On bias due to observer distribution in the analysis of data on natal dispersal in birds

Natal dispersal, the displacement from site of birth to site of reproduction, is an important process. It determines the spatial scale of population dynamics as well as the genetic structure of populations. Although some inferences can be made about dispersal from the measurement of genetic parameters, direct data on individuals marked at their site of birth and recorded at their site of breeding are scarce. In principle, such data could be collected on a large scale through bird ringers concentrating on the recapture of breeding birds. However, the analysis of such data has pitfalls. The raw frequency distribution of observed distances is strongly affected by the spatio-temporal distribution of observers. A strategy for reducing these effects of observer distribution on the observed dispersal pattern is proposed and tested by way of a simulation model. Whereas the results presented are preliminary, there are prospects to obtain a description of dispersal which is relatively independent of the limitations on its observation. [KEYWORDS: Great tit; survival]

Keeping up with a warming world; assessing the rate of adaptation to climate change

The pivotal question in the debate on the ecological effects of climate change is whether species will be able to adapt fast enough to keep up with their changing environment. If we establish the maximal rate of adaptation, this will set an upper limit to the rate at which temperatures can increase without loss of biodiversity

Measuring dispersal as distance-dependent recruitment rates: testing the performance of DDRR on simulated data.

Dispersal is an important process in ecology, but its measurement is difficult. In particular, natal dispersal— the net movement between site of birth and site of first reproduction—is important, since it determines population structure. Using simulated data, I study the claim that measuring dispersal in terms of distance-dependent recruitment rates filters out many problems. Using several dispersal rules and several spatial distributions of breeding sites, it is shown that distance-dependent recruitment rate (DDRR) estimates are independent of the spatial distribution of breeding sites and are sensitive to differences in dispersal rules. These simulations were carried out with sample sizes of 200 individuals, which is a number exceeded in many studies. Variation in clumping of breeding sites (colony sizes) also has little effect on the resulting DDRR estimates. The effects of individuals entering and leaving the study area was simulated by assuming that only half the area was observed. Comparing the ‘‘observed’’ movements with the total distribution of distances dispersed shows that the shape of the DDRR is not affected, although the absolute values are, of course, lower. Thus, DDRR estimates will allow us to start studying dispersal behavior independent of the peculiarities of the study area and independent of the distribution of observer effort.