First of all, frogs have external fertilization and development. Therefore, any fertilized eggs are unprotected and vulnerable to drying out or being eaten by predators. Therefore, although thousands of eggs are released during spawning, not all become fertilized, and of those that do, not all complete their development to become tadpoles. Amphibians are especially vulnerable to pollutants in the environment. They are skin breathers and any pollutants in the air or water can be dangerous to frogs. All over the world, the numbers of Amphibians have been decreasing due to pollution. Habitat destruction is another possible reason for the declining numbers. Their decline is a major threat to biodiversity. Ozone layer destruction has allowed harmful ultraviolet rays to penetrate the lower atmosphere and this in turn is very damaging to amphibian's skin and eyes. Increasing temperatures due to global warming is another possible reason for the decline of amphibians. However, this problem is currently being studied.