Birds dropped out of the sky, dead fish covered miles of the surface of rivers and bays, the media started digging up any reference to mass animal deaths they could find, and the public voiced a growing concern about what it all meant.

These kinds of die-offs are unusual but not unheard of in the nature, and so the good news is that while alarming, they don’t mean the world is ending and probably won’t have too much impact on the overall survival of the species that have experienced them.

While most wildlife experts see little cause for significant concern with these events, there are some mass wildlife deaths that we really should be worrying about.

5. Colony Collapse Disorder

Colony Collapse Disorder (CCD) causes honey bees to mysteriously abandon their hives and die. First reported in the United States in 2006, scientists are still trying to figure out the exact causes. While not native to North America, honey bees are critically important for the pollination of over 100 crops that both people and our livestock rely on for food. Eighty percent of all crop pollination service in the U.S. is provided by honey bees, which means that one-third of all the food we eat is directly the result of these insects. Honey bees also play an important role pollinating wild plants that wildlife depend on for survival. This mass die-off of honey bees could have significant economic and ecological repercussions.

Theories for the cause of CCD include infestation by exotic mites, viruses, a fungus, pesticides or other chemical pollutants, global warming, stress on hives from industrial beekeeping practices, or a combination of these factors that is suddenly pushing millions of honey bee hives over their tipping point and ultimately to death. While native pollinators can help fill in the gap caused by honey bee CCD, unexplained mass die-offs in several native bumble bee species are now also being reported.

4. White-Nosed Syndrome

North American bats are dropping like flies as a result of this mysterious ailment, which is characterized by the growth of a white fungus on the face of bats that hibernate in colonies in caves during the winter. The bats repeatedly wake up from their hibernation and fly about despite the cold temperatures and lack of insect food. In doing so, the bats burn off critical calories and ultimately die. Some bat hibernation caves have experienced mortality rates as high as 99 percent and since 2006 millions of bats have succumbed.

Scientists don’t know if the fungus is the cause of the odd behavior and killing bats directly, or if it is simply a secondary symptom of some other problem. One thing is certain, White-Nosed Syndrome has spread rapidly across the country, adding additional threat to endangered species such as the Indiana bat and drastically reducing once-common species like the little brown bat. Scientists are still searching for a clue as to the cause of these devastating mass bat deaths.

3. Global Amphibian Decline

Amphibians are often considered to be ecological “canaries in the coal mine” because their sensitive skin allows for the exchange of gas and liquids, making them particularly vulnerable to pollution and other disturbances to their habitat. As a result, amphibians are often the first group of animals to die out in disturbed or polluted environments. Dying out is exactly what amphibians are doing all around the world, and scientists don’t know why.

As with Colony Collapse Disorder, any number of causes could be at work either by themselves or in concert, including air and water pollution, global warming, habitat destruction, invasive species and most notably the type of chytrid fungus known as Batrachochytrium dendrobatidis or “Bd” for short. This fungus was discovered in 1999 and has been rapidly spreading and killing mass numbers of amphibians on several continents, including both North and South America, Europe and Australia. As with White-Nose Syndrome in bats, it’s not known whether this chytrid fungus is a new, random pathogen or if it has always been present and is only now spreading because of other, as-yet-unknown reasons. The statistics are frightening: thirty percent of amphibian species on the planet are listed as either threatened or endangered and another six percent are listed as near threatened. Scientists don’t know the status of another twenty-five percent.

2. Gulf Oil Disaster

The official wildlife body count of the Gulf Oil Disaster is 5,686 dead birds, 546 dead sea turtles, and 96 dead dolphins and whales. And that’s just the animals that rescue workers were able to recover in the vast area of the Gulf of Mexico affected by the millions of gallons of oil that gushed into the Gulf’s waters and coastal wetlands when BP’s Deepwater Horizon rig exploded. It’s doubtless that the wildlife death toll is more likely in the millions when you factor in open ocean species such as sperm whales or whale sharks that sink when dead or that might not immediately die but eventually succumb to slow poisoning as they eat contaminated food, as well as the fish and marine invertebrates that have also died but for which no one has a count. Even worse, judging from previous oil disasters such as the Exxon Valdez in Alaska, we can expect wildlife will continue to die for months, years or even decades as a result of this disaster.

1. Global Warming

The scale of the impact that global warming is predicted to have on wildlife across the planet can’t be understated.

We are already experiencing the rapid melting of glaciers, more severe storms, an increase in droughts, wildfires and flooding events, the spread of invasive species, and the record decline in Arctic sea ice making the long-term survival of species such as ringed seals and polar bears uncertain.

Countless other wildlife species around the globe will be negatively affected as global warming destabilizes ecosystems unless we act quickly to change the root causes.