When in Spring 2010 Eyjafjallajökull’s volcanic eruption grounded most of the western world, Icelandic Whimbrels kept pushing North through the dust cloud.
The effects of habitat type and volcanic eruptions on the breeding demography of Icelandic Whimbrels Numenius phaeopus. Katrínardóttir, B., Alves, J.A., Sigurjónsdóttir, H., Hersteinsson, P. & Gunnarsson T.G. 2015. PLOS One 10(7): e0131395. doi: 10.1371/journal.pone.0131395
Our most recent tracking of Whimbrels’ migration reveals that arrival dates in Iceland span from the 25 April to the 14 May. In 2010, when Western Europe was shaded by a massive ash plume resulting from Eyjafjallajökull’s eruption in late April, Whimbrels found their way to Iceland, but there was another dark cloud looming in the air.
Iceland is home to ca. 40% of the Whimbrels in the world! An estimated 250.000 breeding pairs invade Iceland annually, concentrating the subspecies islandicus almost entirely in this country, with few pairs breeding elsewhere, in the Faeroes (2,500), UK (500) and Greenland (50-100) (Gunnarsson, et al. 2006, Thorup 2006, Delany et al. 2009). Whimbrel densities in Iceland are also the highest recorded anywhere in the world, with 29-45 pairs/km2 breeding in some areas (Gunnarsson 2000, Katrínardóttir et al. 2015). The population here is thought to be stable, contrary to what has been reported from the Faeroes and Shetland, as well as for the other 4 subspecies across the Whimbrel’s range (Delany et al. 2009, Perkins 2014, Pierce-Higgings et al. in prep). So why is Iceland such an attractive and seemingly favourable place for this species?
Although the Whimbrel breeding range is quite widespread, their preference for nesting sites with sparse, scrubby or low vegetation is very strong (Ballantyne K & Nol 2011). This type of habitat is quite common across lowland Iceland, where geomorphological characteristics and harsh climate create a landscape constantly at early successional stages. For instance, desert sand areas crafted by high levels of erosion are often colonized by pioneer plant species (in some cases planted by the Soil Conservation Service of Iceland) providing an opportunity for Whimbrel nesting (Fig. 1). Another dynamic but more productive habitat are lowland river plains, which are totally or at least partially fed by glaciers or winter snow that melt in sync with temperature and solar exposure. This habitat is used by an estimated quarter of Iceland’s breeding Whimbrel population, making it the single most used breeding habitat, followed by heathland and grasslands habitats. In river plains, Whimbrels breed at very high densities and can achieve very high productivity with 60-100% of pairs successfully fledging young, whereas this was only 1-19% in heathland (Gunnarsson 2000). However, in our study we found no significant differences in breeding success (as the proportion of pairs fledging young) between river plain and heathland & grassland habitats!
As it happens, we measured the proportion of pairs fledging young during years in which volcanic eruptions sent plumes of ash into the atmosphere, negatively affecting Whimbrel productivity across all habitats. Eyjafjallajökull’s eruption in April 2010 and to a lesser extend Grímsvötn’s in May 2011, distributed volcanic ash over south Iceland (and beyond) which caused a collapsed or delayed peak abundance of insect species. During these years, Whimbrel productivity and chick survival were very poor, most likely because chicks faced a relatively low abundance of invertebrate prey (Fig. 2) and few survived. Whilst these eruptions most likely masked habitat differences in the proportion of pairs fledging young, it is very clear that river plains supported a significantly higher density of breeding pairs and broods than heathlands & grasslands. Interestingly, however, these differences are reduced in years with volcanic eruptions.
Figure 3 “Effect of distance to Eyjafjallajokull volcano on the proportion of successful pairs. 2010 is shown with black symbols & line (y = 0.0035x + 0.455) and 2011 with grey symbols & line (y = 0.0084x−0.088). River plains are shown with squares and grassland/heathland with circles”. (from Katrínardóttir et al. 2015, reproduced with consent from the authors).
The significant positive effect between distance from eruption site and the proportion of successful Whimbrels pairs was clear in both 2010 and 2011 but with a stronger effect recorded in the second year (Fig. 3). Grímsvötn’s eruption in 2011 was further from the study sites (152-196 km) than Eyjafjallajökull’s (22-86 km) in 2010 but still added to the amount of ash deposited in the study areas. This potential additive ash effect might also be responsible for the scarcity of invertebrate prey in the second year, either as a direct effect or because crashes in the previous year affected insect reproduction and thus their abundance on the subsequent year. Such short-term disruptive events affecting reproduction may highlight how underlying variation in habitat quality can be masked by cascading processes originating in preceding seasons. Having recorded data on breeding season parameters during the volcanic eruption years allowed investigating how stable long-term habitat suitability can interplay with short-term stochastic events affecting habitat quality to influence demography at different scales.
Volcanic eruptions might have a temporary negative and localized effect on productivity, but they are also responsible for high system productivity on much larger temporal and spatial scales. In fact, dust deposition onto vegetated land has a strong positive effect on breeding waders across Iceland in both dry and wet habitats, with abundance tripling between the lowest and highest dust deposition areas (Gunnarsson et al. 2015). Furthermore, and given that many Icelandic volcanos sit under glaciers or snow caps, volcanic eruptions frequently trigger flash floods in the lowlands, with water clearing vegetation on river banks and promoting early successional habitats which are favoured by Whimbrels. Over the recent decades river impoundment across Iceland has changed the dynamics of glacial rivers, but at the same time grazing has increased in some areas (mostly by horse and sheep) which also helps keep vegetation in check. This recent increase in farming activities might not always be beneficial to Whimbrels and other breeding birds, particularly when natural land is converted, but also because some farm animals tend to feed on eggs (Katrínardóttir et al. 2015; Fig. 4)
Current studies show that breeding Whimbrels in the river plain study sites are back to high levels of productivity. On some of the same river plain sites studied by Borgny between 2009 and 2011, hatching success in 2015 was a good proxy for fledging success, suggesting that food was available for chicks to grow. Predation by natural causes is likely to be the limiting factor of productivity rather than food availability for young Whimbrels. Therefore, it seems that as the mythological Pheonix, so can Icelandic Whimbrels rise from the ashes!
References and further reading
Ballantyne, K. & Nol, E. 2011. Nesting Habitat Selection and Hatching Success of Whimbrels Near Churchill, Manitoba, Canada. Waterbirds 34: 151–159. DOI: 10.1675/063.034.0203 View
Delany, S., Scott, D., Dodman, T. & Stroud, D. 2009. An atlas of wader populations in Africa and Western Eurasia. Wetlands International, Wageningen. pp. 524.
Gunnarsson, T.G. 2000. Stofnvistfræði spóa á Suðurlandi. (Population ecology of the Whimbrel Numenius phaeopus in S-Iceland). University of Iceland.
Gunnarsson, T.G., Gill, J.A., Appleton, G.F., Gíslason, H., Gardarsson, A. & Watkinson, A.R. et al. 2006. Largescale habitat associations of birds in lowland Iceland: Implications for conservation. Biol Conserv 128: 265–275. DOI: 10.1016/j.biocon.2005.09.034 View
Gunnarsson, T.G., Arnalds, O., Appleton, G., Méndez, V. & Gill, J. 2015. Ecosystem recharge by volcanic dust drives broad-scale variation in bird abundance. Ecol Evol 5: 2386–2396 DOI: 10.1002/ece3.1523 View
Katrínardóttir, B., Alves, J.A., Sigurjónsdóttir, H., Hersteinsson, P. & Gunnarsson T.G. 2015. The effects of habitat type and volcanic eruptions on the breeding demography of Icelandic Whimbrels Numenius phaeopus. PLOS One 10(7): e0131395. DOI: 10.1371/journal.pone.0131395 View
Pearce-Higgins, J.W. et al. (in prep) A global threats overview for Numeniini populations: synthesising expert knowledge for a group of declining migratory birds.
Perkins, A. 2014. Determining the causes of Whimbrel declines in Shetland. Website. Accessed 9 September 2015. View
Thorup, O. 2006. Breeding waders in Europe 2000. pp. 60–61. Info
Check out the near real time GPS tracking of another Icelandic (and continental) migrant, the Black-tailed Godwit View
Featured image – Whimbrel © Andreas Trepte / photo-natur.de; Fig 1: Whimbrel © Tómas Gretar Gunnarsson
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