THÔNG TIN TÀI LIỆU
Freshwater Ecosystems Changes in biodiversity in freshwater ecosystems can arise from many other disturbances Introduction of certain competitively superior species can result in marked losses of biodiversity The infamous example of introduction of the Nile perch into Lake Victoria of East Africa resulted in the extinction of over 200 species of its endemic cichlid fish taxa in two decades There are many other examples of introductions of exotic plant and animal species that resulted in either direct destruction of prey or inferiorly competitive species or indirect alteration of habitats required by many species Dense, floating macrophyte communities and other eutrophicationassociated excessive plant productivity often result in deoxygenation and reduction in habitat and elimination of many plant and animal species Largely based on terrestrial studies, the Eltonian diversity–stability hypothesis suggested that, because of the many different traits of multiple species, ecosystems with more diverse habitats would likely have species that will survive and expand during and following an environmental disturbance and compensate for those species that are reduced by the disturbance Therefore, a more species diverse freshwater ecosystem should be more resilient to disturbances than a less biodiverse system Because genetically based physiological differences and tolerances among species can be small, the individual interactive strengths of some species in a freshwater ecosystem can become saturating at high biodiversity A point can be reached where increasing species may be functionally redundant and have reduced individual impact on the ecosystem processes On the basis of both theoretical and experimental grounds, only a small fraction of species manipulations have strong influences on food web structure Species redundancy implies that an appreciable functional resiliency exists in which the ecosystem can compensate in its collective metabolism and biogeochemical cycling when disturbed Although the population dynamics become progressively less stable as the biodiversity and the number of competing species increases, biodiversity can enhance the resiliency of many community and ecosystem processes in the rate that the system metabolism returns to equilibrium states following a disturbance There is very little storage capacity for organic carbon within the higher trophic levels Low residence times among the higher trophic levels results in rapid cycling of carbon and nutrients of food web components Such rapid cycling and recycling result in a reduction in the resiliency of the higher trophic levels Most of the storage of organic carbon occurs in the dissolved organic carbon compartment in the open water and in the paniculate organic carbon deposited in the sediments In both of these compartments, the soluble organic carbon of the pelagic areas of lakes or running water of streams and the organic carbon of the sediments, the cycling of carbon is slowed That rate of cycling is slowed in the pelagic by the recalcitrant chemical composition of the dissolved organic carbon emanating largely from higher plants In the sediments, cycling is further impeded by the anoxic conditions that prevail almost universally among aquatic sediments The reduced rates of cycling and recycling result in an inherent increase in resilience stability of the ecosystem Any factor that influences the rates of nutrient and carbon cycling in freshwater ecosystems will influence the resilience of 567 the ecosystem and its biodiversity to disturbances Changing sources of organic matter, as discussed in the following, and hence bacterial metabolism and nutrient cycling thus change resilience and biotic stability A wealth of limnological data from a spectrum of hundreds of lake ecosystems of differing productivity suggests that with a shift in nutrient loadings, concomitant shifts occur in the development of photosynthetic producers and loadings of organic matter During the common sequential development of lake ecosystems over long periods of time (centuries, millennia), shifts in the ratios of higher vegetation versus algal dominance can occur Increased relative organic loading from higher vegetation results in proportionally greater loading of recalcitrant dissolved organic carbon, which can suppress nutrient cycling and increase the resilience of the ecosystem In addition, the development of higher vegetation in littoral and wetland combinations increases the habitat heterogeneity enormously, often by a factor of 10 or more, in comparison to lakes with limited littoral development Species diversity nearly always increases under these circumstances by at least an order of magnitude among nearly all major groups of organisms, particularly among the lower phyla Greater biodiversity may have a greater collective effect by improving the capacity of ecosystems to recover from large disturbances Reduced biodiversity increases vulnerability by reducing the total collective physiological tolerances of the community to large habitat changes Recovery after a major or catastrophic disturbance would be slower with a reduced aggregation of residual physiological ranges within the remaining species Recovery then must depend to a greater extent on slower fortuitous methods, such as importation of species, rather than generation from residual surviving species, or slow recolonization processes such as from remnants in resting stages or seed banks In some cases, such as in many ponds, streams, and reservoirs in clay-rich regions where high turbidity often occurs with successive rain events, photosynthetic productivity within the water is intermittently but repeatedly suppressed Biodiversity is likely also suppressed under these conditions or restricted to species with high reproductive potential that utilize improved conditions in periods between turbidity events As indicated earlier, disturbance to freshwater ecosystems can occur in many forms and to different extents Certain perturbations can be catastrophic, such as overwhelming a lake or stream with an organic or inorganic poison in which most of the biota are eliminated Many disturbances, however, are more gradual over long periods of time (months, year), such as nutrient enrichment, or irregularly episodic and often of short duration (days, weeks), such as severe flooding and the scouring of a section of river Biodiversity is coupled to ecosystem stability and the type and extent of disturbances A model of the responses of organic productivity of lake ecosystems to changes in nutrient loading from the drainage basins has been abundantly verified by nutrient and comparative primary productivity data of phytoplankton, attached algae, and macrophytes from hundreds of lakes in different stages of ontogeny (Figure 4) The differences in plant productivity result in very different amounts and types of chemical composition of organic matter loaded to lakes Because of the large amounts and relative chemical recalcitrance of dissolved
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