|Journal||Master of Science Thesis in Natural Sciences|
During 2016 and 2018 four giraffe (Giraffa camelopardalis) mortalities occurred in the Karoo potentially caused by acute hydrogen cyanide poisoning. Plants have various defence mechanisms to protect themselves against herbivory, including the production of secondary metabolites such as condensed tannins and hydrogen cyanide. This study quantified condensed tannin and hydrogen cyanide production in selected Karoo plant species that giraffe may browse, to assess the possibility of acute hydrogen cyanide poisoning and condensed tannin intoxification. Condensed tannins and hydrogen cyanide concentrations were explored in both spatial and temporal scales. The spatial assessment was performed at macro-scales (different locations within the Karoo), whereas temporal assessment was performed at seasonal scale. The effect of water availability and herbivory on condensed tannin production in Vachellia karroo trees was also investigated. Condensed tannin concentrations were high throughout seasons and did not differ significantly among the study sites in plant species giraffe primarily browse. In winter, V. karroo leaves were unavailable and secondary plant species increased in dietary importance. The most preferred plant species in the giraffes’ diet, V. karroo, contained high levels of condensed tannins in mature leaves as well as in new-growth plant tissue. Condensed tannin concentrations increased significantly in several evergreen tree species during winter, including Schotia afra var. afra and species of the Rhus genus, which may indicate an increase in dietary importance during winter season. Schotia afra var. afra contained lower condensed tannin concentrations than V. karroo throughout the study. Condensed tannin production increased significantly in three of the four treatment groups of V. karroo trees that received simulated herbivory regardless of the browsing intensity. Both treatment groups which received water, increased in nitrogen contentment, whereas trees from the browsed and not watered treatment decreased in nitrogen content value and palatability. The high condensed tannin concentrations seem to be a fixed defence response by Karoo plants to browsing, or a response when sufficient water is available. The high condensed tannin concentrations may reduce the available browse as giraffe and other herbivores may reject leaves high in condensed tannins. However, giraffe have the ability to partially degrade condensed tannins and will therefore not be as susceptible to tannin intoxification than other herbivores. The higher browsing pressure caused by giraffe may therefore be detrimental to other herbivores utilising the same plant species in the Karoo, that do not have the ability to degrade condensed tannins. Therefore, careful considerations should be taken when introducing large game species into the Karoo. Only focusing on vegetation composition and abundance may be insufficient in predicting carrying capacities in semi-arid environments such as the Karoo without taking chemical composition into account. None of the plant species, except for one Eucalyptus cladocalyx tree, contained any measurable hydrogen cyanide, therefore making the probability of acute hydrogen cyanide poisoning highly unlikely. However, various other poisonous plants occur in the Karoo, these plants need to be investigated to determine whether they form part of the giraffe diet during times of limited browse, and how these plants may respond to browsing.
Keywords: Giraffe, Herbivory, Condensed tannins, Vegetation, Plant defences, Extralimital populations
Author: Eugene Marais
Journal: Master of Science Thesis in Natural Sciences