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

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 considerably influence the succession process.

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

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

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 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**

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 on bare ground [32].

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

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

For herbivores, the presence of a suitable food plant is crucial. While the moss-eating beetle *S. metallica* was found in the first moss patches on a 3-year-old ground, another moss-eating beetle, *B. fasciatus*, was not trapped until on a 63 years old ground. The beetle *Chrysomela collaris* feeds on *S. herbacea*, which occurs throughout the foreland. However, this beetle colonised late, and was found after 79 years. Clearly, other factors than the presence of the food plant determined the colonisation rate of some herbivorous beetles [26].

Both macro- and microarthropods were split into two main successional pathways: a dry and a wet succession. Due to patchy distribution of dry and moist habitats in the foreland, specialist on dry or moist sites had to overcome dispersal over unfavourable ground. The carabid beetle *C. vaporariorum*, which prefers dry ground, has a disadvantage by its inability to fly, due to rudimentary wings.

#### **6.4. Age about 10,000 years: mature soil**

The number of oribatid species increased clearly in this very old soil compared to 200-year-old soil, maybe facilitated by a deep organic layer. However, the increase was small for springtails, which were more efficient in colonising the foreland.
