The snow, the aphid and climate change

The snow, the aphid and climate change

Top image: The endemic aphid Acyrthosiphon svalbardicum feeding on the flowering shoots of Mountain Avens (Dryas octopetala). Photo: Steve Coulson/UNIS.

There are over 500 species of insect or mite or other creepy crawlies in Svalbard. Despite this large number of species it is clear that not all are found everywhere. The reason might be the weather conditions in winter.

11 July 2012
Text: Steve Coulson, UNIS Associate professor in terrestrial ecology

Distributions may be very limited but the precise controlling factors restriction the range of an animal may be difficult to untangle, as with the aphid Acyrthosiphon svalbardicum (see image above). This aphid is only known from Svalbard, not found in any other region; it is a Svalbard “endemic”. The aphid feeds on Dryas (Mountain Avens or reinrosa) but not every patch of Dryas contains aphids. Why is this? Why should some patches of the plant be suitable for the aphid to feed on and not others?

Oddly, part of the answer lies in winter conditions rather than summer. For much of the year there is a surface cover of snow in Svalbard. This snow insulates the ground against low air temperatures. Soil temperatures along the tops of wind-blown ridges may approximate air temperature yet in the hollows, where snow accumulates, soil temperatures may rarely go below -10°C. It would appear obvious that most invertebrates would prefer to pass the winter tucked up under a blanket of snow. Yet the aphid does not seem to do this.

Aphids have a complicated lifecycle with several different body types; that is, morphs. The aphid overwinters as an egg. No adults survive. So in order to persist in a patch of Dryas the aphid must hatch from the egg, become adult and produce eggs all before the onset of late autumn during the short Arctic summer. If the aphid fails in just one year the population in that patch will become extinct. Recolonisation of that patch is a slow process.

Snow and the distribution of the aphid
An initial study in the 1990’s implicated snow depth in constraining the local occurrence of the aphid. The team studied patches of Dryas up a small slope. They observed that the age of aphids in each patch became progressively older up the slope. That is, young aphids at the bottom, old at the top. How could this be?

This observation was explained as being the result of differences in winter snow accumulation. At the bottom of the slope the snow is deep. At the top the wind blows the snow clear. The result is that in the spring the ground clears of snow first. Here the eggs can hatch and the aphids start to develop on their way to producing eggs in the race against the oncoming winter. At the bottom of the slope the ground does not clear until late in the summer and hence the summer period is reduced compared to conditions at the top of the slope.

Therefore at a particular date aphids collected at the top of the slope will always be older than those at the bottom. At a critical snow depth the summer period becomes too short for the aphid to complete its annual lifecycle. Here the aphid cannot exploit the Dryas patch. However, despite becoming a classic example of Arctic ecology, the original study was on an atypical slope in the middle of Ny-Ålesund. Hence we set about testing the hypothesis on a larger scale.

During the springs of 2009 and 2010 snow depths were measured along the ridges at Gåsebu and Gluudneset in Kongsfjorden. The position of each measurement was logged at centimetre accuracy via DGPS. In the subsequent August each year the snow depth locations were revisited and inspected for the presence of Dryas and the aphid. At this larger scale we did indeed observe a relationship between aphid and snow depth. Aphids preferentially exploited Dryas in areas with shallow winter snow depths.

However, it was not a perfect relationship. Winter snowfall is highly variable between years. Moreover, the pattern of snow accumulation also is dependent on prevailing wind directions. Hence snow depth at any one location will be highly variable between years. Other factors such as winter surface ice thickness, the rate of development of the plant each year or nutrient status or predation pressure will also play a role in determining local distribution of the aphid.

Environmental change indicator?
This aphid-Dryas-snow system can be exploited as a model system by which to observe the effects of projected climate change on the ecology of Svalbard.

Winter precipitation is forecasted to increase. Temperatures are also expected to continue to rise with difficult to foresee effects on snow accumulation and the timing of snow melt.

Elevated summer temperatures may enable the aphid to accelerate its rate of development and colonise Dryas patches currently in areas with too short a summer. Increased summer temperatures may also increase the production of flower shoots by the Dryas. The aphid prefers to feed on the shoots with their nutrient rich sap. Increase in plant nutrient status may enable the aphid to develop more rapidly.

Consequently there are many interactions to consider and the result is far from easy to predict. Nonetheless, with an accurately mapped distribution and ease of access from Ny-Ålesund this system presents an excellent opportunity for climate related studies on an Arctic terrestrial ecosystem.

More information:
Ávila-Jiménez M.L & Coulson S.J. (2011). Can snow depth predict the distribution of the high Arctic aphid Acyrthosiphon svalbardicum (Hemiptera: Aphididae) on Spitsbergen? BMC Ecology 11:25 (Pdf)

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