The study of mutualism once mainly involved noting that plants and pollinators were pretty important, then moving on to the next topic. Now, it's clear that mutualisms are extremely widespread. All eukaryotes are the result of an ancient mutualism between bacteria which became mitochondria and an early forerunner of today's eukaryotic cell.
Chloroplasts are also former bacteria.
Even if you exempt those mutualistic relationships from consideration, it's hard to find a plant or animal not involved in some sort of mutualistic relationship. The relationship between flowering plants and pollinators is the most well-studied of all mutualisms. We could devote a whole semester to the topic, but we won't. Mycorrhizal fungi Mycorrhizal fungi form a mutualistic relationship with plants. The fungi increases the absorption of nutrients in the soil, chiefly phosphorus.