Biodiversity

Biodiversity, short for biological diversity, is the variety and variability of life on Earth. It is the outcome of ongoing evolutionary processes that began about 4 billion years ago. The term gained currency in the 1980s as people became more aware of human-driven mass extinction and the many costs to society that are associated with rapidly diminishing biodiversity. The concept encompasses not only genetic and species diversity but also the multiple dimensions, dynamics, and processes of organisms and their ecological systems or ecosystems. Thus, biodiversity science investigates the past and present forms, patterns, [Page 32]functional traits, and interactions of ecological components, including genetic sequences, organisms, populations, communities, ecosystems, and landscapes. The global biodiversity crisis is one of the greatest challenges facing humanity. Solutions to this complex problem require human adaptive responses coordinated with strategies to address the other major sustainability challenges of our time, including global climate change, human population growth, energy demand, food insecurity, and poverty.

Biogeography is the study of the geographical distribution of biodiversity over space and time. The highest concentrations of biodiversity have been found in tropical ecosystems such as tropical rainforests and coral reefs. Habitat heterogeneity has also been associated with high levels of biodiversity. Species richness and species diversity are commonly used biodiversity metrics. Species richness is the number of species per unit of habitat. Species diversity is a measure of both species richness and species abundance for a given unit of habitat. At this time, our global inventory of plant, animal, and microbial species remains far from complete. Despite this uncertainty about the total number of living species that exist worldwide, estimates range in the tens of millions. To date, taxonomists have classified and catalogued fewer than 2 million living species.

Given these limitations of our biodiversity knowledge, there is also a great deal of uncertainty about the current rate of species extinction on Earth. Nevertheless, recent estimates of species loss per year range in the tens of thousands. Species extinctions and other types of biodiversity loss have occurred throughout the Earth's natural history. Evidence suggests that at least five mass extinction episodes occurred long before the emergence and spread of human influence on ecosystems and the environment. Although the biosphere has been transformed by human action for the past tens of thousands of years, the current human-induced mass extinction episode is unprecedented. Direct and indirect human impacts on the biosphere have greatly expanded and accelerated over recent decades.

Cities can contribute to biodiversity loss through direct disturbance of ecosystems, as well as the consumption of products manufactured or grown in a manner that damages ecosystems. As the global human population continues to grow, and urbanization continues, these impacts are expected to magnify. Some of the major threats to biodiversity posed by contemporary human activities and urban development are:

• Habitat alteration and loss resulting from land-use changes (e.g., deforestation, agricultural modernization, and urbanization)
• Over-exploitation and depletion of natural resources (e.g., overfishing)
• Alterations to biomes and hydrology as a result of anthropogenic climate change (e.g., glacial retreat and rising sea levels)
• Introductions of invasive species
• Nutrient loading and pollution of aquatic systems

To support biodiversity management and decision making, more research is needed to identify the conditions and processes that generate and enhance biodiversity. Indeed, some researchers have already been seeking to improve understanding of the human activities that contribute to biodiversity renewal and enrichment. Scientists are developing novel approaches for integrating multiple measures of biodiversity. For example, in addition to determining species richness and species diversity, biodiversity can also be measured by identifying and classifying the functional traits, roles, and relationships that are present or absent in a given ecosystem.

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