From “Ecological Principles and Environmental Issues”
by Peter J. Jarvis
Systems, Gaia and the Biosphere
The basis of life on Earth is energy from the Sun. This life (Ecology Box 1.1) can be viewed, at different scales, as systems. All systems possess four attributes: a structure; a set of functional relationships (structure reflecting function and vice versa); mechanisms that tend to maintain the system’s equilibrium; and dynamism, which can modify or alter the system to allow adjustment to changing circumstances or to function more efficiently. The extent to which a system is successful can be viewed in terms of its persistence, although its form and function may alter through amendment or some form of development.
Cell and sub-cellular features are systems with these attributes. So too are collections of cells which make up tissues and organs. These in turn form a higher-level system, the individual organism. At this stage, we move from biology into ecology, since organisms of the same species make up a population system. Populations of different organisms make up a community system. Add the non-living component of the environment to a community and we have an ecosystem. A collection of ecosystem of similar features (frequently defined in terms of general climatic and vegetation characteristics) is a biome, for example tundra, hot desert, tropical moist forest and the deep ocean. At the larger scale of study there is the biosphere, the thin skin of life on our planet found between a few hundred metres below the Earth’s crust and a few thousand metres above, all treated as one giant system. This reflects what James Lovelock has called ‘Gaia’ – named after the Greek goddess Mother Earth – the self-sustaining, self-regulating system that is our planet, equivalent to a single super organism (see Ecology Box 1.2)
Gaia possesses exactly the same characteristics as other systems; its structure and function are interrelated; it has an inherent stability, but it has also evolved. There are feedback mechanisms that allow it to do this, and the whole system is much greater that the sum of its component parts. The biosphere is at the surface of Gaia, and is the component possessing this interdependence, or symbiosis, between living and non-living elements.
The interconnection of different components of the biosphere, and ways in which events have knock-on implications, can be exemplified by looking at the forest fires that raged in Southeast Asia during 1997 and 1998.
Homeostasis and Feedback Mechanism
It is important to emphasize that while ecosystems have structures, functions and an ability to maintain a state of dynamic equilibrium via feedback mechanism, they are not ‘living things’ as such: ecosystems possess properties of living things as inherent components of them.
The ability of ecosystems to maintain themselves through self-regulation is a critical property. The term applied to the tendency for ecosystems (and systems generally) to resist changes and to remain in (or return to) a state of equilibrium is homeostasis. Ideas from cybernetics (the science of controls) are useful in providing a way looking at mechanisms of homeostasis. If an ecosystem is viewed as a control system, then control depends on feedback, which occurs when output (or part of it) feeds back as input. When this feedback is positive it accentuates or accelerates the output, much as in banking, where compound interest is generated by the principal sum of money but in turn becomes part of the principal. This positive feedback, being ‘deviation accelerating,’ is necessary for an organism’s growth, but is not by itself beneficial to equilibrium at the ecosystem level. To achieve homeostasis there must also be negative feedback, or ‘deviation counteraction.’
An everyday example of negative feedback loops is the thermostat/air conditioning in a home heating system. If the thermostat is set at a desired temperature of, say 20 oC and the room air temperature rises above this, then the thermostat will signal the air conditioning to cool the air until the desired temperature is reached once again. Similarly, if the room cools below 20 oC the thermostat will start the heating unit, cutting off when 20 oC is reached. The thermostat counteracts change, keeping the air temperature stable.
Feedback can be illustrated in an ecological context in a simple (if unrealistic) density-dependent, two-species predator-prey system. Low prey numbers means low supply of food for predators, so predator numbers are also low. Low predation, however, allows prey numbers to increase; increasing prey numbers then allow predator numbers also to increase: these are positive feedbacks by both populations. But as predator numbers increase, so greater predation pressures are placed on prey numbers, their population growth decelerates then eventually declines, and the population level begins to drop; this is reciprocated by the predator population, which has less to feed on, until both population sizes are low once again: these are negative feedbacks. The cycle starts again. There is in this way a cyclic state of stability--homeostasis exists, but it oscillates around a mean population state for both species. In nature, such simple system would not exist. For instance, the prey species (whether itself a herbivore, carnivore or omnivore) will also depend to an extent on its food sources; the prey will probably be consumed by more than one prey species; the predator will almost certainly have more than one prey species; predator numbers and predator population growth rate will probably be lower than those of the prey; and the influence of external environment fluctuations cannot be ignored. But the principle of such feedback mechanisms holds, however complex is shown diagrammatically in Figure 1.13.
The more complex the system the greater the number and interaction of the feedback loops and the more inherently stable the system tends to be, although complexity itself is not the cause of this. Rather good homeostatic control tends to be associated with ecosystems whose components have had a long evolutionary history during which structural adjustments and functional fine-tuning have taken place. Recently evolved ecosystems, including those human origin such as plantation forestry ecosystems and agro-ecosystems, tend to lack this inherent stability to adjust and are therefore more susceptible to disturbance and perturbation.
This can be seen in the rainforest example outlined earlier. The rain forest ecosystem usually has a number of checks and balances. Rain forest has a long evolutionary history, and the complex of interactions maintains a state of dynamic equilibrium. In Sabah, unlogged rain forest has been estimated to contain 400 Mg (metric tons) of biomass per hectare, 17% of this being underground plant material falls to the ground each year. This means that in this forest there is about 3.3 Mg of litterfall each year in each hectare. The Malaysian example may represent a rather low amount of litterfall. Near Manaus, in central Amazonia, for example, the estimated litterfall is 11 Mg ha-1 year -1.
Figure 1.13 Positive and negative feedback relationships between soil nutrients system factors. Weathering and leaching are likely to be high in the wet tropics, and plant productivity, litter decomposition, microbial activity and nutrient cycling all high under warm, damp environmental conditions such as are found in tropical rain forests.
This litter, however, is rapidly assimilated by the soil microorganisms to become available as nutrients to the forest plants. Nutrients cycle through the animal sector, as well as the microbial community, as they feed on plant material and on each other (see Chapter 4 on energy flow, nutrients cycling, productivity and food webs). The rain forest can withstand disturbances from treefall gaps – indeed, like all forests, it relies on such gaps for its long- term survival as a dynamic system with a series of patches of different ages, reflecting different times since treefall on that site.
Small clearing for agriculture are analogues of such these small-scale perturbations. With large-scale clearance, however, whether burned or not, and whether for logging or conversion to agriculture or pasture, the system breaks down. The feedback mechanisms can no longer return the system to equilibrium. Ideas on ecosystem stability, resilience and recoverability are discussed and exemplified in Chapter 2.
The problem of creating stable, self-perpetuating ecosystems can be seen with the failure of Biosphere 2. Biosphere 2 aimed at providing a self-sustaining microcosm of key ecosystem found on Earth (i.e. the biosphere, or Biosphere 1) with a 1.8 ha glass-and-steel construction in the Sonoran Desert, Arizona. An attempt was made to construct self-regulating ecosystem representing tropical rain forest, savanna thorn scrub, hot desert, freshwater marsh and a marine system, together with an area devoted to intensive agriculture, and comprising 3800 plant and animal species together with a microbial community of unknown size. In September 1991, eight ecologists/biologist were sealed into Biosphere 2. Shortly after, however, carbon dioxide levels began to fluctuate rapidly and, overall, to exceed the capacity of the system to recycle the gas. By March 1993, oxygen concentrations had dropped from the normal level of 20.9 to 14.5% and, despite pumping extra oxygen into the microcosm, levels continued to decline by around 0.25% each month until the end of the experiment in the following September. Much had been learned, including a lesson on how difficult it is for us to repeat on a small scale what Biosphere 1 does naturally.
Ecosystem Stability, Resistance, Resilience and Persistence
So far, we have examined how pollution can destabilize populations, communities and ecosystems without actually defining what we mean by stability. The stability of a system is a measure of its sensitivity to disturbance or perturbation. Stability reflects the ability of a system to return to an equilibrium state after a (temporary) disturbance. The faster the return, with the least fluctuation, the greater the stability. The actual return to an equilibrium state of any specific example depends in part on the external pressures placed on the system, but the degree of stability is an intrinsic property of the system itself.
Stability closely reflects and its often confused with resistance, resilience, elasticity and persistence. Resistance reflects the extent to which a system avoids change or prevents it from having an impact. Resilience is a measure of the ability of a system to absorb changes (Holling, 1973). Begon et al. (1996) argue that resilience describes the speed with which a system returns to its former state after it has been perturbed and displaced from that state, but this characteristic is better described by the term ‘elasticity’ (Orians, 1975). Strong resistance and resilience generally lead to persistence of extinction (Holling 1973).
Stability is also often linked with the diversity and maturity of a system, the latter (as discussed in Chapter 1) commonly reflecting greater opportunity for feedback mechanism to have evolved. For example, spatial environmental diversity (heterogeneity) usually increases species diversity; temporal environment heterogeneity may increase or decrease diversity and stability (Jacobs, 1975). Environmental stress is often negatively correlated with diversity, i.e. increasing the stress reduces the number of species that can tolerate such conditions. Increased diversity of resources will lead to an increased resistance to changes in those resources but no significant change in resilience.
The relation of resistance and resilience to each other depends both on the underlying characteristics of negative feedback to population change and on the sensitive of the growth rate to the environment. Strong feedback mechanism that are independent of the environmental factor causing the stress will increase both resilience and resistance to that stress. If the feedback mechanism makes the population growth rate sensitive to the environment factor, however, the amount of resilience and resistance under changed in that factor will be inversely related.
It was at first believed that there is a direct causal effect between system complexity and system stability, but recent work has suggested that, at least in community ecology, stability is linked with complexity simply because of the way the analysis is undertaken. Studies showing positive stability-diversity relationships have generally used a single class of stability (or, rather, instability) measure, viz. temporal variation in aggregate community properties such as biomass or productivity. Doak et al. (1998) demonstrate that for such measures stability will always rise with species diversity simply because of the statistical averaging of the fluctuations in species abundances.
Orians (1975) sees persistence as reflecting the survival time of a system, or some component of it. He also defines a number of terms that are useful in exploring how system respond to environmental stresses or other changes. Constancy reflects the lack of change in some parameter of a system, for example the number of species, taxanomic composition, lifeform structure of a community, or feature of the physical environment. Inertia is the ability of a system to resist external perturbations; it takes into account elements of both resistance and resilience. Amplitude is defined as the range of parameters or states (‘ecological area’) over which a system is stable. Amplitude can be used to distinguish between local stability, which describes the tendency of a system to return to its original state following a small perturbation (low amplitude), and global stability, where the same tendency occurs after major perturbation (high Amplitude). Cyclic stability is the property of a system to cycle or oscillate around some central point or zone. Trajector stability is the property of a system to move toward some final end- point or zone despite different ‘starting points’ and is seen, for example, in some models of vegetations succession.
As ecosystem or community that is stable only within a narrow range of environmental conditions, or for a very limited range of species characteristics, is said to be dynamically fragile, in contrast to a system that is stable within a wide range of conditions and characteristics, which is dynamically robust (May, 1975).
Many of these ideas can be incorporated into the characterization of what might be called ‘ecosystem health,’ defined in terms of system organization (diversity and number of interactions between components), vigour (using measurement of activity, metabolism or primary productivity) and stability, especially resilience. Ecosystem health has been particularly useful in exploring the responses of regional ecosystems to various kinds and combinations of human impacts on the environment, and also in examining the consequences of ecosystem health on human activity, including human health, economic opportunity and sustainability (Rapport et al., 1998).