A test of the mechanisms behind avian generalized individuals-area relationships

Abstract

Aim Questions related to abundances of organisms are central to ecological research. A priori, a scale independent estimation of abundances would be expected. However, we find estimates of numbers of bird individuals from all over the world to increase less than proportionately with increasing plot size. At the whole assemblage level, the pattern holds across biogeographical regions and habitats. The slope of the interspecific and, for the majority of species, the intraspecific individuals-area relationship is also significantly shallower than 1. The question arises as to which mechanisms cause these patterns. Location Global.Methods At the assemblage, interspecific and intraspecific levels, we tested three mechanisms that could be responsible for these patterns by comparing the slope of the individuals-plot area relationship for subsets of a database compiled from the literature. Spatial autocorrelation was controlled for. ResultsThere was no evidence for an influence of plot area choice in order to sample a constant number of individuals. Evidence for higher survey efficiency was available only with increasing number of visits at the intraspecific level. Evidence for influences of habitat heterogeneity was present at the assemblage, interspecific and intraspecific levels. This mechanism can work only if small plots are delimited non-randomly in homogeneous habitat. Main conclusionsAvian population size estimates without indication of the area over which they were obtained are of substantially less value than those coupled with that information. Ecologists planning to compare avian abundances between plots varying in some other factor of interest should minimize variations in their areas and/or account for them in data analyses. Population viability analyses, regional and global population size estimates, site prioritization and the scaling of ecosystem and species energy utilization need to address the plot area effect on assemblage and individual species abundances.

Are the living collections of the world’s botanical gardens following species-richness patterns observed in natural ecosystems?

“…Analyses were run in SAS 9.1. Mixed models with exponential covariance structure were used to test the null hypotheses described above controlling for spatial autocorrelation (Pautasso and Gaston 2006). Results from spatial and non-spatial models are generally consistent, but it is important to take into account the spatial non-independence of data (Kühn 2007).…”

Merging wildlife community ecology with animal behavioral ecology for a better urban landscape planning

“…doi:10.1016/j.landurbplan.2011.02.001 low breeding output, the overall success of these species might be very low, although results are still controversial. However, more information about the spatial scaling of factors affecting breeding success and mortality in and around towns is necessary, because density estimates might be a misleading indicator of habitat quality and are highly dependent on the area over which they are estimated (Pautasso and Gaston, 2006). In addition, urban centers are not necessarily islands for animals; a city itself could affect the surrounding areas and vice versa.…”

Negative density–area relationships: the importance of the zeros

“…The scale dependence of abundances is well established for mammals and birds (see e.g. Smallwood & Morrison, 1999; Smallwood & Smith, 2001, and references cited in Pautasso & Gaston, 2006). Yet low densities from large plot areas, and high densities from small plot areas, have been reported also for a beetle (Matter, 2003) and a butterfly and its associated host plant (Matter et al ., 2003).…”

“…Gaston et al ., 1999). This is the case whether they are expressed as absolute figures (numbers of individuals) or as densities (numbers of individuals relative to some kind of unit of area) (Pautasso & Gaston, 2006). In the latter case, densities tend to decline with an enlargement of plot areas, a pattern that is described in the literature as a negative density–area relationship.…”