Effects Of Competition On The Keystone Species, Redundant Species And Resource Allocation Of Floating-leaved Aquatic Plant Communities

The aquatic plants and their communities have until recently been among the most neglected components in the ecological studies of aquatic ecosystems even though they play an important role in regulating the structure and functioning of the aquatic systems. According to functional group-species redundancy hypothesis, in the same functional group of aquatic plants, there exist the keystone species and redundant species since different species act as different parts and niche different location in communities. Based on our understanding about theory of competition in determining distribution and dynamics of plant communities, keystone species-species redundancy hypothesis, and species diversity-community stability hypothesis, we choose the floating-leaved macrophyte Nymphoides peltata (Gmel.) O. Kuntze as a main keystone species to assign different community types, and have designed a series of experiments to investigate the competitive abilities and growth traits of N. peltata grown in different communities; to determine if N. peltata, under competitive stress, has the capacity of morphological plasticity. If so, in what way competition affects morphology, and to test Grime's C-S-R model and Tilman' theory; to evaluate the invasibility, resistance and resilience of N. peltata community to different speciesadditions, and to detect whether there exist redundant species in the same functional group.Our results showed that the increase of density of Z. latifolia resulted in apparent decrease of total biomass, relative growth rate (RGR), leaf area ratio (LAR) and mean leaf area per plant of N. peltata. N. peltata allocated above-ground biomass to shoots and roots and decreased the ratios of above-ground to below-ground biomass ( Ab/Bb) with increasing density of Z. latifolia. The competition experiment with different growth forms revealed that Z. latifolia can inhibit the growth of N. peltata, and N. peltata is a more successful competitor than T. bispinosa and M. spicatum. When competing with T. bispinosa and M. spicatum, N. peltata allocated most of its biomass into above-ground parts, i.e., photosynthetically active parts (leaves), forming a dense canopy on the water surface, thus acquiring a considerable advantage in competition for light.N. peltata can plastically adjust its energy conserving strategy in morphology to help it to escape from the stress condition quickly. N. peltata escaped the inhibited stress via developing stronger roots and shoots system (below-ground parts) when it was grown with a stronger competitor (such as emerged species). In contrast, when it was grown with a rather weaker competitor (such as some floating-leaved species or submerged species), N. peltata invested more energy to its leaves and petioles (above-ground parts) and escaped with rapidly-developing leaves and branchings which were useful to form a leaf canopy to cover the water surface, and thus captured more solar radiation and restrained the growth of other species. The significance of these adaptive mechanisms was to enable the plant to grow in a wide range and to show certain competitive advantages over other species. Our findings also demonstrated that differences in floating leaf surface coverage and leaf surface area are not due to the differences in plant vigour, but the factor that N. peltata can plastically adjust its energy allocation to major functional structures. The results of our study were well in accordance with Tilman's theory and model. N. peltata showed competitive advantages over T. bispinosa and M. spicatum which are well inaccordance with our observation of N. peltata grown under natural conditions. Additionally, our study results were concordant with what Gaudet & Keddy observed, and furthermore confirmed the application of Grime 's C-S-R model in floating-leaved aquatic plant communities.N. peltata exerted an strong interspecific competitive effect on T. bispinosa, but T. bispinosa did not exert an significant interspecific competitive effect on N. peltata. There existed an intraspecific competitive effect of T. bispinosa on itself, however, the interspecific competitive effect of N. peltata on T. bispinosa was stronger than the intraspecific competitive effect of T. bispinosa on itself. The removal of N. peltata had a positive impact on growth and performance of T. bispinosa, whereas no apparent impact was found in the removal of T. bispinosa on the residual individuals of itself. Our experiments also showed that the removal of N. peltata had a positive impact on growth and performance of T. bispinosa, whereas no apparent impact was found in the removal of T. bispinosa on the residual individuals of itself. When grown with N. peltata, T. bispinosa allocated most of its biomass to root and shoot fibre. However, when N. peltata had been removed, T. bispinosa tended to invest its energy to produce more and more fruits which enable T. bispinosa to reproduce and expand quickly.Our results also showed, the additions of M. spicatum and H. vertidllata did not significantly influence the growth of keystone species N. peltata, but only a little effects on V. spiralis. In contrast, addition of T. bispinosa resulted in a relative weak competition between itself and N. peltata, but no apparent influences on V. spiralis. As N. peltata and T. bispinosa belonged to the same functional group, compared to N. peltata, T. bispinosa was a redundant species. Similarly, M. spicatum and H. vertidllata were both redundant compared to V. spiralis within functional group. The present study demonstrated that competition for resources and spaces within functional groups is more intensive than competition among functional groups. The community consisted of N. peltata and V. spiralis was resistant to invasion ( referred to species addition) of M. spicatum, H. vertidllata and T. bispinosa. Slight differences in morphology, physiology, or life history traits can reduce competition betweensimilar species by partitioning their shared spectrum of resources in character, time, and/or space. When different species were added, the coexistence of three competitors in communities was mainly attributed to their niche partitions and sufficient resources in environments. The results were well in accordance with niche theory and those results obtained by Paine, Connell and Dudgeon.In conclusion, in present study, we have investigated the competitive abilities, morphological plasticities and traits of resource allocation in N. peltata grown in different communities using microcosm systems. According to our testings and understandings on some relative theories, we expected to provide foundmentaJ documents for better understandings on ecological theories such as effects of competition on communities, functional group hypothesis, species redundancy and keystone species hypothesis and so on.