Ecosystem engineer hiding under the shade: Mutualistic association between Num-num (Carissa bispinosa) and Snouted harvester termites (Trinervitermes trinervoides) in a semi-arid savanna

Abstract

In many ecosystems one individual, or species may alter the environmental conditions in such a way that a stressful habitat becomes more hospitable for other individuals. Mutualism is a relationship between two organisms of either the same species or different species that enhance their survival or growth. Much of the diversity in the world is due to the mutualistic relationships that organisms form with one another. However, very few of these mutual associations have been documented in the semi-arid savanna region of Africa. Our study used seasonal transect surveys to determine the levels and benefits of association between Carissa bispinosa, a fast-growing medium sized ever green thorny shrub, and Trinervitermes trinervoides, a nocturnal termite species, at a semi-arid savanna setting in Nylsvley Nature Reserve, South Africa. The aim was to understand whether the relationship between C. bispinosa and the T. trinervoides termitaria: (1) benefits both species (mutualistic in nature); or (2) if it was only beneficial to one entity; and (3) determine to what extent the relationship benefits the species involved. We hypothesised that the termite mounds provide favourable conditions for vegetation and as a result, num-num shrubs are better established on termite mounds. In return, the shade, thorns and “cagey” shrubs of num-num trees provide termite mounds with a thermoregulation buffer and protection from predation. Our findings show that plants growing on mounds were significantly taller with a wider canopy compared to stand alone plants. Plants in association with mounds also remained greener in the dry season and bear more fruits in winter compared to standalone trees. On the other hand, termite mounds under plants were less prone to be damaged by aardvark and pangolin compared to mounds in the open. Termite mounds in shade had a significantly wider diameter than exposed mounds. Termite mounds under num-num plants maintained a better constant and higher mean internal temperature during winter compared to exposed mounds. The magnitude of this difference increased with mound volume and amount of plant cover. We therefore conclude that the association between Carissa bispinosa (plant) and Trinervitermes trinervoides termite mounds is mutualistic in nature and may play a significant role to ensure persistence of both species in these semi-arid environments. There are several animal-plant associations that have been documented in other parts of the word, but none are reported for southern Africa. Scientists are increasingly studying termite behaviour and the role they play as ecosystem engineers. In this study emphasis was placed on the newly observed survival mechanism that termites employ to escape extreme temperatures and predation.