Effects of Plant Species Diversity and Biomass on Grazing Patch Selection by Semi Free-Ranging Cattle
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Tanzania Journal of Science 47(1): 282-295, 2021 ISSN 0856-1761, e-ISSN 2507-7961 © College of Natural and Applied Sciences, University of Dar es Salaam, 2021 Effects of Plant Species Diversity and Biomass on Grazing Patch Selection by Semi Free-Ranging Cattle Samora M. Andrew Department of Ecosystems and Conservation, College of Forestry, Wildlife and Tourism, Sokoine University of Agriculture, P. O. Box 3010, Morogoro, Tanzania. E-mail address: [email protected] Received 5 Aug 2020, Revised 2 Jan 2021, Accepted 4 Jan 2021, Published Feb 2021 Abstract Studies examining forage selection by large herbivores have generally focused on single herbivore-plant relationships and few studies link plant species diversity with large herbivore feeding preferences, particularly in species-rich ecosystems. A study was therefore conducted to examine how herbaceous plant biomass, species richness, diversity and evenness, together with elevation and distances to houses, agricultural fields and cattle holding-pen areas (kraals), predict cattle grazing intensity at small (0.5 x 0.5 m) and large (20 x 50 m) patches by semi free-ranging indigenous cattle in Kilombero Valley Floodplain wetland Tanzania. Results showed that cattle grazed patches with high species richness and intermediate level of biomass at a small spatial scale, suggesting that selection of small patches is a trade-off between food quality and quantity. On a larger scale, cattle grazed patches with a higher abundance of palatable plant species with high biomass, but with less plant species diversity. Moreover, grazing intensity in large patches was positively related to elevation and distance from agricultural fields. Overall, this study shows that considering both plant community properties and physiographic variables in forage selection studies facilitates a better understanding of grazing ecology of large domestic herbivores in species-rich ecosystems. Thus, maintenance of plant species richness and biomass in wetland rangelands is important to improve domestic herbivore production and conservation of biodiversity. Keywords: Domestic herbivore; Forage selection; plant-animal interaction; Rangelands; Species richness. Introduction for herbivores to meet their optimal nutritional Large herbivores utilize food resources requirements, diet selection must focus on a unevenly by feeding more intensively in some variety of plant species with different areas than others in heterogeneous nutritional quality and quantity, and diet environments (Bailey 1995, Bailey et al. 1998, choices therefore operate at different spatial Wang et al. 2010a, Milligan and Koricheva and temporal scales. Since a single plant 2013). Several theories have been proposed to species will not meet the nutritional demands describe foraging behaviour and associated of animals, large herbivores usually switch grazing patterns among herbivores. According between alternative plant species to diversify to optimal foraging theory, diet selection is a their diet during foraging (Simpson et al. 2004, trade-off between benefits of ingesting a Wiggins et al. 2006). According to the nutrient particular food resource and the cost of balance hypothesis, dietary diversity allows obtaining it (Stephens and Krebs 1986). Thus, individual herbivores to balance the intake of 282 http://journals.udsm.ac.tz/index.php/tjs www.ajol.info/index.php/tjs/ Tanz. J. Sci. Vol. 47(1) 2021 different nutrients (Westoby 1978), which grazing patterns at large scales. Overall, this consequently improves their performance and suggests that the two sets of factors (plant growth (Simpson et al. 2004, Unsicker et al. community properties and physiographic 2008). Switching among plant species during variables) potentially explain the spatial foraging may also buffer the animal against variations in grazing intensities, and that large accumulation of digestion-inhibiting herbivores are more likely to visit species-rich compounds or toxic secondary metabolites that patches and sites close to their congregation could be present in the consumed foliage points more frequently than species poor (Freeland and Janzen 1974, Milligan and patches and distant sites (Bailey 1995, Bailey Koricheva 2013). et al. 1998, Tarhouni et al. 2010). For small invertebrate herbivores (e.g. Studies on forage selection by large arthropods), changes in plant community herbivores have generally focused on properties, particularly reduced species relationships of single species of herbivores diversity, may negatively affect their and plants and studies linking plant species abundance, diversity and performance diversity with large herbivore feeding (Scherber et al. 2006, Unsicker et al. 2008). preferences are only starting to emerge. Two Okullo et al. (2013) found that diversity of notable exceptions are the study by Milligan small savannah mammals increased with and Koricheva (2013), who found that herbaceous species evenness and diversity, and browsing moose (Alces alces) preferred that their biomass increased when few plant species-rich sites, and Wang et al. (2010b), species dominated the herbaceous species who documented an asymptotic relationship assemblage. Moreover, the maintenance of between sheep (Ovis aries) food intake and plant species richness is critical to improve plant species richness. In this study, the nutritional status, performance and overall determinants of foraging decisions by bulk production of large herbivores (Gordon and feeding semi-free ranging cattle were Prins 2008, Wang et al. 2010b, Milligan and investigated in the Kilombero wetlands in Koricheva 2013). This suggests that plant Tanzania. Specifically, plant community community properties affecting forage quality properties (biomass, species richness, evenness and quantity, such as plant biomass, species and diversity) and physiographic variables richness, diversity and evenness, may (elevation, distances to houses, agricultural determine feeding site selection by large fields and kraals) were related to cattle grazing herbivores. intensity at large (20 x 50 m) and small (0.5 x Besides plant community properties, 0.5 m) feeding sites. It was predicted that: (a) physiographic factors, such as slope and plant community properties would be distance to congregation areas, may also important for grazing intensity at a small influence feeding site selection by large spatial scale, while physiographic variables herbivores (Bailey et al. 1998). In general, explain variations in grazing intensities at large domestic herbivores tend to avoid steep slopes patch sizes; consistent with Bailey et al. and feeding sites far away from congregation (1996); (b) since plant species richness offers areas, such as camp sites and water sources an opportunity for large herbivores to optimize (Bailey et al. 1996, Tarhouni et al. 2010). their nutrient balance (Wang et al. 2010b), Moreover, the synthesis of mechanisms behind grazing intensity at small spatial scale would be the grazing distribution patterns of large explained best by species richness, evenness herbivores, Bailey et al. (1996) pointed out that and diversity, and (c) since biomass is plant community properties associated with significant important to bulk feeding cattle forage quality and quantity is the main (Belovsky 1997), it would be an important determinant of grazing patterns at small spatial determinant of spatial variations in grazing scales, while physiographic factors predict intensities at the small spatial scale. 283 Andrew - Effects of plant species diversity and biomass on grazing patch selection … Materials and methods short herbaceous vegetations, were the focus of Location of the study area this study because they form a substantial The study area, Kilombero Valley portion of the wetland. Agro-pastoralists have Floodplain wetland, is situated in the Morogoro grazed their cattle in these grasslands at least Region in Ulanga and Kilombero Districts, in over the last two decades. The marginal the south-central Tanzania and found between grasslands contain a large number of plant longitude 8° 32′ 0′′ S and latitude 36° 29′ 0′′ E species which is 33% of all species reported in (Figure 1). The Kilombero wetland is the the wetland including economically important largest lowland freshwater wetland in East fodder species (Andrew et al. 2012). Africa rich in biodiversity that made it be declared a Ramsar site in 2002 (Ramsar 2020). Vegetation sampling and physiographic The wetland is also ecologically important as it variables adjoins the Great Selous Ecosystem, a World Following a preliminary survey conducted Heritage Site. It covers an area of 7,967 km2 at in 2009/2010, 60 plots of 20 x 50 m each from an elevation of 210-400 meters (Ramsar 2020). different areas of the marginal grasslands were The area experiences sub-humid tropical randomly selected to assess the livestock climate with a mean annual temperature of 26 grazing pattern in relation to plant community C and a mean annual rainfall of 1200-1400 properties and physiographic variables. Using a mm (Kangalawe and Liwenga 2005). At table of random numbers and compass bearings Kilombero, light and sporadic rainfall occurs (1–360°), twenty 0.5 x 0.5 m quadrats were usually between December and February, while established in each plot. In each quadrat, heavy and regular rainfall occurs between species cover and identities were recorded March and June. The Kilombero wetland using the point intercept method (Bråthen and supports diverse livelihood activities, including Hagberg 2004). A point