Island Biogeography and Avoiding the Next Mass Extinction

This is a geology class, and biodiversity may seem to be a bit far-afield, but there are many, many links between our class and biology, so let’s take time for a quick detour. We saw that there have been mass extinctions in the past—times when many living types became extinct in a short interval. We are making decisions now that will control whether a geologist far in the future will identify our time as another mass extinction, the end of the Cenozoic and the start of the Anthropocene.

Early humans were surprisingly hard on biodiversity. Wherever humans arrived with their efficient tool kits—in Australia, New Zealand, other islands, the Americas—extinctions of large animals followed. Direct human hunting, or competition from the rats, pigs, dogs, and others that arrived with the humans, likely contributed. The Smithsonian Museum of Natural History Hall of Deep Time shows that arrival of modern humans had smaller effects where humans and animals had long coexisted (extinction of 7% of large animal species—more than 50 kg or 110 pounds—in Africa and 18% in Eurasia), but huge impacts where animals were not already familiar with us (extinction of 74% of large animal species in North America, 82% in South America, and 97% in Australasia).

Some people don’t like the idea that early humans were hard on biodiversity. Many people, including good scientists, have argued that the extinctions of large animals in the Americas were caused by climate change, which happened to occur at about the same time as human arrivals in some places. Dr. Alley has listened to talks in which data he helped produce were used to argue that the climate changes were so large and rapid that they must have been responsible. But, the work by Dr. Alley and others showed that the animals lived through dozens of such abrupt climate changes before going extinct just after modern humans arrived. Climate change probably did reduce populations of many species, but that also occurred at each of the earlier abrupt climate changes that did not cause extinction, so a human role in the extinctions is unavoidable.

The earlier extinctions were mostly of large creatures. Since the industrial revolution, “modern” humans have contributed to the extinction of various creatures. And, the rate of extinction may pick up soon as we increasingly occupy the planet. To see why, let’s take a little detour into island biogeography.

Island Biogeography

If you were to visit a lot of different-sized islands that are more or less the same distance from the mainland, you would find that the bigger islands have more species. Roughly, an island with ten times the area of another will have twice as many species. If you visited islands of about the same size at different distances from the mainland, you would find that those closer to the mainland have more species.

At least some of what controls these observations is not too difficult to understand. If you have a small island, it can hold only a few individuals of a species. From year to year, populations go up and down depending on food supply, predators, and other things. With a small population, a small drop can hit the absorbing boundary of zero individuals and cause extinction, but a large population can survive a small drop. So, extinction is more likely on a smaller island, causing smaller islands to have fewer species. The mainland is there to supply new individuals to islands to replace those that die, swimming across or floating across on logs or in other ways, and repopulation is easier for islands closer to the mainland, so those islands closer to the mainland have more species.

These patterns of island biogeography are well-established. Studies of the repopulation of islands sterilized by volcanic explosions, and even of very, very tiny islands that were deliberately depopulated and then allowed to come “back to life,” have shown that this is the way the world works naturally.

Now, think about Yellowstone. Originally, the boundaries drawn for the park separated wilderness inside from wilderness outside. Today, as shown in the satellite photo, some of the park boundaries are easy to see from space because loggers outside the park work right up to the boundaries. Yellowstone remains connected to other wilderness regions in other directions; it is not an island (yet), and indeed, some of the logging shown here is being replaced by regrowth of trees that have not yet been logged again.

But what if Yellowstone were an “island,” as some other parks are or soon may be? Suppose a park becomes surrounded by farmland, which is used to feed humans and keep us alive. Farmland does not support a lot of wild orchids or other rare species. Farmland is impoverished in biodiversity, with just a few species, carefully selected to feed us. A park surrounded by farmland is in some ways an island, because many species have great difficulty crossing the farmland just as many species have difficulty crossing the ocean. And, from the well-established principles of island biogeography, the isolation of a parkland from other wilderness will cause extinctions in the park. Perhaps more worrisome, if the only remaining wilderness is in parks, there is no longer a “mainland” to replace species lost to local extinction on the island—extinction in the park is then extinction from the world, as shown in the video below.

We know that as the climate changed in the past, plants and animals migrated long distances to stay with their preferred climate. As the climate changes in the future, migration will be required but may be impossible if the parks become isolated.

We can use a cartoon terrarium to illustrate some of the basics of island biogeography, and how isolating Yellowstone and Glacier from each other could cause extinctions in both. Have a look at this short video.

Video: Small-Scale Biodiversity (1:51 minutes) 

Let's take a look at some charismatic macro fauna

So, Who Cares?

One can ask whether biodiversity is worth preserving. This is proving to be a difficult topic and one that will be discussed much in the future.

To start, we can easily list many reasons why biodiversity is good and should be preserved.

• Many of our medicines have come from plants, and if many plants become extinct before we can study them carefully, we are likely to lose many possible medicines.
• Engineers and designers are increasingly using “biomimetic” techniques—mimicking nature. Evolution has worked over vast times to select the most successful biological patterns, and we can learn from them, if they are here to be learned from.
• More diverse ecosystems seem to be a little more productive and more reliable (if you have hot-loving and cold-loving and wet-loving and dry-loving types in a region, then something will grow well no matter what weather arrives; if you have only one type, and the weather is bad, so is the crop), so if producing more is good, biodiversity seems good.
• Living things have frequently served as “canaries in coal mines”. Miners would take a canary along in the mine, not only for companionship, but because the birds were more sensitive to bad air than were people, and a sick or dead bird would warn miners to get out before the miners became sick or dead. Birds of prey served that function for us with DDT. This chemical was being used to kill pests, increase crops, and wipe out diseases—until the falcons, hawks, eagles, and other predatory birds started disappearing. A little DDT on a plant led to more DDT in an insect that ate lots of plants, and still more DDT in a bird that ate a lot of those insects, and became so concentrated in a falcon that ate the birds that the falcon’s eggs broke and young ones couldn’t be raised. It became clear that such “bioconcentration” threatened humans as well—the other living things gave us a warning. Loss of biodiversity means loss of warning sensors.
• Many people like diversity, and pay a lot to go see it in different countries and in zoos, supporting an important industry.
• And, for many people, this is an important moral or religious issue—do we really have the right to terminate the existence of other living things, that some people believe were put here by a deity that likes those things?

Some planners today are trying to establish corridors connecting wilderness areas, so that the parks do not become islands and lose species. How successful this plan will be remains to be seen. The “simple” answer is that to maintain many species on Earth, we have to maintain much wilderness. And that in turn has implications for how we humans choose to behave. The 30 by 30 initiative has been adopted by the US Government as of 2021, seeking to protect 30% of land and of sea habitat by the year 2030.