Population dynamics

Genovart, M. , Oro, D. and Tenan, S. (2018), Immature survival, fertility, and density dependence drive global population dynamics in a long‐lived species. Ecology, 99: 2823-2832. doi:10.1002/ecy.2515

Disentangling the influence of demographic parameters and the role of density dependence on species’ population dynamics is a challenge, especially when fractions of the population are unobservable. Additionally, due to the difficulty of gathering data at large spatial scales, most studies ignore the global dynamic of a species, which would integrate local heterogeneity dynamics and remove the noise of dispersal. We developed an integrated population model (IPM) at a global scale to disentangle the main demographic drivers of population dynamics in a long‐lived species. We used 28 yr of Audouin’s Gull demographic data encompassing 69 local patches (comprising 90% of the world population). Importantly, we took into account the unobservable fraction of non‐breeders and also assessed the strength of density dependence for this fraction of the population. As predicted by life histories of long‐lived organisms, temporal random variation in survival was highest for immature individuals (1.326, 95% credible interval [CRI] 1.290–1.940) and lowest for adults (0.499, 95% CRI 0.487–0.720). Large temporal fluctuations in the probability of taking a reproductive sabbatical would partly explain the consistency in adult survival, with individuals most likely refraining from breeding when environmental conditions were harsh. Immature survival and fertility were the main drivers of population dynamics during the study period (r2 = 0.83, 0.77–0.87 and 0.73, 0.63–0.79, respectively). We found strong evidence of density dependence, not only due to the number of breeders (r2 = −0.34, −0.43 to −0.24) but also due to individuals on sabbatical (r2 = −0.18, −0.33 to −0.01). From a conservation point of view, the species shows a 5% annual global decrease during the last 10 years, and we propose an update of its conservation status. Even though population dynamics of long‐lived organisms are very sensitive to changes in adult survival, we show here that, in the absence of strong environmental perturbations affecting this vital rate, fluctuations in population density are mainly driven by variations in survival of immature individuals and fertility. Integrated models based on long‐term monitoring at a global scale may enhance our ecological and evolutionary understanding of how demographic drivers influence population dynamics.

Photo credit: Daniel Oro

The EURING 2017 has been held in Barcelona, Spain, at the Natural History Museum.

The EURING conference is primarily focused on the development, understanding and integration of new methodologies in the analysis of capture-mark-reencounter data, commonly used to estimate population parameters. However, this time the conference included also applications of comparable methods to unmarked individuals, such as occupancy modelling and estimation of detection probabilities in monitoring programs.

The conference program is available here.

We gave a talk entitled ‘Modelling density-dependent population growth rate from individual encounter data‘, within the ‘Population Dynamics and Dispersal’ session. The work we presented was carried out in collaboration with Giacomo Tavecchia and Daniel Oro of the Population Ecology Group of the IMEDEA (CSIC-UIB, Spain), and with Roger Pradel of the CEFE/CNRS (France).

Photo: Ana Sanz-Aguilar

In our talk, we presented a new hierarchical formulation of the temporal symmetry approach (aka Pradel model, available in program Mark) that represents a model-based approach to formally test and quantify the strength of density dependence directly on population growth rate (as well as related vital rates) using capture-mark-reencounter data on open populations. In addition, the framework allows the estimation of temporal variance unexplained by density dependence, and can thus be used to quantify the relative contribution of density-dependent (intrinsic) and density-independent (extrinsic) factors affecting fluctuations of animal populations.

We illustrated the modelling approach with simulated data, to investigate the effectiveness of detecting density dependent effects on population growth rate, but also using real data on the Audouin’s gull, marked and resighted at the Ebro delta colony (Spain) by the Population Ecology Group of the IMEDEA.