28 August 2008

It's in the trees! (no it's not)

Many tonnes of bait containing the poison 1080 (sodium fluoroacetate) are dropped on native forests every year in New Zealand to control possum numbers. ERMA (the Environmental Risk Management Authority) has recently reassessed the use of this compound and found that it is still the most useful poison that we have to deal with our major pest problem in New Zealand. There are, however, still many concerns expressed about the use of 1080. One issue is that 1080 may leach into soil and then be absorbed by plants. Of particular concern is whether 1080 is being absorbed into native plants used by Maori for food and medicines, as these plants grow and are harvested in areas that may be included in 1080 drops.


Lincoln University researchers Shaun Ogilvie and James Ataria, with collaborators from Ngai Tuhoe and Department of Conservation, have published in Ecotoxicology the results of experiments on 1080 uptake in plants. In association with Ngai Tuhoe (of the eastern central North Island), experiments were done in a forest block just south of Lake Waikaremoana. Two plant species were selected for the trials. Pikopiko (hen and chicken fern, Asplenium bulbiferum) is a species of fern where emerging shoots are used as food. Karamuramu (Coprosma robusta) is a small tree species used as an internal and external medicine. Ten healthy specimens of pikopiko were enclosed in wire mesh cages and a single cereal 1080 bait was placed at the base of all the plants of both species in a smaller mesh cage to prevent disturbance by rodents. Samples from each plant were taken several times from 0-56 days after the bait was placed. The samples were then analysed for 1080 concentrations using gas chromatography. No 1080 was found in any pikopiko samples. 1080 was detected in one karamuramu plant on days 7 and 14 at between 2.5 and 5 ppb (parts per billion), representing about 0.0004% of the 1080 present in the original bait. The researchers calculated that to reach a lethal dose of 1080 for a 70 kg human that the person would need to consume about 28 tonnes of karamuramu! It was concluded that the risk to humans from 1080 absorbed into these plant species was negligible.

27 August 2008

Life on a southern beech

The southern beeches of New Zealand (Nothofagus species) make up a major forest-type which is extensive throughout the North and South Islands. Studies in beech forests usually focus on the roles of the trees as canopy for forest-floor ecosystems. A new study by John Marris and Rowan Emberson (Lincoln University) in association with James Johnson (University of Idaho) looked at the trees themselves as a habitat. More specifically, they examined the insect community on sooty mould - the characteristic black fuzz seen on the trunks and branches of some southern beeches. The mould grows on the sugary excretions of scale insects (Ultracoelostoma species), often forming a thick mat that provides a rich environment for invertebrates.


In a paper published in The Coleopterists Bulletin Marris, Emberson and Johnson describe how they collected sections of beech branches from near Oxford, Canterbury, and placed them into rearing containers. After a week numerous beetle specimens were collected in the trap jar, or by hand as branches were manually searched. Underlining the hidden diversity of New Zealand ecosystems, seven different beetle species were collected from one Oxford branch. The most exciting find was of the beetle Metaxina ornata, the only member of the family Metaxinidae, which is unique to New Zealand. This species is rarely collected but was found to be common in their samples. Collections from Kaikoura and Craigieburn gave similar results.

How can such a small and uniform ecosystem like the branch of a beech tree support so many different species? The beetles probably take advantage of the scale insects, harvesting both the honeydew and the scales themselves. Several of the predatory Metaxina larvae were found inside scale tests (shell-like structures surrounding the scale insect), presumably after eating the former occupant. The pink-tinged gut contents of the larvae matched the pink colouration of the scales - smoking gun evidence of predation. Studies like this are fascinating for how they show just how little we know about the natural history of the hidden world of invertebrates.

15 August 2008

The fall and rise of New Zealand

There are two things that we know for certain about the geological history of New Zealand. First, the land area that is now modern New Zealand was part of Gondwanaland until about 83 million years ago. Second, modern New Zealand is isolated from other large landmasses by thousands of kilometres of ocean. What happened in between is not so clear. A recent study by researchers from Lincoln University, Geological Nuclear Sciences, Massey University and University of Otago has thrown some light on a key episode in the history of the region. The study, published in the Geological Magazine (The Waipounamu erosion surface: questioning the antiquity of the New Zealand land surface and terrestrial fauna and flora. Landis, Campbell, Begg, Mildenhall, Paterson & Trewick), has important implications for the history of New Zealand’s flora and fauna. It has long been known that after the separation from Gondwanaland, the crust under the New Zealand region (or Zealandia) started to thin, which lead to a gradual submergence of Zealandia. Most of Zealandia (an area the size of India) is now two kilometres under water. The sinking became very acute around 25 million years ago (in the period known as the late Oligocene) when maximum submergence was reached. Following this 'Oligocene Drowning', tectonic activity along the Pacific and Australian plates led to crustal thickening and uplift in the New Zealand region. There has been much debate about how much of modern New Zealand was left above water during that time.


Landis and his colleagues looked at the evidence for dry land during the Oligocene period in order to better estimate the location and proportion available for the flora and fauna to have survived on. One major piece of evidence that they used was the Waipounamu erosion surface. These surfaces are flat areas in the landscape that represent wave cut platforms created as Zealandia was slowly inundated by sea. They also used sedimentary deposits, terrestrial fossil sequences and geomorphological evidence to reach a conclusion that there was no hard evidence for ANY land during the late Oligocene. The authors argue that the New Zealand region may have completely submerged or, at most, persisted in tiny remnants of land during the peak inundation. This has obvious implications for New Zealand’s biology as it suggests that all, or at least the vast majority, of species have colonised New Zealand over the last 22 million years across an ocean gap rather than simply persisting here for 80 million years in splendid isolation.

So much for the geology, what does the biology tell us? We know that the New Zealand biota can change very quickly. New Zealand’s forest composition has changed remarkably over the last tens of thousands of years. Fossils from a lake bed at St Bathans from 16 million years ago show evidence of a more tropical fauna complete with crocodiles, parrot and bat species not found in New Zealand today and even a mammal bone! So 22 million years is long enough for all sorts of things to happen. Molecular data, which can estimate when common ancestors lived, also overwhelmingly shows that closest relatives to taxa living in New Zealand (generally found in Australia) diverged from them well within the last 20 million years, implying that they arrived and colonised here. The only biological evidence that challenges the idea of submergence is that of ancient taxa, like tuatara, leiopelmatid frogs and the recently extinct moa. Their presence in New Zealand and nowhere else suggests that they inhabited Gondwanaland and were isolated here for 80 million years. Tuatara and the frogs are both groups that can persist on small islands and may represent true ‘ghosts of Gondwana’ that made it through the submergence. Moa are species of ratites and we know that another ratite group, the kiwi, did successfully colonise New Zealand. Perhaps moa did the same.

08 August 2008

Our favourite plants, next generation's weeds?

New Zealand's agricultural, horticultural, and forestry industries are battling an increasing number of weed species that reduce their productivity. Similarly, wildlands in New Zealand, like in many parts of the world, are coming under increasing pressure from invasive plants (weeds) that harm native biodiversity. Ecologists are working hard to try to figure out why a certain minority of introduced plants become troublesome weeds while most remain well behaved. If we could accurately predict which introduced species will become weeds, they could be controlled or eliminated before they became too widespread to stop.

Recent Lincoln University masters student and Fulbright scholar, Kelly Gravuer, tackled this problem of how to predict invasive plants, using the well-documented history of clover (Trifolium) introductions in New Zealand. Her results were recently published in the Proceedings of the National Academy of Sciences (USA).

Why clover? While there are no native clover species in New Zealand, nobody is leaping about with a backpack of herbicide killing the many introduced clover species. Quite the opposite. As nitrogen-fixing plants, clovers are vital to pastoral farming. Clover roots contain clover-specialised Rhizobia bacteria, also introduced into New Zealand, which convert nitrogen from the air into nitrogen fertiliser for the soil. It is exactly because clovers are such useful plants that their biology and introduction into New Zealand have been exceptionally well documented, far better than weeds. Kelly concluded that clovers offered an excellent opportunity for ecologists to test and refine their methods of predicting plant invasion success.

Kelly's sleuthing revealed that of the world's 228 clover species, 16 clover species that had never been intentionally introduced here are now wild in New Zealand. These were likely introduced accidentally with soils and other seeds. A further 56 clover species were intentionally introduced to New Zealand, only nine of which are now found in the wild in New Zealand. Kelly also compiled the distributions of all 25 wild clover species in New Zealand.

With this information and a small encyclopedia's worth of information about the biology of each species, Kelly set to work searching for traits that separated the wild (naturalised) species from the cultivated species, and the widespread naturalised species from the rare ones. What she found was surprising.

What separated the successful invaders from the other clovers was not a particular combination of biological traits but largely the extent of species' association with people and human activities. This is despite an impressive diversity of biological traits among the New Zealand clovers. The clover species most likely to have been introduced to New Zealand, most likely to have naturalised, and most likely to be widespread, were all those species closely associated with people.

Of the nine intentionally introduced species that have naturalised, eight were widely grown commercially in agriculture. Those naturalised species that had not been intentionally introduced were typically species that occurred naturally in Britain, the main source of early European settlers for New Zealand, and were commonly recorded as seed contaminants in the early decades of New Zealand agriculture. Those naturalised species that became widespread in the wild had similarly strong associations with agriculture. Just knowing the biology of the species was of little use in predicting which species would be successful invaders. You had to know the extent and type of association each species had with people.

What does this mean? One message from Kelly's work is that it is going to be difficult to predict which cultivated plants will become future weeds without carefully taking into account what people do with these plants. For example, popular garden plants will be more likely to become weeds than unpopular plants, even if their popularity is solely determined by human aesthetics like flower colour or international gardening fashions and is unrelated to the plant's growth and seed production.

Most future weeds are likely to be very familiar to us—like Agapanthus, wilding pines, and kiwifruit—rather than obscure species that accidentally slipped across the border and spread without our help.

Why didn’t the Pukeko cross the road?

We all know the joke about the chicken crossing the road. It’s probably up there with “What time is it when an elephant sits on your fence?” as one of the earliest jokes you learn as a kid. There is a serious side to road crossing, however. Roads are a dangerous place if you are a bird and there are no guarantees that by crossing them you will, in fact, get to the other side. Roads are estimated to affect as much as 19% of the total area of the USA and 28% of the land area of New Zealand! That’s a lot of habitat and wildlife species have to deal with roads on a regular basis. One species that is commonly sighted near, and unfortunately on, roads all around New Zealand is the pukeko (Porphyrio porphyrio melanotus).

A recent study by Clare Washington (Lincoln University) published in New Zealand Natural Sciences looked at the roadside behaviour of Pukeko. The study was based at Otakaikino Reserve, just north of Christchurch, a wetland surrounded by motorways, with a resident population of pukekos who must regularly deal with roads. Clare spent 18 months observing the birds. She banded the resident birds and found that, although they were regularly on the roadside, they were not the birds that were struck by cars during the study; the dead birds seemed to be those that were wandering through the area.

So why were the birds by the road? Roadsides can also be resources for birds. Clare found evidence that foraging was important near the road. Roadsides are regularly mowed and pukeko prefer new shoots of grass (wet weather and spring conditions also bring them to the road verge to feed on the flush of new growth). Pukeko also need grit for their gizzards. Gizzards are pouches in the digestive system where coarse food is ground up using pebbles to help with digestion. There are few sources of grit in the wetland and the birds were able to search along the roadside for hard rocks, like quartz. Finally, roadsides are very open spaces compared to wetlands and the birds tended to congregate in social groups far more than would be randomly predicted. Perhaps the roadside is a good place to look for potential mates, establish dominance or just gossip!

The roadside appears to offer both danger and resources to local birds and this study underlines how little we actually know about this critical habitat.