Species decline and extinction has gone on since the beginning of life on Earth. In some cases, species are extincted or in decline as far as humans can observe them, but have simply migrated or retreated to more remote and inaccessible locations. It is a simple matter to look at some common factors that cause life to thrive and some common factors that cause life to falter, then fail.
Thriving factors include food, fresh water, non violent weather, stable land, appropriate temperature, chemistry and light levels in aquatic environments, light levels on land, soil quality, waste management, defenses against predators and defenses against disease. Also included are proper balances between predator and prey where there is not an overload of food that can make some populations too large to sustain both the food source or habitats that sustain life.
Faltering and failing factors include overgrazing and soil depletion, imbalances between predators and prey that come from overeating, or lack of natural defenses that kill some predators and allow some prey to over populate until they decimate the food sources.
Disease, microbial and fungal spreads that vary in virulence or a specie's lack of ability to produce natural protections or adaptations clearly contribute to decline of species. Changes in seasonal, aquatic or atmospheric temperature that life forms cannot adapt to can cause migration to less survivable areas, failure to thrive or death. Some species are sensitive to a few degrees of change or temporary changes in temperature alone.
Weather can decimate land forms that serve as habitat when violent storms, heat waves, flooding, lightning fires and other natural occurrances upset the delicate balance of life and biome.
Half of the marine oil intrusion is natural, from seepage of oil under the water or sea. Life forms have a way of adapting and thriving in such environments, especially those that exist in the abyssal zones as scavengers of any nutrients that show up.
New human or anthropogenic factors that lead to the decline of species are being discovered every day, it seems. There is deforestation that either kills or forces life into already populated areas, then leads to soil depletion through erosion. There are the unnatural anthropogenic oil spills that, combined with chemical dispersant, create rapid and life ending challenges to the marine and ocean riparian habitats. Species simply cannot adapt to such substances and the entire food chain is decimated, making for catastrophic cases of species decline.
Pollution from pesticides, fertilizers, human and animal wastes and even chemicals that are run into waterways through drainage systems have wreaked havoc in riparian freshwater and marine environments, causing the decline of species in many ways.
Species decline from pollution can come from reproductive and metabolic problems that chemicals cause. From damage to reproductive organs, sperm and eggs to damage at the genetic level, pollution can wreak reproductive havoc that is irreparable, but not always easy to observe or detect in the undomesticated animal kingdom. Metabolic problems are simply poisoning that happens when the substances either interfere with, stop, or damage essential systems that cannot break down the substances and eliminate them through waste.
In humans, plants, aquatic life, birds and animals, there is either natural capacity to control spawning, an environmental or disease factor, or synthetic forms of birth control, that lead to abortion, abandonment, failure to gestate, killing of offspring and other factors contribute to either low birth rates or failure to make it to maturity, which leads to declines from generation to generation in some species.
Finally, there is species decline from introduction of alien or invasive species by humans, by natural occurrences or by forced migration. When the indigenous or native species has no defense against a new, aggressive or predatory plant, animal or microbe, then the native species will suffer great decline, either from the new predator, loss of habitat, or inability to migrate or adapt in time.