Population regulation of territorial species: both site dependence and interference mechanisms matter

Abstract

Spatial patterns of site occupancy are commonly driven by habitat heterogeneity and are thought to shape population dynamics through a site-dependent regulatory mechanism. When examining this, however, most studies have only focused on a single vital rate (reproduction), and little is known about how space effectively contributes to the regulation of population dynamics. We investigated the underlying mechanisms driving density-dependent processes in vital rates in a Mauritius kestrel population where almost every individual was monitored. Different mechanisms acted on different vital rates, with breeding success regulated by site dependence (differential use of space) and juvenile survival by interference (density-dependent competition for resources). Although territorial species are frequently assumed to be regulated through site dependence, we show that interference was the key regulatory mechanism in this population. Our integrated approach demonstrates that the presence of spatial processes regarding one trait does not mean that they necessarily play an important role in regulating population growth, and demonstrates the complexity of the regulatory process.

Figure 1.

Schematic representation of the relationship between a vital rate and population size at the site level under different density-dependent mechanisms. The dotted and the solid lines represent the relationship at a low- and a high-quality site, respectively. Model names are described in the text.

Figure 2.

Density-dependent decline of breeding success at the population level in the Mauritius kestrel. Average number of fledglings produced by 1-year-old breeders (open dots) and older breeders (filled dots), and estimated relationship between the number of fledglings and population size in 1-year-old and older breeders (dashed and solid line, respectively) over the period 1991/1992–2007/2008. Note that the breeding success is estimated for unmanaged pairs only while population size refers to the total number of breeders.

Figure 3.

Density-dependent decline of juvenile survival at the population level in the Mauritius kestrel. Inter-annual fluctuations in juvenile survival (filled dots) and estimated relationship between juvenile survival and population size (solid line) represented by the model with a plateau: p(a) S(a₁ × (plateau_(<25), N_(>25)) + a₂) (model 10, electronic supplementary material S3) over the period 1988/1989–2006/2007.

Figure 4.

Influence of density dependence on population dynamics in the Mauritius kestrel. Trajectories of the number of breeding pairs in the population for a full density-dependent model (density dependence in both fecundity and juvenile survival; solid line), a model with density-dependent fecundity and constant survival (dashed line), a model with density-dependent juvenile survival and constant fecundity (dot-dashed line) and a density-independent model (dotted line). Simulations were run using the stage structure of the full density-dependent model at equilibrium as a starting point (see electronic supplementary material, S2 for details). The shaded area represents the observed number of breeding pairs in the Mauritius kestrel population at stability.