Around 6 million people moved to South Florida between 1940 and 1965. With a thousand people moving to Miami each week, urban development quadrupled.[50] As the human population grew rapidly, the problem of exotic plant and animal species also grew. Many species of plants were brought into South Florida from Asia, Central America, or Australia as decorative landscaping. Exotic animals imported by the pet trade have escaped or been released. Biological controls that keep invasive species smaller in size and fewer in number in their native lands often do not exist in the Everglades, and they compete with the embattled native species for food and space. Of imported plant species, melaleuca trees (Melaleuca quinquenervia) have caused the most problems. Melaleucas grow on average 100 feet (30 m) in the Everglades, as opposed to 25 to 60 feet (7.6 to 18.3 m) in their native Australia. They were brought to southern Florida as windbreaks and deliberately seeded in marsh areas because they absorb vast amounts of water. In a region that is regularly shaped by fire, melaleucas are fire-resistant and their seeds are more efficiently spread by fire. They are too dense for wading birds with large wingspans to nest in, and they choke out native vegetation.[51] Costs of controlling melaleucas topped $2 million in 1998 for Everglades National Park. In Big Cypress National Preserve, melaleucas covered 186 square miles (480 km2) at their most pervasive in the 1990s.[52]