Plant species composition and diversity depending on piospheres and seasonality in the southern rangelands of Kenya

Abstract

Introduction: Scarcity of water in the rangelands of Kenya has led to the introduction of piospheres. Previous research has however produced contrasting results on the effects of piospheres and seasonality on ShannonWiener's diversity index, and therefore, this information is still deficient. In this study, the impact of these piospheres on plant species composition, diversity, and richness was assessed. Methods: Vegetation sampling was done during both the long rains (April) and the dry season (August). Three piosphere types (dam, trough, and a seasonal river) were studied using 0.25 m 2 quadrats to sample vegetation at intervals of 20 m along 100-m transects. Four 100-m transects were used per piosphere (north, east, south, and west directions). Shannon-Wiener's diversity index was used to determine species richness and composition. Twoway ANOVA was used to determine if piospheric distance had an effect on species diversity, richness, and evenness using GenStat 15th edition. Results: A total of 22 grasses and 29 forbs were recorded in the study area. The most abundant grasses near the piospheres were Eragrostis tenuifolia (12.9%) and Cynodon dactylon (10.6%). E. tenuifolia is an increaser species and was therefore most abundant in areas of severe grazing while C. dactylon has been known to be tolerant to grazing. The most abundant forbs were Crotalaria brevidens (37.5%). The Shannon-Wiener diversity index significantly increased (F = 25.07, P = 0.001) with distance from the three piospheres owing to the high grazing intensity near the piospheres and was significantly different between piospheres, being higher (F = 10.05, P = 0.001) at 20 m from the river (1.2 ± 0.1) compared to a similar distance from the dam (0.9 ± 0.1) and the trough (0.8 ± 0.2). This was probably because the trough was smaller in size compared to the other piospheres, thereby concentrating more grazing animals per unit area and causing a greater impact on plant species. Conclusions: The study demonstrated that species diversity, richness, and evenness were low near the piospheres due to heavy grazing. Similarly, the lowest species diversity, richness, and evenness were recorded near the trough compared to the dam and the river. This study therefore recommends that range reseeding and rest period be done to rehabilitate degraded areas and facilitate plant regeneration. Larger piospheres should also be used to minimize animal impact per unit area.

The effects of herbivore aggregations at water sources on savanna plants differ across soil and climate gradients

“…2005, Jawuoro et al. 2017), likely because of its ability to produce both stolons and rhizomes (Dong and de Kroon 1994). We postulate that these opposite responses can be attributed to variation in dominant plant reproductive and growth traits whose trade‐offs become apparent across a gradient ranging from high competition (high abiotic resources and low herbivory) to elevated stress (low abiotic resources and high herbivory).…”

“…Meanwhile, we found the opposite pattern on arid sandy soils; Cynodon dactylon, a globally important grazing species, was the most common species on 60% of water source transects. C. dactylon is able to propagate in heavily grazed areas (van der Westhuizen et al 2005, Jawuoro et al 2017, likely because of its ability to produce both stolons and rhizomes (Dong and de Kroon 1994). We postulate that these opposite responses can be attributed to variation in dominant plant reproductive and growth traits whose trade-offs become apparent across a gradient ranging from high competition (high abiotic resources and low herbivory) to elevated stress (low abiotic resources and high herbivory).…”

“…2003) and diversity (Jawuoro et al. 2017) near water, and others finding mixed or no significant effects (Stumpp et al. 2005, Cheng et al.…”

“…However, plant responses as a function of proximity to water sources vary based on environmental variables such as habitat, year (Wesuls et al 2012), distance to alternative water supply, soil, and prior rainfall (Thrash and Derry 1999). Plant diversity patterns are more variable, with some studies reporting a steep decline in species richness (Landsberg et al 2003) and diversity (Jawuoro et al 2017) near water, and others finding mixed or no significant effects (Stumpp et al 2005, Cheng et al 2011. Underlying site differences, such as mean annual precipitation, soil type, and grazing history could explain why some plant responses are observed in one location, but not another (Stumpp et al 2005, Wesuls et al 2012).…”

Heavy Grazing Altered the Biodiversity–Productivity Relationship of Alpine Grasslands in Lhasa River Valley, Tibet

“…However, with increasing grazing intensity, vegetation coverage decreased dramatically but had no effects on species richness, Shannon-Wiener index, and Pielou evenness index in Gangcha County, Qinghai Province (Chu et al, 2019). In the southern rangelands of Kenya, heavy grazing decreased species diversity, richness, and evenness (Jawuoro et al, 2017). Dingaan et al (2016) found no dramatic difference in species richness between L.G.…”