inst/non_lin_models/Mod_powerR.R

In txm676/mmSAR2: Fit and Compare Species-Area Relationship Models Using Multimodel Inference

# POWER MODEL BIS (ROSENSWEIG 1995)
model <- list(
  name = c("PowerR"),
  formula = expression(S == f + c*A^z),
  exp = expression(f + c*A^z),
  shape = "convex",
  asymp = function(pars)FALSE,
  custStart = function(data)c(5,.25,0),
  #limits for parameters
  parLim = c("R","R","R"),
  #initials values function
  init = function(data) {
    if (any(data$S == 0)) {
      log.data <- data.frame(A = log(data$A), S = log(data$S + .5))
    }else{
      log.data <- log(data)
    }
    res <- stats::lm(S~A,log.data)$coefficients
    res <- c(exp(res[1]), res[2])
    res <- c(stats::lm(S~A,data)$coefficients[1], res)
    names(res) <- c("f","c","z")
    return(res)
    }
)