Heather moorland is a habitat that is relatively common in the UK but rare elsewhere in the world.

The diagram shows an example of biomass transfer in a heather moorland ecosystem.

The numbers below the arrows represent the percentage of biomass transferred to the species shown in the next trophic level.

The hen harrier is the top predator on heather moorland in the UK$.$

Scientists are concerned about a recent decrease in the population of hen harriers.

The current estimate of the hen harrier population in the UK is $545$ pairs. This represents $71\%$ of the estimated population in $2004.$

i$.$ Calculate the estimated population of UK hen harriers in $2004.$

Estimated population $=$ pairs $[2]$

ii$.$ Since $2004,$ the population of red grouse in the UK has been relatively stable and it is not thought that the population has been affected by changes in climate.

Suggest an explanation for the decrease in hen harrier numbers since $2004.$