Population regulation by habitat heterogeneity or individual adjustment?

Abstract

Summary1. The habitat heterogeneity (HHH) and individual adjustment (IAH) hypotheses are commonly proposed to explain a decrease in reproduction rate with increasing population density. Higher numbers of low-quality territories with low reproductive success as density increases lead to a decrease in reproduction under the HHH, while more competition at high density decreases reproduction across all territories under the IAH. 2. We analyse the influence of density and habitat heterogeneity on reproductive success in eight populations of long-lived territorial birds of prey belonging to four species. Sufficient reliability in distinguishing between population-wide, site-specific and individual quality effects on reproduction was granted through the minimal duration of 20 years of all data sets and the ability to control for individual quality in five of them. 3. Density increased in five populations but reproduction did not decrease in these. Territory occupancy as a surrogate of territory quality correlated positively with reproductive success but only significantly so in large data sets with more than 100 territories. 4. Reproductive success was always best explained by measures of territory quality in multivariate models. Direct or delayed (t)1) population density entered very few of the best models. Mixed models controlling for individual quality showed an increasing reproductive performance in older individuals and in those laying earlier, but measures of territory quality were also always retained in the best models. 5. We find strong support for the habitat heterogeneity hypothesis but weak support for the individual adjustment hypothesis. Both individual and site characteristics are crucial for reproductive performance in long-lived birds. Proportional occupancy of territories enables recognition of highquality territories as preferential conservation targets.

Multi-Scale Effects of Nestling Diet on Breeding Performance in a Terrestrial Top Predator Inferred from Stable Isotope Analysis

“…cold and rainy days) in spring 2009 that negatively affected the productivity of Bonelli’s eagle, which was remarkably low (0.85, n = 40), while the productivity of successful pairs on that breeding season (1.71, n = 17) was similar or even higher than in other years. Apparently, pairs holding good territories are able to rear chicks even in bad years [70], but this does not necessarily implies that the environmental and food supply conditions that these pairs experience are the same over years, as suggested by our results. In particular, the population-year with the highest mean productivity (i.e.…”

Pathogenic bacteria and timing of laying

“…Krüger et al. (2012) showed that poor-quality territories of the goshawk were generally only occupied in years with high population density, while only core territories were occupied in years with low population density. Our findings provide limited evidence of accumulation of microorganisms across years being measurable at the level of adults or nestlings.…”

Individual Quality Explains Variation in Reproductive Success Better than Territory Quality in a Long-Lived Territorial Raptor

“…Accordingly, individuals may benefit by recognizing spatial variation in habitat quality and settling accordingly and it is widely assumed that both clutch size and reproductive output in territorial birds depend on the quality of their territories [4], [10], [11]. In fact, habitat heterogeneity and despotic settlement have been postulated as the main mechanism of population regulation for territorial birds [10], [12], [13], [14]. Under an ideal free distribution equal competitors select habitats to maximize their individual fitness, whereas under an ideal despotic distribution or more complex distribution models, individuals are unequal competitors and the resources or territories of highest quality are monopolized by the strongest competitors [4].…”