Modelling the Impacts of Grazing and Trampling on Grassland Ecosystems in the Semi-arid Environments

Abstract:

In semi-arid regions, rainfall is limited and unreliable, and pastoralism is the only viable economic activity and a major source of livelihood. Small-holder farmers mainly practice it with households having livestock ranging from one to 10 head of cattle. Coupled with the low cattle off-take rates in this small-holder sector, there is an ever-growing livestock population, causing over-utilization of the grassland ecosystem. This over-utilization of the grasslands directly affects the structure and functions of the grassland ecosystem, leading to environmental degradation.

In this study, we modelled the effects of grazing and trampling on the grassland ecosystem using the delay differential equations in which weaning is the time delay/lag. The control strategy used in the study is pulsed harvesting, which aims to reduce the livestock population.

Numerical simulations were applied on a 10-hectare piece of land with five variables: Juveniles, Adults, Gestation, Forage and Intrinsic growth rate.

Numerical results showed that without harvesting, there is over-utilization of forage resources, and thus pulsed harvesting was adapted as an off-take strategy to reduce the livestock population.

This harvesting enabled us to obtain a range of 45-79 animals for sustainable grazing and productivity of the enterprise. The first is realized by keeping 35 juveniles and 15 adult livestock and harvesting a cumulative biomass of 109730 kg, and the second is attained when 12 juveniles and 67 adults are maintained with a cumulative harvested biomass of 107916 kg. Furthermore, the study revealed a need to harvest more adult livestock in the first harvesting scenario and more juveniles in the second to address the pastoral community's ecological, economic and social needs.  Besides, there is a need to allow the before animals are taken back to graze again.

We conclude that strategic harvesting coupled with close monitoring of the grassland ecosystem and fallowing of grazed areas should be practiced if we are to sustainably use the grazing resources to minimise overgrazing, hard trampling and, consequently, rangeland degradation.