The Process of Succession
Ecosystems undergo ecological succession, which refers to the natural process of change over time brought about by progressive replacement of one plant (or animal) community with another in response to changes in environmental conditions. This process starts off with the pioneer community and eventually leads to the development of a stable mature community, referred to as the climax community. The process of succession can halt in a pre-climax stage when some factor is limiting, such as when the organism needed to bring about the necessary changes that lead to the creation of the following community, is absent. Limiting factors may also be abiotic (non-living), such as lack of water.
Succession can be of two types. Primary succession begins with the colonisation, by pioneer species (such as mosses and lichens) of barren substrate (rock, sand or soil), which has never supported any vegetation. On the other hand, secondary succession occurs in areas where natural vegetation has been disturbed or destroyed. The latter type is generally less species rich.
Natural habitats in the Maltese Islands appear in different stages of ecological succession. In certain localities it is easy to differentiate between the different stages, though in other cases, the habitats occur as a mosaic of the different stages of ecological succession. There are four principle stages of ecological succession in the Maltese Islands. These are, steppe (Maltese: Steppa); garrigue (Maltese: Xagħri); maquis (Maltese: Makkja) and woodland (Maltese: Masġar). Succession is most commonly secondary.
Valley Watercourses (Maltese: Il-Widien)
Valley watercourses are one of the most species-rich habitats on a national scale. Yet, they are considered as one of the most endangered habitats in the Maltese Islands.
The biotic (living) communities of valleys can be divided into two groups: those growing on valley sides and those growing along the watercourse.
In gently sloping valleys, the watercourse community is similar to that of the valley sides, whereas in steep-sided valleys there is a clear distinction between communities along the watercourse and those vegetating valley-sides. Where the terrain permits, the valley sides are terraced and cultivated. The construction of man-made dams in certain valley systems has intentionally retarded the water flow for irrigation purposes. Such dams have created new freshwater habitats where a variety of aquatic and semi-aquatic species thrive.
The watercourse community is by nature dynamic and its integrity depends on the amount and frequency of rainfall as well as other abiotic factors such as the rate of siltation. Valleys are dry for some months of the year and water only flows during the wet season. However, some local valleys drain springs originating from the perched aquifers and retain some surface water even during the dry season.
In general, the greater part of local plant and animal species reliant of water during some part of their life cycle are found in valley watercourses. Various annual and perennial plants colonise the watercourse, some of which are rare on a national scale because of the restricted distribution of their habitat. An example is the very rare perennial Willow-leaved Knotgrass (Scientific: Persicaria salicifolia; Maltese: Persikarja tal-Baħrija).
Plants that grow in watercourses include herbaceous perennials such as the Water Plantain (Scientific: Alisma plantago-aquatica; Maltese: Biżbula ta' l-Ilma) and the Water Speedwell (Scientific: Veronica anagallis-aquatica; Maltese: Veronika ta' l-Ilma). Perennials, unlike annual plants, are able to withstand periods of dryness. Watercourse plants require a good underground system of roots or rhizomes for anchorage to the unstable waterlogged substrate of watercourses. Attached watercourse vegetation mainly comprises grasses, sedges and rushes, while algae thrive in the open water, like species of Spirogyra and Zygnema. One of the most common plants to colonise valleys is the Giant Reed (Scientific: Arundo donax; Maltese: Qasba Kbira). Encroachment by this reed results in reduction of water current, however when the water passes through the rhizomes of this plant, the water is filtered from nutrients. The Giant Reed is often replaced by the Common Reed (Scientific: Phragmites australis; Maltese: Qasbet ir-Riħ) at the mouth of valley watercourses where freshwater feeds into the sea.
Remnants of riparian woodlands, located on the bank of a watercourse, still exist along a few watercourses where water flow is abundant. Examples of trees growing along watercourses include the rare White Poplar (Scientific: Populus alba; Maltese: Siġra tal-Luq), the Mediterranean Willow (Scientific: Salix pedicellata; Maltese: Żafżafa ż-Żgħira) and the Grey-leaved Elm (Scientific: Ulmus canescens; Maltese: Siġra tan-Nemus). Different subtypes occur in different localities.
Watercourses provide habitat and food to various animals, the most well known being the only amphibian found in Malta, the Painted Frog (Scientific: Discoglossus pictus pictus; Maltese: Żrinġ), and the legally protected endemic Maltese Freshwater Crab (Scientific: Potamon fluviatile lanfrancoi; Maltese: Qabru). A huge variety of insect and other invertebrate fauna also thrive in local valleys, such as dragonflies and damselflies, semi-aquatic grasshoppers, mayflies, aquatic and semi-aquatic beetles, such as the Large Predacious Diving Beetle (Scientific: Dysticus circumflexus; Maltese: Wirdiena ta' l-Ilma), water-associating flies, bees and wasps, small crustaceans and many others. Some of these are only found in these habitats and some are only known from one or a few localities in the Maltese Islands.