The big picture

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This subtopic looks at three key areas of systems:

• The laws of thermodynamics and how they govern energy flow.

• Equilibria or balance in systems, and how that can be maintained or lost.

• Tipping points.

Figure 1. Balance and harmony.

Thermodynamics is a branch of physics that studies heat and temperature and how that relates to energy and work. The idea is that matter has variables such as energy, entropy and pressure and these are subject to general controls. The laws of thermodynamics set out some of these indisputable rules by which the universe works. 

The first and second laws of thermodynamics have significant impacts on ecological systems. As energy moves through a food chain, entropy increases and the amount of energy available to do work becomes more and more limited. So as you move up the food chain, less energy means fewer individuals and a higher concentration of organic poisons in the tissues and organs of the organisms (biomagnification and bioaccumulation).

Figure 2. Order to chaos to order.

Natural, undisturbed systems tend to be in a state of equilibrium or balance, which is maintained by feedback loops. Negative feedback maintains the status quo of a system and keeps it functioning within certain limits. Positive feedback amplifies change and may cause the system to find a new equilibrium. Positive feedback can be good or bad - it drives ecological succession. For example, primitive plants grow and die enriching the soil with organic matter, richer soil can support more plants, more plants put more nutrients into the soil and so on. On the other hand, the addition of too many nutrients in an aquatic systems sets of the processes of eutrophication and the system collapses.

If a system is experiencing positive feedback it is likely to reach a tipping point, a critical point at which the system has experienced so much change there is no way it can return to its original state and a new equilibrium is reached. For instance, if you push a heavy object up a slope to a peak, there is a point at which you can stop pushing. 

Figure 3. Tipping point.

The laws of thermodynamics

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The laws of thermodynamics explain the rules that energy flows in a system. There are two laws that concern ecosystems - the law of conservation of energy, and entropy and the second law. 

The law of conservation of energy

The law of conservation of energy is the first law of thermodynamics, which states that energy can neither be created nor destroyed. The total amount of energy in an isolated system does not change but the energy may transform from one type to another. This is demonstrated very clearly in all food chains.

Figure 1. Plant energy.

Theory of Knowledge

Scientific laws are based on inductive reasoning. How reliable is inductive reasoning? 

Food chains and the first law

Figure 2 shows that energy enters the system as light. This is then transformed into chemical energy (carbon bonds) during photosynthesis. The chemical energy passes along the food chain (in the direction of the arrow) as consumers eat producers or other consumers, some chemical energy is converted into mechanical energy during respiration so that it can be used to fuel life processes. It is then transformed into heat which releases into the atmosphere.

Figure 2. The food chain and the first law of thermodynamics.

Energy production and the first law 

As the laws of thermodynamics are universal, you can also see the first law at work in energy production systems. In a traditional thermal power station (Figure 3) coal is burnt to alter the chemical bonds and release heat, that heat turns water from a liquid to a gas (steam), that spins a turbine (kinetic energy), which drives an electrical generator to produce electricity (electrical energy). As with the food chain, the original energy source is solar energy. Millions of years ago plants used sunlight to photosynthesize and grow. Some of those plants died and fell into the swamps where they eventually formed coal.

Figure 3. Coal-fired power station. 

It is a similar process in renewable energy production. We use one type of energy to make another type, usually electricity because it is convenient and clean. For example, instead of burning coal to heat the water to produce steam to spin the turbine, wind does the job of spinning the turbines directly. 

Figure 4. Wind turbines. 

Implications of the first law

1. In an open system such as an ecosystem, once energy has entered it will never increase. Energy has to keep entering to keep the ecosystem functioning.

2. In a food chain, energy transforms from light to chemical to heat energy. This increases entropy (second law) so there is less available to do work, therefore at higher trophic levels there are fewer animals. 

3. Animals at higher trophic levels must eat a large number of smaller animals, so if there are non-biodegradeable toxins in the chain they will become progressively more concentrated the higher up the food chain you go. (Bioaccumulations)

4. We can never create energy for our use. We have to take what is available and transform it into a form that is most useful to us. 

5. No new energy is being created in the universe.

Entropy and the second law

Entropy is the increase in disorder and randomness in a system. In energy terms it means that an increase in entropy means a decline in the amount of energy available to do work. The second law of thermodynamics states that the entropy of a system increases over time; the only way to avoid entropy is a continuous input of additional energy. Imagine your bedroom - if you do not put your stuff away (and neither does anyone else) then your room falls into chaos and disorder, until you can't find anything and have to tidy up - the tidying up is adding energy to the system so entropy is pushed back for a while. In nature, organisms are kept from disintegrating and being subject to entropy by the continual input of energy into the system. If an animal stops eating, it dies and decomposes into its constituent parts. 

Figure 5. Entropy in a green pepper.

The second law, entropy and food chains

The most useful energy in an ecosystem is light energy, because it is low entropy energy and can be used to do work – photosynthesis. The chemical energy in the carbon bonds of organisms is also useful because it allows organisms to move around and perform life processes. The problem is that the use of chemical energy is not 100% efficient and some of it is converted to heat (high entropy energy) that dissipates into the atmosphere where it is useless for work purposes. So as energy changes form, it becomes less and less concentrated and there is less available to do work. 

Figure 6. The food chain and the second law of thermodynamics.

The second law, entropy and energy production

The process for the production of energy was discussed under the first law. At every stage of our energy production, systems energy becomes less concentrated and more and more is released into the atmosphere as heat. 

Implications of the second law

1. Entropy will always increase, so in order to keep a body together organisms must continually put in energy, such as food and sunlight.

2. Increase in entropy is reduced by an input of food as a source of energy. All living organisms respire, even at rest and energy stops them disintegrating into a puddle of inorganic chemicals.

Feedback and tipping points

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Watch the following video as an introduction to feedback loops in nature: 

In the section What is a system, we looked at the idea that systems have inputs, processes and outputs, and that part of the output may re-enter the system as feedback. Feedback may be positive or negative.

Be aware

For some students, the use of the terms positive and negative implies value - we intuitively think positive is good and negative is bad. DO NOT fall into this trap; the terms are not value statements about how good or bad something is.

Negative feedback

This type of feedback promotes stability in a system as it reverses the change and returns the system to the original state of equilibrium. There are many examples of negative feedback in the world: 

1. Predator prey relationship

2. Human body temperature

3. Toilet flush

Feedback loops can be given in words or shown as a diagram (Figure 1).

Predator prey relationship

An increase in the rabbit population (prey) gives more food for the foxes (predators). More food means more fox cubs will survive, therefore increasing the fox population. Too many foxes will increase the predation rate on rabbits and the rabbit population falls. Less rabbits mean less food for foxes, fewer cubs survive and the fox population falls.

Figure 1. Predator prey negative feedback loop.

Human body temperature regulation

The average temperature for humans is around 37^°C. If the body deviates too far from that we die, so negative feedback is essential. As the temperature increases we sweat. The evaporation of the sweat removes heat from the body and we cool down. If the temperature drops and we shiver, shivering generates heat from respiration and we warm up again.

Figure 2. Human body temperature regulation.

Toilet flush

The cistern of the toilet holds water ready for flushing. When the toilet is flushed that water is then released in order to wash away the waste products. Once empty, the ballcock drops and allows water into the cistern, the ballcock then floats up and as soon as the cistern is full the flow of water is cut off. If that feedback mechanisms breaks down there is a lot of water to deal with.

Positive feedback

This type of feedback amplifies the change in the system and keeps it going in the same direction. So a small disturbance in the system causes an increase in that disturbance. For example in climate change, more CO₂ in the atmosphere causes rising temperatures, which causes permafrost to melt. That releases methane (a powerful greenhouse gas) and so temperatures continue to rise. 

There are many examples of positive feedback loops - some are beneficial while others are not. Deforestation causes many problems, one of them is soil erosion – Figure 3 shows how positive feedback can amplify the problem. 

Figure 3. Positive feedback of deforestation.

Tipping points

A tipping point is part of a system that kick-starts self-perpetuating positive feedback loops that push the systems to a new state of equilibrium. Ecosystems are essential to the well-being of every human on the planet. As we push the environment harder and harder we need to understand what could happen if any part of the environment reaches a tipping point. If tipping points are reached in the natural environment, a number of problems may occur:

1. Environmental support services could collapse – for example, water cycle regulation, clean air, pollination, soil conservation.

2. The land's food production capacity will deteriorate.

3. The seas’ food production capacity will be compromised.

4. Climate may spiral into a positive feedback cycle and become unsuitable for human existence.

The problem with tipping points is that in large systems, feedback loops may be slow and the impact of a particular action will not be seen immediately.

Gerald Marten published an article in the Journal of Policy Studies (Japan, 2005) about ecological tipping points. The article contains three case studies:

• An Environmental Tipping Point Story: Cooking Fuel, Deforestation, and Biodigesters

• Apo Island: A Story of Fisheries Collapse and Salvation

• Deforestation and Reforestation in Japan

The case study that follows is a brief outline of the first of these three case studies. When reading it consider:

1. How long the feedback loops may take to develop and have an impact.

2. The complexity of the whole system and how the tipping point came about. 

3. The wide ranging impact of the tipping point.

Theory of Knowledge

Extension

Tipping points have been observed in small scale systems; how can we be sure that the theory can be applied to global systems?

Which ways of knowing are the most useful to determine how to be sure?

Climate change deniers claim that climate change does not exist and, therefore logically, that a tipping point cannot be applied to global systems. To examine this connection in further depth, this article attempts to systematically refute the arguments of climate change deniers.

Prescribed Title Explorations

PT May 2017 #6: Humans are pattern-seeking animals and we are adept at finding patterns whether they exist or not (adapted from Michael Shermer). Discuss knowledge questions raised by this idea in two areas of knowledge.

Equilibria and stability

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Equilibria

Systems have inputs, processes, outputs and feedback. If everything is in balance, the system is said to be in equilibrium. It does not mean that there are no changes, but that the impact of the change varies over time and is dependent on the type of equilibria - static or steady state.

Static equilibrium is really only applicable to non-living systems and the components of the system remain constant over a long period of time (Figure 1). 

Figure 1. Static equilibrium.

Our atmosphere has been in static equilibrium for around two billion years. When life first started to evolve four billion years ago, the atmosphere started to change in composition (Gaia hypothesis). Then eventually two billion years ago, the proportions of the gases settled to what they are today (section 6.1.1) – 21% oxygen, 78% nitrogen and 1% others. The 1% “others” is now cause for concern. The CO₂ proportion is rising and has gone from around 370 ppm to over 400 ppm, a figure that by many is considered to be a tipping point. So static equilibrium can be disturbed.

Table 1. Comparison of stable and unstable static equilibrium.
	Stable equilibriumUnstable equilibrium
Figure 2. Stable and unstable static equilibrium.
Figure 3a. Stable equilibrium.	Figure 3b. Unstable equilibrium.
If the green ball in Figure 2 is disturbed it will return to where it started.	If the red ball in Figure 2 is disturbed it will not return to its original state but find another one.
It is possible that our atmosphere could be in a state of stable or unstable static equilibrium. 400 ppm atmospheric CO2 is considered by many scientists as the tipping point at which catastrophic changes could be expected. If that is true the atmosphere is in a state of unstable equilibrium and the disturbance will push it to a new state. If those scientists are wrong then the disturbance will not be permanent and things will return to the starting point.

International-mindedness

The atmosphere is a global system. The actions of one country will impact many, for example industrial inputs in North-West Europe cause acid deposition in Norway and Sweden. 

Steady state equilibrium has many small changes over shorter periods of time and the changes occur within limits. Small changes in part of the system will be countered by negative feedback and the system is bought back to the same state as before (Figure 4). The human body has an average temperature of 37^°C but there will be minor fluctuations around that average temperature.

Figure 4. Steady state equilibrium.

Most ecosystems are in steady state equilibrium. Natural disturbances are part of the cycle of life - negative feedback comes in to play and the system is is returned to its original state, for example, in a pond ecosystem (Figure 5). At certain times of the day and year matter and energy will enter the ecosystem – water, soil, plant debris and animals can all enter the system by various routes. At these times there will be more matter and energy in the system. At other times, matter and energy leave the system and there is less. Over a longer period of time there is an average state of the system.

Figure 5. A pond ecosystem.

Stability

Stability is the ability of an ecosystem to remain in balance. There are two components of stability. Resistance is when the ecosystem continues to function during a disturbance. Resilience is the ability of the ecosystem to recover after a disturbance.

Disturbances may be natural (flooding, fires or volcanic eruptions) or human induced (deforestation, pesticides or introduced species). If the disturbance occurs over an extended period of time, over large areas or is of sufficient severity then any ecosystem may reach a tipping point. If that happens it becomes unstable and normal patterns cannot be maintained. Such disturbances are often the result of human activity, loss of biodiversity, pollution (Figure 6) and climate change. These factors are all putting a strain on ecosystem resilience and the resultant ecosystems are often degraded and lack stability and resilience.

Figure 6. Plastic pollution in an aquatic system.

Different ecosystems have very different stability. A number of factors have been identified that account for this:

• Climate and limiting factors: An ecosystem that has an equable climate (with no extremes of temperature or rainfall) that supports vegetation growth with few limiting factors will be more stable (all other things being equal). Desert ecosystems have a hostile climate, numerous limiting factors and low stability. Climate change is likely to push some ecosystems past their tipping point.
   

• Biodiversity: Higher biodiversity ensures a complex ecosystem with many interconnecting parts, so the collapse of part of the system is supported by another part. If a species disappears, the niche they vacate will be taken over by another species and the ecosystem continues. Tropical rainforests have high biodiversity and good stability. As discussed in "How do humans influence biodiversity?" we are reducing biodiversity on a global scale. 
   

• Trophic complexity: Many trophic levels create complex food webs that support greater biodiversity. Organisms from the same trophic level may carry out different tasks or organisms from different levels carry out the same task. So the loss of one organism has less of an impact. Many ocean ecosystems have a high level of trophic complexity and are very stable. If we are losing biodiversity we are losing trophic complexity. 
   

• Nutrient stores: The size of nutrient stores, the relative distribution of nutrients in the stores and the rate of nutrient cycling are all important. If all the nutrients are held in a single store and that store is lost then the system can collapse very quickly. Deforestation in the tropical rainforest removes a large percentage of the nutrients and the system collapses. 
   

• Frequency and intensity of disturbances: Small, infrequent disturbances can be tolerated and overcome whilst large and/or frequent ones cause problems. In the tropical rainforest the traditional agriculture system is one of slash and burn. A small patch of forest (garden) is cleared, crops are grown for a few years then when yields start to decline due to falling soil fertility, the garden is abandoned and left to the forest. The natural rainforest is re-established very quickly. On the other hand, large scale deforestation de-stabilizes the system and the natural vegetation is unable to re-establish (Figure 7).

Theory of Knowledge

Extension

Slash and burn practices are based on indigenous knowledge. To what extent, though, is there a persistent perception that modern scientific knowledge is better than indigenous knowledge? How has indigenous knowledge been proven scientifically sound in the modern and Western and/or Eastern worlds?

To explore these questions further, consider how this article outlines the ways in which indigenous knowledge is being sought to address preserving biodiversity.

To what extent does this article change your perspective on this topic and/or on the ways of knowing that you consider valid?

Case studies in stability

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Watch the following video as an introduction to stability:

This section will look at the resilience of a number of ecosystems and the impact humans have on that resilience. For background details of each ecosystem go to Biomes, Zonation and Succession.

Systems are made up of stores and flows. Humans interrupt these in a number of ways – all of which impact the resilience of the system. A convenient way to view the stores and analyse how human impact is by looking at the Gersmehl nutrient cycles.​

Figure 1. Gersmehl nutrient cycle.

Figure 1 is a generic cycle that shows what the stores and flows are. It also shows the stores and flows as being equal. However, the size of the store, the flows between them and the speed of the flows, will vary depending on the ecosystem - forest, grassland or desert.

Theory of Knowledge

Extension

P.F. Gersmehl's nutrient cycles offer a model to depict stores and flows of nutrients in ecosystems. Are models like Gersmehl's really useful when predicting human impacts? What knowledge, including ways of knowing and/or areas of knowledge, limits our thinking in coming to conclusions to these questions?  Do (past) models quickly devolve as (current) data and situations change?

Prescribed Title Exploration

Specimen Title #3, 2014 IBO Publication: To what extent are areas of knowledge shaped by their pasts? Consider with reference to two areas of knowledge.

Important

You may be given one of Gersmehl's nutrient cycles similar to the one in Figure 2, with no names of the stores and flows shown. It is therefore important that you understand these. 

Temperate grasslands: North America Prairies

Temperate grasslands have some of the most fertile soils in the world and they are the biggest store in the nutrient cycle (Figure 2). This is caused by the numerous grass roots that grow and decay there. The roots hold the soil together and provide humus, which then retains water. This comes together to form the basis of some of the most productive agriculture lands, such as the wheat belts of the USA and Australia.

Figure 2. Nutrient cycle for temperate grasslands.

A small proportion of the nutrients move from the soil into the biomass. Then because grass has an annual die-back most of those nutrients go into the litter store and back to the soil. Fire (started by lightening) is part of the natural cycle and sweeps through these areas on a regular basis. Fire moves quickly and burns the above-ground vegetation, leaving the massive root systems unharmed. The burn releases the nutrients in the vegetation and litter to the soil - the cycle is balanced and the system resilient.

Humans have cleared the vast majority of the natural grasses and replaced them with cultivated ones, such as wheat. Agricultural crops remove the nutrients from the soil as natural grasses would but they are harvested and so the nutrients are removed from the system. Applications of herbicides kill “weeds” (plants humans do not want to grow) and pesticides control insect populations, so biodiversity is reduced. Almost all animals have been eliminated from the system, so there is no addition of nutrients to the soil from defecation, death and decay. Fertilizers can be applied to the soil but they merely replace nutrients not organic content and the structure to the soil is lost.

Figure 3. Wheat field of the Prairies, USA.

If this practice continues the tipping point is reached and the system spirals out of control until it finds a new state. This was demonstrated in the 1930s in the USA and Canada, when ecology and agriculture of the Prairies was destroyed and the Dust Bowl was the result. It has taken decades to restore soil fertility and productivity.

Figure 4. Soil drifting over a farm building: 1935.

​

Tropical rainforests: Madagascar

In contrast to the temperate grasslands, the tropical rainforest has the majority of its nutrients held in the biomass. This is due to the fact that the rainforest has multiple layers of vegetation, the majority of which are 30-meter-high trees. The tropical rainforest does not have a seasonal die-back, so the flow of nutrients into the litter store is very limited. The high temperatures ensure that decomposition is rapid and the nutrients are released from the litter store very quickly. High annual rainfall of over 200 cm/year would wash away the nutrients, but the lateral root systems of the trees have evolved to catch these nutrients before they are washed away.

Figure 5. Nutrient cycle for tropical rainforest.

The biodiversity of the tropical rainforests is one of the highest in the world. Some scientists estimate that over half the terrestrial species live there. This is a delicate balance, but rainforests are resilient to small-scale damage of anything up to 10% of the area.

Initially when explorers first saw the tropical rainforest they thought it would make excellent agricultural land. They believed that such lush vegetation was a consequence of fertile soils, so started the clearance of the rainforest. Since then it has been realised that the majority of the trees are very valuable hardwoods that are good for making furniture and flooring. That tropical rainforest areas contain vast amounts of mineral wealth - gold, silver and diamonds to name a few. 

Figure 6. The removal of hardwoods for furniture and flooring.

Hence deforestation has been widespread and rainforests are not resilient to large-scale damage. When the tipping point is crossed the ecosystem is unable to maintain or restore balance. Once a large area of rainforest is cleared a number of things become unbalanced:

1. The majority of the nutrients are removed from the system so there are very few left for re-growth.

2. Heavy rainfall washes away the leaf litter, removing yet another store of nutrients.

3. Soil is exposed to the torrential tropical rain and is washed away – more nutrients leave the system.

4. Loss of the trees means that the recycling of water is reduced. Water that is usually taken up by the trees and transpired back into the atmosphere runs into the rivers and leaves the area. That reduces rainfall in other areas, which can cause problems beyond deforestation.

5. Loss of biodiversity of plants and animals. Removal of the vegetation takes away vast numbers of species but it also removes the food source and habitat for others. 

Figure 7. Aerial view of deforestation in Madagascar.

International-mindedness

Many tropical rainforests cover several countries (Amazonian Rainforest cover nine countries in South America). The decision of one of these countries to deforest may impact the water supply in others.