Local giant

Forest and Bird, Issue 282, 1 November 1996, Page 50

Local giant

Ann Graeme

UR NATIVE EARTHWORM fauna is diverse, big in number of species and sometimes big in size. The largest, Spenceriella gigantea, grows up to 1.4 metres long and can be as thick as your thumb. It lives in the subsoil of native forests of North Auckland and offshore islands like Great Barrier. This giant worm burrows as deep as three metres below the surface, extracting nourishment from the humus, fungi and invertebrates it swallows along with the dirt. Spenceriella is big, but some Australian earthworms are bigger, up to four metres

long. Length has its limitations — the drag factor for a start — but the major constraint is diameter. An earthworm breathes by taking in oxygen and releasing carbon dioxide by diffusion through the moist layer around its skin. (This explains why dry earthworms die.) This diffusion of gases is passive, and can only effectively reach cells fairly close to the skin. So an earthworm can be long, but it can only be as stout as the limits of oxygen diffusion allow.

species. But try and imagine the same dispersal of our native earthworm species. Earthworms are not carried by the wind or by birds (alive, that is) and they cannot survive in seawater. Time alone saw them burrow their way throughout New Zealand. Their slow spread and subsequent adaptation have resulted in 173 species, each with its distinct and sometimes highly specialised niche. Native earthworms are mostly found in the leaf litter, topsoil and subsoil of native forest and tussockland. Others have developed lifestyles under the bark of trees or high in the forest canopy. Like all earthworms, each breaks down organic matter allowing it to be better utilised by smaller invertebrates and ultimately providing nutrients for plants. Those earthworms that live in leaf litter don’t make permanent burrows. They are small, relatively quick (to avoid the beaks of hungry birds), and darkly coloured to protect them from the damaging ultra-violet rays of the sun. Further down, earthworms of the topsoil are less vulnerable to predators, and are larger, less active and less pigmented than leaf-litter species. They make permanent burrows. Even further below the surface, subsoil earthworms like Spenceriella (see box) are very large and sluggish, reflecting their safe, albeit boring lives, far beyond the reach of predatory birds or the destructive rays of the sun.

Our specialised native earthworms began to disappear when forest clearance, fertiliser and stock trampling altered their soil homes and the more generalist introduced earthworms moved in. This latter group are members of the most recently evolved family, the Lumbricidae and are the worms of your lawns and gardens. They have followed the spread of human development around the world, dominating and excluding the local endemic worms, providing an important horticultural service but reducing biological diversity at the same time. UR PRESENT DAY giant weta, contemporaries of the dinosaurs, are little changed from their fossilised ancestors of 190 million years ago, when New Zealand and Australia were part of Gondwana. They have evolved in this country to become giants among insects. Being big is energy efficient, and, with less likelihood of being eaten as is often the case in simpler island ecosystems with fewer predators, there are few countervailing disadvantages to being large and conspicuous. Animals therefore often evolve into larger forms on islands than they do elsewhere, Problems begin when a suite of predators such as rats are introduced and you suddenly find that being very big is being very vulnerable. But why didn’t weta become as big as elephants — or more modestly, as big as

cats? The limit is imposed by the breathing system. In animals with backbones — fish, amphibians, reptiles, birds and mammals — oxygen and carbon dioxide are carried to and from the body cells by the blood vessels, powered by the beating heart. In insects, the blood system is not involved in respiration. Air is piped through tubes opening from ten — ’ ‘ } ERROS ~ ee mol Se paired portholes along the thorax (> and abdomen — these openings can (>.i-~ easily be seen in a weta or even a (.°y>cricket. The air tubes branch again yp es and again, and the very finest Sas capillaries are filled with fluid. Oxygen in the air flows directly {> along the larger tubes, dissolves in F~--45 the fluid of the small capillaries and y~.. --4 then diffuses into the insect’s tissues. ° Although some muscular pumping can be exerted, mostly the air travels slowly by passive diffusion. It could not reach the inside of a large animal fast enough to support rapid or strenuous activity. So giant weta, the fattest insects alive, are not renowned for speed. The other factor limiting insect size is the external skeleton. This armour, which has contributed to the enormous variety and success of the insect group, also curtails an insect’s size. It is humbling to remember, though we may unconsciously think that big is beautiful, that the mammals of the world today, which include most of the large land animals, number only about 4,100 species. By comparison, 400,000 different beetle species have been identified and many more await discovery. A theologian once asked the famous biologist J.B.S. Haldane what inference could be drawn about the nature of god from a study of his works. Haldane replied: "An inordinate fondness for ©

beetles."

ANN GRAEME 1S the national coordinator of Forest and Bird’s Kiwi Conservation Club.