of their life-history dragonflies are also a source of food for other creatures. Water
beetles and water-bugs will prey upon smaller nymphs. Waterside birds such as Herons.
Coots and Water-rails consume large numbers of nymphs, whilst other birds such as
Swallows, Flycatchers and Wagtails can catch and consume the adults in flight. One
rare bird of prey, the Hobby, also frequently takes dragonflies on the wing. The common
amphibians and reptiles feed on dragonflies, along with some species of spiders and
even an insectivorous plant, the Sundew. Dragonflies are at their most vulnerable to
these predators whilst they are still teneral.
As well as availability of food at all stages in their life history, dragonflies are affected
by the acidity or alkalinity of their aquatic habitats (more species favouring acidic
rather than alkaline waters), salinity or its absence (some dragonflies are quite tolerant
of brackish water), and various kinds of pollution (especially agricultural run-off).
Some species, such as Ischnura elegans are quite tolerant of pollution, whereas others,
such as Platycnemis pennipes appear to be vulnerable to even slight pollution. Finally,
the aquatic sites used may be still-water habitats such as ponds and lakes, slow-moving
waterways such as canals and some rivers, or more swiftly flowing rivers and streams.
Some species are relatively specialised in their aquatic habitat requirements, whereas
others inhabit a wide range of habitats. Calopteryx virgo (the Beautiful Demoiselle), a
species of relatively swift streams and rivers, belongs to the former category. Ischnura
elegans to the latter.
The plant-life of a site is also of great importance to the well-being of its dragonfly
fauna. Obviously, the vegetation of the site will be important indirectly as the ultimate
food source of the invertebrate prey of the dragonflies. But in addition nymphs
frequently live in association with the root-systems of some marginal plants, and use the
stems and leaves of emergent vegetation for their own emergence to the adult stage. The
adults of various species may use the leaves of floating plants such as water-lilies as
'basking' sites, and also as a perch during egg-laying. Many species, especially among
the Zygoptera also lay their eggs in the tissue of submerged or marginal water-plants.
Projecting stems or leaves of marginal plants may be used by adult dragonflies as
perches from which to make sorties after prey, or for basking in sunshine. Rank grasses,
hedgerows, or scrub in the vicinity of the emergence site also is important for many
species as shelter during bad weather, and during the vulnerable 'teneral' stage.
Finally, it has already been noted that many of the aquatic habitats used by
dragonflies are transitory. This is especially true of still-water sites. This means that the
long-term survival of species dependent on these habitats requires effective means of
dispersal to other sites. In some species, such as Libellula quadrimaculata (the Four-
spotted Chaser), and several Sympetrum species, long-distance migration can play a
part in this. As some sites cease to be usable as breeding sites, others may be newly
formed - nowadays especially as a by-product of human activity. Relatively newly
formed aquatic sites, such as freshly dug drainage ditches or flooded pits, are suitable
for colonisation by species such as Orthetrum cancellatum or Enallagma cyathigerum
which seem to require little or nothing in the way of emergent or marginal vegetation
Other species colonise later as more species of plants and associated invertebrates
arrive.
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