The interacting effects of forestry and climate change on the demography of a group-living bird population

Demographic consequences of mutualism disruption: Browsing and big-headed ant invasion drive acacia population declines

Across the globe, biological invasions have disrupted mutualisms, producing reverberating consequences for ecosystems. Although invasive species frequently trigger mutualism disruptions, few studies have quantified the demographic mechanisms by which mutualism breakdown may generate population effects. In a Kenyan savanna, the invasive big-headed ant (Pheidole megacephala) has disrupted a foundational mutualism between the monodominant whistling-thorn tree (Acacia drepanolobium) and native ants (Crematogaster spp.) that deter browsing by large mammalian herbivores. We conducted experiments to quantify the demographic consequences of this mutualism disruption in the presence and absence of large mammalian herbivores. Invasion by P. megacephala exacerbated population declines of A. drepanolobium, primarily through decreased survival and reproduction of adult trees. However, these fitness reductions were small compared to those resulting from the presence of large mammalian herbivores, which negatively impacted growth and survival. Contrary to expectation, the expulsion of metabolically costly Crematogaster mutualists by P. megacephala did not result in higher population growth rates for trees protected from large mammalian herbivores. Our results suggest that invasive P. megacephala may impose a direct metabolic cost to trees exceeding that of native mutualists while providing no protection from browsing by large mammalian herbivores. Across landscapes, we expect that invasion by P. megacephala will reduce A. drepanolobium populations, but that the magnitude and demographic pathways of this effect will hinge on the presence and abundance of browsers.

Forest management affects seasonal source-sink dynamics in a territorial, group-living bird

The persistence of wildlife populations is under threat as a consequence of human activities, which are degrading natural ecosystems. Commercial forestry is the greatest threat to biodiversity in boreal forests. Forestry practices have degraded most available habitat, threatening the persistence of natural populations. Understanding population responses is, therefore, critical for their conservation. Population viability analyses are effective tools to predict population persistence under forestry management. However, quantifying the mechanisms driving population responses is complex as population dynamics vary temporally and spatially. Metapopulation dynamics are governed by local dynamics and spatial factors, potentially mediating the impacts of forestry e.g., through dispersal. Here, we performed a seasonal, spatially explicit population viability analysis, using long-term data from a group-living territorial bird (Siberian jay, Perisoreus infaustus). We quantified the effects of forest management on metapopulation dynamics, via forest type-specific demography and spatially explicit dispersal, and how forestry impacted the stability of metapopulation dynamics. Forestry reduced metapopulation growth and stability, through negative effects on reproduction and survival. Territories in higher quality natural forest contributed more to metapopulation dynamics than managed forests, largely through demographic processes rather than dispersal. Metapopulation dynamics in managed forest were also less resilient to disturbances and consequently, may be more vulnerable to environmental change. Seasonal differences in source-sink dynamics observed in managed forest, but not natural forests, were caused by associated seasonal differences in dispersal. As shown here, capturing seasonal source-sink dynamics allows us to predict population persistence under human disturbance and to provide targeted conservation recommendations.

Hot droughts compromise interannual survival across all group sizes in a cooperatively breeding bird

Climate change is affecting animal populations around the world and one relatively unexplored aspect of species vulnerability is whether and to what extent responses to environmental stressors might be mitigated by variation in group size in social species. We used a 15-year data set for a cooperatively breeding bird, the southern pied babbler Turdoides bicolor, to determine the impact of temperature, rainfall and group size on body mass change and interannual survival in both juveniles and adults. Hot and dry conditions were associated with reduced juvenile growth, mass loss in adults and compromised survival between years in both juveniles (86% reduction in interannual survival) and adults (60% reduction in interannual survival). Individuals across all group sizes experienced similar effects of climatic conditions. Larger group sizes may not buffer individual group members against the impacts of hot and dry conditions, which are expected to increase in frequency and severity in future.

Remotely sensed forest understory density and nest predator occurrence interact to predict suitable breeding habitat and the occurrence of a resident boreal bird species

Habitat suitability models (HSM) based on remotely sensed data are useful tools in conservation work. However, they typically use species occurrence data rather than robust demographic variables, and their predictive power is rarely evaluated. These shortcomings can result in misleading guidance for conservation. Here, we develop and evaluate a HSM based on correlates of long-term breeding success of an open nest building boreal forest bird, the Siberian jay. In our study site in northern Sweden, nest failure of this permanent resident species is driven mainly by visually hunting corvids that are associated with human settlements. Parents rely on understory nesting cover as protection against these predators. Accordingly, our HSM includes a light detection and ranging (LiDAR) based metric of understory density around the nest and the distance of the nest to the closest human settlement to predict breeding success. It reveals that a high understory density 15-80 m around nests is associated with increased breeding success in territories close to settlements (<1.5 km). Farther away from human settlements breeding success is highest at nest sites with a more open understory providing a favorable warmer microclimate. We validated this HSM by comparing the predicted breeding success with landscape-wide census data on Siberian jay occurrence. The correlation between breeding success and occurrence was strong up to 40 km around the study site. However, the HSM appears to overestimate breeding success in regions with a milder climate and therefore higher corvid numbers. Our findings suggest that maintaining patches of small diameter trees may provide a cost-effective way to restore the breeding habitat for Siberian jays up to 1.5 km from human settlements. This distance is expected to increase in the warmer, southern, and coastal range of the Siberian jay where the presence of other corvids is to a lesser extent restricted to settlements.

Ecological markers to monitor migratory bird populations: Integrating citizen science and transboundary management for conservation purposes

Countries share responsibility for the management and conservation of migratory bird species. However, a limited understanding of population dynamics hampers the implementation of harvest and transboundary management. Age-ratios and population density can be useful indicators to assess population dynamics to improve management and conservation actions. Here, the dynamics of an Atlantic population of Common quail Coturnix coturnix, using 32,508 quail samples and 4814 hunter questionnaires over a 20-year period (1996-2016) served as a comparative study for examining age-ratio patterns related to different geographic zones, population density and weather parameters. Results show that age-ratios varied over zones and years, specifically age-ratio 1 (AR1), used as an index of late breeding attempts, varied from 0.1 to 0.21. Age-ratio 2 (AR2), a surrogate of central recruitment, varied from 0.16 to 0.66. Finally, age-ratio 3 (AR3), used as an indicator of the population's annual breeding success, varied from 3.69 to 6.68. Age-ratio is linked to internal and external factors (i.e. effect of rainfall, variations over time and density-dependent relationships) depicting how quail age groups make segregated migration in time and space. Quail age groups perform a complex pattern of migration because of entwined changes in abundance, migration routes and timing, influencing population connectivity and dynamics. Our findings highlight the relevance of citizen science and transboundary agreements to improve management and conservation measures of migrant species. Administrations and policy-makers in developed and developing countries must coordinate efforts to engage hunters in a participatory management systems to achieve sustainability.

Decreased Precipitation Predictability Negatively Affects Population Growth through Differences in Adult Survival

Global climate change is leading to decreased climatic predictability. Theoretical work indicates that changes in the climate's intrinsic predictability will affect population dynamics and extinction, but experimental evidence is scarce. Here, we experimentally tested whether differences in intrinsic precipitation predictability affect population dynamics of the European common lizard (Zootoca vivipara) by simulating more predictable (MP) and less predictable (LP) precipitation in 12 seminatural populations over 3 years and measuring different vital rates. A seasonal age-structured matrix model was parametrized to assess treatment effects on vital rates and asymptotic population growth (λ). There was a nonsignificant trend for survival being higher in MP than in LP precipitation, and no differences existed in reproductive rates. Small nonsignificant survival differences in adults explained changes in λ, and survival differences among age classes were in line with predictions from cohort resonance. As a result, λ was significantly higher in MP than in LP precipitation. This experimentally shows that small effects have major consequences on λ, that forecasted decreases in precipitation predictability are likely to exacerbate the current rate of population decline and extinction, and that stage-structured matrix models are required to unravel the aftermath of climate change.