Because genetically based physiological differences and tolerances among species can be small, the individual inter-active strengths of some species in a freshwater ecosystem can become
Trang 1Changes in biodiversity in freshwater ecosystems can arise
from many other disturbances Introduction of certain
com-petitively superior species can result in marked losses of
bio-diversity The infamous example of introduction of the Nile
perch into Lake Victoria of East Africa resulted in the
extinc-tion 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
eutrophication-associated excessive plant productivity often result in
de-oxygenation and reduction in habitat and elimination of
many plant and animal species
Largely based on terrestrial studies, the Eltonian
diversi-ty–stability hypothesis suggested that, because of the many
different traits of multiple species, ecosystems with more
di-verse 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
inter-active 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
popu-lation dynamics become progressively less stable as the
bio-diversity and the number of competing species increases,
biodiversity can enhance the resiliency of many community
and ecosystem processes in the rate that the system
metabol-ism 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
sedi-ments 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
pe-lagic 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
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, mil-lennia), 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 lit-toral and wetland combinations increases the habitat hetero-geneity 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 ag-gregation of residual physiological ranges within the re-maining 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, photo-synthetic productivity within the water is intermittently but repeatedly suppressed Biodiversity is likely also suppressed under these conditions or restricted to species with high re-productive potential that utilize improved conditions in per-iods 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 com-parative primary productivity data of phytoplankton, attached algae, and macrophytes from hundreds of lakes in different stages of ontogeny (Figure 4) The differences in plant prod-uctivity 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
Freshwater Ecosystems 567