**7. Remarks**

**6. Driving forces in various phases of animal succession: facilitating and** 

In early succession theory, facilitation, inhibition and tolerance were central concepts [49]. They were all used in a biotic context, and it was assumed that succession was driven by the way species interacted with one other. Early occupants could modify the environment in a way that influenced 'late-successional' species in three possible ways: (a) make the habitat more suitable for other species (facilitation) and (b) less suitable (inhibition), or early occupants had little or no effect on subsequent recruitment of species (tolerance). In the following presentation of four characteristic phases of succession, we use the terms facilitation, inhibition and tolerance in both biotic and abiotic contexts. We want to show that animal succession is only partly driven by the development of vegetation, and that abiotic factors may consider-

Wind facilitated transport of invertebrates, prey, algae and mosses into newly exposed ground [28]. In a foreland at Svalbard, aerial dispersal of midges and ballooning spiders was

The glacier itself facilitated the pioneer community by producing ponds, in which chironomid larvae assimilated ancient carbon. Within ponds, chironomid larvae were eaten by predatory diving beetles. Adult midges transported ancient carbon to terrestrial predators [29, 30]. The presence of predators before visible plants, often referred to as the 'predator first paradox' [13], can to a large degree be explained by local production of chironomid prey from young ponds. Cold-adapted species, like the springtail *A. bidenticulata* and the ground beetle *N. nivalis*, were facilitated by proximity to the glacier. However, a glacier retreat around 20 m

A high soil humidity due to much silt facilitated the colonisation of several plants and animals. Small patches of *S. herbacea* initiated the production of an organic layer. The moistloving carabid beetle *P. septentrionis* colonised the ground. Pitfall catches documented a high surface activity among larger springtail species, not only within vegetated patches but also

A closed vegetation created shelter, reduced wind and maintained humidity. Web-building spiders were favoured by a three-dimensional vegetation. The pioneer ground beetle *B. hastii* disappeared when the vegetation became closed, but a local population survived on a 75-year-old bare patch [21]. The gradually deeper organic soil layer was positive for soil-

per year means that they had to migrate continuously to remain in the cold zone.

**6.2. Age about 30–40 years: patchy pioneer vegetation and much open ground**

**inhibiting factors**

168 Glacier Evolution in a Changing World

on bare ground [32].

ably influence the succession process.

**6.1. Age 3–7 years: bare ground or only scattered pioneer vegetation**

even assumed to add nutrients to virgin soil [12, 14].

**6.3. Age about 60–200 years: mainly closed vegetation**

living springtails and mites (**Figures 6**–**8**).

Glacier forelands offer unique possibilities for the study of succession. We are beginning to understand patterns of arthropod succession by comparing studies from Norway, Svalbard, Iceland and the Alps [26, 32]. Several species or genera among arthropods are common pioneers in Norway and the Alps. However, glacier foreland chronosequences are both variable and complex. More case studies are needed, both to reveal local variations in pioneer communities and succession patterns and to look for general patterns.

Sample size is a critical factor. **Figure 18** shows how 12 soil samples within one plot gradually increased the cumulative number of mite taxa, but none of the samples contained all taxa. Ideally, sample numbers should be so high that the cumulative species number stabilises. If species numbers in different sites shall be compared, corresponding sampling effort should be used in all sites. To cover local variation in species composition, it may be better to take several small samples instead of a few larger covering the same area. During sampling with a soil corer, large, surface active springtails may escape by jumping. Some pitfall traps in addition may give valuable information.

Pitfall traps are much used for beetles and spiders in comparative studies, but the number of traps is often low. Even in the present study, with 20 traps operating during 2 years at each site, several species were taken in very few specimens [26]. Traps should be operated throughout the snow-free season, since certain species may have restricted seasonal activity.

The term 'primary succession' is questionable when both aquatic and terrestrial pioneer communities use ancient carbon released by the glacier. Young ponds acted as 'biological oases'

**Figure 18.** In plot no. 18 (age 180 years, see Figure 1), 12 soil cores were taken. This example shows how the cumulative number of mite taxa increased with increasing number of cores. However, none of the single cores contained all taxa (columns).

where ancient carbon was assimilated by Diptera larvae, mainly Chironomidae. In a foreland without ponds, a possible release of ancient carbon can be checked by radiocarbon dating chironomid larvae from the glacier river. A peculiar thing is that if invertebrates, which had assimilated old carbon, had been recovered as subfossils and radiocarbon dated, their age had been overestimated by up to 1100 years [29]. Since several pioneer species were herbivores on biofilm or mosses, the present succession did not fit with the 'predator-first' hypothesis. Although pioneer species may be ecologically very different, the pioneer community is surprisingly predictable, both within Norwegian forelands and in the Alps, and several genera are in common [32].

We need to improve our knowledge about the autecology of the individual species to better understand their position and functional role in the succession process. From each species´ point of view, colonising the foreland is a question of fulfilling minimum ecological demands. For instance, analyses of gut content were the key to understand the pioneer food web in the present foreland [29, 31]. Experimental studies involving transportation and re-location of species could be rewarding, but would it for the sake of science be ethically acceptable to move species within a 'natural laboratory' that should develop in a natural way?

A negative and special aspect by melting glaciers is that their meltdown will threaten coldadapted invertebrates which live near glaciers. Especially when it comes to endemic, coldadapted species, melting glaciers represent an extinction threat, as in certain mountains of the Southern Alps [50].
