Plants that eat insects!
Geoff Marsh examines the range of insectivorous plants in Dorset’s bogs and heaths
Published in March ’11
Standing on a small rise of land in Morden Bog Nature Reserve, looking south across one of the best preserved areas of lowland heath in Dorset it is, for a moment, difficult to appreciate the importance of what one is looking at. To the uninitiated, it can seem very repetitive – there is nothing but acres and acres of heather and sphagnum bogs, which is not terribly exciting at first glance. However, although it may seem to lack the aesthetical attractions of rolling hills, chalk downlands, reed beds or broadleaved woodland, this is an internationally-important habitat and, furthermore, it carries the highest level of wildlife protection.
So what it is exactly that makes this visually uninspiring vista so special? In terms of the flora and fauna, the answer is just about everything. The species that live on our lowland heaths are very habitat-specific; the sandy, acidic soil is very nutrient-poor – so poor, in fact, that just about everything has had to evolve specifically to cope with these harsh conditions. Where plants have evolved to live on the poor soil, insects have had then to adapt to those plants. Even some reptiles and bird species have evolved adaptations specific to the conditions prevalent. All areas of heath land are generally nutrient-poor but there are distinct different types of habitat. The main controlling facture is consequently how wet or dry these areas are. This can range from very dry to very wet and everything between the two extremes. All the insectivorous plants prefer the wetter areas.
From a hillock where the ground is dry and well drained, the sandy soil supports a thick covering of Ling and Bell Heather, along with Dwarf Gorse. As one descends the slope, there is a subtle change; first there is an intermingling of different plants, and then, plunging deeper inwards, the cross-leaved heath (a kind of heather) dominates. These areas are not so dry and, at wetter times of the year, can have standing water for a time.
After pausing a while to take stock of what is visible, when taking a closer look at the forms of plants around, one plant in particular stands out. It is rather small but, because of its different shape, colour and form, it is difficult to ignore; this is round-leaved sundew, the first of our native insectivorous plants.
It grows in the damper patches of soil, and its leaves – which are unlike leaves of species to be found outside of the wet heathlands – with their long, sticky hairs, make it quite distinctive. The sticky hairs, which trap insects that are attracted to them and foolish enough to alight upon them, then curl inwards, drawing the insect in to the centre of the plant allowing the plant to slowly digest the victims. Why does it need to do this? The answer is simple: the soil is so nutrient-poor that the plant needs the extra goodness that the insects’s digested corpses provide. Two other sundews also live on the Dorset heaths: oblong-leaved sundew and great sundew, which like to grow near, or even on, the very wet sphagnum bogs; they too trap insects in their leaves.
The round-leaved sundew is a strangely attractive plant, whether it is the sticky round leaves, spread out like a rosette, or its macabre mode of life, it is hard to say, but once you find one it is difficult to resist looking for more. It was whilst looking for more sundews that I first came across my first pale butterwort on Hartland Moor. Its lilac-pink flowers, although small, are quite attractive and distinctive, but again it is its sticky leaves that trap small insects which dare to land on in. Once an insect is trapped, the plant slowly absorbs the goodness from the insect’s body. Its close relative, the common butterwort, a plant found fairly frequently on northern moorland, was once found rarely on Dorset heaths but, possibly due to global warming, has not been seen for many years.
The most bizarre family of insectivorous plants found in Dorset must be the bladderworts. They too are associated with our nutrient-poor heath lands, but in acidic nutrient-poor ponds and lakes, rather than on land. There are three species of the bladderwort: common bladderwort, lesser bladderwort and intermediate bladderwort. All three have rather unimpressive yellow flowers on stems that protrude above the water. Floating on, and just below, the water, there are stems with very small leaves and tiny bladders. It is the bladders that trap minute pond life, and through which the plants absorb the nutrients, using the badders – as opposed to using roots – as their main source of nutrition.
Walking on Hyde Heath a few years ago, going as close to a large area of sphagnum bog as I dared, while looking for Oblong-leaved Sundews, I came across several clumps of what must be our oddest insectivorous plant. It is a plant that should not be there, an alien, the pitcher plant (Sarracenia purpuria ). It has been introduced from North America and looks a little out of place on our heath land. The pitchers in question attract insects which, following a descent towards what seems like a meal, turns out to be just that, only for the plant, whose digestive juices dissolve and absorb the insects’ nutrients for the benefit of the plant. The pitcher plant has quite large flowers that look just as odd and out of place as the rest of the plant.
Insectivorous plants are an evolutionary masterpiece; one part of nature developing a new method of deriving sustenance to make up for its natural habitat’s inability to provide it through the normal root route. Size appears to be no barrier for the insectivorous plants, either. Damselflies, hardly the smallest of insects, can find themselves stuck on the sticky hairs on the leaves of an oblong-leaved sundew. It seems impossible that such a small plant could subdue such a large insect, but such is the imperative for survival of one of the most extreme evolutionary routes taken by any group of plants, that nothing is impossible where nature is concerned.