John K. Mworia

Resilience of soils and vegetation subjected to different grazing intensities in a semi‐arid rangeland of Kenya

Abstract The resilience of rangeland soils and vegetation to different levels of grazing is still poorly understood. A study was conducted to determine the recovery of a rangeland grazed at different intensities and allowed a two‐year rest period. The following treatments were applied to 0.5 hectare plots: 0, 4, 8 and 16 heifers per hectare, hereafter referred to as CL, X, 2X and 4X respectively. At the end of the grazing period, the highest stocked treatments (2X and 4X) had lower herbage biomass, higher soil bulk density, lower soil moisture and lower herbaceous cover than the lower stocked treatments (CL and X). Drought in the rest period caused an increase in bulk density and decline in soil moisture in all the treatments. Even after the two‐year rest period, the more heavily grazed treatments had higher bulk density and lower soil moisture than the more lightly grazed treatments. Similarly, the herbaceous biomass in the 2X and 4X treatments did not recover after the two‐year rest period and was lower...

Influence of cultivation, settlements and water sources on wildlife distribution and habitat selection in south-east Kajiado, Kenya

In Kenya, lands surrounding wildlife protected areas (PAs), referred to as dispersal areas, have undergone widespread land use changes, but these have been little studied. This study investigated impacts of different land use types on wildlife distribution and composition. Transect data from stratified random sampling based on land use and vegetation type were analysed using correlation and canonical correspondence analysis (CCA). Household density and cultivation intensity were negatively correlated with grass cover and were greatest on small-scale farms and lowest in a dedicated PA. Three patterns of wildlife distribution were identified. Wildlife density in communal grazing and the PA was significantly higher than on other land use types. While most wildlife used pastoral ranches in the wet season, larger herbivores moved to the PA during the dry season. Wildlife density along the grass cover gradient, which was a disturbance gradient, was dome shaped, indicating that wildlife tolerated moderate levels of disturbance. The primary factors influencing wildlife distribution were vegetation type and proximity to water sources in the dry and wet seasons, respectively. The apparent anomaly in the wet season is attributed to wildlife moving from Chyulu, which lacked seasonal ponds, to the lowland Masaai ranches, which had plenty of ponds. In both seasons, cattle density was the most important secondary factor. To mitigate declining wildlife trends, management should ensure a heterogeneity of vegetation types is maintained and wildlife retain access to seasonal water sources.

Patterns of seed dispersal and establishment of the invader Prosopis juliflora in the upper floodplain of Tana River, Kenya

Invasive species in Africa have important impacts on food security and biodiversity conservation. African floodplains in arid areas are critical wildlife habitats in addition to crop production and dry season livestock grazing. The study aimed to understand the patterns of spread of the invader Prosopis juliflora in a typical African floodplain characterised by both multiple dispersers and habitats. Data was collected on faecal seed density, establishment and vegetation composition, and subjected to non-parametric tests and regression analysis. The results showed that both livestock and wildlife species played a critical role in dispersal of Prosopis juliflora, especially yellow baboons (Papio cynocephalus), bush pigs (Potamochoerus larvatus), donkeys and goats. Habitat preference of wildlife dispersers and livestock herding patterns influenced the spatial pattern of invader seed influx. Establishment of Prosopis juliflora was enhanced by ecological disturbance such as in rested crop fields and habitats outside conservation areas as compared to those inside. Establishment was also higher inside the floodplain than outside probably due to higher seasonal soil resources. Lastly, indigenous woody species diversity declined significantly as the density of Prosopis juliflora increased. We conclude that patterns of spread of Prosopis juliflora were related to ecological disturbances, type of disperser and flooplain effect.

Invasive Plant Species and Biomass Production in Savannas

Savannas are the second largest biome accounting for c. 30% of terrestrial production. Tropical savannas are distributed largely in Africa, Australia and South America occurring between tropical forests and deserts. It is the coexistence of trees and grasses that make savannas unique. The structure of savannas or the ratio of trees to grasses which has important implications on ecosystem productivity is determined by resource availability (rainfall and soil nutrients) and disturbances (fire and herbivory) also referred to as ‘drivers’. Resources influence the distribution and productivity of savanna vegetation while fire can alter vegetation structure via effects on the woody layer. Herbivory influences savannas structure and composition through its effects on nutrient cycling, seed dispersal and physical defoliation effects and may lead to expansion of the shrub layer. While ecologists agree the four drivers determine tree-grass balance the exact mechanisms are still debated with one school of thought emphasizing the importance of resources as ‘primary determinants’ in what are referred to as ‘competition models’ which basically invoke the classic niche separation mechanisms in resource acquisition. The other school of thought referred to as ‘demographic bottleneck models’ emphasizes the role of disturbances as the primary determinants through their effects on life history stages of trees. It’s been shown however that at low levels of mean annual rainfall, precipitation governs the cover of trees and above a critical value disturbances prevent trees from forming a closed canopy. Invasive species are considered to be non-native species that have been introduced outside their normal range and are expanding in range causing ecological and economic harm and can drastically alter the structure and composition of savannas. Most non-native species introduced in savannas were for well intended commercial and ecological purposes such as pasture and fodder improvement or rehabilitation of degraded areas. Even though patterns of invasion can not be easily generalized, a trend is that African C4 grasses such as Melinis minutiflora and Andropogon gayanus make up the most obnoxious invaders in the South American and Australian savannas while in contrast neotropical trees and shrubs are among the most successful invaders of African and Australian savannas such as Prosopis spp and Lantana camara. Ecologists have persistently attempted to answer the question ‘what makes a community susceptible to invasion’? Plant characteristics of the invader is an important factor, plants introduced in savannas for improvement of pasture/fodder are generally selected for aggressiveness/competitiveness compared to native species. Selected shrubs for example tend to have fast growth, easy to propagate and often N fixers while grasses

Impact of the invader Ipomoea hildebrandtii on grass biomass, nitrogen mineralisation and determinants of its seedling establishment in Kajiado, Kenya

The invasive coloniser Ipomoea hildebrandtii aggravates the problem of inadequate grass forage in Kajiado district, Kenya. To test its impact on grass biomass, nitrogen (N) mineralisation and seedling establishment, grazing and coloniser density were controlled using experimental exclosures and weeding treatments separately in a factorial design. Grass biomass increased 47% in weeding treatments and 117% with protection from grazing. Ipomoea hildebrandtii removal also led to decline in soil moisture at at a depth of 5 cm and an increase at 30 cm, and lower soil compaction. Grazing lowered soil moisture and increased soil compaction. Mineralisation of N was highest under the dominant grass Chloris roxburghiana followed by I. hildebrandtii and bare ground. Weeding increased N mineralisation whereas grazing lowered it. Multiple regression showed that I. hildebrandtii seedling establishment was significantly higher with low grass biomass, high soil moisture at a depth of 30 cm and higher soil N nitrification. Thus weeding, which caused an increase in the latter two factors, led to increased seedling establishment. Grazing, which lowered soil moisture and mineralisation, led to lower seedling establishment despite reduced competition from lowered grass biomass. This shows invasibility by I. hildebrandtii in rangelands increases when lowered competition is accompanied by an increase in soil resources such as moisture and nitrogen, thus supporting the resource fluctuation theory.