Variety in the natural world is an inherent part of its beauty and interest. But it can also be a critical factor in the survival of whole ecosystems. Biodiversity, defined as the variety of species living in an ecosystem as well as the genetic diversity that exists within populations of each species, provides stability to ecosystems, especially when they face changes. Factors that threaten biodiversity must be mitigated to help keep ecosystems and their members intact.

In a forest ecosystem, living members are interdependent, and they are also dependent on abiotic, or nonliving, factors in the environment, such as:

• water
  
• light
  
• temperature
  
• space
  
• topography
  
• soil type
  
• chemicals
  
• nutrients
  
• other factors

If something in an ecosystem changes drastically or rapidly -- for example, if fire sweeps through, if there is a sudden change in weather or if disease breaks out -- the changes could cause the death of many organisms, or even entire species. The resiliency of an ecosystem depends on having a diversity of species with varied adaptations to survive changes and help the ecosystem recover. Fire-hardy plant species will continue to live after a fire and can help keep soils intact and provide food for the surviving animals. Disease-hardy varieties of a species will pass on their genes after an epidemic, helping make the population stronger.

Since the organisms in forest ecosystems are interdependent, if one or more species or populations of a species disappears, it can have harmful effects on the rest of the ecosystem. Taking large populations of plant species from forests, such as trees for lumber, can greatly impact the survival of species that depend on trees for food, nesting or cover. Cutting down all mature trees from an old-growth forest can threaten populations of owls or other creatures that require the larger trees for nesting spaces. Even removing old logs or brush can reduce necessary cover that some animals depend on. Over-hunting or trapping carnivores can cause herbivore populations to explode, eventually resulting in a shortage of edible plants for the herbivores and possibly starvation. To help preserve biodiversity in forests, the harvesting and exploitation of forest resources -- old-growth trees, other plants and animals -- must be limited to sustainable levels that will help keep the ecosystem in balance.

Introduced or invasive species -- non-native organisms, including diseases, that are introduced to an ecosystem from other locations – can greatly disrupt ecosystems by killing, out-competing or even interbreeding with native species. For example, a non-native fungus, chestnut blight, wiped out millions of American chestnut trees after the disease was brought to North America, and the emerald ash borer, an Asian beetle, threatens ash trees across North American. Laws and practices that limit the unnatural dispersal of species to other areas can help reduce loss of biodiversity in forests. In addition, targeted manual removal of invasive species or removal by careful biological controls, such as the mottled water hyacinth weevil, which has had excellent results in controlling water hyacinth, can help give native species populations a chance to recover.

Pollution can damage organisms in a forest and cause loss of biodiversity. Acid rain, caused in part by pollution from coal-burning power plants, has weakened and destroyed many tree species, especially trees in high altitudes such as the Appalachian Mountains of North America. In addition, global warming, intensified by rising levels of carbon dioxide emissions from burning of fossil fuels, has been changing climate patterns and threatening biodiversity in forests. As global temperatures rise and complex changes occur in ecosystems, including changes in precipitation levels and shifts in species' geographic ranges, species adapted to cooler climates suffer and may die off. Reducing the overall “carbon footprint” – the burning of fossil fuels – by reducing energy use and using non-polluting energy sources such as solar, wind and other forms of "clean" energy, can help reduce global warming and help forest species to survive.

Cutting down forests for development or agricultural purposes obviously reduces their biodiversity. While some forest ecosystems can survive limited development within their boundaries or edges, taking down whole forests or causing their fragmentation can result in the loss of other species. For example, mountain lions, or cougars, require a large habitat range to hunt their prey or corridors between pieces of habitat. Fragmentation of that habitat results in mountain lions infringing on human spaces or having trouble finding mates. In addition, some animals, including northern goshawks, require large stands of mature forest with a closed canopy. And because forests, especially tropical rainforests, absorb large quantities of carbon dioxide from the atmosphere, cutting down large swathes can contribute to global warming, reducing biodiversity worldwide. By keeping as much of a forest system as intact and undisturbed as possible and avoiding "suburban sprawl" -- that is, by centralizing human development, preserving large, undisturbed natural areas around cities and neighborhoods and finding alternatives to destroying tropical rainforests -- habitat loss and fragmentation and the resulting loss of forest biodiversity can be minimized.