Department of Environmental Studies
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Item Comparative performance of invasive alien Eichhornia crassipes and native Ludwigia stolonifera under non-limiting nutrient conditions in Lake Naivasha, Kenya(Hydrobiologia, 2010-09) Njambuya, Josephine; Triest, LudwigThe ability of Ludwigia stolonifera to thrive in the presence of Eichhornia crassipes was investigated in Lake Naivasha, Kenya. L. stolonifera (indigenous) and E. crassipes (invasive alien) were grown in outdoor experimental boxes in monocultures and mixtures under non-limiting nutrient conditions. An additive series design with eight combinations of planting densities and four replicates was used. Competitive interactions between the two species were determined by assessing the final total biomass and above–below-ground biomass allocation after 98 days of growth. Biomass accumulation and allocation were significantly affected by competition in relation to species, with L. stolonifera accumulating more biomass than E. crassipes. ANOVA analysis indicated that there was no significant difference in Relative Growth Rate (RGR) and root/shoot ratio between monocultures and mixtures with E. crassipes. However, significant differences in RGRs were observed between monocultures and mixtures of L. stolonifera. Multiple regressions on species RGRs revealed that increasing initial biomass of a conspecific neighbour resulted to a greater reduction in species RGR in relation to increasing initial biomass of a hetero-specific neighbour. Thus, a stronger intrathan inter-specific competition coupled with the significantly higher RGR of L. stolonifera relative to that of E. crassipes enabled L. stolonifera to outperform E. crassipesItem Competitive abilities of invasive Lagarosiphon major and native Ceratophyllum demersum in monocultures and mixed cultures in relation to experimental sediment dredging(Aquatic Botany, 2011-06) Steirs, Iris; Njambuya, Josephine; Triest, LudwigCompetitive abilities of Lagarosiphon major (Ridley) Moss (invasive in Belgium) and native Ceratophyllum demersum L. were assessed experimentally in relation to sediment dredging. We mimicked these conditions by taking undisturbed sediment (‘before dredging’ treatment) and by using restored sediment where the uppermost nutrient rich top layer was removed (‘after dredging’ treatment). Both the species were allowed to grow for seven weeks in monocultures and mixed cultures at different planting densities. Overall, invasive L. major performed better than native C. demersum independent of the characteristics of the growth environment. L. major achieved a higher relative growth rate (RGR) in both treatments based on total length (0.17–0.21 week−1) and weight (0.10–0.19 week−1) compared to C. demersum (length: 0.04–0.07 week−1; weight: 0.03–0.17 week−1). The better performance of L. major was due to a high plasticity under stressful conditions of low free CO2 and high pH. Intraspecific competition and niche partitioning were observed between the two species indicating that species coexistence is favoured instead of competitive exclusion. L. major performed better in the ‘after dredging’ treatment. Consequently, we deduce that sediment dredging will not lead to a decline of the invasive L. majorItem Competition between Lemna minuta and Lemna minor at different nutrient concentrations(Plant Biology and Nature Management (APNA) Elsevier, 2011-02) Njambuya, Josephine; Stiers, Iris; Triest, LudwigWe investigated the differential responses of invasive alien Lemna minuta and native Lemna minor to nutrient loading as well as the mechanism of competition between the species. The role of nutrients, species identity, species influence in determining the outcome of competition between the species was estimated using the Relative Growth Rate Difference (RGRD) model. The two species differed in their response to nutrient loading. The native L. minor responded indifferently to nutrient loading. The species Relative Growth Rate (RGR) was 0.10 d−1, 0.11 d−1 and 0.09 d−1 in high,medium and low nutrients, respectively. On the other hand, the invasive L. minuta responded opportunistically to high nutrient availability and had an RGR of 0.13 d−1, 0.10 d−1 and 0.08 d−1 in high, medium and low nutrients, respectively. As a result, the invasive species was dominant in high nutrient availability but lost to the native species at low nutrient availability. The invader formed approximately 60% and less than 50% of the stand final total dry biomass in high and low nutrient availability, respectively. Species RGR were reduced by both intra- and interspecific competition but intraspecific effects were stronger than interspecific effects. On the overall, the species significantly differed in their constant RGR. These differences in RGR between the species (species identity) and the differential response to nutrient loading were the main determinant of change in final biomass composition of these species in mixture. Species influence (competition) only had a small influence on the outcome of competition between the species. The observed species response to nutrient loading could be targeted in management of the invasive species. Lowering nutrients can be proposed to reduce the impact of the invasive L. minuta.