The human species has thrived on the planet Earth. Our population has soared to 8 billion strong and counting, and the ingenuity and development that has accompanied this growth to support ourselves is extraordinary by any measure. From establishment and expansion of agricultural systems to development of new transportation modes and corridors, we have substantially modified the planet to conform to our needs and desires. We are creative, resourceful, and enterprising.

We are also destructive.

One consequence of this development is a considerable loss and degradation of natural habitats that support the diverse plants and animals (biodiversity) on our planet. At present, almost 40% of Earth’s land surface is covered by human agriculture and over 70% of the remnant forests are within 1 km of a road or other human disturbance. As a direct consequence of such disturbances, the rate at which species go extinct is now 100 – 1,000 times greater than before humans dominated the planet. Compared to 50 years ago, there are 3 billion fewer birds in North America, insect populations have declined by nearly 50%, and the average vertebrate population (mammals, birds, fish, amphibians, and reptiles) has declined by nearly 70%.

Although there is a moral argument to be made about this biodiversity loss in terms of our responsibility to the other denizens of Earth, there is also a pragmatic one. As a matter of fact, we rely on other species to provide services that help support human populations and societies. Pollination of our foods, dispersal of our seeds, purification of our water, and even filtering of the very air we breathe is done by other organisms with whom we share the planet. Every resource we use is drawn from an ecosystem that relies on its components (i.e., its species) to function. In losing species we are thus degrading the very systems on which we depend.

Although biologists and ecologists don’t necessarily agree on how much space is required to support the remaining species, most estimates are in a similar ballpark. The late E.O. Wilson, one of the greatest naturalists in recent memory and a specialist in the field of biodiversity (among many other things), argued that if we protect half of the Earth as natural areas, we can expect to support 80% of the remaining biodiversity. The United Nations’ Convention on Biological Diversity, however, has set a more conservative goal of protecting 30% of the land, waters, and seas by the year 2030. More locally, this 30% target has been codified in the United States under the America the Beautiful conservation initiative. Regardless of the specific target, biologists generally agree that more unspoiled natural areas are better.

Scientists do not, however, agree on how these protected areas should be distributed in space. Since the 1970s, ecologists have been arguing over whether a single large conservation reserve is better for supporting biodiversity than multiple smaller reserves. A recognizable (if overly simple) example of this would be to envision a situation in which your county has a finite amount of money to purchase exactly 10 acres of land to set aside as a grassland preserve. The county has the option of purchasing a single 10-acre grassland plot that is centrally located, or 5 parcels that are 2 acres in size and spread relatively evenly throughout the county. Ignoring, for a moment, the sociopolitical considerations of the two decisions, which approach would be best for supporting the plants and animals that rely on local grassland habitat? The answer may depend on which scientist you ask.

At the heart of the matter is whether fragmentation has a positive or negative effect on biodiversity, a subject that scientists have been studying for decades. Fragmentation is the process of breaking apart large, contiguous tracts of a habitat type into smaller, more isolated patches. Fragmentation occurs, for example, when a road is built through a forest or when a housing development results in small, isolated pockets of natural habitat intermixed with houses and yards. In each of these scenarios, the loss of habitat accompanying development is inarguably bad for the species that rely on that habitat type. Whether the separation of the remnant patches into smaller units is, itself, detrimental remains controversial.

This disagreement remains because data from all over the world have yielded inconsistent results. In some cases, researchers have found that because smaller patches contain smaller populations, each individual population is more vulnerable to local extinction. Fragmentation can also make it more challenging or dangerous for individuals to move among suitable patches of habitat (think about a snake or turtle crossing the road). On the other hand, more fragmented and isolated populations cannot be wiped out as easily by disease outbreaks or a single major disturbance such as a fire or hurricane. Further, when patches are more spread out, they tend to be more diverse because they are exposed to more variable forces such as climates and weather patterns which can result in systems that support a more diverse group of species overall.

So, is a single large reserve better than several small reserves for supporting biodiversity? At present, the answer remains ambiguous, but scientists across the world (including at the USGS) are working diligently to uncover general patterns. The ultimate goal is to devise a set of principles that can help policy makers, land-use planners, and conservation organizations make the best decisions about how to spatially distribute protected areas. With careful planning based in sound ecological principles, we hope to develop a well-distributed network of protected areas that will maximize their conservation value.

The ONB features articles from Cooperative Fish and Wildlife Research Units across the country. Working with key cooperators, including WMI, Units are leading exciting, new fish and wildlife research projects that we believe our readers will appreciate reading about. This article was written by Jonathon Valente, jvalente@usgs.gov or jjv0016@auburn.edu, Assistant Unit Leader, USGS Alabama Cooperative Fish & Wildlife Research Unit, Auburn University, College of Forestry, Wildlife, and Environment.