Ecosystems Under Stress

Producers in an ecosystem fix 90 000 kJ per square metre per year (NPP), and the primary consumers store 9000 kJ per square metre per year. Calculate the efficiency of energy transfer to the primary consumers and explain the loss.

1. 01Set up the calculation

   Efficiency = (energy transferred / energy available) x 100 = (9000 / 90 000) x 100.

2. 02Calculate

   This gives 10 per cent.

   $$\dfrac{9000}{90000} \times 100 = 10\%$$
3. 03Explain the loss

   The other 90 per cent is lost as heat from respiration and movement, and in parts of the plants not eaten or not digested - which is why so little energy reaches the next level.

Result: The transfer efficiency is 10 per cent; the remaining energy is lost to respiration, movement and material not eaten or digested.

An area of heather moorland shows no sign of developing into woodland despite the climate being suitable for trees. Explain why, using the idea of a plagioclimax.

1. 01The natural climax

   The climate would naturally support a climax of woodland, so if left alone the moorland would in time succeed to trees.

2. 02The arresting factor

   The moorland is regularly burned and grazed (for example for grouse and sheep), which removes tree seedlings and the taller vegetation.

3. 03The result

   This human management arrests the succession at the heather stage, so the moorland is a plagioclimax - held below its natural woodland climax by the arresting factor.

Result: Regular burning and grazing arrest the succession below its woodland climax, maintaining the heather moorland as a plagioclimax.

Explain why, despite its enormous biomass, a tropical rainforest is easily and lastingly damaged when it is cleared for agriculture.

1. 01Where the nutrients are

   The Gersmehl model shows the nutrients are held mainly in the living biomass, not the soil, and are cycled rapidly through fast decomposition and uptake.

2. 02Effect of clearance

   Removing the trees removes the main nutrient store and stops the recycling; without the canopy, heavy rain leaches the exposed, already-poor soil.

3. 03The lasting damage

   The land quickly loses its fertility and cannot sustain cultivation, and because the store was in the lost vegetation, recovery is slow - so the damage is lasting.

Result: The nutrients are in the biomass, not the soil, so clearance removes the nutrient store and exposes poor soil to leaching, causing rapid and lasting loss of fertility.

Explain how the introduction of an invasive predator to an island ecosystem can reduce its biodiversity and disrupt its functioning.

1. 01The introduction

   An invasive predator arrives with no natural controls (predators, parasites or competitors) and native prey have not evolved defences against it.

2. 02Direct effect

   The predator eats native species faster than they can reproduce, so their populations crash and some may become extinct - a direct loss of biodiversity.

3. 03Knock-on effect

   Removing those species disrupts the food web (their prey may boom, their own predators lose food), reducing the ecosystem's stability and the services it provides - so the impact spreads beyond the species directly eaten.

Result: The uncontrolled invasive predator drives native species down or to extinction and disrupts the food web, reducing both biodiversity and ecosystem functioning.

A coral reef is protected by a marine protected area with fishing controls, yet it continues to decline. Evaluate why the local management may not be enough.

1. 01What local management achieves

   The marine protected area and fishing controls reduce local pressures - destructive fishing, pollution and physical damage - which genuinely helps the reef's resilience.

2. 02The global driver

   The reef is also stressed by warming and acidifying oceans caused by global greenhouse-gas emissions, which cause coral bleaching and which no local rule can control.

3. 03Reach a judgement

   A defensible conclusion is that local management is necessary and buys resilience, but it cannot succeed alone against a global driver - so sustainability requires local action combined with global emissions reduction.

Result: Local protection reduces local stresses but cannot address the global driver of warming, so the reef's sustainability depends on combining local management with global climate action.