Biodiversity is simply the variety of life. This can mean anything from the microbes in a few grams of soil to all the organisms that inhabit the earth. In practice, to assess how much diversity we have and what it does, we need to be more specific about the aspect of biodiversity we are concerned with, and the area and time frame over which we want to measure it. For example, we might consider the types and relative abundances of species of trees in a forest, or the genetic diversity associated with the individuals of those species, or even how the number and composition of forests across a biogeographic region have changed over the past century. This hierarchy of organizational levels is implicit in the definition developed by the UN in their Convention on Biological Diversity, which states that biological diversity is 'the variability among living organisms from all sources, including, inter alia, terrestrial, marine, and other aquatic ecosystems, and the ecological complexes of which they are part: this includes diversity within species, between species and of ecosystems'. However, the human population is rising and wildlife reserves don’t work if they hinder local people making a living. The poaching crisis for elephants and rhinos in Africa is an extreme example. Making the animals worth more alive than dead is the key, for example by supporting tourism or compensating farmers for livestock killed by wild predators.