Applied Ecosystem Services, LLC

The Environmental Issues Doctor

Photo of Factors Limiting Species Populations: 2


Estimated reading time: 2 minutes

Habitat use is one of the first factors considered when determining limitations on species abundance and distribution. For species being considered for some level of protection there are existing data describing habitats in which they have been found as well as abundance estimates.

When projects are proposed in areas with potential habitats for the species it is common to survey these habitats for the species’ presence. The survey methods seek data to answer this question: What is the probability of the species occupying a site if it is not observed during a visit?

This question cannot be answered using frequentist (null hypothesis significance testing) statistical models because these measure frequencies of long-term averages rather than predicting the probability of a single observation.

Because data exist for the presence of the species in the habitat type even when they are not observed, these data are used to calculate the probability of the species being present but not observed. This prior information is applied using Bayes’ Rule of conditional probability: the probability of the species being present but unobserved given that it has been seen in these habitats in this, or other, locations. The proportion of prior observations of the species in this habitat is the prior probability of its presence in the one being surveyed.

This work was originally published on the Applied Ecosystem Services, LLC web site at

It is offered under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International license. In short, you may copy and redistribute the material in any medium or format as long as you credit Dr. Richard Shepard as the author. You may not use the material for commercial purposes, and you may not distribute modified versions.

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    Explaining Environmental Data


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    Most people are familiar with statistical hypothesis tests such as the t-test and ANOVA to analyze whether two or more samples (from a parametric distribution) came from the same population. The nonparametric equivalents (Wilcoxon and Kruskal-Wallis tests) are less familiar but equally robust. What is not always clear is that these models are applied to one or more response variables; e.g., chemical concentrations that result from natural or anthropogenic causes. They do not answer the question of why these values were observed.
  2. Photo of Factors Limiting Species Populations: 1

    Factors Limiting Species Populations: 1


    Estimated reading time: 1 minutes

    There are many plant and animal species considered to be threatened, endangered, or of special concern to regulators and the public. Correctly estimating population sizes, relationship to habitats, and potential effects of anthropomorphic activities is crucial to making informed policy and regulatory decisions. Environmental conditions affecting species populations are the limiting factors. Quantifying limiting factors is fundamental to developing policies and practices that are most likely to create the desired future conditions for the species and its habitats.

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