Why does the species richness not continue to increase over time?

This question has created a stir among conservation biologists and ecologists that has lasted for 15 years. New insights into the debate are critically reviewed in an editorial by the editors of the international scientific journal Biological Conservation, and a separate article in the same journal. The evidence affects the core tenets of conservation biology: How do humans influence biodiversity? And how do we set and measure conservation goals?

Many recent ecological studies have found the surprising result that the number of species (i.e., species richness) at sites around the world has remained stable on average or is even increasing. These sites include wide range of species and habitats-plants and animals; and forests, grasslands, freshwaters, and oceans.

"These findings highlight the need for conservation biologists to avoid oversimplification when making the case for conservation and selecting indicators of success," says Dr. Richard Primack, lead author of the editorial and an editor at Biological Conservation. "'More species is good' is a seductively straightforward and attractive argument. However, it ignores scale (biodiversity can increase locally, but decline globally), it ignores ecological processes and interactions, and it ignores many other aspects of biodiversity-such as changes in ecosystem services and loss of genetic variation-less obvious to non-biologists. It also misses the ethical, cultural, and aesthetic values of certain species and ecological communities, such as monarch butterflies and redwood forests. These aspects of biodiversity can still be damaged or lost, even as species richness remains steady or increases."

Some ecologists argue that the trends should not be considered as true "global" patterns of changes in local biodiversity. The reason is that the studies exclude sites that were paved, turned to farmland, or recovering from past disturbance (e.g., abandoned farmland returning to forests), and include sites that are not evenly distributed around the world. These ecologists argue that a fair analysis would conclude that biodiversity is generally declining both globally and locally.

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