainesii, of which there are only about 500 and 250 individuals left in the wild respectively.

The authors briefly mention an interesting statistic regarding rarity: Nine out of 11 of the most threatened mangrove trees are considered rare or uncommon, but five out of the rest are also considered uncommon, bringing up an important distinction. There is definitely a tendency for the two factors – rarity and threat level – to be tied for obvious reasons, but it’s not a necessary linkage. In the case of uncommon, least concern organisms, their rarity can be explained by physiological, reproduction or ecological factors like dispersal or certain competitive pressures that are normal for the organism. An uncommon organism might be rarer because of its distribution relative to other, comparable species or it might very well be under certain immediate threats, but is able to reproduce and disperse with greater efficiency than its peers.

This paper was also covered over at Conservation Bytes, where Corey details some of the essential services mangrove forests provide.

Polidoro, B., Carpenter, K., Collins, L., Duke, N., Ellison, A., Ellison, J., Farnsworth, E., Fernando, E., Kathiresan, K., Koedam, N., Livingstone, S., Miyagi, T., Moore, G., Ngoc Nam, V., Ong, J., Primavera, J., Salmo, S., Sanciangco, J., Sukardjo, S., Wang, Y., & Yong, J. (2010). The Loss of Species: Mangrove Extinction Risk and Geographic Areas of Global Concern PLoS ONE, 5 (4) DOI: 10.1371/journal.pone.0010095

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