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.
Caves (Maltese: Għerien)
In spite of the calcareous nature of Malta's rocks, deep caves are not frequent. Maltese caves are inhabited by organisms, which are adapted to live in such habitats and therefore, have a very restricted distribution. The best-known cave dwellers are bats but there are many other species, particularly invertebrates. Moreover, a number of these species are endemic to the Maltese Islands and therefore, of great scientific interest.
Cliffs and Boulder Shores (Maltese: L-Irdumijiet u l-Komunitajjiet Rupestrali)
Cliffs found mainly along southern and western shores of Malta, Gozo and Comino, represent an important natural habitat because they harbour many interesting species of flora and fauna including endemic forms.
Rupestral plant communities mainly consist of halophytic shrubs and also comprise endemic species that are restricted only to this habitat, such as the Maltese Cliff-orache (Scientific: Cremnophyton lanfrancoi; Maltese: Bjanka ta' l-Irdum) and the National Plant of Malta, the Maltese Rock Centaury (Scientific: Palaeocyanus crassifolius; Maltese: Widnet il-Baħar), both belonging to monospecific genera. The cliffs in Gozo, support rupestral species that are not present in Malta, namely the Gozo Hyoseris (Scientific: Hyoseris frutescens; Maltese: Żigland ta' Għawdex) and the Maltese Everlasting (Scientific: Helichrysum melitense; Maltese: Sempreviva ta' Għawdex). Plants found on cliffs need to be resilient to the harsh abiotic conditions such as lack of water, strong winds, sea spray and little soil. Nonetheless, the inaccessibility of cliffs has meant that they have received little relative interference by man.
Cliffs provide shelter and breeding habitat for many bird species, such as the Cory's Shearwater (Scientific: Calonectris diomedea; Maltese: Ċiefa), the Mediterranean Shearwater (Scientific: Puffinus yelkouan; Maltese: Garnija) and the Maltese National bird, the Blue Rock Thrush (Scientific: Monticola solitarius; Maltese: Merill) .
The South Western Cliffs of mainland Malta provide a vital habitat to one of the rarest animals in the Maltese Islands, the Maltese Door Snail (Scientific: Lampedusa melitensis; Maltese: Dussies ta' l-Irdum). Other rare endemic snails also have their distribution restricted to only a few cliff-side localities, such as the Filfola Door Snail (Scientific: Lampedusa imitatrix gattoi; Maltese: Dussies ta' Filfla) and the Għar Lapsi Top Snail (Scientific: Trochoidea ghar lapsi; Maltese: Żugraga ta' l-Irdum).