The symbiotic relationship that hummingbirds have with nectar producing plants is unique.  North America alone has more than 200 nectar producing species that are pollinated by insects, birds and bees.  Of those 150 species depend of hummingbirds for dispersion of their pollen.    Add Central and America and the number flower species climbs into the thousands.  Without this pollination, the flower species would die out.  

Unlike bees, hummingbirds cannot smell.  They spot their targets by the vibrant colors, so flower species that depend on the hummingbirds are usually bright colored reds, oranges, pinks, yellows and purples.  Many of the flowers have adapted to specific hummingbirds. There is even one type of heliconia that only will be pollinated by two species of hummingbirds.  Many blossoms hang downwards, making them ideal for the hummingbirds and suited to the way they hover.

In order to consume nectar from flowers, hummingbirds must visit hundreds of them every day.  When they feed, their beak and the top of their head will rub against the pollen inside of each flower.  They will then carry this from flower to flower, spreading pollen as they go.  

Thousands of flowers depend on hummingbirds for pollination.  All over the Western Hemisphere flowers and hummingbirds have evolved in a symbiotic relationship. 

Plants get their energy directly from the sun.  A plant which produces sugar (either by producing nectar or sugar cane which is converted to granulated sugar and placed into feeders) then transfers this energy to the hummingbird.  The hummingbird might obtain energy from an insect that obtained it by eating a plant.  

Hummingbirds have many predators that will move the energy up the food chain.  Small predatory birds such as Sharp-skinned Hawks, American Kestrels, Merlins, Mississippi Kites and Loggerhead Shrikes will target hummingbirds. Owls might spot a hummer in torpor on a branch at night.  

Large spiders such as the Orb Weaver and the Praying Mantis will kill and eat hummingbirds.  These insects might then be eaten by a bird, again moving the energy up the food chain.  Snakes, lizards and frogs can catch them or target their eggs.  Bats will target young hummingbirds and their eggs.  In the tropics, omniverous birds such as the toucan will target baby birds.  Corvids (jays, crows and ravens) will also target them.