The Menu of Moa

Forest and Bird, Issue 306, 1 November 2002, Page 6

The Menu of Moa

BILL LEE.

—BILL LEE

Experiments with ostriches and emus show the extinct moa didn’t do the damage goats and deer do.

says

he suggestion that deer, +... Cattle, goats and sheep are suitable replacements for the extinct moa is often made in conservation debates about the management of native vegetation. However, experiments using emus and ostriches, by William Bond an ecologist based at the University of Cape Town and myself, have shown that many shrubs and trees have special features to cope with the way the big birds fed which are largely useless for defence against introduced mammals. The dominance of birds in the pre-human vegetation of New Zealand has long fascinated ecologists interested in plantanimal interactions. In other countries many woody plants have spines and distasteful or toxic leaves to reduce plant feeding by large grazing mammals. In New Zealand these features are relatively rare amongst shrubs and trees. Moa were our biggest plant eaters, and were voracious feeders. A 100-kilogram bird would require approximately 10 kilograms of fresh plant material daily. Subfossil gizzard contents show that the birds ate seeds, fruits, leaves, and shoots of woody plants. Across the landscape they would have had a strong influence on plant composition and form. There are, however, woody growth forms and features that could have developed to defend plants against large browsing birds. In New Zealand, shrubs and the juvenile phase of several trees commonly have canopy exteriors comprising dense, interlaced branches with small leaves, often hidden in the interior. These are called divaricates, and have been widely cited as an example of a growth form that may have deterred browsing birds. In the

1970s Ian Atkinson and Michael Greenwood pointed out that the divaricate hedge-like structure would have limited moa access to leaves. However, others have argued that similar growth forms occur elsewhere in the world, in the absence of browsing birds, and divaricates often grow in climatically distinctive parts of the landscape. For example, perhaps the growth form helps them to cope with valley-floor frost, or strong wind, and has little to do with animals. Using experiments with emus and ostriches, we re-examined the interaction between woody plants and large browsing birds. Close relatives of the extinct moa, these birds have only become available in New Zealand in the last decade. We fed shoots off a range of woody species to domesticated birds in Otago. On farms they graze grass but mostly survive on mash pellets. However, younger birds were keen browsers of local native woody plants. The emus and ostriches have a characteristic feeding technique. They plucked, tugged, and stripped leaves and often stem material from shoots, and easily demolished some shrubs in a few minutes. However, the birds lacked the ability to cut even small stems. The divaricate form common in New Zealand shrubs and some juvenile trees. The exterior consists of dense interlaced branches.

They were messy eaters, unable to manipulate shoots with their beak. Much of the plant material they pulled off merely fell to the ground. (Plant chemistry was occasionally important, as emus ignored miro and kowhai, while ostriches defoliated both.) To defend themselves against this type of feeding, plants would need special properties such as small leaves (hard to pluck), narrow strong stems (hard to hold and break), elastic branching patterns, and flexible stems (difficult to gain a purchase). These are the key features that distinguish divaricate juvenile and adult growth forms in trees such as lowland ribbonwood and kowhai. Importantly, these characteristics also distinguish many shrub species, including some of the native brooms. We call these trees and shrubs that can resist browsing by birds ‘wire plants’ in recognition of their tough, sinuous stems and their uniqueness to New Zealand. In experiments, we presented the emus and ostriches with shrubs exhibiting a wide range of growth forms. The amount of material eaten by birds was significantly reduced in wire plants. Typical large-leaved shrubs (e.g. Coprosma lucida) provided excellent fodder and the birds would have been satisfied after about 8-10 hours feeding. However, wire plants were much more difficult for

the birds to handle, and consumption was low. If the birds had only wire plants available, they would have to feed for over 40 hours per day to get their requirements! Clearly this is impossible and they would have been forced to forage on other growth forms. Sadly, the wire-plant architecture works against browsing birds but is useless against mammals, which readily cut through the thin stems, manipulating long shoots with their lips and tongue, and then swallowing all the plant material. Contrary to what some deerstalkers have suggested, mammals are definitely no substitute for the lost moa. The feeding and rate of plant consumption of mammals are quite different from birds, and vastly more destructive in the New Zealand context. The majority of wire plants need protection from stock and wild mammals to survive. They are overrepresented on the threatened plants list, and most favour fertile floodplain soils, which have been widely developed for agriculture. A very few wire plants are able to cope with mammals, and this accounts for why they remain common. These have spines (for example matagouri) and the ability to form impenetrable hedges. What sets wire plants apart from other woody species in New Zealand, and the woody flora of the world, is their phenomenal stem tensile strength, which makes them difficult to break. Wire plant are perhaps best recognised by the tugging test — take a shoot of less than three millimetres in diameter, and pull. If it is hard to break then it is probably a wire plant!

is an ecologist with

Landcare Research in Dunedin.