Q: What was the chief role of humans in causing extinctions?

HJ: My particular focus is on island extinctions. There have been a tremendous number of these, and they're ongoing. They're occurring as we speak. Looking specifically at these island extinctions, the chief role of humans was alteration to habitat -- the removal of the essential factors in the environment that the species needed to survive. Change in the fire regime has been important in many of the island extinctions. Tremendous population growth on certain islands in prehistoric times led to overexploitation of resources for the human population. This was absolutely necessary to maintain the population, but perhaps actually depleted resources in the environment, so that the human population itself began to collapse in numbers. At that stage, I think there was a real crisis -- and this is when probably most of the island extinctions occurred.

In Hawaii, where I have done most of my research, we estimate that maybe as high as 90% of the native vertebrates have indeed gone extinct, or are imminently threatened with extinction. So we need to have had major changes in the environment to cause this kind of extinction. Another way in which island extinctions have differed from continental extinctions is that many small animals have become extinct on islands. On continents, the human-era extinctions mainly involve large animals. It's very difficult for Stone Age people to hunt a small, abundant animal to extinction. Populations of widespread and abundant island animals would simply not be wiped out by using what the prehistoric Hawaiians had available to them as far as hunting: bird lime, bird nets, and so forth. The technology was insufficient to wipe out small animal populations. So I think that drastic environmental perturbations were required in order to cause as much extinction as we've actually observed.

Q: What is the relevance of prehistoric extinctions to our current biodiversity concerns?

HJ: One of our big societal concerns about biodiversity and about the environment at the current time is that we are transforming biodiversity reserves on continents into what are essentially habitat archipelagos. As we convert more and more land -- either for industry or agriculture, or simply for human settlement -- the biodiversity that we have on continents is being restricted to reserves of habitat, which are essentially islands within continents. If we look at the history of humans on islands, as prehistoric humans arrived on and colonized islands, a major biodiversity crisis ensued, and this happened all around the world. Many people don't realize that the biodiversity crisis dates back to biblical times and before. Around 2,000 years ago, when the aboriginal Polynesians arrived in Hawaii, over half of the native species of birds became extinct -- and this story is repeated all over the world on islands. Looking at the island record teaches us to be very concerned that all of our worries are justified, as far as conservation of biodiversity on habitat archipelagos, on continents. If we don't thoroughly investigate and try to learn from the island record, perhaps we will be just as poor managers of habitat archipelagos on continents as our predecessors were on islands.

Q: How would you convince skeptics that extinction rates are indeed high?

HJ: I would respond to that by exhibiting the actual hard evidence, the bone evidence, that many creatures have disappeared very recently. There is a long geological record of past life, and it's been studied for hundreds of years now. So we can document that many creatures have appeared on earth, and there have been periods of major extinctions. Looking at the most recent geological past, the Late Pleistocene -- in other words, the last ice age -- and our current epoch, the Holocene -- the last 10,000 years -- I would bring forth the actual bone evidence of many creatures that were widespread and abundant, and have simply vanished during the human era. On islands, I could show you bones of 200 species of birds that are no longer with us and have disappeared in the last 5,000 years or less This is a very high extinction rate, comparatively, with what has been recorded through most of the history of life. I think that the hard evidence is before us, and we have to accept that extinction rates are extremely high.

Q: Were there any facts from the symposium that would lead you to change your mind about how these extinctions in prehistoric times began?

HJ: I would say that I gained a wonderful appreciation for the complexity of the problem, and I really thoroughly enjoyed hearing the viewpoint and seeing the evidence that my colleagues presented. In particular, I enjoyed the viewpoint of Norman Owen-Smith, who, using modern African ecosystems as an analog for the Pleistocene of North America, explained the relationships of large herbivores to other animals -- that large herbivores can transform the environment in such a way that other animals may be dependent for their existence on the presence of those large herbivores. And I think there are many possibilities for interactions in what we might call the "complex ecological web" that could have contributed to the extinctions.

Q: You seem to have evidence that extinctions in small places could be protracted. Does this need to be considered in more detail by others?

HJ: The radiocarbon record is always extremely important to the study of these extinctions, and it's very instructive because it contests some of our preconceptions or hypotheses. In the Hawaiian instance, as we have gone and dated the actual bones of extinct animals, we have found that there may have been a period of perhaps a thousand years of human coexistence with the animals before even the most vulnerable species disappeared. That has changed my concept of what caused the extinctions. The idea of first contact with a naïve fauna is one that I have left behind in my thinking, because of the radiocarbon evidence. It has turned my thoughts toward what happens with human populations, and how they transform the environment. The human population in Hawaii was free of many of the diseases that control human populations elsewhere. There were more people living on each Hawaiian island -- except for the island of Oahu, where Honolulu is -- at the time that Captain Cook arrived in 1778 than are living there now. These people were not trucking in vegetables from California. They were living off local resources -- terrestrial and marine resources.

So there was a period of population crisis that the archeologists have recorded. It led to warfare, and it probably led to human population decline. It led to the organization of state societies and consolidation of power through warfare. It had large implications for the human population, and I think that it may have had large implications for animal populations -- because, at this time, the aboriginal Hawaiians had to expand their agriculture into marginal areas of the islands. They dispersed across the islands to areas that had not been settled before. Suddenly animal populations were in contact with humans much more than before. They were in contact with disease associated with human settlement; in contact with introduced domesticated animals that the Polynesians brought with them. Every kind of pressure on Hawaiian natural populations would have increased during this period of human population expansion. It's at that point that the major extinction could have been precipitated. And I think that could be a very important insight into understanding human-caused extinctions on islands and elsewhere. It could really have applications to our concerns about continental species, as well -- that it may require real contact with a human settlement before you begin to get extinctions, especially of the abundant small populations.

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