1. To pollinate 87% of plants. 75% crops need insect pollination.
2. Recycle dung, leaves, corpses.
3. Keep soils healthy.
4. Control pests, though they can also be pests.
5. Food for larger animals eg. fish. Crickets are 12 times more efficient than cows in converting vegetation into digestible body mass, produce little or no methane and use 55 times less water.
6. Insects are in all food chains.

Problems –

German nature reserve studies – over 27 years the insect biomass has dropped by 75%. Forest and grassland had a 40% drop over 10 years.
UK butterflies – over 41 years a 46% drop in common species numbers. 77% drop in rare species numbers.
US studies – 70% drop in insectivorous birds in 20 years.

Source:- Dave Goulson “Silent Earth” 2021