Aim Estimates of abundances and densities of birds and mammals have often been shown to be scale dependent, in that population sizes over large areas are overestimated if extrapolated from surveys of small plots. Previous tests of the mechanisms suggested to cause this decelerating scaling pattern found evidence of a biased choice of small plots in patches of homogeneous habitat. Here we show that negative density–area relationships can also arise as result of not considering plots where individuals of the species or assemblage of interest are absent in surveys of differing spatial resolution.
Location We took a complete census of violets (Viola spp.) in 800 m2 of chalk grassland in Wye, Kent, UK, and used human population censuses for Finnish, Swiss and Italian municipalities, English districts, states of the USA and European countries.
Methods We used mixed models of logarithmically transformed number of individuals or densities as a function of area.
Results The census of violets shows that by increasing the survey resolution and by not considering plots without individuals, the effectively occupied area diminishes and a negative density–area relationship arises. The finding that negative density–area relationships are also common for people is evidence that the non-random choice of plots in population surveys of varying areas can be responsible for many observed negative density–area relationships. The shallower slope of the people–administrative area relationship for Switzerland and Finland compared with Italy, as well as for England and the USA compared with Europe, confirms that less than proportionate individuals–area relationships can be the consequence of larger plot areas containing a higher proportion of areas without individuals.
Main conclusions Densities should be reported together with the effective areas for which they were estimated. It should be clearly conveyed whether or not plots where the surveyed species was absent were included in the density estimation.