The stability of ecosystems

Transcription

1 The stability of ecosystems Today's session is linked in particular to two ideas. Firstly, the concept of succession that we examined last week. Secondly, the importance of disturbance as a feature of ecological communities. We will be considering disturbance again over the next couple of weeks. Ecological stability has important practical applications. There is concern about human damage to ecosystems. Understanding their natural degree of stability might help us to understand how much damage they can withstand. Objectives Use examples to understand the meaning of the term stability, and its components: resistance and resilience. Show what factors test the stability of an ecosystem. Appreciate types of disturbances caused by humans. Understand how systems can shift from one stable state to another. Stability and the climax community. When we looked at succession, we saw that some ecologists believed that each ecosystem would, over time, reach a stable state - the climax community. Others argued that natural communities never really stop changing. Given enough time, the latter view must be true - otherwise we might still be hiding from dinosaurs. Stability is probably best regarded as apparent stability - stability for a while. Another term for stability, and a good fashionable one these days, is sustainability. A sustainable ecosystem is one which maintains, over time, features like levels of productivity, processes of nutrient cycling, levels of soil fertility, and its characteristic level of biodiversity. A stable ecosystem, a sustainable ecosystem, is something which keeps working - working in the sense of the processes we have been looking at in the course so far. Stability, in theory at least, can be measured by monitoring these kinds of processes. Resistance and resilience. Stability actually refers to two concepts, and these are useful when we go on to look at the things which challenge natural stability. Resistance measures how much a system resists change. A system which remains the same in spite of disturbance or changes in, for example, nutrient input, has a high resistance. Resilience measures how quickly a system recovers from disturbance and returns to a steady state. These ideas can be illustrated by looking at the response of forests to fire. Forest fires 1. Black spruce taiga The black spruce forest of Alaska burns easily in the summer. Afterwards there is a secondary succession following this path (diagram) 1/6

2 following this path (diagram) What is happening is this: 1/2/2016 Introduction to Ecology. There are immediate physical changes. The blackened soil, without its insulating layers of moss and litter, warms up and the permafrost retreats. Nutrients in previously frozen areas are now available. Black spruce grows from seed, but slowly. In the meantime, windblown birch and poplar establish and thrive in the nutrient-rich, warmer soil. Slowly, spruce and moss grow and the litter layer redevelops. The soil becomes colder again and once the birch and poplar die they are not replaced. Moss and spruce litter decompose very slowly. The litter layer thickens, the permafrost rises and nutrients become bound in the undecomposed material. A resilient system The taiga therefore has low resistance to fire: it burns easily. But the original system is soon restored after the fire. It recovers, it has high resilience. Resilience depends on the disturbance Just because it is resilient in the face of fire, however, taiga is not resilient to any kind of disturbance. If the spruce trees were felled and removed, there would be no cones from which new seeds could establish. The spruce forest might then never recover. Tropical rain forest and fire. Rain forest does not burn easily, because it doesn't dry out as the taiga does in summer. It is therefore resistant to fire, and the only large-scale fires are those caused by humans. However, once burned, it shows low resilience, particularly if burned on slopes, where nutrient leaching is much faster than on level ground. Low resilience means that the original system may never recover. The extent of resistance and resilience therefore depends on both the nature of the ecosystem, and the type of disturbance. Testing the stability of ecosystems. The forest fire examples discussed above show that both natural factors and human influences can test or challenge the current state of an ecosystem. Look at the diagram of possible factors acting in the Serengeti. 2/6

3 The direction of the various interactions is shown by the arrows, and each is marked as to whether it is positive or negative. Elephant browsing, for example, has a negative effect on the amount of woodland community present. The most important factors affecting stability in any ecosystem can be summarised under a number of headings. Climate. Obviously this has to be of central importance. Clements, remember, called the climax community the climatic climax. There are only two things which might be worth reminding you of - - climate changes naturally over time, and no ecosystem can resist that for ever. Now, however, humans are changing the climate too, and so even this factor comes under the human list. - climate is more than temperature. Levels of carbon dioxide, levels of ultraviolet radiation: all these things should be included, and are again, of course, things that we can affect. Disturbance in nutrient flows. We have already seen that nutrient cycling is an important component of ecosystem function. Disturbance to the movement of nutrients might therefore be expected to threaten the stability of an ecosystem. This was studied by looking at models of ecosystems where nutrient movements had been estimated. An example of one of the models, that for tundra, is shown. In all, eight types of ecosystem were used. The disturbance considered was a 5% increase in outflow from the system. The immediate effect would be a decrease in the pool of available nutrients. However, the models predicted a return to a new steady state. The effects over 3000 years were calculated, and the results are shown in a diagram. If resilience is a measure of how quickly a system reaches a steady state, in what order could the systems here be put in terms of resilience? 3/6

4 Resistance can be measured by the size of the change suffered by the system. On that basis, which is the most resistant system? The disturbance regime This means the size and frequency of disturbance events, usually things like fires or storms that cause localised damage or killing. You will look at the effects of these in your exercise on intermediate disturbance. This factor is worth noting because it reminds us that a stable ecosystem is not one which is completely placid. A stable ecosystem suffers constant change, but recovers. Human-induced disturbance We have now joined the list of factors that affect ecosystem stability. As we have mentioned already, sometimes we do so indirectly through our influence on, for example, climate. We can also act directly, through pollution. Examples of pollutants that can affect whole ecosystems are oil and pesticides. Some examples of important oil pollution events are described at this Environmental Protection Agency site, and the Sea Empress spill in Wales is described by Swansea University. Notice that although great damage was caused, ecosystems in these cases appear to have recovered. Of course, there may be subtle effects which are difficult to detect. Perhaps, however, these spectacular events tend to distract us from the fact that oil is actually a chronic rather than an acute pollutant: that is, it is constantly released rather than just in occasional accidents. More oil is released through ordinary activities and in waste water than through disasters. Pesticides are a particularly good example of pollutants that can disrupt the stability of ecosystems. The diagram shows one of the effects of DDD (related to DDT) on Clear Lake in California. The pesticide was sprayed here to kill gnats that were annoying anglers. Figures show ppm of the pesticide. Problems were first noted when large numbers of dead grebes were found. They had been poisoned: the birds had accumulated a lethal dose of the pesticide, and this concentration of the chemical along the food chain is the first example of how it can affect the whole ecosystem. The maximum permissible dose of DDD for people was 7 ppm, so you can see that the fish caught in the lake were now inedible. The gnats, by the way, were completely unaffected. 4/6

5 Other ecosystem effects were noted: nutrient enrichment and reduced levels of fish allowed increased algal growth, and the once clear waters turned cloudy. Changes in stable state. Sometimes ecosystems appear to shift from one stable state to another. This means that the ecosystem has been pushed beyond the point from which it can recover. It is thought that there may be a critical limit to a change that an ecosystem can tolerate. Once beyond that, it settles in the new stable state. This is termed a catastrophic change. Catastrophic change in Lake Victoria. Lake Victoria was remarkable for its cichlid fish. These are a family of fish where the eggs incubate in the mouths of the adults. In Lake Victoria, there were more than 300 species, only found in the lake. The lake was like a tropical rainforest: there was a great variety of species, each occupying extremely specialised niches. The clam crusher, for example, crushed clams in its mouth, a method of feeding used by no other species. Between 1970 and 1990, this remarkably diverse community collapsed to three species. One of the native species is left. Of the remaining two, one is the Nile tilapia, introduced for food, and the Nile perch, a piscivore also introduced to the lake. The Nile perch was introduced in the late 50s, and did nothing for a while, before undergoing a population explosion from The Nile perch now makes up 90% of fish biomass in the lake, and survives, now that there are no native fish left, by eating shrimps and each other. The population explosion of the perch corresponds with the disappearence of the cichlids, and at first it was assumed that the perch had eaten them all. It now appears that the root cause of the catastrophic shift that has occurred is eutrophication. Lake Victoria became steadily more eutrophic from the 1920s onward, with a rapid increase in the 1960s. It has been suggested that the presence of the cichlids helped resist the increased nutrient load: the nutrients were held in the cichlids, whose biomass made up most of the nutrient load in the lake. The nutrients were therefore kept out of the water. the cichlids moved about in huge numbers and kept nutrients moving from the bottom to the upper water. This prevented them accumulating on the bottom, where the resulting increased oxygen demand would have deoxygenated the lake. The lake was therefore in a fragile state (low resistance) when the Nile perch arrived. They may then (helped by overfishing) have reduced the cichlid populations to such a level that the lake could no longer cope with increased nutrient loads. The lake is now physically different as well as different in its fish community. There are frequent algal blooms and episodes of deoxygenation in the deeper water, which results in fish kills. There is now much concern that the Nile perch population will collapse and take the local fishing industry with it. Because species endemic to Lake Victoria have been lost, this change is an irreversible one. Summary. 1. There are two types of stability: resistance and resilience. 5/6

6 2. Some communities are resistant, some resilient. 3. Communities may be stable in the face of some pressures, but not others. 4. Communities can switch from one stable state to another: a catastrophic change. 6/6