2.4 Dynamic equilibrium
Homeostasis is the term used to describe the dynamic equilibrium that maintains living systems. Homeostasis could be described as the perfect blend of positive and negative feedback cycles in order to maintain living systems.
Homeostasis occurs at all levels of organisation within living systems. Individual cells are constantly pumping chemicals across their membranes in order to maintain the appropriate chemical composition for crucial functions such as metabolism and DNA repair. An organism moves around in order to position itself within a favourable environment: sufficient food; appropriate temperature; low levels of toxic wastes. Ecosystems often result in dynamic equilibriums through their food webs. If a species becomes too successful, it will soon use up its resources and/or become surrounded by its wastes.
The key term here is dynamic equilibrium. A famous French proverb captures the essence of dynamic equilibrium: ‘Plus ça change, plus c’est la même chose’ (the more things change, the more they stay the same). Take for example, the tropical rainforests. These are ecosystems which have survived along the terrestrial zone of the equator for millions if not billions of years. Their apparent stability is the result of complex and frantic feedback mechanisms. Gaps open up as mature trees die and suddenly, dormant saplings in the sub-canopy are engaged in frantic positive feedback. Intense light finally reaches their few leaves, and a higher rate of photosynthesis kicks in. This produces the building blocks for new leaves. The more leaves it has, the more it can photosynthesise, and the more it can photosynthesise, the more leaves it has. Thus, you can imagine the vertiginous growth of a small sapling into a mature tree. This positive feedback gives rise to ever increasing exponential growth, but the tree does not go on growing forever at the same exponential rate. Above a certain size, negative feedback kicks in which starts reducing its growth rate. In the case of the tree, it is its ability to transport nutrients from its root system up to the leaves. This works by the evaporation of water from pores in its leaves which produces a vacuum, which in turn helps to suck up more water through the tree. But the longer the distance between the roots and the leaves, the more fragile this suction mechanism becomes, so eventually the tree stops growing taller. Tree growth is now in homeostasis – it has achieved a balance between the number of leaves and the overall height.
The limits set by negative feedback mechanisms can be both internal (e.g. size and density of a tree’s water transport vessels) and external (e.g. amount of water in the soil). The ‘internal’/‘external’ boundary is a significant one when looking at systems: ‘internal’ refers to properties of the living system itself, while ‘external’ refers to properties of larger systems within which the living system is situated. Identifying the boundaries of a system is not always as simple as this internal/external divide. Social systems are notorious for their fuzzy boundaries: what is ‘internal’ to a family: institution; social movement; social event? Establishing boundaries has therefore become a major task of many systems methodologies focusing on social systems. On the other hand, the natural sciences don’t seem to have many problems with a clear establishment of boundaries. Pick up any text on environmental systems and you will soon be introduced to distinct boundaries, the clearest being between rock (lithosphere), air (atmosphere) and water (hydrosphere). But clear boundaries within the natural sciences may also be misleading, as there are many feedback mechanisms across boundaries. The carbon cycle is an example where the basic carbon atom can be found within, and moves between, the lithosphere, the atmosphere and the hydrosphere (see Figure 4.4).
Homeostasis operates at all levels of organisation within living systems. The exuberant biomass of tropical rainforests is partially maintained by their ability to recycle water. For example, the Amazon basin is surrounded by the Llanos savannas of Venezuela to the north and the Cerrado savannas of Brazil to the south. If the humidity coming from the Atlantic Ocean were to flow directly back, the Amazon ecosystem would only be a small fraction of its current size. In fact, the giant trees of the Amazon rainforest rapidly pump humidity back into the sky which then falls back on to the rainforest, thus preventing significant amounts of water from draining away into the ocean. Again, here we have two feedback cycles in operation. A positive feedback which promotes large volumes of water to stay within the Amazon basin (the more rain falls down, the more water there is to pump up, which in turn creates more rainfall), and the negative feedback which drains water away from the Amazon basin (the more rainfall there is, the more water manages to escape down the rivers into the ocean). Thus, these two feedback mechanisms play a major role in determining the exact extent of the Amazon ecosystem. Removal of forest cover significantly weakens the positive feedback mechanism. Some scientists believe that after a critical threshold of deforestation, the water recycling mechanism will break down, resulting in extremely rapid advancement of the two savanna regions into the Amazon basin.
The human body also depends on homeostasis; we have a vast range of positive and negative feedback systems to promote our growth in childhood and to maintain us in a healthy state. The levels of glucose, sodium and water in our blood are all regulated by feedback mechanisms. Just a small variation of these compounds would place the growth of a child in jeopardy, if not place their life at risk.
We need to eat in order to provide energy for the various bodily functions which help us to grow and survive, but a sudden influx of nutrients could easily overwhelm the fine chemical balance we have in our bodies. This is the case for people suffering from diabetes. In healthy individuals, the sudden increase in blood sugar levels after a high calorie meal is controlled by the release of insulin. This is a perfect example of negative feedback; the more insulin is produced, the lower the blood sugar level becomes, the lower the blood sugar level becomes, the less insulin is produced, etc., until the body achieves the appropriate blood sugar level. Insulin enhances the capacity of our muscles and liver to absorb the sudden surge in sugars. Individuals with diabetes have lost the capacity to produce sufficient quantities of insulin. They therefore have to inject themselves with artificial insulin after each meal. Without the injections, they risk slipping into a hyperglycaemic coma and eventual death as the body cannot control the sudden influx of sugars in the bloodstream.
When we talk about health, we are basically trying to represent the idea of homeostasis. For humans, the World Health Organization (WHO) defines health as: ‘a complete state of physical, mental and social well-being and not merely the absence of disease or infirmity’ (WHO, 1946). In 1986, the Ottawa Charter for Health Promotion added that ‘the fundamental conditions and resource for health are peace, shelter, education, food, income, sustainable resources, social justice and equity’ (WHO, 1986). Thus, it seems as if humans need the above conditions in order to maintain homeostasis within individuals and society at large.