Over 99.9% of species that have ever lived are extinct. What causes the extinction of a species? Our dawn redwood page discusses extinction in terms of three species added to the park in March of 2025. We don't know what caused the disappearance of the Franklinia in the wild, but some general causes of extinction can be listed.

First, habitat destruction is the single biggest threat to biodiversity. This could be due to human activity, but it can also occur naturally. The cold-loving Franklinia may have been pushed south during the last ice age, where it survived in ice-free valleys with favorable microclimates. Once a small population of a species decreases in numbers, the genetic variation within the population decreases due to inbreeding. The loss of genetic variation makes it hard for that population to adapt to any further environmental changes. In addition, trees can't individually move. If a population can't gradually move north with the climate warming, possibly due to physical barriers, then it may suffer the consequences of having to stay put. Franklinia seems to be doing fine when planted in northern latitudes like Philadelphia, though attempts to reintroduce it into Georgia have been unsuccessful. Perhaps the Franklinia was able to move gradually south with the advancing ice sheet but was unable to return north to the cold environs to which it was adapted.

Second, humans may overexploit a species. The flightless dodo on the island of Mauritius was first encountered by sailors in 1598. By 1662 it was extinct due to its being easy prey for food. As discussed on our honey locust page, animal extinction can result in a plant species extinction. The North American megafauna were killed off when humans crossed the Bering strait land bridge 15,000 years ago. The megafauna were the dispersers of trees with large seed pods like the honey locust. The honey locust range was severely restricted as a consequence of the decreased populations of megafauna.

Third, invasive competitors or pests can wipe out a species. There is an American elm on the northeast corner of  20th and Hamilton Street that was planted three years ago and is really doing well. This specific cultivar seems to be resistant to the fungus that causes Dutch elm disease. This fungus came to the American continent in the 1940s and has almost wiped out elms in the wild. Invasive species can also outcompete native species to death. The trees along the Callowhill Cut on the other side of the fence on the south side of Baldwin Park are all invasive species, as discussed on our page Darwin in Baldwin Park. The Callery pear, Siberian elm, paulownias, white mulberry and catalpa there are all considered invasive in our region.