Using behavioural and state variables to identify proximate causes of population change in a seabird

Abstract

Changes in animal population size are driven by the interactions between intrinsic processes and extrinsic forces, and identifying the proximate mechanisms behind population change remains a fundamental question in ecology. Here we report on how measuring behavioural and state proxies of food availability among populations experiencing different growth rates can be used to rapidly identify proximate drivers of population trends. In recent decades, the Cape gannet Morus capensis has shown a major distributional shift with historically large colonies in Namibia decreasing rapidly, whilst numbers at South African colonies have increased, suggesting contrasting environmental conditions in the two regions. We compared per capita growth rates of five of the six extant colonies with foraging range (using miniaturised Global Positioning System loggers), foraging work rate, food delivery rates and body condition of breeding adults. We found significant associations between the rate of population change, individual behaviour, energetic gain and body condition that indicate that recent population changes are associated with extrinsic effects. This study shows that behavioural and state data can be used to identify important drivers of population change, and their cost-effectiveness ensures that they are an appealing option for measuring the health of animal populations in numerous situations.

Implications for seabirds off South Africa of a long-term change in the distribution of sardine

“…As in most population studies [e.g. Lewis et al 2006] we observe the number of breeding birds at the peak of the breeding season. Using census data from 20 years , including the critical moment of the eastward shift in prey (late 1990s), we investigated the regional trends …”

Jellyfication of Marine Ecosystems as a Likely Consequence of Overfishing Small Pelagic Fishes: Lessons from the Benguela

“…6B) and was estimated at 5.86 kJ g −1 in the SB in the more recent period, whereas it was slightly less than 5 kJ g −1 in the NB (between 21% and 24% less than in the other studies). The Cape Gannet is probably the most specialized predator considered here, consistently deriving the largest portion of its diet from pelagic fishes (Berruti et al 1993), but also able to scavenge offal behind vessels fishing for hake (Lewis et al 2006, Pichegru et al 2010. Of the predator species discussed here, the Cape Gannet also has the largest potential feeding range.…”

“…As a result, although its food in the SB is entirely dominated by anchovies and sardines when they are available, in the NB, the sardine that dominated gannet diet in the 1950s has not been replaced by the low-energy bearded goby, which on average contributed <4% to the diet. Instead, gannets in the NB make longer feeding trips (Lewis et al 2006) and derive a higher percentage of their diet from trawler offal. Recently, Cape Gannets off South Africa's west coast have also turned to feeding on lower-energy discards from the hake fishery after the eastward shift in relative distribution of sardines and anchovies and have not been able to sustain breeding at the levels they achieved when more anchovies and sardines were available (Pichegru et al 2007).…”

Conservation implications of the apparent mismatch between population dynamics and foraging effort in French northern gannets from the English Channel

“…These field techniques have been used on several occasions for cape gannets Morus capensis and in African penguins Spheniscus demersus , Lewis et al 2006, Pedersen et al 2006 with no measurable impact on the behaviour and the wellbeing of the animals. We nonetheless compared foraging trip durations of birds equipped with data loggers with those of non-equipped control birds.…”

“…It has recently been suggested that in long-lived species such as seabirds, foraging parameters recorded during the breeding season could be used to better understand the status of populations, based on the premise that foraging performance influences reproductive success, and ultimately recruitment (Lewis et al 2006). This approach is appealing from a management perspective, as studies of foraging behaviour can be performed within a much shorter timeframe than population studies, and results from the former can be quickly used as a base for management strategies.…”