Stability in Ecosystems (HL IB Biology)

• Stable ecosystems have the following features
  • Efficient nutrient cycling which allows the system to be self-supporting
  • High biodiversity
  • Stability, i.e. resistance to change
    • E.g. consumer population sizes do not change significantly so resources are not overused
  • High levels of photosynthesis
• Some tropical rainforests, e.g. the Amazon rainforest in South America and the Congo rainforest in Africa, have remained in their current state for tens of millions of years; they are
  
  • Highly diverse, e.g. the Amazon rainforest his thought to contain millions of invertebrate species, tens of thousand of plant species, thousands of bird and fish species, and hundreds of mammal species
  • High levels of light and moisture mean that photosynthesis rates are high
  • Organic matter is cycled by detritivores such as termites, slugs and worms, and by decomposers such as fungi; the nutrients are then taken up again by the trees
  • Water is cycled within the ecosystem as it is lost from trees by transpiration before condensing and falling again as rain
• Note that while healthy ecosystems are highly stable, as described above, they are not entirely static; natural selection, leading to evolutionary change, will always be acting on species

CC BY-SA 2.0, via Wikimedia Commons

The Amazon rainforest is millions of years old

• For an ecosystem to be considered stable, there are several features that it must have

Supply of energy

• A reliable source of energy must be present
• For most ecosystems this energy source is sunlight
• Light energy is converted into chemical energy by photosynthesis
  • Many photosynthetic organisms must be present, e.g. plants or algae
• Stored chemical energy is then passed up food chains through consumption

Recycling of nutrients

• In order for an ecosystem to support itself, the cycling of nutrients is essential
  • If nutrients are not recycled then the supply of nutrients will run out
• Nutrients are cycled when decomposers such as bacteria and fungi break down the carbon compounds, e.g. proteins and nucleic acids, in the tissues of dead organisms or waste matter
  • Carbon is released into the atmosphere in the form of carbon dioxide
  • Minerals such as nitrates and phosphates are released into the soil
• The nutrients released by decomposers can then be taken up again by producers and re-enter the food chain
• The conditions in an ecosystem must be suitable for decomposers
  • There needs to be enough oxygen and moisture, and temperatures need to be suitable; an ecosystem that is too hot or dry will have reduced nutrient cycling and so will be less productive
• If nutrients are removed from an ecosystem then cycling will be interrupted and productivity will be reduced, e.g.
  • Trees that fall are removed for timber rather than left to decompose
  • Crops are harvested

Genetic diversity

• Genetic diversity is one aspect of diversity; it can be defined as

The number of different alleles of genes present in a population

• High levels of genetic diversity mean that natural selection can act on favourable alleles, providing a population with the potential to adapt to changes in the environment
  • If a population has a limited number of alleles then the chances of one allele being favourable if the environment changes is reduced, and a population will be unable to adapt
• Genetic diversity allows populations to resist the effects of change in their environment

Climatic variables remaining within tolerance levels

• Genetic variation will only allow populations to resist the effects of change up to a point; if extreme environmental changes occur then they may be outside the tolerance levels of a species
  • Climatic changes to the environment include factors such as temperature and rainfall
• Changes that are outside tolerance levels will mean that a species needs to either migrate or face extinction
• Human activities are causing climate change at such a rapid rate that climatic variables are changing beyond tolerance levels in some ecosystems

• The stability of an ecosystem can be investigated using a model ecosystem known as a mesocosm
• A mesocosm is an experimental container in which a naturally occurring ecosystem is simulated
• Mesocosms can be used to study the response of an ecosystem to changes in specific factors such as nutrient and light levels
• Unlike a real ecosystem, it is possible in a mesocosm to control all of the factors other than the variable being studied
• Mesocosms can be set up in many different ways for many purposes
  • Water tanks can be set up on land to study the effect of sewage pollution on ponds or lakes
  • Underwater enclosures can be built in coastal waters or lakes to study the effect of temperature change or dissolved carbon dioxide on ocean ecosystems
  • Trees can be planted in large greenhouse-like buildings to replicate a rainforest to investigate the passage of carbon through this ecosystem
• Mesocosm experiments can be considered unrealistic due to their enclosed nature and the level of control that can be achieved
  • Realism can be improved by designing large mesocosms that share more of the features of a real ecosystem e.g. enabling mixing of layers of water in a large ocean mesocosm

Building a mesocosm in the lab

• It is possible to build small mesocosms in the laboratory
• Factors to consider:
  • The container should be transparent to enable sunlight to reach producers inside the mesocosm
  • Autotrophs should be included so that light energy can be converted into chemical energy inside the mesocosm
  • Small primary consumers such as zooplankton or other small invertebrates could be included, but it is important to consider whether the mesocosm is likely to be large enough to support them
  • Do not include secondary consumers in a mesocosm because there will not be enough energy in the food chain to sustain them for long, and it could be considered unethical to allow the primary consumers to be eaten in this way
• Mesocosms can be set up as open systems, i.e. without a lid, but sealed systems are more controlled, and therefore more useful for experimental purposes
  • Sealed systems prevent organisms and substances from entering or leaving
• In the lab, a mesocosm can be set up and then a known factor can be altered to assess its effect
  • E.g. different light levels, different temperatures etc.
• In order to assess the impact of changing one factor, a control mesocosm must be set up at the same time
  • A control mesocosm will be exactly the same as the experimental mesocosm, but the altered variable will not be changed
  • The purpose of this is to demonstrate that any change in the mesocosm is due to the altered factor and not another factor

Terrestrial mesocosm

• Place drainage material such as gravel in the bottom of a clear container
• Add a layer of charcoal on top of the drainage layer; this can help to prevent the growth of mould
• Place a layer of sphagnum moss or filter paper on top of the charcoal to provide separation between the base layers and the organic matter above
• Add a layer of soil or compost above the separation layer; this provides organic material and micro-organisms to aid with nutrient cycling
• Plant slow-growing producers such as healthy mosses and ferns in the growth medium
• Water the growth medium before sealing the container with a lid
  • The mesocosm may need watering while it establishes, but avoid excessive watering; once the mesocosm has stabilised, the plants should release enough water vapour during respiration to maintain moisture levels
• Place the container in a light location, and ensure that the temperature is stable

Aquatic mesocosm

• The base layer of the mesocosm should consist of organic substrate from the bottom of a lake or pond; this will provide naturally occurring nutrients and microorganisms
• Add lake or pond water; this ensures that it contains the required microscopic organisms and avoids chemicals from tap water
• Add healthy aquatic plants to produce carbohydrates and oxygenate the water
• Small aquatic organisms such as water fleas or water snails can be added, but not more organisms than the size of mesocosm can support
  • Only primary consumers should be used
• Place the container in a light location, and ensure that the temperature is stable

Mesocosm diagram

Mesocosms can be either terrestrial or aquatic

NOS: Care and maintenance of the mesocosms should follow IB experimental guidelines

• The IB policy on animals in schools states that investigations should only involve animals where no alternative options are available, and that any investigations that must involve animals should not be cruel, and should include measures that remove potential causes of animal distress
• For a mesocosm experiment, this may mean removing animals entirely
  • This is the most ethical approach, as not all mesocosms need animals to be sustainable
• If animals are required then the guidelines may mean only including a limited number of herbivorous animals in a carefully controlled environment
  • I.e. enough food should be available, the mesocosm should not get too hot or too cold, the investigation should not continue for too long, and the animals must be returned to their natural environment at the end