Biostatistics with R An Introductory Guide for Field Biologists

Although there are many ways of describing nonlinear relationships among variables, this chapter focuses primarily on the polynomial regression, which is related to the multiple linear regression model. We pay particular attention to models using the second-order polynomial. These models are often employed in the field of community ecology to describe unimodal changes of species abundances along environmental gradients. The downsides of using polynomial regression are also addressed. We bring this chapter to a close by touching on the non-linear least-squares regression models and the appropriate context in which they should be applied. The methods described in this chapter are accompanied by a carefully-explained guide to the R code needed for their use, including the nlme package.