On simplification and substitution Agroecosystems provide a particularly good example of how we have substituted the services provided by natural ecosystems for those provided by organisms of particular interest to humankind. It would seem, on the face of it, that comparisons of the diversity-function relationship would be easy between natural and managed ecosystems. However, in agroecosystems, no matter how simple or intensive, the services lost, such as nutrient and water regulation, are compensated for by human-provided substitutions, often at a considerable energy cost.
The data available suggest that, for a given function such as productivity or organic matter accumulation, it does not take many species to provide full services. However, the few analyses available are generally unidimensional in nature and do not consider all functions and their interactions in a system context at a given time, much less through time. Clearly, there is ample opportunity to explore more fully the role of genetic, species and landscape diversity in ecosystem functioning in agroecosystems versus natural systems.
One message that emerges clearly is that in agroecosystems, landscape diversity is an essential component of sustainability,
On simplicity due to dispersal Islands, of course, do not represent a special ecosystem type since virtually all of the world's major ecosystems can be found on islands in one place or another. What is special about them is that they generally represent a special case of any ecosystem, in that they are simpler than their continental counterparts. This generally means that they have fewer representatives of a given functional group, or may even have whole groups missing, it is thus not surprising that islands are of particular value in studying the role of species in ecosystem functioning. Most of our knowledge on these issues come not from observing deletions, or species extinctions, but from documenting the effects of additions brought about by successful invasions. Most of the spectacular cases of deletions, such as flightless birds, have preceded the era of scientific inquiry. On the other hand, there are a number of well-studied examples of the ecosystem consequences of relatively recent species additions which have shown the dramatic effects that can result, particularly if the addition represents a new functional type. It is clear that islands will most certainly be utilized as the testing ground for emerging hypotheses on the role of diversity and function.
On simplicity due to limiting water Arid ecosystems share with islands the characteristic that they have few representatives of any functional group. In this case, however, climatic severity rather than dispersability is the filter on diversity. The results are apparently the same, however. Some of the most dramatic examples of the consequences of species removals and additions come from these arid systems, where resulting major shifts in functioning have been documented with the alteration of species composition. Also, the fact that the structural dominance of desert ecosystems is dependent on only a few species makes any loss result in cascading effects on the whole system. The removal of a single arboreal species, or a shift from grasses to a single tree being dominant, totally alters the structure and function of the entire ecosystem.
On simplicity due to cold temperature As in deserts, arctic and alpine systems have both structural and taxonomic simplicity. Because of evolutionary constraints, entire functional groups are missing from the extremes of cold-dominated ecosystems. Humans have also been responsible for deletion of many of the large grazers. The arctic, because of the ease of performing certain types of ecosystem manipulation, has been an important testing ground for the diversity- function issue.
Responses to massive impacts Fresh water bodies, and the organisms that inhabit them, have been impacted more by humans than virtually any other system on Earth. In a direct sense humans compete, and win, against organisms for limiting fresh water. Rivers are massively dammed or diverted, and lakes arc extensively utilized for recreation. The biotic composition of water bodies has been greatly impacted by these activities, as well as others which include deliberate biotic introductions and the effects of pollutants, including acid deposition.
Lakes in particular provide excellent examples for examining the consequences of changes in biotic composition on ecosystem functioning. They are relatively clearly circumscribed systems, and limnologists, by training, generally have a more holistic view of their systems than terrestrial eeolo-gists. Many important insights about ecosystem functioning and population dynamics have come from lake studies.
Chapter 12 in this volume illustrates these issues and system advantages. The enormous and complex impact of a single species addition, such as the cases of the opossum shrimp and the Nile perch, has been well documented. These effccts have been mainly through food web alterations, or trophic cascades. At the same time, lake systems have been shown to undergo dramatic shifts in species structure under stress conditions, and yet certain ecosystem processes, such as primary productivity, have shown little change at first because of species compensations.
Because of the ease of experimentally manipulating iake systems, they offer particularly powerful models for deriving general rules of where species additions or deletions will, or will not, have a major impact on ecosystem processes. Unfortunately, we already have thousands of uncontrolled experiments in progress on biotic additions and deletions to lake and river systems, the consequences of which are poorly monitored. We should certainly make the effort to remedy this as soon as possible.
Keystones and compensations It was in an intertidai system that the presence of a keystone species was first experimentally demonstrated. Since then, many other examples of keystones have been illustrated, and extensions of the concept have been made. One important finding is that the role of a species may vary along with its distribution; it may play a strong keystone role in one place but not in another, since the complex of associate species changes in widely distributed species. Also, in intertidal systems it has been shown how humans themselves play a keystone role.
Where keystones are lacking, species compensations are evident, with function being maintained after species removals by replacement of the activity by the remaining species.
Complexity Coral reefs represent a remarkable collection of organisms, many of which have co-evolved commensal relationships. Thus it is no surprise that dramatic instances of major ecosystem rearrangements have been noted by either the deletion or increase of one species or another. These systems also provide strong support foT the notion of the cascading influence of the loss of a single guild, such as algal grazers, on the health of entire coral systems, and in turn on the loss of such ccosystem services as coastal protection and attributes of interest to tourists. Since these systems are bathed in water and colonized by larvae, the distances between reef systems and the currents between them are crucial. The importance of virtually all dimensions of biodiversity, from genes to seascapes, is readily demonstrated in coral ecosystems.
Low diversity and low redundancy Boreal systems illustrate many aspects of diversity-function relationships. Low species richness translates into low representation in any functional type. Thus, the impact of the removal of any single species can be great. No doubt the characteristic boom and bust cycles of many animal grazers in these systems is related to system simplicity, and thus intrinsic instability. There are many examples of large influences by single species, not only on local systems but on whole landscapes, as in the case of beaver. Also remarkable because of system simplicity is the dramatic ecosystem impact of a single trait within the small functional group of tree species, depending on whether they are evergreen or deciduous. It is the boreal forests, along with the deserts and tundra, which provide the best evidence for the importance of functional group diversity in maintaining ecosystcm stability.
Where experiments are most tractable Temperate grasslands have provided most of the available experimental evidence on the relationship between species diversity and ecosystem functioning. The relatively short life-span of grasses or their small size may partially explain the concentration of manipu lative experiments in this biome. Results of these experiments, in conjunction with new conceptual models, suggest ways of predicting the effects of different species on ecosystem functioning. A common feature of most ecosystems is that a few species account for a large fraction of a given ecosystem process (e.g. primary production), but account for a small fraction of system diversity. Removal of grass species has a different impact on ecosystem functioning depending on the abundance of the removed species in the original ecosystem. Removal of subdominant species is generally compensated by the remaining species, but removal of the dominant species does not result in full compensation, at least in the short time-span of most experiments.
Grasslands have also provided experimental evidence for the relationship between species diversity and ecosystem stability. Long-term monitoring of a large set of grassland plots with differing diversities, in conjunction with the impact of a severe drought, provided the evidence to show the importance of species diversity on ecosystem resistance and resilience. The most diverse plots showed the least reduction in productivity during the drought, and were the plots which recovered their full capacity the fastest.
It was in the savannas of Africa that the first good evidence was gathered showing the importance of species richness to ecosystem resilience. The large numbers of species within a single functional type, grasses, provides enormous buffering against environmental perturbation. It is also in savannas that a clear differentiation in the ecosystem role can be seen among certain functional types, such as in the deep-rooted but sparse trees and the continuous cover of shallow-rooted herbs.
Diversity and function over evolutionary time The temperate forests of the world are remarkable in that each continent has not only different species dominating them, as would be expected, but also a large difference in the numbers of dominants they have, which is related to the glacial history of these continents. The temperate forests of China have the greatest number of dominants, followed by the northeastern United States and then western Europe. The slim evidence we have now would indicate comparable flux rates of water and nutrients, as well as other functional similarities. Thus, in evolutionary time, comparable growth forms will utilize all of the available resources. The SCOPE project, however, focused on the impacts of humans on diversity and the results of these perturbations - a very different issue from evolutionary niche partitioning. Results from other biomes predict that the responses to, and recovery from, species losses would potentially be greatest in the lcss-rich forests of Europe. We are now seeing in Europe a very substantative shift in species composition due to the effects of nitrogen and acidic deposition. These shifts are resulting in a breakdown of ecosystem functioning, with consequent resources being lost.
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