Species extinctions

Of all the factors harming frogs and other species, the most ubiquitous and devastating is habitat destruction. Humans are radically altering more habitats than ever. Forests are cut down for lumber and turned into pastures or agricultural lands. Grasslands are plowed for agriculture leading to erosion and subsequent siltation of streams and lakes. Reservoirs flood streams and overuse of surface water and groundwater dries vital habitats. Wetlands and ponds are drained or filled for other uses. Urbanization, suburbanization, and exurbanization (domestic development outside of the suburbs) are destroying habitat for many species. Most land suitable for cultivation has been converted to cropland. Aquatic and terrestrial habitats are under assault every place on earth with even moderate human population densities.

These trends of global impact are global, with loss of most of the area of tropical rain forests (one of the top areas with biodiversity) predicted in the next few decades. The habitat destruction is causing extinctions at far greater rates than evolution can replace the species. I argue in my book, Humanity’s Footprint, that the pressures on natural habitats will intensify as population grows and our insatiable appetite for ever greater resource uses increases. Without drastic changes in the way we treat the natural habitats, extinction is inevitable for many species.

One of the strongest results of ecological theory and field research is that the number of species found decreases as the area of the habitat is decreased. Small islands have few species relative to continental regions. We know if we decrease the total habitat by 90%, we will loose around half of the species. Habitat destruction, even if it does not destroy the entire habitat, creates small islands of habitats. Ecological principles tell us that these small islands cannot support as many species as one very large area of natural habitat.

Frogs are but one of the species threatened by habitat loss; habitat destruction is a primary cause of a very rapid ongoing major extinction of plants and animals on Earth. This extinction is one of the 6 mass extinctions in Earth’s 3.5 billion year history. About 65 million years ago an asteroid slammed into the earth causing loss of about half the species in the oceans and many terrestrial groups of organisms, including the dinosaurs. Around 200 million years ago massive volcanic activity caused another mass extinction, and 250 million years ago 95% of Earth’s species were lost (the exact cause has not been determined). About 360 million and 440 million years ago there were two more mass extinctions.

Back to the species area calculation and how this influences extinction. Considering it is likely that we will decrease habitat by 90% in the speciose tropics, and that carbon dioxide increases in our atmosphere will continue to acidify the oceans and ultimately make it difficult for corals to survive in a warmer ocean, humans are causing an extinction that will rival other major catastrophes on earth.

The loss of amphibians has been compared to the loss of the dinosaurs. That was the last time a major taxonomic group was almost completely removed from the entire Earth’s fauna. The amount of terrestrial and aquatic habitats humanity conserves will ultimately determine the number of species that survive into the anthropocene (the current geological era of the Earth’s history where humans dominate the biosphere).

Amphibians arose around 400 million years and made it through four all four major extinction events that occurred since they arose. Modern frogs arose during the period dominated by the dinosaurs and survived the extinction that eliminated the dinosaurs. Now, amphibians, including frogs, are some of the hardest hit species of the current (human-caused) mass extinctions.

The Global Amphibian Assessment, the effort of a large international group of herpetologists, estimates that one third of the 5,918 known species of amphibians are endangered or extinct. In Panama, 55, or over 1/4th of the species, are threatened or extinct. Habitat loss and degradation affect almost 4000 species globally. Pollution is the second most common threat, and unknown causes, fires, and invasive species follow. Most of these causes are controllable or reversible. Disease is not.

No one factor can account for the disappearance or decline of frog species around the world. In some areas that seem pristine, there have still been substantial losses of frogs. In other areas the effect of chytrid fungal disease, UV exposure, pesticides and herbicide pollution, or habitat destruction are obvious. In most cases multiple impacts are occurring. Whatever the cause, frogs are becoming rare or extinct.

Amphibians are excellent indicators of ecological health. They are commonly surveyed by field biologists to indicate if an environment is degraded. Frogs and salamanders are among the first organisms to disappear when environmental damage occurs because most of them need both terrestrial and aquatic habitats, and their eggs develop in such an exposed fashion. Lakes, streams and wetlands where eggs and tadpoles develop integrate all the processes occurring in the stream.

Humanity should be deeply troubled that an entire group of organisms is disappearing from our planet, if nothing else this could indicate that the fundamental ability of earth to support us is being damaged. A broader moral interpretation, that other species have the right to exist, or for some, that humanity was placed on earth in part to take care of creation, provides reasons, other than the purely utilitarian reasons, to be concerned about amphibian declines