Brent Cornell

    
A limiting factor is an environmental condition that controls the rate at which a process (e.g. population growth) can occur

• Limiting population factors can exert their influence via either top down or bottom up control

Top Down Control

• Top down factors are pressures applied by a higher trophic level to control the population dynamics of the ecosystem
• The top predator either suppresses the abundance of its prey or alters its behaviour to limit its rate of population growth
• Top down control results in an oscillating trophic cascade (suppression at one level increases numbers at the next level)
• Keystone species commonly exert top down control by preventing lower trophic levels from monopolising essential resources

Bottom Up Control

• Bottom up factors are pressures that limit the availability of resources to lower trophic levels (e.g. producers)
• A lack of resources at lower trophic levels suppresses the abundance of organisms at higher trophic levels
• Population growth will be reduced for all higher levels as the suppression of the 'bottom’ restricts energy supply to the ‘top'
• Human activity can often limit resource availability and hence inadvertantly exert bottom up pressure on an ecosystem

    
An algal bloom is a rapid increase or accumulation in the population of algae in a water system (e.g. lake or ocean)

• Algal blooms are typically caused by the sudden enrichment of nutrients in the water due to run-off (eutrophication)

Algal blooms generally have a detrimental effect on the wider aquatic ecosystem:

• The spread of algae will block out sunlight below the surface and reduce photosynthesis by phytoplankton and seaweeds
• The reduction in light will cause algae to respire instead of photosynthesise, reducing levels of dissolved oxygen in the water
• As algae begin to die, an increase in the numbers of bacterial decomposers will further reduce levels of dissolved oxygen
• Without adequate levels of light or dissolved oxygen, most aquatic organisms within the environment will struggle to survive

Algal blooms can be limited by measures that exert either bottom up control or top down control

• The most success will be had if bottom up and top down control measures are used in combination

Bottom Up Control:

• Algal blooms can be reduced by limiting the supply of nutrients such as nitrogen and phosphorus in the water
• This may involve reducing the use of fertilisers for agricultural practices to limit the nutrient input from surface runoff
• Nutrient reduction can be expensive to implement and difficult to police, as it requires a concerted community effort

Top Down Control:

• Algal blooms can be reduced by introducing piscivorous (fish-eating) fish into the aquatic ecosystem
• The piscivores will feed on zooplanktivores – and by reducing their numbers, will increase the number of zooplankton
• Zooplankton (such as Daphnia) feed on algae, and hence will reduce the population of algae via herbivory
• Introducing piscivores can have unintended consequences on food webs and should be done with caution

Controlling Algal Blooms