The ecological importance of rheas in the grasslands of South America is similar to that of the ostrich in Africa, emus in Australia, and kiwis in New Zealand. That is, they act as keystone species by eating seeds and fruits that have fallen on the ground, thus allowing the vegetation to thrive. They also eat insects and arthropods, keeping everything tidy. Rheas also disperse seeds throughout their territory via their droppings.

Sheep, cattle, and other introduced livestock have replaced rheas as a food source for humans over the past two hundred years. This has resulted in a decline of rhea numbers due to hunting and competition for grazing land. To make matters worse, farmers regularly shoot rheas as pests because they raid crops. Some people believe that the rhea (and other large flightless birds) are responsible for spreading seeds from weeds onto farmland; however, it is more likely that livestock are the cause of weed spread.

Rheas, large flightless birds native of South America, are the ecological equivalent of large grazing mammals. They play a key role in the functioning of grassland ecosystems, helping control plant growth and facilitating nutrient recycling.

Rhea populations have declined drastically in the last few decades due to hunting and habitat loss. In response to this decline, many rhea conservation efforts have been initiated throughout the birds' range.

Rheas are large ground-dwelling birds that belong to the same family as ostriches and emus. Though they cannot fly, rheas can run up to speeds of 35 miles per hour (56 kilometers per hour). Rheas can weigh between 27 and 70 pounds (12 and 32 kilograms), with males weighing more than females on average.