Demographic modeling to fine‐tune conservation targets: importance of pre‐adults for the decline of an endangered raptor

Abstract

Large, long-lived species with slow life histories and protracted pre-breeding stages are particularly susceptible to declines and extinction, often for unknown causes. Here, we show how demographic modeling of a medium-sized raptor, the Red Kite Milvus milvus, can aid to refocus conservation research and attention on the most likely mechanisms driving its decline. Red Kites' survival and reproduction increased through three sequential stages for 1-2, 3-6, and 7-30 yr of age, mainly corresponding to individuals that are dispersing, attempting to gain a territory, and breeding. As typical of long-lived species, elasticities were highest for adult (≥7 yr old) survival, but this was high, with little scope for improvement. Instead, the declines were driven by an extremely low survival of pre-adults in their first years of life, which weakened the whole demographic system by nullifying the offspring contribution of adults and curtailing their replacement by recruits. For example, 27 pairs were necessary to generate a single prime age adult. Simulation of management scenarios suggested that the decline could be halted most parsimoniously by increasing pre-adult survival to the mean levels recorded for other areas, while only the synergistic, simultaneous improvement of breeding success, adult and pre-adult survival could generate a recovery. We propose three actions to attain such goals through selective supplementary feeding of both breeding and non-breeding individuals, and through mortality improvement by GPS remote-sensing devices employed as surveillance monitoring tools. Our results show how improving demographic models by using real, local vital rates rather than "best guess" vital rates can dramatically improve model realism by refocusing attention on the actual stages and mortality causes in need of manipulation, thus building precious time and resources for conservation management. These results also highlight the frequent key role of pre-adult survival for the management of long-lived species, coherent with the idea of demographic systems as integrated chains only as strong as their weakest link.

Density‐dependent age of first reproduction as a key factor for population dynamics: stable breeding populations mask strong floater declines in a long‐lived raptor

“…However, as we show here, and as also demonstrated by Sergio et al. (2020), a focus on breeding individuals alone fails to account for substantial decreases in the juvenile and non‐breeding age classes of the red kite. If this more comprehensive view is taken, and cryptic population declines inferred from demographic data are considered, the major part of the European red kite population in Germany is still decimated well below its numbers from 30 years ago.…”

Favoring recruitment as a conservation strategy to improve the resilience of long‐lived reptile populations: Insights from a population viability analysis

“…The variance in a parameter may contribute as much or more to the population growth rate, due to the propensity of natural selection to stabilize a trait strongly related to fitness closer to its optimum (Pfister, 1998;Stearns & Kawecki, 1994). Our retrospective analysis suggests that the population trajectory of Chancel iguanas may have been more strongly influenced by the large variation over time we detected in recruitment than by constant adult survival, a pattern often found in other long-lived species (Gaillard & Yoccoz, 2003;Sergio et al, 2020).…”

“…The smallest increase in recruitment rate necessary to reverse the iguana population decline on Chancel Islet would require a substantial and sustained conservation effort (Figure 5b ). Recruitment depends on multiple parameters, from reproductive success to the survival of the different immature cohorts (Gaillard et al., 1998 ; Sergio et al., 2020 ). In addition, long‐lived species usually mature late, at the age of 2 to 3 for the Lesser Antillean iguana (Knapp et al., 2014 ).…”

“…Conversely, focusing conservation actions on improving the less canalized demographic parameters may prove more effective at halting or even reversing population decline (Manlik, 2019;Mortensen & Reed, 2016;Sergio et al, 2020). A growing body of literature emphasizes the key role of reproduction and juvenile demographic parameters in long-lived species conservation.…”

“…A strong implication of the canalization hypothesis is that improving the population conservation status in long‐lived species may not be responsive to management actions targeting adult survival if this parameter is already high and fairly constant. Conversely, focusing conservation actions on improving the less canalized demographic parameters may prove more effective at halting or even reversing population decline (Manlik, 2019 ; Mortensen & Reed, 2016 ; Sergio et al., 2020 ). A growing body of literature emphasizes the key role of reproduction and juvenile demographic parameters in long‐lived species conservation.…”