2.7. Indirect methods (relative estimates)These are many measures which may correlate with population density: trap catches, visual counts, counts of animal products (frass, nests), proportion of infested hosts (for parasites, in broad sense). Indirect measures can be related to population density using regression analysis (linear or nonlinear).Let us remember the basic concepts of regression analysis.
Linear regression:The least square method is most often used to draw the "best" line through a cloud of points. This method adjusts the values of regression parameters (a and b) so that the residual sum of squares (=sum of square deviations of points from the line) reaches a minimum. The residual sum of squares is:
The Least Square Method means that we find such parameter values a and b that the value of is minimized. It follows from the calculus that derivatives at the minimum point are equal to zero:
After substituting of and simplification: The total sum of squares is . The sum of squares for the factor effect: , where the covariance is estimated using equation:
The residual sum of squares: . Rsquare is .
Important things in regression analysis
Polynomial regressionThis is a nonlinear function, but least square estimation leads to a system of linear equations. Thus, this regression is analyzed by a linear method. This is NOT a nonlinear regression! Note: use stepwise regression when you test the significance of nonlinear terms in the polynom. The effect is significant if the increment of Rsquare is large enough according to Fstatistics: where, in the numerator, there is a difference in Rsquares estimated in two consecutive steps, and are corresponding degrees of freedom (d.f.= number of regression coefficients minus 1). The difference  is equal to 1 because one term is added at a time.
Nonlinear regressionNonlinear regression is estimated numerically. Residual sum of squares is a function of model parameters. Thus the minimum can be found as a lowest point on the response hypersurface:Several methods are used to search for a minimum. Examples are:
It is desirable that the equation represents some theoretical model of a real system. Then regression coefficients have biological interpretation. Example. We return back to indirect population measures. Binomial sampling is a method when instead of counting organisms in each sample, we count the number of samples where organisms were present. For example, the density of Colorado potato beetles can be reconstructed from the proportion of potato plants where at least one insect was found. This is a faster method than counting all insects. If we assume the random (poisson) distribution of beetles, then the proportion of infested plants is equal to the zero term p_{o} of the poisson distribution:
where M is the mean number of individuals per plant. It is clear that the mean density, M, can be estimated as the negative logarithm from the proportion of infested plants, p_{o}. An alternative theoretical model can be derived from the assumption that beetles are aggregated on host plants and that their distribution is negative binomial. The zero term of the negative binomial distribution is: where k is the aggregation parameter. To test, which model is better, it is necessary to use the nonlinear regression and then to compare Rsquare.
