Vicia glaucopsyche, 2015

The nitrogen cycle represents one of the most important nutrient cycles found in terrestrial ecosystems since it is essential in the formation of amino acids, proteins, and nucleic acids.

This cow vetch (Vicia cracca) is a member of the family Fabaceae, along with Arctic Lupine and White Sweetclover.

Many species of Fabaceae have developed a symbiotic relationship with nitrogen-fixing bacteria. The bacteria propagate within the specialised root nodules and receive carbohydrates, in return for which the plant gains nitrogen.

Butterflies in the family Lycaenid, like these Silvery blues (Glaucopsyche lygdamus), have a long evolutionary relationship with Fabaceae and will preferentially lay their eggs singly on the flower buds of these plants. When the caterpillars emerge, they feed on newly opening flowers (as shown in Lupinus glaucopsyche).

Plants are not defenceless; however, and actively produce secondary metabolites. These compounds are not necessary for survival, but serve as defence against grazing insects and also attract pollinators.

In an evolutionary twist of fate, some insectivorous plants like northern bog violet plants have evolved to gain nitrogen from their insect prey.