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Species Diversity
At small scales, like a single pond or patch of forest, it is possible to conduct an exhaustive survey of the animal and plant life. At the level of a single habitat, biodiversity is often measured using species diversity. Species or taxonomic diversity is a measure of the number of species in an ecosystem (species richness) and the ̶0;evenness̶1; of those species. Evenness is the relative abundance of the organisms -- a community with similar numbers of all species present would be very even. If an ecosystem has high species richness and evenness, then it has high species diversity.
Ecosystem Diversity
At larger scales, biodiversity isn't just a measure of the number and evenness of species; it also includes information about ecosystem diversity. Ecosystem diversity is a measure of the variety of habitats present in a landscape. Areas with particularly rich habitat types can support more types of species, as well as species that require different habitats for different parts of their life cycle. For large-scale assessments, investigators put together the information that already exists and also conduct new surveys. Information that is particularly important includes full lists of species and their locations in the area. Researchers also assess the extent and rate of change of the different types of habitats present in the study area.
Genetic Diversity
Genetic diversity is a measure of how much genetic variation exists in a population of interbreeding individuals. Different individuals will usually have some different versions of genetic information, and the more differences there is between individuals, the more genetic diversity the population has. If a population has too little genetic diversity, harmful recessive genes may become more commonly expressed in the population. This leads to something called "inbreeding depression," where the offspring of each generation are less fit. If left to continue its course, inbreeding depression may lead to a population going extinct. Scientists measure genetic diversity of small populations by sampling individuals, sequencing their DNA and comparing the sequences.
Sustainability
"Sustainability" has two meanings: sustainable development and sustainable resource use. The former refers to development that ensures future generations will have the same opportunities and resources that we do today. The latter meaning refers to resource use that does not reduce the future use potential of resources. Sustainable resource use for a species can only be estimated if you have accurate data on current population size, harvest rates and a reliable population model. In developing countries, accurate population data about rare species may be hard to come by. Harvest rates may also be under-reported, increasing the uncertainty about the accuracy of a sustainable harvest assessment.