The importance of nutrient hotspots for grazing ungulates in a Miombo ecosystem, Tanzania

Abstract

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, in the same system. 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 used more often 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.

Fig 1. Mean grass height (in cm) and grass biomass (in g) measured as moving away from the influence of termite mounds (A, C) and grazing lawns (B, D), respectively.

N = 10 for termite mounds, and N = 6 for grazing lawns. Boxplots show the mean (a square within boxes) and ranges from 25% and 75% quartile (boxes), and the tips of the whiskers indicate standard deviations. Boxes with different letters indicate significantly different means as tested by Fisher LSD at P = 0.05.

Fig 2

Mean of (A) average Cynodon dactylon grass growth height, (B) average greenness score ranging from scorched grass (1), pale green (2), green (3) and deep green (4), Nitrogen (C) and Phosphorus (D) contents in pots containing soils from termite mounds, grazing lawns and control sites after 61 days of the experiment. N = 30. Termite mounds (TM), control (CONT) and grazing lawns (GL). Boxplots show the mean (a square within boxes) and ranges from 25% and 75% quartile, and the tips of the whiskers indicate standard deviation. Boxes with different letter(s) are significantly different by Fisher LSD at P = 0.05.

Fig 3

Grass leaf N contents (A) and P contents (B) before setting experimental plots (BEF), in fertilized plots (FERT) vs control (CONT) plots. Herbivore presence using average tracks (C) and dung (D) in fertilized (FERT), clipped (CLIP) and control plots (CONT). Boxplots show the mean (a square within boxes) and ranges from 25% and 75% quartile, and the tips of the whiskers indicate standard deviation. Boxes with different letter(s) are significantly different by Fisher LSD at P = 0.05.

Fig 4

Average (A) grass height and (B) tuft usage between fertilized (FERT), clipped (CLIP) and control plots (CONT). Boxplots show the mean (a square within boxes) and ranges from 25% and 75% quartile, and the tips of the whiskers indicate standard deviation. Boxes with different letter(s) are significantly different by Fisher LSD at P = 0.05.

Fig 5

Grass tuft usage estimates in % when moving away from (A) termite mounds (TM) and (B) grazing lawns (GL). Herbivore presence averaged using tracks (C) and dung (D) between termite mounds (TM) vs control (CONT). Boxplots show the mean (a square within boxes) and ranges from 25% and 75% quartile, and the tips of the whiskers indicate standard deviation. Boxes with different letter(s) are significantly different by Fisher LSD at P = 0.05.

Fig 6

A. Herbivore presence averaged using dung between grazing lawns (white box vs controls (grey box) and herbivore presence using tracks between grazing lawns (white box) vs controls (grey box). Capital letter and small letters has been used to differentiate dung and tracks. B. Herbivore visitation frequency, hartebeest (grey box), reedbuck (black box) and roan antelope (white box) in grazing lawns and termite mounds. Capital letters indicate significant differences between reedbuck visitations, small letters with asterisk compare Roan antelope visitation and small letters compare means of hartebeest visitations. Boxplots show the mean (a square within boxes) and ranges from 25% and 75% quartile, and the tips of the whiskers indicate standard deviation. Boxes with different letter(s) are significantly different by Fisher LSD at P = 0.05.

Fig 7

Herbivore presence using tracks (A) and dung (B) on termite mounds (solid lines) vs controls (dashed lines) and tracks (C) and dung (D) between grazing lawns (solid lines) vs controls (dashed lines) over the year.