Evidence for an ideal free distribution in a migratory bird population in experimentally altered habitat

Background/Question/Methods

The distribution of individuals in a mosaic of habitat types and the effects of habitat selection on reproductive success are central aspects of population ecology.  According to the ideal despotic distribution (IDD), individuals acquiring territories of higher quality gain a fitness advantage over subordinate individuals.  The ideal free distribution (IFD) suggests that local densities are adjusted according to habitat quality, resulting in territories of similar quality among different habitat types.  Nonideal habitat selection has also been shown in a variety of systems.  Although these scenarios have contrasting consequences on population dynamics and can have important conservation implications, few researchers have conducted habitat-scale experiments testing predictions from these conceptual models, especially along a temporal sequence of habitat change.  In this study, we measured the demographic response of the Ovenbird (Seiurus aurocapilla), a neotropical migratory songbird, to selection harvesting (30-40% tree removal) over a 6 year period (1 year pre- and 5 years post-harvest; 783 territories monitored) to test predictions from the IDD and IFD, and eventual variations in their relevance as harvested stands regenerate. 

Results/Conclusions

We found significant year × treatment interaction effects on density and productivity per unit area.  Density and productivity per unit area were ca. 40% lower in treated plots than in controls during the first year post-harvest and, by the fifth year post-harvest, the difference decreased to 10 and 25% for density and productivity per unit area, respectively.  Interestingly, both the abundance and biomass of soil invertebrates followed the same pattern.  Finally, there was no evidence for a treatment effect on per capita productivity.  These findings are consistent with the IFD, whereby individuals in habitats of contrasting suitability adjust the size of their territory as a function of habitat quality, resulting in similar contributions to population dynamics on a per capita basis.  Future studies should compare survival rate of individuals in habitats of varying quality to determine the influence of this parameter on population growth estimates.