Sunday 10 December 2017

Understanding the dispersal of Rockwood and Spoonwood seeds in the South African Fynbos.

The Fynbos of the Western Cape, South Africa, is a dry shrubland ecosystem, prone to frequent fires, which play an important ecosystem function, clearing old growth and stimulating growth of the seeds of fire adapted trees and shrubs such as Mountain Cypress, Widdringtonia nodiflora, and various Protea species. Within this ecosystem there are patches of rocky habitat, such as cliffs, rock outcrops and scree slopes, where non-fire tolerant species such as Rockwood, Heeria argentea, and Spoonwood, Hartogiella schinoides, dominate. Unlike Cypresses and Proteas, these species produce large, fleshy fruits, which are not fire tollerant, and which appear to only survive fires by nestling deep within rock crevices, a location which they are unlikely to reach on their own, suggesting an animal vector is placing them there. It has been suggested that the Rock Hyrax, Procavia capensis, may be responsible for dispersal of the large fleshy seeds of the Rockwood, while the smaller, as redder, seeds of the Spoonwood are probably dispersed by a Bird (the colour red is often associated with dispersal of seeds or pollination by Birds, since few Mammals or Insects have receptors for this colour in their eyes).

In a paper published in the South African Journal of Science on 27 November 2017, Joseph White and Jeremy Midgley of the Department of Biological Sciences at the University of Cape Town, describe the results of a series of tests intended to determine the animals responsible for the dispersal of the seeds of the Rockwood and Spoonwood trees.

 Intact and cross-sectioned fruits of (a), (b) Heeria argentea and (c), (d) Hartogiella schinoides, showing the thin, fleshy pericarps and chlorophyllous endosperm/embryo. White & Midgley (2017).

White and Midgley placed Rockwood seeds in front of camera traps in both Fynbos shrublands and rocky areas within theses shrublands. 

The seeds placed in the rocky areas were ignored by Rock Hyrax, suggesting that this animal is not responsible for the dispersal of these seeds, while 66% of the seeds were removed by the smaller Namaqua Rock Rat, Micaelamys namaquensis. The Cape Genet, Genetta tigrina, and Cape Grey Mongoose, Galerella pulverulenta, were also seen near seeds in these rocky areas, but took no notice of them, which is unsurprising as neither species usually consumes seeds.

A Namaqua Rock Rat, Micaelamys namaquensis. Andrew Deacon/iSpot.

Four species of seed-eating Mammals were seen around the seeds placed in the Fynbos shrublands, the Four-striped Grass Mouse, Rhadbomys pumilio, the Vlei Rat, Otomys irroratus, the Cape Spiny Mouse, Acomys subspinosus, and the Cape Porcupine, Hystrix africaeaustralis. Of these only the Four-striped Grass Mouse paid any attention to the seeds, and this species removed only a very small number.

Searching for the removed seeds revealed a number of seeds with their pericarps (outer fleshy parts) removed, having apparently been gnawed off by a Rodent or similar animal. Experimentation with seeds revealed that 60% of those with the percarp removed germinated, while none of those with it left intact did so, suggesting that this is a natural part of the plant's life cycle, providing a free meal to the Rock Rats, which probably retreat into rock crevices with the seeds to enjoy a meal out of sight of predators such as the Genet and Mongoose.

A Heeria argentea seedling emerges from a dark, rocky crevice at Limietberg Nature Reserve. White & Midgley (2017).

The same procedure was repeated for the seeds of the Spoonwood tree, though without the camera trap. As predicted, some of these seeds were observed being removed by Red-wing Starlings, confirming that these smaller seeds are available to Birds, but a search for the seeds in crevices again found most of the removed seeds, which again showed signs of having the pericarps removed by a Rodent, suggesting that Rats are likely to be the most important distributors of this species as well.

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