Understanding nutrient hotspots for grazing ungulates in a Miombo ecosystem, Tanzania

<b>Abstract</b>While movement patterns of grazing ungulates are strongly dependent on forage quality their use of nutrient hotspots such as termite mounds or grazing lawns has rarely been quantified, especially in savanna ecosystems where soil-nutrient quality is low. Additionally, few experiments have been conducted to determine the role of termite mound- and grazing lawn-derived soils in improving forage quality in the field. We studied wild ungulate grazing activities around ten termite mounds, six grazing lawns and their respective control sites in a Miombo system of Issa Valley, western Tanzania. We used indirect observations (i.e., dung, tracks) to identify seasonal and spatial variations in habitat use of various wild mammalian grazers. Grazer visitation rates were nine and three times higher on termite mounds and grazing lawns, respectively, compared to control sites. During the rainy season, termite mounds were more frequently used than grazing lawns while the latter were more frequently used during the dry season. In an additional pot experiment with soils derived from different areas we found that Cynodon dactylon in termite mound-derived soils had twice as high Nitrogen and Phosphorous contents and biomass compared to grasses planted in grazing lawn soils and control site soils. We highlight that both termite mounds and grazing lawns play a significant role in influencing seasonal nutrient dynamics, forage nutrient quality, habitat selectivity, and, hence, grazing activities and movement patterns of wild ungulate grazers in savannas. We conclude that termite mounds and grazing lawns are important for habitat heterogeneity in otherwise nutrient–poor savanna systems<b><br></b><br><b>Methods</b>We selected ten active termite mounds, covered with grass, that were not close to water bodies (i.e., more than 100 m away from water source) or from big trees that were on average (±SE) more than 9.0 ± 0.3 m tall and had a canopy radius of 5.5 ± 0.2 m (27) to avoid potential confounding factors. We further selected ten respective control sites 100m away from the mounds , away from big trees. We established transects radiating away from each termite mound centre in all four cardinal directions (N, S, E,W), placed a 1 x 1 m2 quadrat at 2 m, 12 m and 22 m distance away from the base of the mound. In each quadrat, we surveyed grass communities, i.e., species and their respective basal cover, on and around the mound. Additionally, in each quadrat, we measured standing biomass by harvesting all grasses to ground level and recording their dry weight between May 2016 and October 2017. Grass biomass was measured in February, May and September. In each quadrat, there were 4 sub-quadrats of 50 x 50 cm2. At each sampling event, a new location of the four subplots was used. We followed the same procedure for six grazing lawns at distances of 20 m, 40 m and 60 m away from the grazing lawn centre and their respective control sites were at a distance of 100 m away from the grazing lawn edge. Grass identification in all study plots was done with the assistance of a botanist and published literature. We measured vertical grass height at four different points within each sampling quadrat, thereafter averaged the height measurement for all sampling plots. We assessed grass tuft use and estimated use as a percentage in all quadrats. Grass tuft usage was estimated by placing a quadrat of 1 m2, within which we counted all tufts and visually estimated (in %) the number of grass tufts that were eaten partially or fully by the mammalian herbivore. Grass preference indices were assessed within each quadrat as 0 (no grass available), 1 (no grazing, i.e., none of the grass tufts showed bite marks), 2 (moderate grazing/very light grazing, i.e., partially eaten), 3 (heavy grazing) and 4 (intense grazing). Additionally, we visually assessed grass greenness as dry grass (a), pale green (b), green (c) and (d)deep green. Grass assessments were done in February, May and September. However, for termite mounds, September data were not included in the analysis due to the effect of fire. In grazing lawns, which were moister than termite mounds in September, only edges were affected by fire and, hence, February, May and September data were included in the analysis.Additionally, to tease apart the factors that might contribute to the attractiveness of nutrient hotspots, we created experimental plots in the field that were at least 100 m away from shade or water bodies to avoid confounding factors. Ten plots of 5 x 5 m2 each were subjected to three treatments: i) fertilized with NPK fertilizer (ETG Input NPK 17-17-17 400 g/m2) irrigated with 10 l/m2 of water once; ii) cut to ground level and iii) a control treatment (no treatment), summing up to 40 experimental plots in total. Fertilizer was applied and irrigated once (as in treatment iii) to avoid pellets being eaten by animals. In addition, we sampled the most common grass in the area, <i>Hyparrhenia hirta</i>, before and after the experiment to assess its nutrient status (Nitrogen (N) and Phosphorus (P) content). Further, we visually assessed the extent of grazing (as % of grass tufts eaten, see above) on each plot. The presence of different wildlife species was determined by recording cumulative dung depositions (graded as 1-fresh, 2-recent, 3-old) and tracks, i.e., footprints, whereby a series of tracks for an animal that was moving in one direction was considered as one event. After recording evidence, we removed signs to avoid re-counting. From five termite mounds, five grazing lawns and five control sites, we collected composite soil samples, mixed the soil of each category separately, distributed each composite sample across 10 different pots and planted a common grass species, <i>Cynodon dactylon</i>, in the soils derived from those three categories, making a total of 30 pots.<b>Ethical statement</b>This study complied with Tanzanian Wildlife Research Institute (TAWIRI) ethical regulations permission was granted from TAWIRI and the Tanzanian Commission for Science and Technology (COSTECH). <b><br></b>