The leaves of most plants absorb carbon dioxide (CO2) and oxygen (O2) from the air and capture the solar energy required for photosynthesis, while their roots draw water and dissolved minerals from the soil. Nitrogen compounds, phosphorus, calcium and many other minerals that plants need are contained in essential constituents or serve as enzyme activators. Some habitats – acidic peat bogs and marshes, for instance – are lacking in these substances. To overcome this lack, many plants form associations with fungi to form mycorrhizae on their roots.
In this symbiotic arrangement, the plant provides the fungi with sugars and other substances in exchange for substances that the fungi can more easily extract from the soil. But some plants have developed adaptations that allow them to use another source of nourishment: the insects that so often alight on their leaves. These carnivorous plants are found in nutrient-poor soils like Quebec’s acidic peat bogs.
These plants are not strictly carnivorous, though. Insects form only part of their diet. In fact, laboratory studies have proven that most (perhaps all?) of them can survive even without insect prey (Slack, 1979), although they become less vigorous and often produce fewer seeds. How much they decline depends on the quantity of nutrients available to their limited root systems. Even though they can survive on this diet, it is unlikely that they could resist strong competition in their natural habitat for long.
Reference: Slack, A. Carnivorous Plants, Mass., MIT Press, 1979, 240 p.