Effect of Domestic and Wild Herbivores on Vegetation Quality, Cover, Species Turnover and Diversity Response to Fire in an East African Savanna

Abstract

The primary effects of fire on forage attributes such as quality, vegetation community structure (species diversity, turnover and vegetation cover) have been shown to be short lived, diminishing within a few months after fire for forage quality or a few years for cover and composition. There is evidence that herbivores that are attracted to and feed on previously burned areas are able to maintain such pastures in a highly post fire nutritious state for longer thus prolonging effects of fire on the pastures. However, these herbivore effects may differ amongst different herbivore guilds thus there is need for experimental designs that are able to tease out such differences. In this study, therefore, we investigated the separate and interactive effects of cattle and wild herbivores on post-fire herbaceous vegetation nutrient quality, cover and species composition, and how these effects change with time after burning. The study was conducted at the Kenya Long-term Exclosure Experiment (KLEE) site that was established in the year 1995 and is located at Mpala Research Center in Laikipia County, Kenya. The KLEE plots were ideal for this study due to various reasons: First, the plots are designed in a way that the effect of different herbivore species and or groups can be clearly determined; secondly, the study plots are inhabited by a diverse group of herbivores in a way that simulates various savanna ecosystems in Africa; and lastly, the site also boasts of diverse vegetations which are a forage resource to these herbivores. The KLEE setup consisted of six herbivore treatment plots that: 1) exclude all large herbivores(O); 2) allow only cattle grazing (C); 3) allow herbivory by wild mesoherbivores (W); 4) allow combined herbivory by cattle and wild mesoherbivores (WC); 5) allow wild mesoherbivores and megaherbivores (elephants and giraffes) (MW); and 6) allow access to all herbivores (MWC). Each herbivore treatment plot was established in a 4-ha plot and the plots replicated three times in a randomized block design creating a North, Central and South Blocks. Within each plot, controlled burning was applied to a 30m by 30m sub-plot in 2013 and a separate similar-sized sub-plot in February 2018. A similar-sized sub-plot in the unburned matrix, located at least 50 m away from the burned sub-plot was delineated to serve as a control sub-plot. Leaf samples of two main grasses, Brachiaria lacnantha and Themeda triandra were obtained from each of the sub-plots burned in 2018 and each corresponding control (unburned) subplot at one, four, seven and fifteen months after burning giving a total of 288 grass samples (72 samples per sampling season). The samples were separately analyzed for forage nutritional quality (crude protein [CP], crude fiber [CF], in vitro dry matter digestibility [IVDMD],

Potassium [K], Phosphorus [P] and Sodium [Na] contents). These two grass species were selected as they are among the most common grass species at the study site. Vegetation cover, species turnover and diversity were also estimated from pin hit data obtained from vegetation surveys carried out in the sub-plots burned in 2013 and corresponding controls every July since 2013 to 2017. Linear mixed models and GLMM were used to test for the effects of herbivore treatment, fire, grass species and time after burning and their interactions on the measured vegetation attribute. Herbivory did not have significant effect on how forage quality and herbaceous vegetation species diversity responded to controlled burning regardless of the amount of time after fire and the herbivore guild grazing on the pastures. Herbivory however caused a 20%-29% reduction in vegetation cover in the burns of plots O, W, MW, WC and MWC burns compared to controls. Species turnover rate in W, MW, WC, and MWC burns were higher than non-burn controls by 12%-20%. These changes in vegetation cover and species turnover rates however were not demonstrated in plots C and MWC. Therefore, both wild and domestic herbivores have similar effect on post-fire forage quality and species diversity. However, wild herbivores, separately or while foraging with cattle improve species turnover (except when megaherbivores forages with both cattle and the mesoherbivores) while reducing post-fire vegetation cover. The current land uses that simulates the experimental plots thus still remains feasible with regards to vegetation quantity and species diversity. However, more research should be conducted to establish ways of preserving post-fire vegetation cover especially under herbivory by wild herbivores.