How Many Habitats In Our Biosphere?
Donald A. Windsor
Many more than realized — some multiple of the total number of free-living species. Because every free-living species hosts one or more species of parasite. To a parasite, its host is its habitat. Parasites with complex life cycles dwell in several habitats. Moreover, some parasites host their own parasites.
A host provides several habitats. Each organ or tissue could be a habitat for parasites. Parasites that enter the gastrointestinal tract and travel to a specific tissue have to pass through other tissues to get to their final destination. So, the total number of habitats is a multiple of the total number of hosts.
Some free-living species host large numbers of parasite species. The American Robin (Turdus migratorious) is the host for at least 94 parasite species (1). The European Starling (Sturnus vulgaris), a cosmopolitan bird, is host to at least 175 parasite species (2). These are just the species I could document in the literature. There are likely to be many more yet to be discovered.
In addition to all the biological habitats are the non-biological habitats.
The upshot is that while the exact number of habitats on Earth may be too elusive to calculate, it is safe to proclaim that the number is astronomical.
1. Windsor, Donald A. Biocartel of the American Robin (Turdus migratorious). Archives of the SciAesthetics Institute 2000 August; 1(1): 13-18.
2. Windsor, Donald A. Biocartel of the European Starling (Sturnus vulgaris). Archives of the SciAesthetics Institute 2000 August; 1(1): 19-28.