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Quantitative Habitability Theory (QH Theory)


The purpose of QH Theory is to explain the distribution, abundance, and productivity of life. It integrates elements from Habitat Suitability Models (HSM), the Metabolic Theory of Ecology (MTE), Population Dynamics, Macroecology, Biogeography, and Ecophysiology. QH Theory can be applied to any life, from prokaryotes (bacteria and archaea) to eukaryotes (i.e. plants). The theory is scalable in space and time from microenvironments to planetary scales. 
The following postulates are the basis of this theory:

Postulate I: The habitability of an environment for a species is defined as the normalized metabolic rate (NMR) of the species as function of the environment state. 
This postulate is the definition of the quantitative habitability. It is similar to the Habitat Suitability Index (HSI) in ecology but with some important differences. It is a number directly related to metabolic rate between -1 and +1 that weighs up the effect of physical, chemical or biological environmental variables. The scale is relative to a specific species or community but its value always means the same thing. Positive values are interpreted as habitable environments (potential for activity and growth) while negative values as inhabitable environments (potential for decomposition and death). Zero habitability is interpreted as a marginal environment for life (survival with no activity or growth).

Postulate II: Complex habitability functions of many environmental variables can be constructed as the product of simple ones. 
This postulate states a method to construct complex habitability functions from simple ones. This might not be always the case for all variables, but serves as a starting point for more complex developments. Similar compounded constructions have been used to study the simultaneous effects of temperature, pH and water activity on microbial growth, primary productivity of plants and phytoplankton, and the HSI.

Postulate III: Maximum growth rates take place during maximum metabolic activity. Therefore, measurements of growth rate can be used to estimate the habitability. 
In practice, direct or indirect measurement of metabolic rates might be difficult. Changes in biomass or productivity related to growth can be used to calculate habitability. This relative growth rate has been used and called sometimes the performance or fitness in the field of microbial ecology.

Postulate IV: The habitation of an environment by a species is defined as the normalized biodensity of the species. 
This postulate is the definition of the quantitative habitation. It is a value between 0 and 1 that can be interpreted as the fraction of population or biomass compared to the potential maximum (carrying capacity).

Postulate V: The habitation can be estimated from the habitability of an environment from a growth equation and an initial habitation. 
This postulate states the relation between habitability and habitation. It can be used to model the effects of the environment on the distribution, abundance, and productivity of one or multiple species including population interactions.


  • Méndez, A., and Hoehler, T. (2010) Habitability Metrics for Astrobiology. [in preparation]