Temporal and spatial composition of arboreal insects along the Omaruru River, Namibia

[摘要] English: Insects play a major role in any ecosystem and are also of extreme importance to the well-being of humans. Amongst others, they are pollinators that indirectly determine food security. On a more negative side, they can cause great crop damage and act as vectors for many diseases. It is thus of utmost importance to understand their biology. In this context a study was undertaken to analyze the temporal and spatial composition of arboreal insects along the Omaruru River in central Namibia. This river is one of the ephemeral rivers in Namibia, running along an east-west rainfall and altitudinal gradient.Three typical Namibian tree species (Acacia erioloba, Acacia tortilis and Faidherbia albida) were selected as host species and their canopies sampled over a period of one year. An anaesthetising insecticide, Pyrethroid, was used to fog the tree. Insects dropped onto plastic sheets suspended underneath each sample tree. This material was collected, stored in 70% Ethanol and then sorted and identified into relative taxonomic units (RTUs).The data obtained was used to compare the insect diversity and composition of the three selected host species. Results indicated that there are no statistical differences regarding canopy associated insects between the three tree species. Not only do they have similar numbers of RTUs, but they also share a high percentage (50% and higher) of the same RTUs.The influence of aridity was also investigated by comparing the different sampling stations with each other. The stations lie within different rainfall regimes and show an increase in mean temperature and a decrease in humidity from east to west. The effect of rainfall was eliminated because the trees make use of year-round groundwater to fulfil their requirements.Results indicated that the sampling stations differ from each other with regard to their insect diversity. The further apart the stations are from each other the less similar they are regarding recorded arboreal insect diversity. There was a noticeable decrease in RTU numbers from east to west.Lastly the effect of seasonality was also investigated. Samples were taken on a bi-monthly, basis allowing comparison of the three main seasons (pre-rainy, rainy and dry). Results indicated an increase during and after the rainy season and a decrease during the dry season.Finally, when deciphering the temporal and spatial composition of arboreal insects along the Omaruru River in Namibia, seasonality seem to be the most meaningful determining factor, followed by locality (site), in turn followed by host (tree) species.These results are largely in accordance with the results of other studies and, when linked to climate change, can provide valuable information to decision makers on various levels. An increase in temperature can cause a shift in insect distribution into areas presently not occupied by them, changing ecosystem function of the area (e.g. insect-plant interaction and disease transmission) in the process.

[发布机构] University of the Free State