Measuring ecosystem diversity is not easy, and ther e is no definite index for its measurement. It is generally assessed by the measure of species diversity in an ecosystem, often based on the diversity of vegetation in a particular area. This may involve the assessment of the number of species in an area as well as their relative abundance. If all species have equal numbers, the area will be more diverse than one in which one, or few, species predominate. Also, an area with more size classes of the vegetation, different trophic levels (i.e. positions of different species in a set of linear feeding relationships) and taxonomic groups will be considered more diverse. Thus, a hypothetical ecosystem with ten species of plants only would be considered less diverse than an area with seven species of plants, two herbivores and one predator.
Further, an area with relatively low species diversity, but having a large number of endemics (i.e. species limited to a particular area, such as a country) make an important contribution to overall biodiversity. For example, small oceanic islands and continental montane regions have relatively low species diversity, but high levels of endemism.
It also has to be borne in mind that the scale of measurement needs to be given serious consideration when comparing diversity between two different ecosystems. For example 1 m2 of European chalk grassland can have much more plant species than a comparable area in an Amazonian rainforest! At the scale of 1 km2, however, the situation will be quite the reverse.