How Invertebrates Colonize Deglaciated Sites

Mitopus morio (Source: Javier Díaz Barrera/Flickr).
Mitopus morio (Source: Javier Díaz Barrera/Flickr).

Scientists have long wondered how species colonize sites after deglaciation. A recent study by Amber Vater and John Matthews in the journal The Holocene of invertebrates–animals without backbones—on a number of sites in Norway advances the understanding of this colonization. It pays particular attention to succession, the processes of change in the species composition of ecological communities over time. The invertebrate groups which were studied include insects, spiders and mites, as well as harvestmen, also known as daddy longlegs.

To study the process of succession, Amber and Matthews collected invertebrate samples from pitfall traps in 171 locations across eight glacier forelands, which deglaciated over the last few centuries, in the Jotunheimen (high altitude) and Jostedalsbreen (low altitude) subregions in southern Norway. Jotunheimen is the highest mountain in Europe north of the Alps and west of the Urals, while Jostedalsbreen is the largest ice-cap in Europe outside Iceland. These forelands represent different ecological regions and areas that have been deglaciated for periods of different length. A variety of geological and biological evidence allowed the researchers to establish the precise timing of glacier retreat across their sites. The researchers identified the organisms by taxa—the species, genus or family to which they belong—since species identification was difficult in some cases.

The location of the eight glacier forelands in southern Norway (Source: Vater and Matthews/Sage Journals).
The location of the eight glacier forelands in southern Norway (Source: Vater and Matthews/Sage Journals).

Several major findings were derived from this study. Firstly, invertebrates arrive fairly quickly after the retreat of glaciers, within a decade or two. In particular, initial colonization is faster and dispersal is more effective at high altitudes, where glacier forelands are small, reducing the distance from established communities to new sites; in addition, the strong winds in such areas can carry organisms further. The flying insects, such as flies, aphids, bees, wasps, stoneflies, caddisflies and flying beetles, arrived earlier than the ground-active non-flying species, such as spiders, harvestmen, mites, ants, and non-flying beetles. Moreover, the communities grow more complex over time. In the first stage, lasting about 20 years, 11-31 taxa were found; this number increased to 21-55 in the fourth and final stage, over two centuries later. The authors found as well that invertebrate communities tend to be more diverse at low altitudes, where environmental conditions are more favorable.

Jotunheimen from southern Norway (Source: Thomas Mues/Flickr).
Jotunheimen from southern Norway (Source: Thomas Mues/Flickr).

Vater and Matthews summarize their findings by stating “invertebrate succession on the glacier forelands is viewed as driven primarily by individualistic behavior of the highly mobile species with short life-cycles responding to regional and local abiotic environmental gradients”.

Amara quenseli (Source: Chris Moody/Flickr).
Amara quenseli (Source: Chris Moody/Flickr).

This research calls into question earlier studies of succession. Previous studies, often based on plant species rather than invertebrates, have emphasized that nearly all taxa occur only in some of the stages of succession. By contrast, Vater and Matthews find that most of the taxa that first appear remain all the way till the final stage—65-86%, depending on the site. The authors describe their results as an ‘addition and persistence’ model (because taxa remain, once they arrive) rather than the more established ‘replacement-change’ model, in which different taxa replace each other over time. This ‘addition and persistence’ model seems to be more applicable in severe environments.

This research offers some insights into the regions that will become exposed as glacier retreat continues. It brings the positive finding that lands that appear after glacier retreat will not remain barren for long, since invertebrates are likely to colonize these sites soon. However, the new areas at higher elevations may have only a small number of specialized invertebrate taxa instead of a wide range of them.

For more details on invertebrates living on glaciers, look here.

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