When human beings first arrived on the terrestrial scene, more than 8,000 species of mammals and more than 21,000 species of birds existed. Today, the number of extant species of mammals stands at 5,020 and fewer than 11,000 species of birds remain.
Ecologists have long known that birds and mammals face high extinction risks. Much evidence indicates that most, though not all, of the recent mammal and bird annihilations are caused by human activity. By examining what specific human activity resulted in extinction events, ecologists can gain an increased understanding of the natural (nonhuman) environmental processes that lead to extinction and the time scale over which such processes generate extinction events. This kind of research can determine whether or not evolutionary mechanisms are capable of generating new species at a rate adequate to compensate for extinctions. In this manner, competing creation/evolution models can be put to the test.
Three European evolutionary biologists examined geographical variation as a predictor of mammalian extermination risk.1 They showed that extinctions were highest where large-scale land conversion (development) occurred, especially for high body mass species. In the process of their research, they found evidence for two correlations. First, the greater the degree and size of the land conversion the higher the rate of extinction. Second, the greater the body mass the higher the extinction rate. The biologists also discovered that the smaller the habitat size for a particular species, the higher its risk of imminent extinction. In terms of the immediate ecological situation, the researchers found that large body mass species "are at higher risk only in tropical regions."2 There are three obvious reasons for this result. First, land conversion as a result of human activity occurred decades and centuries ago in Earth’s temperate zones, whereas presently such conversion is most aggressive in the tropics. Second, habitat sizes for mammals tend to be smaller in the tropics than they are in temperate regions. Third, mammals follow Bergmann’s Rule, which states that the number of large-bodied animals and the average weight of those animals increases with higher latitudes.
The three evolutionary biologists concluded that "the interactions we describe between biological traits and anthropogenic threats increase understanding of the processes determining extinction risk."3 In their paper the three decline to comment on exactly what strictly natural extinction risks are implied or on how these risks challenge evolutionary models.
In biology there is no agreed upon definition of a species. Some definitions are so narrow that the different breeds of dogs and cows would qualify as distinct species. If such extremely narrow definitions are discounted (as is the case for the species numbers quoted above), then it is true that humans have never directly observed the appearance of a new mammalian species. That is, during the human era, 3,000 species of mammals have gone extinct but no new ones have appeared.
While the rates at which strictly natural land conversion occurs are typically slower than the those generated by modern human activities, they are not so slow as to give even the most optimistic evolutionary models opportunities to produce enough new species to counterbalance the animals driven to extinction. For example, ice ages occur every hundred thousand years, super-volcanoes every half million years, and super-wildfires every thousand years. As for the degree of land conversion change, human impact is certainly no greater than the impact produced by strictly natural causes.
Thus, at least for mammals, some kind, or kinds, of supernatural interventions are needed to explain how thousands of different species have been maintained upon Earth for the past sixty-five million years. As for the lack of such maintenance over the past fifty thousand years, Genesis 1 explains that after God created Adam and Eve He rested, that is ceased, from His work of physical creation.