thefuzzysasquatch: Wonders of Nature: Indian Ocean Endemics

Socotra

Socotra really is a treasure trove of unique biodiversity. The four Yemeni islands (Socotra, Abd al Kuri, Samhah and Darsa), lie ca. 240 kilometres east of the coast of Somalia and about 380 kilometres south of the Arabian Peninsula.

Socotra is one of the most isolated continental landforms on Earth as unlike most deep-sea islands it is not of volcanic origin but was once part of Gondwana. The archipelago became detached during the Miocene, in the same rifting event that opened the Gulf of Aden to its northwest. In the 20 million years since Socotra became an island much of its flora and fauna has developed into forms unknown anywhere else on Earth, with 30% of its plants being endemic. The main island has three geographical zones: the narrow coastal plains, an upland limestone plateau of karst and the central spine of the Hajhir Mountains.

Dracaena cinnabari or Dragon Blood tree belongs to a genus with 60-100 individual species. It is a remnant of the Miocene-Pliocene sub-tropical forest that once girdled north Africa, Sudan, Eritrea, Ethiopia and Somalia. These forests are now largely extinct due to desertification.

Named for its economically valuable red sap used in medicines, dyes, and makeup it is only one of six species from its genus to grow in a tree-like habit. The dense umbrella shaped crown being thought to reduce transpiration in the arid climate, with morning mists being the chief source of its moisture. This is especially important as it grows on thin soils at mountainous elevations. It also grows in thick stands, with trees close together which allow young trees to prosper on the shaded ground below.

Current threats include global warming, over-grazing, habitat loss due to development and tourism. A worrying lack of recruitment of young trees has been noted in recent decades by botanists.

Through the daily mists: Hajhir Mountain sunrise with Dracaena cinnabari seen in the foreground from White (2012).

Dragon’s Blood tree forest from White (2012).

Dragon’s Blood Trees on thin soils at altitude. Source: unknown.

Three Dragon’s Blood Trees and local goat herder, from Aspindur (2020)

Adenium socotranum or Dessert Rose is an endemic succulent that has the habit of a small baobab and is actually a member of the dogbane family. Some authorities name it Adenium obesum subs. Socotranum but it has a number of fundamental, differences from Adenium obesum.

A. socotranum, is the characteristic plant of rocky slopes, forming extensive patches of succulent shrubland in favourable locations. It grows among stones in grit or on other well drained soil. It displays several morphological and physiological adaptations to cope with the dry climate and fierce monsoonal winds. A. socotranum has a special cell sap cycling within the caudex which prevents overheating. Plant bodies are globular or columnar, with reduced surface areas that decrease transpiration. Glaucous wax surfaces and micro-anatomical epidermal emergences reflect radiation. It blooms after the rainy season in March-April and only when they are well past flowering do the fruits release their air-borne seeds, leaving them to be dispersed by the heavy monsoon storms. Then the dry season begins with (perhaps) a million adenium seedlings waiting for the next rainy season. The most common and regular associate in the Adenium shrub layer is the endemic Jatropha unicostata. This shrub is perhaps one of the most common species on Socotra. The close association between the two species is probably due to their apparent preference for, or ability to cope with, coarse rocky substrates. Numerous regeneration niches exist in such substrates, offering the seedlings protection from trampling and affording them some degree of shade. The seeds of both species germinate readily after rainfall. Furthermore both species are highly toxic to domestic livestock and as such are rarely browsed.

Desert Rose in fresh leaf from Reese (2015)

Desert Rose in bloom from Wos (2017).

Dessert Rose clings to a mountainside overlooking the Indian Ocean from White (2012)

Dendrosicyos socotranus, the cucumber tree, In Soqotri, its name is qamhiyn.

D. socotranus is the only member of a monotypic genus in the plant family Cucurbitaceae. The species is endemic to the island of Socotra in Yemen, there are however a few trees on the island of Samhah.

A recent molecular phylogenetic analysis of the family Cucurbitaceae found that the Dendrosicyos lineage is about twice as old as the island, and thus seems to be an island relic of a progenitor lineage that became extinct on the mainland.

It has a bulbous trunk and a small crown. The leaves are nearly round, covered with fine bristles, and slightly toothed. The yellow flowers, males and females are present on the same plant for cross pollination. It reproduces only by seed. Fruits are green, turning brick-red when ripe.

Seedlings subjected to overgrazing and regeneration may be compromised over time, except for seedlings protected from goats by Cissus subaphylla. It is quite abundant on the dry parts of the island of Socotra, associated with Croton socotranus in the plains, and on calcareous soils to 500m elevation. The species is well-adapted to dry sites. It is widely distributed in several vegetation types but has a rather fragmented distribution; over large areas there are only isolated trees or small relict populations, whilst in other areas it is relatively abundant. The species is considered vulnerable.

Cucumber Tree on the slopes of the Hajhir Mountains from PRC (2013).

Cucumber Tree in coastal scrubland from Melnik (2020)

Cucumber Tree on the karst plateau from Sudeten (2012)

Seychelles

The main Seychelles Islands are a microcontinent underlain by Precambrian granites. The islands themselves are exposures of that basement rock, and of Cenozoic acidic volcanic rocks. The Seychelles are part of the Mascarene Plateau, which along with the Madagascar Plate, broke off from the Indian Plate approximately 66 million years ago.

Thus, like Socotra, they have been isolated from the evolutionary trajectory of Africa and Asia, for an extremely lengthy period. Thus, many endemic plants, birds and animals have developed on the island. Being much further south than Socotra a different set of genera, such as tropical forest palms and pandus species show endemism. (sources: Feagan (2020);

Geographical location of the Seychelles and main islands adapted from Huff Post (2011) and World Maps (2020)

Lodoicea maldivica or Coco de Mer is an endemic palm only found on Seychelles’ islands of Praslin and Curieuse. I first became aware of this tree at 18 years old, in the bedroom of my then girlfriend. On a high shelf was an enormous nut! It took some while for her to tell the tale. She was an undergraduate at Oxford and had been to the island to illustrate a botanical guidebook and brought it back as a souvenir. So, if you’re reading this Anne, I have you to thank for a lifelong interest in endemic plants.

Now the science bit.

Adapted from Wikipedia (2020): Lodoicea, commonly known as the, Coco de Mer, is a monotypic genus in the palm family. The tree generally grows to 25–34 m with the tallest measured, after felling, was 186 feet (56.7 meters) in height.

The leaves are fan-shaped, 7–10 m long and 4.5 m wide with a 4 m petiole in mature plants. It is dioecious, with separate male and female plants. The male flowers are arranged in a catkin-like inflorescence up to 1 m long which continues to produce pollen over a ten-year period. The mature fruit is 40–50 cm in diameter and weighs 15–30 kg, and contains the largest seed in the plant kingdom. The fruit, requires 6–7 years to mature and a further two years to germinate.

While the functional characteristics of Lodoicea are similar to other trees of monodominant forests in the humid tropics, its unique features include a huge seed, effective funnelling mechanism and diverse community of closely associated animals. These attributes suggest a long evolutionary history under relatively stable conditions. Of the six monospecific endemic palms in Seychelles (see below), Lodoicea is the only true case of island gigantism among Seychelles flowering plants, a unique feature of Seychelles vegetation. It holds five botanical records: It produces the largest wild fruit so far recorded, weighing up to 42 kg, the mature seeds weighing up to 17.6 kg are the world's heaviest. The seed upon germinating, produces the longest known cotyledon, up to four meters (13 feet). The female flowers are the largest of any palm. and Lodoicea is the most efficient plant known at recovering nutrients from moribund leaves.

This tremendous reproductive, outlay of nutrients, for a plant growing in poor soil, has long puzzled scientists. Recently Edwards et al. (2015) elucidated the variety of mechanisms the plant employs to ensure survival of its offspring. A nice summary appears in the New Scientist (2015): “They found that the leaves have only about one-third of the nitrogen and phosphorus concentrations seen in the leaves of other trees and shrubs growing on the Seychelles. Also, before old leaves are shed, the palm efficiently withdraws most of the nutrients from them and recycles them. Investing so little into the foliage means the palm has more to invest in its fruit.

But that’s not the only way the foliage helps fuel fruit growth. The huge, pleated leaves are remarkably effective at funnelling water down the trunk during rain showers. Kaiser-Bunbury and his colleagues showed that this stream of water also picks up any nutrient-rich detritus on the leaves – dead flowers, pollen, bird faeces and more – and washes it down into the soil immediately around the base of the palm. Consequently, the nitrogen and phosphorus concentrations in the soil 20 centimetres from the trunk were at least 50 per cent higher than in the soil just 2 metres away.”

The New Scientist also speculates that original, back in the time of the dinosaurs, the seeds were dispersed by animals.. Now doesn’t that paint an enthralling picture!

Fruits of Lodoicea maldivica or Coco de Mer from the New Scientist (2015)

Male inflorescence or catkin of the Coco de Mer, from Wikipedia commons (2020)

Coco de Mer leaf funnelling debris to the base of its trunk to give seedlings nutrients, from Edwards et al. (2015)

The true size of the Coco de Mer nut from Amla (2015). Original caption reads: A Seychellois boy holds a mature inner shell of a Coco-de-Mer while gazing up at smaller nuts, still encased in their green husks on the palm tree (Gerard Larose, STB)

There are 5 other endemic palms from the Seychelles, each from a different genera. These are: Deckenia nobilis Cabbage Palm; Nephrosperma vanhoutteana Creole Name: Latanier Millepattes; Phoenicophorium borsigianum Thief palm; Roscheria melanochaetes Creole Name: Latanier Hauban and Verschaffeltia splendida Creole Name: Latanier Latte. There are also species of Pandanus (Screw Pine) and at least 68 other endemic species of plant.

References:

Amla, H. (2015). Surviving almost impossible odds – how the Seychelles Coco-de-Mer palm parents its seeds at: http://www.seychellesnewsagency.com/articles/2360/Surviving+almost+impossible+odds++how+the+Seychelles+Coco-de-Mer+palm+parents+its+seeds accessed 24.07.20

Aspindur (2020) “Socotra: The Island of Strange Plants” at: http://aspundir.blogspot.com/2012/05/socotra-island-of-strange-plants.html accessed 24.07.20.

Edwards, P.J., Fleischer‐Dogley, F. and Kaiser‐Bunbury, C.N., 2015. The nutrient economy of Lodoicea maldivica, a monodominant palm producing the world's largest seed. New Phytologist, 206(3), pp.990-999.

Feagan, S. (2018). The Seychelles: An Ecological Overview at: https://www.workingabroad.com/blog/the-seychelles-an-ecological-overview/ accessed 24.07.20

Huffington Post (2011) at: https://www.huffingtonpost.co.uk/entry/seychelles-map-population_n_860481?ri18n=true accessed 24.07.20

Melnik, V. (2020) at: https://photosight.ru/users/827/ accessed 24.07.20

New Scientist “The secret of the world's largest seed revealed” https://www.newscientist.com/article/dn26930-the-secret-of-the-worlds-largest-seed-revealed/ accessed 24.07.20

PRC (2013), at: https://www.thelivingmoon.com/forum/index.php?topic=5373.0 accessed 24.07.20

Reese, N. (2015). Socotra Plant: Cucumber Tree at: https://socotraplants.wordpress.com/2015/03/20/socotra-plant-cucumber-tree/ accessed 24.07.20

Sudeten, A. (2012) at: https://www.flickr.com/photos/andy_sudeten/8281113898 accessed 24.10.20

White, M. (2012) “Where the Wild Things Are” at: https://www.nationalgeographic.com/magazine/2012/06/socotra/ accessed 24.07.20

Wikipedia (2007) https://en.wikipedia.org/wiki/Lodoicea#/media/File:CocoMale.jpg accessed 24.07.20

Wikipedia (2020) at: https://en.wikipedia.org/wiki/Lodoicea accessed 24.07.20

World Maps (2020) at: http://ontheworldmap.com/seychelles/seychelles-islands-map.html accessed 24.07.20

Wos (2017) Adenium obesum ssp. socotram at: https://worldofsucculents.com/adenium-obesum-socotranum-socotran-desert-rose/ accessed 24.10.20