RESTIOS IN PEACE...

For this mini-assignment, I narrowed in on two genera of Restios, namely Mastersiella and Rhodocoma, each represented by a single species on the Cape peninsula (Cape Point area). I was most interested to discover that of the 350 known taxa that make up the Restios, 107 of them are found on the Cape peninsula, that's just over 30% of all taxa! Only the genera Mastersiella and Rhodocoma out of all the other genera that are encountered sympatrically, are represented by a single species. I wondered why...

This is Mastersiella digitata:

Mastersiella digitata is typically 0.2 - 0.7m tall and does not have spreading rhizomes, therefore it is also vulnerable to fire and will not resprout after a fire event. The distinguishing characteristic of this species from the other species within the genus is shape of the cone-like spikelet of the male flower, which are also reflexed. This species is distributed along coastal mountains from 20m - 50m above sea-level, so it is generally a low-altitude species. Its seeds are dispersed by ants who make use of the elaiosome through a mutually beneficial relationship which sees the role of the ant as the "seed planter."

Mastersiella purpurea (named after its purple colour) has a different growth form to M. digitata. M. purpurea is typically erect, while M. digitata is short and tufted. Mastersiella purpurea is also taller and grows to 0.5m - 1.5m tall. Mastersiella purpurea seed dispersal strategy is the same as M. digitata, as is its vulnerability to fire, but M. purpurea is found at a higher altitude range of 300 - 1600m above sea-level. Mastersiella purpurea is not found on the Cape peninsula.

Mastersiella spathulata (named after its shape, similar to that of a spoon) is similar in height to M. digitata at 0.2m - 0.6m tall, and is also tufted, not erect. It is found commonly on acid coastal sand as far East as Bredasdorp. It also has a wide altitude range of 50m - 1900m above sea-level. This species however is also not found on the Cape Peninsula.

WHY?

Interestingly, there are a few factors which seem to be important in the distribution of these three species above. The reason why M. digitata is found solely on the Cape Peninsula is linked to the soil-type and winter rainfall. Low altitudes also suggest that M. digitata is more resistant to salt spray from the waves which pound both sides of the Cape Peninsula, and their shorter growth form is probably more efficient in this wind-swept region, which could probably cause damage to taller forms. Mastersiella purpurae is found mostly in the Southern Cape where the area receives Summer rainfall, and at the altitude above 300m, it is placed away from salt spray of the sea and possibly within the mist-belt of the Cape folded mountains which would also provide a higher intensity of year-round precipitation. The taller growth form possibly suggests that wind is not an important factor. Mastersiella spathulata is the most common of the three species and can adapt to a variety of areas. It also prefers slightly wetter climates, but has adapted to drier environments. This species prefers acid coastal sands, which is probably why it is not found on the Cape Peninsula, since its altitude range suggests that it could possibly live here.

A common and very interesting factor between all three species is that the seed dispersal agents (ants) must be extremely diverse and capable of making use of a variety of climatic conditions and environments from low coastal altitudes to alpine heights at the tops of mountains. Ants are extremely successful.

This is Rhodocoma fruticosa:

Rhodocoma fruticosa is 0.4m - 0.8m tall, and grows almost exclusively on cave sandstone substrate. It gows at altitudes of 200m - 1600m above sea-level and is the most widespread of all the Restios in South Africa. It has adapted to live in both Winter rainfall regions as well as Summer rainfall regions. In Winter rainfall regions it is commonly associated with dry Fynbos at the lower altitudes. Rhizomes are present and it can therefore regenrate after a fire event. The seeds of R. fruticosa are "seed surface type" and are not dispersed by ants. The seeds therefore do not possess elaiosomes.

I believe that the only reason why R. fruticosa is the sole representative of the genus from the Cape Peninsula, is because it is the most likely of all the species within the genus to survive there. This must suggest that the Cape Peninsula is either a very specific habitat, or a very harsh environment that restricts the successful recruitment of other species within the genus. Rhodocoma fruticosa is the most common species in the genus and therefore must have adapted to a wide variety of different climates, possibly predisposing it to the conditions on the Cape Peninsula as a survival advantage or adaptation over its related species. Its seed dispersal agent must also be present on the Cape Peninsula.

For both genera, although much work is missing regarding the role of fire in germination of seeds, it seems as though Restios with rhizomes, which are adapted to resprouting after a fire-event, produce either wind-dispersed or larger-animal dispersed seeds (birds?), while those which are killed off by fire because they cannot regenerate, produce hard, woody seeds which are taken underground by a large variety of different ant species whose home is the Fynbos area. In times of fire, the ants would be protected underground as would the seeds of the Restios which they took down with them, and maybe the behaviour of the ant species after a fire, such as nest relocation, or other response to the fire-event, could play an important link in the success and germination of the seeds to support the next generation of these Restios.

All of this from a key...

Cheers

David Vaughan
Senior aquarist, Quarantine
Two Oceans Aquarium
Cape Town, South Africa
+27 21 418 38 23
dvaugan@aquarium.co.za

Image credits:

Mastersiella digitata: Image from Fernkloof Nature Reserve: http://fernkloof.com/species.mv?754 (accessed 12:47; 13 March 2007).

Rhodocoma fruticosa: http://www.wonderlingsbandb.com/restio.htm (accessed 13:47, 13 March 2007.)