Parthenium hysterophorus (Asteraceae), of South American origin, is considered to be one of the worldÕs most serious invasive plants, invading Australia, Asia and Africa. As part of an international collaborative project, this study attempted to improve the understanding of the geographical distribution of P. hysterophorus in eastern and southern Africa. The climate modelling program CLIMEX was used to assist in the selection of survey localities. Roadside surveys of the distribution of the weed were conducted in Botswana, Ethiopia, South Africa, Swaziland and Uganda. Prior to these surveys, only limited P. hysterophorus locality records existed; substantially more records were obtained from surveys. Most infestations were high density (>3 plants m )2 ). Distribution records were used to validate the CLIMEX model, which proved a useful tool. This study increased current understanding of the distribution of P. hysterophorus and developed a baseline from which to monitor future spread and abundance of P. hysterophorus. Additional surveys are required in other countries in Africa which are predicted by CLIMEX to be at risk. This will enhance integrated management decisions for the control of a weed which has implications for food security and human health.

1. Global change, such as exotic invasions, dramatically affects ecosystem functioning. However, the mechanisms behind the impacts are often unclear and despite extensive experimental work, we know little about the importance of biodiversity loss as a component of global change effects in real-world ecosystems. 2. We disentangled several mechanisms by which an exotic invader affected ecosystem functioning in East African drylands in Kenya and Ethiopia. We used structural equation modelling to separate direct effects of a woody invader Prosopis juliflora on a range of ecosystem functions from indirect effects mediated through changes in biodiversity (plant species richness) and ecosystem functioning (herbaceous biomass). We analysed effects on ecosystem functions linked to soil biogeochemical cycling and transfer of energy between trophic levels. 3. We found that the mean size of individual indirect effects mediated by biodiversity and herbaceous biomass was about twice as large as the mean size of individual direct effects of Prosopis on ecosystem functions, showing that indirect effects are an important component of the invader's overall environmental impacts. Changes in both herbaceous biomass and biodiversity were approximately equally important as drivers of indirect effects, indicating that we need to expand our view of indirect effects to consider a wider range of mechanisms. Simple univariate models failed to capture some Prosopis invader effects because positive direct effects on soil stability and predatory invertebrates were counteracted by negative indirect effects on biodiversity or biomass loss. The majority of effects were similar in both study systems. Suggestions that woody invaders are able to | 2661 Journal of Ecology LINDERS Et aL.

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An investigation into the impact of Parthenium hysterophorus infestation was conducted in 2007 in the north-eastern grazing lands of Ethiopia. Data on the above-ground and seedbank species diversity were collected from five areas, each having sites with low, medium, or high levels of weed infestation. A total of 72 species was found in all areas. They were categorized into grass species (23), other species (48), or P. hysterophorus for ease of interpretation. A regression analysis showed a highly significant, but negative, relationship between the aboveground species diversity and evenness with P. hysterophorus abundance. The mean cover abundance for the three infestation levels was 33.4% for P. hysterophorus, 41.0% for the grass species, and 26.5% for the other species.The most dominant grass species under all infestation levels were Cynodon dactylon, Urochloa panicoides, and Chloris gayana, while Andropogon abyssinicus and Eragrostis spp. were dominant under the low and medium infestation levels, respectively, and Hyparrhenia hirta was dominant under the low infestation level. Among the other species, Solanum nigrum was the most dominant under the low infestation level and Datura stramonium and Xanthium spp. were the most dominant under the medium and high infestation levels, respectively. The above-ground dry biomass of P. hysterophorus increased between the low and high infestation levels, while that of the grass or other species reduced in the high, as compared to the low, infestation level. Although the grass species density decreased significantly with successive increases in the P. hysterophorus infestation level, no such trend could be seen for the other species. Within the soil seed bank, the viable seed density for the grass species, other species, and P. hysterophorus were 25.7, 5.8, and 68.5%, respectively. Similarly, the soil seed bank under the low-, medium-, and high-infestation sites was dominated by P. hysterophorus, which contributed 25.1, 65.4, and 87.4% of the viable seed bank, respectively. Although the overall similarity between the above-ground vegetation composition and the soil seed bank was low, it was similar at the low-infested site. Thus, the invasion by P. hysterophorus was found to critically endanger the biodiversity of the grazing lands, particularly for the different grass and forbs species in the area. These changes might adversely affect not only future agriculture, but also food security, unless appropriate practises are developed and implemented for P. hysterophorus management.

Vegetation and environment have been analyzed along an altitudinal gradient in Harena Forest, Bale Mountains National Park, southeastern Ethiopia. Vegetation data include numbers of each tree and shrub species and cover-abundance values of each herbaceous species. Environmental data comprise edaphic factors, altitude and topography. The two vegetation layers data were analysed separately.Probabilistic similarity coefficients were computed between the releves, and these values were used in subsequent computations for classification and ordination. Two sets of stratocoena, comprising 6 types each, derived on the basis of separate analyses of tree-shrub and herb layers of the forest were recognised.