**5. Ecological similarities and differences between pioneer invertebrates**

### **5.1 Specialists or generalists?**

158 International Perspectives on Global Environmental Change

from the glacier snout on 2 year old ground. Sticky traps were placed either just above ground level, or at a height of 1 m. It was concluded that spiders caught 1 m above ground must have been aerially dispersed. The actual spider family, Linyphiidae, is known for their ability to fly by wire, called "ballooning". By raising the abdomen and gradually releasing a thread in the breeze, the spider is finally lifted upwards and can be blown very far away. Holm (1958) suggested that many spider species on Svalbard had originally arrived from Greenland as aerial plankton. The airborne Diptera and Hymenoptera in the glacier foreland represented a food source for the spiders. Another interesting observation was that more than 95 % of the animals caught in sticky traps were taken close to the ground, and very few at 1 m height. The vast majority of animals were dispersing at, or below, 0.25 m. Furthermore, animals were trapped from all directions, despite some prevailing wind directions during the study. It was concluded that these arctic insects appear to make flights of short duration and remain close to the ground where wind velocities are considerably reduced and air temperatures elevated. This behaviour enables them to perform directional

Although several springtails and mites are early colonisers on Svalbard, Coulson et al. (2003) did not catch these groups in the sticky traps. Later, Mangnussen (2010) achieved some springtails and mites in water traps on Svalbard outside glaier forelands. His traps had a sticky rim to avoid crawling into the trap. Interestingly, the airborne transport of springtails seemed to occur at low wind speeds and in periods with high air humidity, indicating a high surface activity during such conditions. Since springtails can jump, they may be taken further by air currents. In Alaska, wind-blown springtails and mites have been collected in suspended plankton nets (Gressitt and Yoshimoto 1974). Elsewhere, they have even been taken as aerial plankton at altitudes of 1,500 m (Glick, 1939; Riley et al., 1995), so

Fig. 6. Sticky trap illustrating activity of various Diptera on a 3 year old moraine at

Midtdalsbreen glacier snout, Norway. Photo: Sigmund Hågvar.

flight largely independent of wind direction (Coulson et al., 2003).

local dispersion by wind seems likely.

Pioneer invertebrates in European glacier forelands comprise both specialists and generalists. Even specialists represent a heterogeneous group, depending on their speciality. Some are "cold-loving", represented by the springtail *Agrenia bidenticulata* (Hågvar, 2010) and certain carabid beetle species of the genus *Nebria*, for instance *Nebria nivalis* (Bråten & Flø, 2009; Gobbi et al., 2007; Kaufmann, 2001; Vater, 2006). Such cold-adapted species may increase their distribution if the area of pioneer ground increases due to an increased melting rate, but may eventually disappear locally if the glacier or snow field melts away. A second group of specialists are those preferring open, barren ground. Some of these, both among microarthropods, beetles and spiders, have an alpine and/or arctic distribution. However, some also occur in lowland areas on various sandy, gravely or stony habitats, for instance carabid species of the genus *Bembidion* (Bråten & Flø, 2009) or the springtail species *Bourletiella hortensis* (Hågvar, 2010).

Ecological generalists from several taxonomic groups are found in pioneer communities. These species tolerate a wide range of habitats, both in the lowland and in mountains. Examples from European glacier forelands are the carabid beetle *Amara quenseli*, the harvestman *Mitopus morio* , the springtail *Isotoma viridis,* and the oribatid mite *Tectocepheus velatus* (Bråten & Flø, 2009; Hågvar, 2010; Hågvar et al., 2009; Hodkinson et al., 2004; Kaufmann, 2002; Vater, 2009). An interesting point is that is rather predictable which "generalists" are present among the pioneers, in the same way as the specialists are predictable. Clearly, only a few "generalists" can extend their ecological niche far enough to thrive on pioneer ground close to a glacier – including the ability to arrive there. Later successional stages may contain several other "generalist species", but they do not have this extra flexibility.

#### **5.2 Parthenogenetic or bisexual?**

Some springtails and mites are parthenogenetic, which means that one single individual can start a local population. This ability is an obvious advantage for a pioneer species if dispersion is a limiting factor. In a glacier foreland in south Norway, Hågvar et al. (2009) found that the two characteristic pioneer mites were parthenogenetic. However, parthenogenetic mites were found along the whole foreland gradient, including some slowdispersing species. Among springtails in the same foreland, the pioneer species were mainly bisexual (Hågvar, 2010). Therefore, among microarthropods, parthenogenesis is not more typical among pioneer species than among later colonisers. This may indicate efficient dispersal of individuals.

Primary Succession in Glacier Forelands:

How Small Animals Conquer New Land Around Melting Glaciers 161

Pioneer foreland communities containing macroarthropod predators have been documented both on Svalbard, in Norway, and in the Alps (Bråten & Flø, 2009; Gobbi et al., 2006a,b, 2007; Hodkinson et al., 2004; Kaufmann, 2001; Kaufmann & Raffl, 2002; Vater, 2006). While spiders represent the pioneer predators on Svalbard, a mixture of carabid beetles, various

spiders and one or two harvestman species are typical on the European mainland.

Fig. 7. Air-borne insects sampled on the surface of the Hardangerjøkulen glacier, south Norway. These specimens have been a part of the air plankton, but low temperatures above

The predator first-hypothesis is at first sight an ecological paradox, but can be explained if the predators are fed by airborne food as for instance chironomid midges. But how stable is the airborn input of suitable and sufficient food to the pioneer ground? As already pointed at by Hodkinson et al. (2002), detritivores such as Collembola can also be eaten by predators such as spiders and carabid beetles. But to what degree is this occurring, and how important are resident Collembola or mite species as a stable food source? Gut content analyses are needed to answer these questions, preferably by recognizing prey items via their specific DNA. Perhaps the input of predators is very high, for instance of ballooning spiders, and that predators to a large degree eat other predators? Are pioneer sites in practice large sinks, where the majority of even predators do not survive? And which of the pioneer beetles,

Recent studies in the foreland of Midtalsbreen glacier snout, south Norway, indicate that chlorophyll-based food chains may start very early. Interestingly, the key organisms in this respect are mosses. On a large moraine which was freed from ice in 2005, twenty pitfall

the glacier have made them fall down. Photo: Marte Lilleeng.

spiders and harvestmen do really reproduce on the barren ground?

**6.1 The predator first-hypothesis challenged** 

### **5.3 Short or long life cycle?**

Pioneer species with a short life cycle might have an advantage compared to species with a long life cycle in establishing a high and permanent population. Most of the typical pioneer springtail species in alpine south Norway have a one-year life cycle, which is relatively "fast" under these conditions (Fjellberg, 1974; Hågvar, 2010). However, the pioneer oribatid mite *Tectocepheus velatus*, is assumed to use two or more years to fulfil the life cycle in the same area (Solhøy, 1975). This species was represented mainly with juveniles in the pioneer site at Hardangervidda, indicating local reproduction (Hågvar, 2010). For this species, a slow development does not seem to be a handicap in colonisation and establishment.

#### **5.4 Resident survivors or continuously colonising?**

High densities of pioneer microarthropods could be due to continuous transport by air. Theoretically, the pioneer ground might be an ecological sink, receiving animals which continuously die. However, filled guts in sampled microarthropods indicate feeding activity on the pioneer ground. Whether pioneer ground may to a large degree be a sink for ballooning spiders, is an open question.

### **5.5 Saprophagous super-pioneers?**

In a glacier foreland in the Austrian Alps, Bardgett et al., (2007) found that pioneer, heterotrophic microbial communities to a large degree used ancient carbon released by the glacier as an energy source. Only after more than 50 years of organic matter accumulation did the soil microbial community change to one supported primarily by modern carbon, most likely from recent plant production. This means that also pioneer microarthropods feeding on fungi and bacteria could use ancient carbon, allowing microarthropods to establish resident populations immediately after the ground is laid free of ice. Inblown organic material will also gradually add substrate for saprophagous food chains. Microbialfeeding animals like microarthropods, rotatoria, tardigrada, nematoda and enchytraeidae may be the first animals which establish viable and resident populations independently of resources from outside. If so, they are the super-pioneers among animals.
