The distribution, abundance and ecological impacts of invasive plant species at Ol-Donyo Sabuk National Park, Kenya

Abstract

Invasive species are a major conservation and management concerns in natural ecosystems and pose a significant threat to many of Africa's conservation areas. The aim of this study was to asses the impacts of invasive plants species on the biodiversity of Ol-Donyo Sabuk National Park. The objectives were to determine the types and distribution of vegetation at the National Park, assess the types, distribution and abundance of invasive plant species, determine the birds and mammal species associated with invasive plant species and examine the factors that were related to the dispersal of invasive plants species. The study area was stratified by analysing an advanced spaceborne thermal emission and reflection radiometer (ASTER) imagery and line transect method was used to describe the vegetation in the different strata. Invasive species impact on biodiversity was quantified through random quadrant sampling method while factors related to the dispersal and spread of invasive plant species were determined through point-count and line transects methods. Data was analysed through calculation of density for the different species and Shannon-wiener diversity (HI) and evenness (JI) indices for each stratum. Student t test was used to compare diversity differences between the invaded and un-invaded sites. Chi-square (X2) test was used to test for trophic differences in mammals and birds associated with disturbed and invaded areas between the wet and dry seasons. Analysis of variance (ANOVA) tests to determine species associations between the different sampling strata within the study and surrounding areas. Results from imagery analysis revealed seven land-cover types with closed canopy forests being the largest and dams the least. Lantana camara L. was the most abundant invader species and invaded areas had lower species diversity but higher plant density. There was significant difference in species diversity between invaded and un-invaded sites (t =2.41, P< 0.05). In the surrounding areas, invasive species were equally distributed (F=0.0065, P<0.05). There was significant difference in the trophic status for birds associated with disturbed and invaded areas between the sampling seasons with higher number of insectivore/gramnivores in dry season and higher number of nectivores in the wet season. (X2= 83.338, P<0.001, d.f =6). Diversity and density also differed with type of disturbances being highest in places with edge effect. Mammals didn't differ significantly between the dry and wet seasons (X2=16.656, P < 0.005, d.f=S). Edge effects were strongly associated with occurrence of invasive species. The study showed that invasive species had negative impact on native species diversity.