Research Of The Compensatory Physical Structure Of Plants In Response To Browsing Pressure

Introduction

African savanna is an essential ecosystem which is characterised by co-dominance of grasses, trees and woody cover and plants responses to browsing which is a chief determinant of ecosystem properties. African savannas can are influenced by a variety of factors including climate, grazing and the plant's responses to browsing pressures. Plants are known to evolve defences to protect themselves against herbivore attacks, which in Africa can be a strain due to the excessive quantity of species richness and density of herbivores. Plants can respond to herbivory through various forms of defences, through morphological, biochemical and physical mechanisms to counteract and offset the effects of herbivore attack. Well, known defences are physical and chemical defences. Physical defences include the production of thorns and spines. Whereas chemical defences include the production of toxic chemicals that deter the development of herbivores. The production of these plant defences aims to reduce the palatability. However, they have disadvantages as it reduces resources needed for seed production. This study aims to investigate the compensatory physical structure of plants in response to browsing pressure. Physical and chemical hypotheses based on allocation theory and herbivore selection patterns predict that defences should increase in species where there is high herbivore guild.

Methods

The study site was the Mankwe wildlife reserve situated in the north-west province of South Africa. In which there were two study sites used one of which is the main area the wildlife reserve itself which majority of the reserve's wildlife occupies. The second being the factory area in which most of the wildlife there is excluded. Which provided the two ideal study areas to compare the impact of large mammals on vegetation at the reserve, which was the ideal situation to investigate vegetation defence as one area provided high-quality browsing and the other low. The method for this survey involved comparing the density and size of a plant's physical defences, in this case, the Umbrella thorn acacia (Vachellia tortilis) in the two areas.

Nine pairs of trees comprising of juveniles and mature plants were sampled for the same species. All collections took place on two consecutive days in September 2018. Two Random clippings were taken from either side of each tree approximately 30cm long at the height of between 0. 75 and 1. 7m, within the browsing reach of impala and other ungulates. The length of each branch was measured, and the density of thorns and hooks was measured by counting the number of thorns and hooks, to provide a maple to calculate the average and then the standard deviation. To be able to quantify the differences between length and thorn density of the two areas.

Results

Low browsed branches produced more thorns than un browsed branches. Low browsing branches also produced more hooks. The ratio of hooks was slightly higher in low browsed species although it did not differ significantly between the two browsing sites. The length of branches in the browsed site was not significantly different from that of the length of branches in the un browsed location. Thorn length was not undertaken in the investigation so shows no impact of thorn length in the results. Because thorn numbers appear to be higher in the low browsed area than the high browsed area, it is suggested that in low browsing areas there is more vegetation defence towards herbivory attack.

Discussion

Browsing in the area where a majority of animals is absent produced results showing a higher number of thorns and hooks suggesting that this area has higher defences against herbivore guild. Which is unusual as most hypotheses suggest that individual plants that are subject to herbivore guild, would increase their defences whether it be chemical or physical. Which this study showed the opposite as it showed high defences of plant species in low browsing areas. This is controversial as many studies provide experimental evidence that increased defences such as thorns were an induced response to large mammal herbivory. A study by Gowda (1997) found that acacia growing within reach of browsing ungulates shows significantly higher spine density and longer thorn length.

Moreover, he found that a study of giraffes and acacia that the spine scene increased when a large density of Giraffe under came the acacia. So that twig growth is reduced by browsing when the spines are removed. He suggested that the production of longer spines was a physical defence and an induced defence, in which the triggering mechanisms being the excess pruning of the shoot tips by the browser, in this case, the giraffe. The increase in the spine density could be an adaptive response to the increase in browsing or an indirect effect of reduced growth on browsed branches.

Another study suggests that plants which are in poor habitats often invest resources into anti-herbivore defence as loses to browsers are costly when growth rates are low. Few plants also invest simultaneously in different categories of defence, suggesting there is a trade-off that exists. Although there is little evidence about how plant species, allocate their resources for defence. Which could explain the slight difference in the low browsing area could be a poor habitat with poor soils so can't afford to lose any leaves to browsers so produce to defences to deter herbivore attacks. Another reason for the lower density of thorns in the high browsing area could be because thorns are permanent investments. However, their production is a slow process and would take several weeks for new thorns to grow to their final size, meaning that the induced response of increased thorn length would only be effective if fluctuations in herbivore pressure are a long-term phenomenon. The presence or absence of herbivores would need to be reasonably constant.

However, this study did not consider the length of thorns which could have improved the results and provide a better indication of the impact of large mammals on vegetation at the reserve.