Last week, a group of scientists announced that declining biodiversity may be tipping us toward Earth’s sixth mass extinction event. Coincidentally, another group of scientists reported evidence that a loss in biodiversity might have prevented the dinosaurs from surviving the last mass extinction event 65 million years ago. [read my article about it in Smithsonian Magazine online ]
Biodiversity is the amount of variation in biological life—diversity among plants, animals and ecosystems. The more varied life forms there are, the easier it is for an ecosystem to adapt to changing environmental conditions such as a warming climate, and the more likely something will survive a cataclysmic event such as the asteroid impact that wiped out the dinosaurs.
Biodiversity provides flexibility and resilience and underpins the stability of Earth’s ecosystems. We can see that on a very small scale when we consider what happens with bees in almond orchards. A study in 2013 showed, among other things, that honey bees don’t fly when it’s windy. That means almond flowers don’t get pollinated. But wild bees continue to fly in windy conditions, so almond orchards with both kinds of bees produced more fruit. More variety of bees means an orchard is more resilient to windy conditions. And that’s not just an interesting point; it could have an impact if climate change leads to a stormier, windier spring season when almonds are flowering.
That’s a very simple example of how the biodiversity of one type of organism (bees), affects the farming of one single tree (almonds) when faced with a change in one single condition (wind). Expand that out to the fields around an almond orchard, or consider that same effect on other flying insects and the things they eat or pollinate as well as the predators that eat them. The impacts can have ripple effects far out into the ecosystem. These dynamics are particularly noticeable because pollinators such as bees play a key role in their environment.
Large herbivores such as elephants and rhinos are also considered keystone species because of the role they play. In fact, the two new studies relating biodiversity to mass extinction suggest that diversity among large herbivores is vital to a healthy system.
The press release announcing that current biodiversity loss may herald the next mass extinction describes a striking example of the importance of large herbivores. The release cited previous studies in which elephants, giraffes and zebras were excluded from patches of land in Kenya. Grass and shrubs grew wild, soil became less compact. Seeds and shelter were more easily available to rodents, which doubled in number, along with the disease-carrying parasites they harbor. The loss of large herbivores clearly poses a significant threat to the stability of an ecosystem. The thing is, for large herbivores to maintain a foothold in a changing world, they need diversity.
According to Smithsonian Paleontologist Matthew Carrano, the 10 km asteroid that hit the earth 65 million years ago would not have been enough to kill off the dinosaurs if they hadn’t already been experiencing a decrease in diversity among large herbivores. Carrano is a coauthor on the recent study of dinosaur diversity mentioned above. Over the 160 million years that dinosaurs walked the earth, there had been other major asteroid strikes, but the dinosaurs survived.
“Dinosaurs come and go for 160 million years at a relatively regular rate,” Carrano said, “and they’ve survived some other extinction events that other organisms disappear at.” At the time the asteroid hit, their numbers were as high as they’d ever been, according to Carrano, but their diversity had been declining for about a million years or so. The dip was small, maybe ten percent, and Carrano says it would likely have begun to rise again as new species evolved. But then a 10 km wide asteroid hit, shaking the earth with tremors 1000 times greater than any earth quake ever known, and plunging the planet into a nuclear disaster-style “impact winter”.
The impact and its aftermath wiped out more than 50 percent of the species on the planet, including all non-avian dinosaurs. According to Carrano and his colleagues, the decline in diversity among keystone herbivores would have reduced their resiliency to such a cataclysmic event. They would have suffered, and their decline would have had ripple effects throughout the ecosystem.
Why herbivores were losing diversity remains unclear, although there’s speculation that extreme volcanic activity at the time had been causing climate changes that threw the ecosystem out of whack. Carrano says there are signs that other animals aside from dinosaurs were showing signs of stress around the time of the impact as well, so there was clearly something going on, and climate change is a good place to look.
In fact, the past is always a good place to look when considering what might be coming in the future. Although we can’t predict when another large asteroid will slam into the earth, human actions alone are causing very rapid changes in both climate and biodiversity that, it could be argued, are tantamount to a cataclysmic event.
Among large herbivores, we’ve hunted many subspecies of rhino to extinction including the Western Black Rhino and the Javan Rhino. African Elephant populations have been decimated from a few million 100 years ago to fewer than half a million today. Among predators, 75 percent of the world’s top carnivores such as lions, wolves and polar bears are in decline.
Globally the picture is astounding. According to the UN Environmental Program (UNEP) Conference on Biodiversity: Species have been disappearing at up to 1000 times the natural rate, and this is predicted to rise dramatically. Based on current trends, an estimated 34,000 plant and 5,200 animal species – including one in eight of the world’s bird species – face extinction.
Many scientists still believe that decimated ecosystems can recover, and the earth will bounce back, but for that to happen, humans have to change the way we live. Maybe it’s time to listen to what the dinosaurs have to tell us.
To learn more about international efforts to improve biodiversity, combat climate change and reduce anthropogenic degradation of the Earth, visit the United Nations Environmental Program, the World Wildlife Fund, Conservation International.