Evan Olson Josh Schneider Chapter 12 – Redundancy in Ecosystems I. Introduction A. Redundant means surplus to requirements. Redundant words in a sentence can be deleted with no loss of clarity of meaning. If a species is removed from an ecosystem but the ecosystem continues to survive then that species is considered redundant. B. Two common questions that are asked are: 1. How much species redundancy is built into an ecological process? 2. To what extent are patterns of biological diversity important in determining the behavior of ecological systems? 1. There are theoretical and empirical studies that might help us answer so fundamental a question are few; nor does there appear to be a clear consensus among ecologists about what the answer might be. But with unprecedented numbers of the earth’s flora and fauna threatened with extinction, getting the answer right is not merely of theoretical interest. C. Rivet Hypothesis 1. All species contribute to the integrity of the biosphere in a small but significant way. 2. You have to look at how many species or rivets you have to take away before the structure of the ecosystem collapses. D. Redundant Species Hypothesis 3. The biomass of primary producers, consumers, decomposers are maintained. 4. These are maintained in general all ecological process will function perfectly well with very few species. Q. Which of these two hypotheses would describe the ability of an ecosystem to survive better? E. There are certain factors that go into determining whether or not a species is redundant. 1. Major biogeochemical cycles that constitute the lifesupport systems of the planet. 2. Key ecosystem processes of primary production, primary consumption and secondary production. 3. Food-web assembly 4. Ecosystem stability and resilience 5. Succession and the whole gamut of processes that structure communities. II. Evidence From the Fossil Record A. Evidence from the fossil record suggests that most of the ecological systems that have existed over the past 600 million years have been conspicuously less species-rich than extant assemblages both in the sea and on land and that these relatively impoverished ancient assemblages suffered occasional mass extinctions. B. Fossil records state two conclusions 1. The current rates of species extinction are several orders of magnitude faster than anything seen in the fossil record, even during the most severe mass extinctions. 2. We often cannot tell from the fossil record, although deposits of coal and limestone clearly point to substantial differences in some ecosystem processes in time past; but again, it is unclear how these differences relate to reductions in species richness of the species composition of particular ecosystems. Q. Do you think we can predict the future of ecosystem survival and species richness of an ecosystem based on the fossil record? III. IV. Patterns of Energy Flow, Biomass and the Structure of Food Webs A. Productivity in Biomass 1. Information exists to suggest that major energy flow and an apportionment of biomass maybe broadly insensitive to the number of species involved. 2. There are trends in species richness within these relationships, with the low productivity systems tending to be from relatively species-poor tundra and some deserts, and the high productivity systems dominated by species-rich tropical forests. 3. Species richness depends on the productivity of the biomass that contains the species. B. Food Web 1. A food web is a map that describes which kinds of organisms in a community eat which other kinds. 2. Common Features of a food web a. Top species have no predators b. Basal species support the web and have no prey. 3. It’s not to say however that certain rules govern all food webs only that the rules are more complicated than current theory and data suggest. 4. Very recent work suggests that scale invariance may be an artifact of poor data, particularly the failure of ecologists to study and document the structure of very large food webs with sufficient rigour and care. Evidence already exists, for example, that linkage density is not constant; rather it probably increases with increasing numbers of species. Q. What happens to a food web if the primary producers of that web become extinct? Does this web continue to thrive? Theoretical Models of Ecosystem Stability and Resilience A. V. A different number of species and different strengths of the links of species to an environment determine an ecosystems stability and resilience. B. Lower diverse systems will be greater negatively affected by loss of a species. C. Biomass stability is not simply generated by having more species but also the interaction of the species. D. More complex systems are affected less by the deletion of a species E. For a given level of connectance, species deletion stability decreases as the number of species in the web increases. F. An example would be a large mammalian grazer changed the total green plant biomass less in high diversity than in low diversity grassland plots. G. Possible modeling approach would be to explore the affects of species deletion on the rates of nutrient cycling and upon the stability and resilience of the nutrient cycle itself. Q. What are certain things that keep an ecosystem stable? Observations and Experiments on Real Systems. A. Having high species richness doesn’t seem to affect the ecosystem; it is the interaction of the species that affects it. B. It is possible that systems with many species fluctuating out of synchrony will show a greater stability of ecosystem function than systems with fewer, similarly fluctuating species. C. Keystone Species 1. Certain species that are contained within an ecosystem are more important to that ecosystem then others. These species are called the keystone species. These species are in a way the “drivers” of the ecosystem and the rest of the species that are contained within are considered the “passengers” of that system. 2. There is no doubt that removal of a keystone species affects the ecosystem. 3. It is not clear by what mechanism keystone species might be expected to exert there effects. In some cases it is trophic-by eating other potentially dominant members of the community. In other cases however it is clearly not. 4. We have no idea what proportion of species in an ecosystem is “key”. There can be assumptions made as to what species could have a greater effect on that ecosystem. However it is too hard to experiment with this because you would have to take every species within the system and remove them one at a time in order to find out what species had the greatest effects. 5. The message in the keystone species concept for an emerging understanding of species redundancy in ecosystems is that all species are decidedly not equal, and that and understanding of redundancy will not be possible until we better understand how to define, identify and measure the impact of key species. Q. What would be a general characteristic of a keystone species? VI. Manipulation Experiments A. General considerations 1. Removal of a species often leads to changes in the distribution, abundance and performance of at least one other species. 2. We often can’t tell how big of an affect takes place due to the idea that our measurements are not accurate enough or easily containable. B. Examples 1. Perturbation experiments a. Removal of plants in a field biomass b. Some of these manipulative experiments also deal with the removal of individual species or functional groups have different effects on the community in terms of species-richness and biomass. c. The magnitude of the effect is not always consistently correlated with the abundance or even the dominance of the species. 2. Changing a keystone species and measuring its results on a nutrient cycle. 3. Any organism that has a major impact on nutrient cycling, as well as the rate or direction of successional change, is a keystone species, and if nothing has else can fill its role, the process has no redundancy built into it. 4. Removal or addition of natural enemies. 5. Use of insecticides Q. Regarding experiments of manipulation, does removal of one species necessarily cause that ecosystem to fail? VII. Conclusion A. Rivet Hypothesis 1. Data tends to go against rather than support. 2. It is difficult to tell from this restatement of the rivet hypothesis whether Ehrlich and Wilson subscribe to it in the extreme form which assumes that the removal of each and every species matters. B. Considerable redundancies are built into ecosystem processes. C. Attention must be shifted to figuring out the minimal level of the species that an ecosystem needs to survive. D. Also a set of problems centres on the difference between average ecosystem processes under normal conditions and the maintenance of ecosystem function under rare or unusual extreme events. E. Namely all species are not equal, that the loss of some species is more important than that of others, and that species loss may be tolerated up to some critical threshold. Any Questions or comments?