Lambdas File

The lambdas file within the Maxent output contains the actual model created by Maxent.

All the but last four files of the file contain the "features" that are used to create the response curves. Each of these lines describes a section of a response curve.

In all of these lines, the last two values represent the minimum and the maximum predictor value that the feature applies to. The value just before the minimum is the lambdas file.

Note that these "features" are really piece-wide equations that first convert each x value to a value from 0 to 1 over the specified range of predictor values and then multiply that value times the lambdas value to scale the result to the correct predictor value.

Below are the definitions and equations for each feature

Linear Feature

• Predictor, lambda, min, max
• f(x)=lambda*(x-min)/(max-min)

 

Quadratic Feature

• Predictor^2, lambda, min, max
• f(x)=lambda*(x*x-min)/(max-min)

 

Product Feature

• Predictor1 * Predictor2, lambda, min, max
• f(x1,x2)=lambda*(x1*x2-min)/(max-min)

 

Forward Hinge Feature

• 'Predictor, lambda, hinge, max (note the quote to define the feature type)
• if x < hinge then f(x)=0
• else f(x)=lambda*(x-hinge)/(max-hinge)

 

Reverse Hinge Feature

• `Predictor, lambda, min, hinge (note the apostrophe to define the feature type)
• if x < hinge then f(x)=0
• else f(x)=lambda*hinge-x)/(hinge-min)

 

Threshold Feature

• (Threshold<Predictor), lambda, 0.0, 1.0
• if (x<Threshold) then f(x)=0
• else f(x)=lambda

Note: BlueSpray will compute the number of coefficients in a lambdas file as three for all features except a threshold because the 0.0 and 1.0 values are not computed as part of the modeling process.

Computing habitat suitability

The last four values in the Maxent file are used to compute the Maxent output values.

• S(x)=Sum of all f(x) - linearPredictorNormailzer
• Q(x)=exp(S(x))/densityNormalizer
• L(x)=(Q(x)*exp(entropy)/(1+Q(x)*exp(entropy))

Q(x) is the raw model output. L(x) is the logit output from Maxent and is scaled from 0.0 to 1.0.

This information was taken from: Guidelines for computing Maxent model output values from a lambdas file.