Îles Kerguelen and Îles Crozet belong to the South Indian Ocean Province, as do the Heard and McDonald Islands, and the Prince Edward Islands (Marion Island and Prince Edward Island) (Figure 1.1). Only Heard Island and McDonald Island are located in the very cold Antarctic waters. The others are located above the Polar Front, separating the very cold Antarctic waters from the sub-Antarctic cold waters.
The Îles Kerguelen are located at 49°21’S, 70°13’E (Port-aux-Français) and Île de la Possession at 46°24’S, 51°46’E (Base Alfred-Faure). They are respectively 3,250 km and 2,800 km from Île de la Réunion, 3,800 km and 2,400 km from South Africa and 1,900 km and 2,100 km from the Antarctic Continent. The Îles Kerguelen and Heard Island are located on the same continental shelf and are only 423 km apart. The Îles Crozet and the Prince Edward Islands are a second, more western subgroup.
The Îles Kerguelen are 130 km long and 120 km wide. Their total area is 7,215 km² including 6,675 km² for the main island which is called Grande Terre (Figure 1.2). The Îles Crozet are composed of an eastern group of islands (Île de l’Est and Île de la Possession) and a western group about 110 km away (Îlots des Apôtres, Île aux Cochons and Île des Pingouins) (Figure 1.3). Île de la Possession, the only inhabited island, is much smaller than Grande Terre, at only 150 km².
1.1.2. Topography
The morphology of the Îles Kerguelen is mainly related to their magmatic origin and glacial erosion. Their geological history is long. The most ancient parts emerged about 35–40 million years ago and the last magmatic events date back to about 30,000 years ago. Mount Ross rises to the South at 1,830 m a.s.l. Part of the western and central regions is covered by an ice cap, the Cook cap, which rises to 1,000 m a.s.l. and covers about 550 km². The North (Péninsule Loranchet) and the South-West (Péninsule Rallier du Baty) are mountainous and rise to 1,260 m a.s.l. To the east, Péninsule Courbet constitutes a vast area of glacial erosion which is at a low altitude and is dotted with numerous lakes (Giret et al. 2003a) (Figure 1.4).
Figure 1.1. South Indian Ocean sub-Antarctic islands
Île de la Possession was formed between 8 million years ago and about 10,000 years ago for the most recent episodes (Delépine 1973; Giret et al. 2003b). The island culminates at 934 m a.s.l. at Pic du Mascarin. Large alluvial valleys (Vallée de la Hébé and Vallée des Branloires) open onto the north-east coast (Figure 1.5).
Figure 1.2. The Îles Kerguelen
Figure 1.3. The Îles Crozet
Figure 1.4. Topography of the Îles Kerguelen
Figure 1.5. Topography of Île de la Possession (Crozet)
A large part of the two islands, Kerguelen and Possession, is mountainous with an altitude of over 200 m a.s.l. Areas below 100 m in altitude represent 32% and 22% respectively on the Îles Kerguelen and Île de la Possession, and areas below 200 m represent 50% and 40%, respectively (Figures 1.4 and 1.5).
1.1.3. Climate
As with all remote sub-Antarctic islands, both the Îles Kerguelen and Île de la Possession have an oceanic cold climate, strongly influenced by the South Indian Ocean. They are located at the same latitude as France and therefore have an equivalent photoperiodic regime, with long days in summer and short days in winter. The mean air temperature for 1951–2013 at sea level was 4.8°C at Îles Kerguelen and 5.5°C at Île de la Possession (Figure 1.6). Seasonal variations are low, between 2°C and 8°C at Îles Kerguelen and 3°C and 8°C at Île de la Possession. Summer temperatures may temporarily reach or exceed 15°C. At the Îles Kerguelen, average temperatures increased by 1.9°C between 1964 and 1981 and then stabilized to around 5.2°C. Concomitantly, the number of freezing days has decreased from 130 days per year in the 1960s to 100 days per year now. At Île de la Possession, the number of freezing days is less than 60 per year.
Air temperature decreases by approximately 1°C per 100 m increase in altitude (Pointe Suzanne and Port-Jeanne d’Arc, Kerguelen; M. Hullé unpublished data). This steep decay means that the thermal thresholds of development of many insects may be reached rapidly with altitude. The mean temperature is below 3°C from 200 m a.s.l., and insect life is consequently mainly concentrated on low-lying coastal areas (Figures 1.4 and 1.5).
The average rainfall of Île de la Possession is high with 2,390 mm per year (Figure 1.6). On the Îles Kerguelen, there is a rainfall gradient from west to east. The west coast receives about 2,000 mm per year, and Port-aux-Français, located to the east, receives about 700 mm per year. At Port-aux-Français, rainfall oscillates between years of drought, for example, 1965 with 350 mm, and wet years, for example, 1987 with 1,150 mm.
Winds are predominantly from the northwest, west and southwest. The average wind speed is 35 km/h on the Îles Kerguelen and 38 km/h on Île de la Possession. Strong winds are frequent. They are mostly from the northwest (Figure 1.7). The strongest gusts can exceed 200 km/h. They reach an average speed of 131 km/h on the Îles Kerguelen and 141 km/h on Île de la Possession. The number of days with winds above 100 km/h is 68 per year on the Îles Kerguelen.
Figure 1.6. Climate at Port-aux-Français (Îles Kerguelen) (left) and Base Alfred-Faure (Île de la Possession) (right): ① Annual mean temperature, ② Monthly temperature (mean in gray, maximum in red and minimum in blue), ③ Annual precipitation (data from Météo France)
Figure 1.7. Direction of the strongest winds (data from Météo-France)
1.2. Landscapes
Macroinvertebrates live mainly in low-elevation areas, which present four major types of landscape: sea-edge, grassland, wetland/mire and fellfield/rock (Davies and Greene 1976). Buildings and huts are also a habitat for some invertebrates.
1.2.1. Sea-edge
The seaside habitats (Figure 1.8) may consist of bare rocky areas such as pebble beach, blocks or shore platforms. They can be also covered with Crassula moschata or with coastal meadows, where native plants such as Leptinella plumosa, Pringlea
