Apparent breeding success drives long-term population dynamics of a migratory swan
The ability of a species to adapt to environmental change is ultimately reflected in its vital rates – i.e., survival and reproductive success of individuals. Together, vital rates determine trends in numbers, commonly monitored using counts of species abundance. Rapid changes in abundance can give...
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Zusammenfassung: | The ability of a species to adapt to environmental change is ultimately
reflected in its vital rates – i.e., survival and reproductive success of
individuals. Together, vital rates determine trends in numbers, commonly
monitored using counts of species abundance. Rapid changes in abundance
can give rise to concern, leading to calls for research into the
biological mechanisms underlying variations in demography. For the NW
European population of Bewick’s swan (Cygnus columbianus bewickii), there
have been major changes in the population trends recorded during nearly
five decades of monitoring (1970-2016). The total number of birds
increased to a maximum of c. 30,000 in 1995 and subsequently decreased to
about 18,000 individuals in 2010. Such large fluctuation in population
numbers is rare in long-lived species and understanding the drivers of
this population change is crucial for species management and conservation.
Using the integrated population model (IPM) framework, we analysed three
demographic datasets in combination: population counts,
capture-mark-resightings (CMR) and the proportion of juveniles in winter
over a period of ~50 years. We found higher apparent breeding success in
the years when the population had a positive growth rate compared to years
with a negative growth rate. Moreover, no consistent trend in adult and
yearling survival, and an increasing trend in juvenile survival was found.
A transient life-table response experiment showed that apparent breeding
success and adult survival contributed most to the variation in population
trend. We explored possible explanatory variables for the different
demographic rates and found a significant association between juvenile
survival both with the water level in lakes during autumn migration, which
affects food accessibility for the swans, and with summer temperatures.
Such associations are important for understanding the dynamics of species
with fluctuating population sizes, and thus for informing management and
conservation decisions. |
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DOI: | 10.5061/dryad.g1jwstqpj |