Paula C. Dias

Sources and sinks in population biology.

The various habitats inhabited by a given species are never of the same quality. When demographic models take into account this habitat heterogeneity, the source-sink concept naturally emerges: a local demographic surplus arises in good quality habitats (source), and a local demographic deficit occurs in habitats of poor quality (sink). Within a landscape, a permanent migration of propagules or individuals from source to sink habitats may lead to a stabilization of the overall demographic system. This simple situation, explored in the recent literature, has surprising properties. In particular, it requires a change in our view of classical concepts such as ecological niche and carrying capacity, it can explain the existence and persistence of local maladaptation and it can improve conservation practice.

Habitat quality as a predictor of spatial variation in blue tit reproductive performance: a multi-plot analysis in a heterogeneous landscape.

Vertebrate studies have rarely investigated the influence of spatial variation in habitat richness on both short-term (breeding) and long-term (offspring recruitment) reproductive performance using simultaneously multi-patch, multi-habitat type and multi-year approaches at landscape level. Here we present results of such an approach using the influence of two oak tree (Quercus ilex, Q. humilis) species on reproductive performance in Corsican blue tits (Parus caeruleus ogliastrae) as a model system. We found that blue tits breeding in rich broad-leaved deciduous patches consistently laid eggs earlier in the season, and produced larger clutches and more fledglings of higher quality, than those breeding in poor evergreen patches. Also, parents, especially males, were in better physical condition in the broad-leaved deciduous than in the evergreen patches. Surprisingly, estimates of long-term effects of reproduction, such as recruitment rates of locally born offspring, did not differ between the two habitat types. Our results suggest that short-term breeding performance and phenotypic quality of both chicks and parents do not necessarily provide reliable information about contributions to following generations at a scale larger than that of the local study plot. Differences in reproductive performance between the two oak habitat types could not be attributed to density-dependent effects, differences in levels of nest predation, or differences in age structure of the birds. We suggest that habitats that are optimal for breeding are not necessarily optimal for survival after the breeding season.

Local specialization and maladaptation in the Mediterranean blue tit (Parus caeruleus)

Although the evolutionary interplay between gene flow and local adaptation of organisms in heterogeneous environments has been widely discussed from a theoretical point of view, few empirical studies have been designed to test predictions on the consequences of habitat patchiness on the evolution of life history traits. Using blue tits in Mediterranean habitat mosaics as a model, we defined two nested levels of habitat heterogeneity: an inter-regional level which compares two isolated landscapes (mainland, southern France vs the island of Corsica), and an intra-regional level which compares two habitat types within each landscape (deciduous vs evergreen trees). Deciduous habitats are more common than evergreen habitats on the mainland whereas the opposite is true on the island. Results suggest that: (1) on a regional scale, each population is specialized to the more common habitat, i.e. life history traits have evolved in such a way that breeding success is high; (2) in the less common habitats within each landscape, birds are clearly mistimed because they mismatch the best period of food availability, and hence their breeding success is lower; and (3) the density of the populations and the morphometry of the birds support the model of ideal despotic distribution. These results, which are supported by preliminary data on the genetic variation and gene flow of populations in the mainland landscape, are consistent with a source-sink model of population structure within each landscape. They are discussed in the framework of metapopulation theory and habitat selection models.

Will Estimates of Lifetime Recruitment of Breeding Offspring on Small-Scale Study Plots Help Us to Quantify Processes Underlying Adaptation?

Reviewing 22 long-term studies of birds that provide information on the individual lifetime recruitment of breeding offspring in local study plots, we estimated local replacement rates of adults by breeding offspring (LRR) on these plots. The studies indicate that on average two thirds of the offspring emigrate to breed outside the study plots and are replaced by immigrants. Because recent studies show that individuals that disperse can differ from those that stay, and parents may produce dispersers and non-dispersers in different proportions, recruitment of breeding offspring in local study plots may not reflect the total recruitment of breeding offspring in the whole population. It is concluded that the identification of adaptations using estimates of lifetime recruitment of breeding offspring in study plots may be misleading and conclusions concerning adaptations are most often premature and extremely complicated to demonstrate quantitatively.

Do harlequin mediterranean environments function as source sink for Blue Tits (Parus caeruleus L.)

We investigate whether a mosaic of habitats of different quality functions as a source-sink system for the Blue Tit Parus caeruleus L. Breeding parameters, especially laying date, clutch size and breeding success have been studied in relation to the food supply in three habitats: two habitats, one rich and one poor, next to each other on the mainland (southern France) and one poor habitat on the island of Corsica. Food resources are more abundant and are available earlier in the season in the rich habitat than in both the mainland and the island poor habitats. The timing of breeding is nicely timed on the food peak of abundance in the rich mainland habitat and in the poor insular one but tits are mistimed in the poor mainland habitat because they start to breed too early in relation to food availability. Such patterns strongly suggest that the rich mainland habitat where birds produce many fledglings functions as a source from which birds emigrate in the poor habitat which functions as a sink. These birds which are genetically programmed to breed in the source habitat become mistimed in the sink. Tits on Corsica which are isolated from any mainland population have adjusted their breeding traits on the local patterns of food availability and abundance. This hypothesis is supported by the existence of a strong genetic component of laying date which has been experimentally proved.

Summergreenness, evergreenness and life history variation in Mediterranean Blue Tits

Using a food chain approach, which includes the leafing patterns of summergreen and evergreen oaks, caterpillars that feed upon them, and insectivorous tits, we examine life history variation of the birds in different habitats of the Mediterranean region. The annual renewal of leaves involves only one third of the whole foliage in the evergreen Holm oak instead of 100% in the summergreen Downy oak and occurs c. three weeks later in the former than in the latter. Caterpillars are nicely synchronized on the leafing process and are more abundant in summergreens than in evergreens. The diet of the Blue Tit includes a large diversity of prey, especially in evergreen oaks, and the way tits compensate for the low abundance of caterpillars is discussed. Blue Tits fairly match the period of food availability in summergreens on the mainland and in isolated habitats dominated by evergreens (i.e. Corsica), but they are mis-timed in evergreen habitat patches within a mainland landscape which includes both summergreen and evergreen habitat patches. We hypothesize that gene flow among sub-populations living in discrete habitat patches of different quality results in a source/sink system whereby the tits breeding in evergeen habitats must be permanently restocked by immigrants from more productive source habitats.