Biological invasions are now recognized as major threat to biodiversity and an important element of global change. These organisms have become a focal interest in ecology, owing not only to the tremendous destruction that they can cause, but because we also don’t yet understand fully how they change from being minor components of their native communities to dominant components of invaded communities. As such, various concepts have been developed in an effort to define what factors and/or conditions that promote invasions and aggravate impacts on native species. Species traits, competition and environmental conditions are some of the factors that can determine species invasiveness and invaders impacts on native species. How environmental conditions (such as nutrient availability and climate warming), species growth traits, and competition influences invasive species performance and aggravate impacts on native species (in particular effects on species growth/ performance and abundance) was the subject of study. Overall, we determined the competitive effects of invasive alien on native macrophytes and the underlying mechanism of competition. Specifically, we examined (1) the impacts of competition (intra- and interspecific competition) on species performance (growth, biomass), (2) how change in environmental conditions influenced both non-native and native species performance and (3) the relative importance of species identity (reflected in the difference in Relative Growth Rate (RGR) between species), species influence (reflected in the difference between a species intraspecific effects on its own RGR and interspecific effects on RGR of another species) and environmental conditions as determinant of change in final biomass composition and abundance of a species. Competition is increasingly recognised as a major means through which non-native species impacts and displace native species. To fully examine the influences of competition in plant-plant interactions, it is important to distinguish between intensity of competition and its importance. The intensity of competition is the absolute decrease or increase in plant performance (e.g., growth, biomass output) as a result of intra- and interspecific competition. Importance of competition on the contrary, describes the role of competition in relation to other processes that may influence the future abundance, density, or species composition of a plant community relative to all other process. Here, we not only investigated the effects of competition intensity but also its importance relative to species growth traits and environmental conditions in determining species abundance. Such a holistic approach not only determines the impacts of invaders but also point out the most important processes that mediate impacts. To deliver on the competitive impacts of non-native species and its importance relative to species growth traits and environmental conditions, we design one field based and three laboratory based experiments. All the four experiment followed response surface experimental design (also referred to as ‘factorial designs’ ‘complete additive designs’) with factorial combination of the competing pairs at two densities, resulting to a total of eight densities (four monoculture and four mixture). Each density was then replicated five times. The field experiment was between the highly invasive Eichhornia crassipes and native Ludwigia stolonifera, which co-occur at the littoral zone in Lake Naivasha, Kenya. In this experiment, we did not apply any environmental treatment and therefore we only investigated the effects of competition (both intra- and interspecific competition) on species performance and its relative importance compared to species identity in determining species relative abundance. The three-laboratory experiment involved three lemnid species; invasive Lemna minuta, non-native Landoltia punctate and native Lemna minor. Lemna minuta and Landoltia punctate are non-native to Western Europe while Lemna minor is a native species in Europe. With the lemnids, we conducted two manipulative experiments on nutrient availability and one on climate warming. In all the experiments, species initial and final dry biomasses were measured and used to determine species RGR. The RGRs were then modeled to assess the effects of intra- and interspecific competition on species performance. Additionally, the difference in RGR between species in mixture was model using multiple linear regressions to assess (1) change in biomass compositions of species in mixture and (2) assess the relative importance of species identity, species influence and environmental condition in determining the change in biomass composition and abundance. In regards to influence of species on one another performance, the overall results indicate that the competing pairs influenced each other performance. The interactions between invasive E. crassipes and native L. stolonifera were both competitive and facilitative. While the invader suppressed the growth of the native species, the presence of the native species enhanced the performance of the invasive species. The invader performed slightly better while in mixture with L. stolonifera than in monocultures and thus initial abundance of the native species did not v negatively influence the species RGR. Intraspecific effects on species own RGR generally reduced both species performance and these effects were more intense in high initial biomass. In interactions between lemnids, intra and interspecific effects between each competing pairs investigated were generally negative and thus reduced each species performance. We however observed one case of intraspecific facilitation in the competition experiment between the non-native L. minuta and L. punctate. While battling each other, each species performance was more enhanced by high initial conspecific abundance, an indication of the Allee effects, where species fitness decreases when conspecific density is low. Overall, all the species investigated here were subject to stronger intraspecific effects than interspecific effects. Stronger intra than interspecific effects indicate that each competing pair can co-exist even though there is interference. In this study, we were also interested in determining the importance of competition relative to species growth traits (species identity) in determining change in biomass composition of two species in mixture. On the overall, species identity was the strongest element that determined change in biomass composition of two species in mixture and thus it was the most important factor that structured final species biomass. Species influence (competition) was less important compared to species identity. Nevertheless, competition did partly influence the final biomass abundance of species. Environmental conditions were generally observed to influence both nonnative and native lemnids performance with differences in response being observed between non-native and native species. Non-native species showed greater plasticity to high than low nutrient availability and to warmed vi than ambient temperature conditions compared to the native species. Such opportunistic response to favorable environment clearly indicates that nonnative species are more plastic and are better able to increase their fitness in favorable conditions compared to native species. In mixture, both nutrient availability and warming differentially influenced non-native and native species growth and consequently played an important role in structuring biomass composition of species. Warming effects are expected to vary with nutrient availability given the interactive effects of warming and nutrient availability. The effects may especially be important in high nutrient availability and might favor the invasive L. minuta. Four general points emerges from this dissertation. Firstly, both nonnative and native species are subject to both intra and interspecific competition, which greatly influences both non-native and native species performance. Secondly, species identity is more important in determining the outcome of interactions than species influence and therefore to draw general conclusions about competitive impacts of non-native species it is necessary to examine the intensity of competition and its importance relative to other process in order to fully understand the role of competition in changing native species biomass abundance. Thirdly, due to context dependence of outcome of species interactions, it is important to include environmental variables in experimental designs in order to determine the role of environmental conditions in changing native species biomass abundance. Finally, to improve our understanding of aquatic plant invasion and their impacts, we need to pay more attention to how both non-native and native species respond to changes in environmental conditions, and how this affects species abundance and mediate impacts. |
