The Great Veronica Hunt —Part 6.

I'm writing this in Melbourne, where I'm about to fly home after a wonderful three weeks in Australia. I wasn't specifically on a Veronica hunt, but kept my eyes open anyway, just in case.

I didn't see any Veronica in Queensland or around Sydney. The first I saw was the introduced V. arvensis in Bega, a small New South Wales town.  Australia has many of the same weedy speedwells that New Zealand does, so I was more interested to see plants of the indigenous species.

Mallacoota inlet, Vic.

We spent a couple of days with friends at Mallacoota in the far east of Victoria, and there came across V. plebeia growing beside a track in coastal forest in the wonderfully-named Croajingalong National Park.

Veronica plebeia, Mallacoota, Vic.

The flowers were closed just as they often are in New Zealand, needing a warm sunny day to open.  If they don't get to open, I assume they self-pollinate, because they always seem to set fruits.

The flower below was photographed on a cultivated plant in New Zealand, where V. plebeia is widespread and considered by some botanists to be native.  It is introduced and weedy in some other parts of the world though, so it does have the ability to be invasive.

Veronica plebeia, from a cultivated plant in New Zealand.

That was it for wild speedwells the whole trip, but my sister-in-law, near Ballarat, had some small plants of another Australian native, Veronica gracilis, ready to plant out in the garden, and one of these was in flower.

Veronica gracilis, cultivated near Ballarat, Vic.

The plants are strongly rhizomatous, and this one even had a shoot coming out of the drainage hole in the bottom of its pot.

Australia has 23 native species of Veronica, classified in section Labiatoides, and they are the sister group to the large New Zealand clade (section Hebe) that includes the hebes and their relatives (Albach & Briggs 2012).  Thus, although they look much more like northern speedwells than New Zealand hebes, they are known to be more closely related to the hebes.  And because of that fact, it's misleading to classify them as Veronica unless you classify our hebes in Veronica as well.

Reference

Albach, D; Briggs, BG. 2012. Phylogenetic analysis of Australian species of Veronica (V. section Labiatoides; Plantaginaceae). Australian Systematic Botany, 2012, 25, 353–363
http://dx.doi.org/10.1071/SB12014

New associations good and bad.

The word weed can be a hard one to define.  Most people accept that a weed is a plant growing where it’s unwanted, something that’s in the way, or that stops the flower or crop you’re trying to grow from growing, or interferes with valued native vegetation.  When you think about it that way, it’s clear that one person’s crop or wildflower can easily be, or become, another’s weed.  Unfortunately, the corollary is that one person’s pest might be another’s treasure.

Ngaio, Myoporum laetum.

Last week I wrote about the common confusion between New Zealand and Tasmanian ngaio, and how in New Zealand the latter is sometimes planted unintentionally in place of the former.  Our native ngaio, although prone to self-seeding in gardens and capable of fast growth, is never really a weed here.  But it is a pest plant in California, along with some others of our native flora, like pōhutukawa and cabbage trees.  This is the story of the rise and fall of ngaio in California, as told in a recent research paper by Jon Sullivan of Lincoln University (Sullivan 2013).

Ngaio was introduced into California as an ornamental tree and widely planted around the middle of last century, mostly using a California-derived cultivar, M. laetum ‘Carsonii’. It’s the 18th most common street tree in San Francisco and is valued for its fast growth and salt tolerance near the sea.  From widespread plantings in Southern California, ngaio has spread into many wild and semi-wild communities from Sonoma County southwards to Baja California in Mexico.  It forms a dense canopy that shades out other plants and the dry woody centres of the trees are considered a fire risk.  The trees even re-sprout after fire or herbicide spray treatment, so they’re hard to get rid of.

The core of Sullivan’s paper describes the effects of the chance introduction of a tiny insect, a kind of thrips (the singular and plural are both thrips).  This thrips, Klambothrips myopori,  feeds on the leaves and shoots of plants of Myoporum and seems to have got there from Australia, where New Zealand ngaio isn't native, but where other species of Myoporum are.  Although it was first described and named from Californian collections, later a small population was discovered on boobialla in Tasmania.  And its closest relative is also in Australia, so it’s likely the insect is a dinkum Aussie and a newcomer to California.  Most likely, Myoporum thrips got accidentally introduced to California, maybe via the airline routes that converge on Los Angeles.  It probably wouldn’t have become established there, except that there were already large populations of planted and weedy ngaio for it to feed upon.  

And it got stuck in.  It's taken it about five years to kill about half the ngaios in Southern California, and the remaining live ones are looking pretty sick.

Thrips are small slender insects with fringed wings.  They mostly feed on plant sap, which they do through mouth-parts that are modified for piercing plant tissue.  A thrips infestation typically produces silvery or bronze patches on shoots and leaves, where sap has been drawn out of the cells.  Affected young ngaio shoots turn brown and the leaves are distorted.  Sullivan found high densities of nymphs and adults on affected trees in California. Other thrips are pollen feeders and are often seen in flowers, where some botanists believe they can be significant pollinators.

This inadvertent spread of thrips to California is an excellent outcome for environmental managers trying to deal with the Californian ngaio outbreak.  To introduce a biological control agent these days involves a paper war of bureaucracy, and rightly so, because they can have unintended consequences.  But in California, nature—or at least accident—had already done the job.  So, all good, you might say.

The success of Myoporum thrips in California seems to support an idea that ecologists call the New Associations Hypothesis.  The idea is that when a host-specialised organism—like a thrips that feeds only on Myoporum—comes into contact with a naive host, one that hasn’t been exposed to it before, then all hell breaks loose (for the host).  The best-known historical examples are probably the human populations that hadn’t ever been exposed to European diseases, like smallpox and measles.  Because long-distance dispersal to islands is a filter that only some organisms get through, it might be that our ngaio and other native plants have evolved in New Zealand without some or all of the parasites and predators that would damage them in their countries of origin.  If they’ve let their guard down, so to speak, then introduction of those parasites and predators by human activity could be a disaster for them.

So, what if this thrips ever makes its way to New Zealand?  We now know it can and will happily eat ngaio, and we know it has the potential to hitch rides in aircraft.  It’s yet another pest we need to watch for at the border.  Presumably in Australia, the thrips and the Myoporum have evolved together and the plants have enough defenses not to be wiped out.  But we can see what might happen here by looking at Hawai'i.  There, the Myoporum thrips has already been introduced, again probably unintentionally and perhaps from California, and it’s taken to their native species of Myoporum, M. sandwicense, with gusto.

A branch of boobialla, M. insulare.

If that calamity happens here, we can only hope the thrips prefer the introduced boobialla or Tasmanian ngaio (M. insulare) to our native ngaio, M. laetum.  My guess, and Sullivan’s too, is it’s more likely to be the other way round, because boobialla is likely to have more tolerance to thrips.  Add that to people planting the wrong species, and in the future we might find our ngaio replaced by boobialla almost everywhere.

Sullivan, Jon J. (2013-08-24) Inadvertent biological control: an Australian thrips killing an invasive New Zealand tree in California. . DOI: 10.1007/s10530-013-0532-x  

Going through the motions: what did moa eat?

Moa were giant flightless birds found in New Zealand (the plural of moa is moa, because the Māori language doesn’t distinguish singular from plural nouns, with one exception).  There were 6 genera and 9 species of moa; the largest, Dinornis, stood well over 2m tall.  They’ve been extinct since shortly after Māori arrived here.  It’s thought they were an easily harvested source of protein and were quickly driven to extinction.  Although everywhere on earth where humans live has extinct megafauna (e.g., aurochs in Europe, giant sloths in South America, giant lizards and kangaroos in Australia), in most places the extinctions happened so long ago that they're very hard to study.  But in New Zealand, the extinction of moa is quite recent, dating from around the 13th century, and there are still traces to be found and studied.  Deposits of regurgitated gizzard stones are sometimes found, and subfossil birds can be recovered from caves.  It's thought that some large trees still alive today might have been dispersed as seeds by moa.  

About 35 years ago, Michael Greenwood and Ian Atkinson (Greenwood & Atkinson 1977) proposed that moa could have been a major influence on growth forms of New Zealand plants.  In particular, they suggested the twiggy wiry tangled small-leaved growth forms that we call divaricating shrubs could have evolved as a defense against moa browsing.  That’s been a very popular and appealing idea, but one that’s had its critics.  While New Zealand botanists have been happy to attribute our unusual flowers to pollination by our depauperate and unspecialised pollinator fauna and our prevalence of small fleshy fruits to dispersal by frugivorous birds, many have been wary of accepting the moa browse hypothesis.

A divaricating shrub, Coprosma cuneata, Campbell Island.

Partly their objections have arisen from concern that these ideas can’t directly be tested, because moa are no longer with us.  Nevertheless, many other purely historical ideas in biology can be tested, by indirect methods at least.  Greenwood & Atkinson’s seminal paper has spawned an industry in New Zealand ecology largely driven by questions about the likely selection pressures of moa on New Zealand plants.  One recent test of moa browsing was a cafeteria experiment (Bond et al. 2004), where two other large ratite birds—emus and ostriches—were offered related pairs of divaricating and non-divaricating plants.  The birds stripped the non-divaricating plants in short order, but had trouble pulling the springy and wiry stems of the divaricates and manipulating the twigs and small leaves in their beaks. 

Another very successful research strategy is coproecology, the gleaning of evidence from fossil droppings, coprolites.  The most recent paper (Wood et al. 2012) by the moa coproecologists has received a lot of press attention because it showed for the first time that moa fed on flowers, as well as on fruits, leaves, and twigs.

The scientists found a pile of poo just inside the entrance of a cave in the Garibaldi Range, South Island mountains.  Dried in sunshine and breezes, but protected from rain, these droppings had lain undisturbed for hundreds to thousands of years.  Taking great pains to avoid contamination, the scientists sampled 35 of the coprolites, collecting DNA to identify the species of moa as well as plant species eaten, macrofossils (seeds, leaves, etc.), microfossils (pollen grains), and measuring organic content of the dung.  They also used radiocarbon dating to estimate when the droppings were dropped.

The dung was all from one moa species, the upland moa (Megalapteryx didinus), a stout bird that stood about 1m tall at the rump.  The oldest droppings were dated from about 6,300 years ago, and the youngest from a bit less than 700 years ago, so they span a good proportion of the time from the last ice retreat to the final extinction of moa.  Interestingly, several of the droppings had identical ages and plant contents and are thought to have been deposited in the same "defecation event". 

The three methods of sampling plant remains (the authors refer to these as proxies) in the droppings—pollen, macrofossils, and DNA— were complimentary.  Of these, pollen could be contamination from outside, especially when it comes from wind-pollinated trees that flower largely out of reach of moa, like Nothofagus (southern beech) or from plants that are highly poisonous, like wind-pollinated Coriaria.  The plot below, from the paper, relates pollen abundance in the coprolites to abundance in the environment; plants above the null distribution line are the ones likely to have been part of the moas' diet.

A range of montane and subalpine plants were found, some (southern beech, buttercups, sedges, grasses and Fuchsia) in all three proxies.  The results show moa were generalists, eating pretty much everything, and they ranged across all the available habitats, as the figure below demonstrates.  

But only a few of the eaten plants might be divaricating shrubs.  These include Myrsine and Coprosma, for which the DNA and pollen evidence can't distinguish if the plants eaten were divaricating or not, and Neomyrtus, which is divaricating.

A divaricating Myrsine, M. divaricata.

A non-divaricating Coprosma, C. foetidissima.

The finding of pollen from bird-pollinated flowers—Phormium and Fuchsia—is especially interesting.  These produce quite large amounts of sweet nectar and are pollinated by birds that are much smaller than moa, such as bellbirds and tūī.  Yet their pollen is not likely to have got into coprolites other than by passing through the gut of the moa.  The authors aren't suggesting moa were pollinating the flowers, rather that they were eating them.  The large fleshy flower stalks of Phormium are probably quite nutritious and the nectar from a single flower is a small but sweet treat for a human.  On the other hand, Fuchsia flowers are produced singly or in small clusters on the twigs or bare trunks of the trees, and it must be quite finicky work to pick these one at a time; they hardly look worth the effort for a large hungry bird.  If moa had a taste for sweet nectar such that they were a threat to flowers, could their grazing have driven the evolution of tall scapes in Phormium and the tree habit in Fuchsia excorticata?  The controversy lives on.

Flowers of tree fuchsia, Fuchsia excorticata.

Flowers and young fruits of mountain flax, Phormium cookianum.

I was surprised to find in this paper evidence that moa ate so many small alpine herbs and small fruits too.  They might have been significant seed dispersers.  This, like the Fuchsia flowers, suggests they might have been capable of choosing tasty morsels.

A previous study by some of the same scientists (Wood et al. 2008) showed the presence of a small buttercup, Ceratocephala pungens, in moa coprolites from Otago.  Ceratocephala is tiny and seasonal.  The plants are ground-hugging rosettes at most a couple of centimetres across, and they grow in bare ground, yet their seeds were found in coprolites from two species of moa.  The genus is otherwise known only from Europe and W. Asia, so when this new species was described from New Zealand, I entertained the possibility that it might not be a native (Garnock-Jones 1984).  Yet here it is, in coprolites produced before humans arrived in New Zealand.

In the past, the deer-hunting lobby in New Zealand has argued that introduced mammals were good for the environment because they replace these extinct giant herbivorous birds.  This study suggests otherwise.  Two very palatable plants that were common in moa diet—Fuchsia and wineberry—are no longer found on the Garibaldi Range, and many others are now confined to inaccessible cliffs and edges of sink-holes.

References.

Bond WJ, Lee WG, Craine JM (2004). Plant structural defences against browsing birds: a legacy of New Zealand's extinct moas. Oikos 104: 500–508.

Garnock-Jones PJ (1984). Ceratocephalus pungens (Ranunculaceae): a new species from New Zealand.  New Zealand Journal of Botany 22: 135–137 (Note the different spelling in this paper; the original spelling Ceratocephala is now preferred)

Greenwood RM, Atkinson IAE (1977). Evolution of divaricating plants in New Zealand in relation to moa browsing. Proceedings of the New Zealand Ecological Society 24: 21–33.

Wood JR, Rawlence NJ, Rogers GM, Austin JJ, Worthy TH, Cooper A (2008). Coprolite deposits reveal the diet and ecology of the extinct New Zealand megaherbivore moa (Aves, Dinornithiformes).  Quaternary Science Reviews 27: 2593–2602.

Wood JR, Wilmshurst JM, Wagstaff SJ, Worthy TH, Rawlence NJ, & Cooper A (2012). High-Resolution Coproecology: Using Coprolites to Reconstruct the Habits and Habitats of New Zealand's Extinct Upland Moa (Megalapteryx didinus). PloS one, 7 (6) PMID: 22768206