Butterf ly Blues

Forest and Bird, Volume 18, Issue 4, 1 November 1987, Page 18

Butterf

ly Blues

Being hybridised into oblivion by an invading, closely related species is probably not very common but is a recognised evolutionary pathway. Victoria University scientist Dr George Gibbs has been studying the fate of the native southern blue butterfly, which, like the black stilt, is being swamped by an aggressive trans-Tasman invader.

ext time you stop by the roadside in the Mackenzie Basin, spare a thought for the little blue butterflies dancing among the stones and dry summer vegetation. This butterfly, one of our smallest, could possibly be facing oblivion by hybridisation with an Australian immigrant species along the same lines as the black stilts which share their habitat. The similarity of their predicament is striking but the blues still have a long time ahead of them before they could be called threatened. This article briefly compares some parallel features of stilts and blue butterflies and attempts to trace the historical events that have led to the present state of affairs with the butterflies.

Species or subspecies?

Black stilts and native ‘‘southern blue’ butterflies share the open river valleys of the Mackenzie Basin. Both are very much like their invading Australian counterparts (the pied stilt and "common blue" respectively) to the extent that competent taxonomists may argue over whether the endemic New

Zealand entity should be called a species or subspecies. Nevertheless they are sufficiently distinct to be recognisable by colour and/or behavioural and structural differences from their Australian relatives and are uniquely New Zealand. Hybrids are readily formed in each case with the Australian relative and can be differentiated from either parent type by colour pattern. Both have widespread near relatives distributed beyond New Zealand and Australia to include Malaysia, India and Africa. In both cases we have difficulty tracing the historical events that led up to their present predicament. The period 1850-1870 could have been a critical one for both the stilts and the blue butterfly. The mid- 1800s were pioneering times — for natural history discoveries and for agriculture. Both are linked in our story. If we research the records of our museums and early zoological literature, we can find reference to the existence of the endemic black stilt (in 1840) and the endemic southern blue butterfly (in 1859). In each case

mention of a second species, with an Australian distribution, does not come until some years later (1869 for the pied stilt and 1878 for the common blue). Does this imply that the more widely dispersed Australian species had just arrived in New Zealand, or had it previously been overlooked? We cannot be sure. Like fossil discoveries, the first record simply means that the animal was there but it tells nothing about when it arrived.

No butterflies reported

However, with our present knowledge of blues and their status as our most common butterflies both inland and on the coast, it comes as a Surprise that no blue butterflies (of either species) were reported from New Zealand until nearly 100 years had elapsed since the date of the first butterfly discoveries (Cook's Endeavour voyage, 1769-70). At the time when the southern blue (Zizina oxleyi) was described from a specimen taken (we think) in Nelson, no fewer than six kinds of butterfly, including red and yellow

admirals, the tussock and two coppers, were known from this country. Why did blues, especially the common blue, get overlooked for so long? The probable answer is that they were nowhere near as common nor as widely distributed as they are today. For the explanation of this we can turn to what was happening as European farming got underway. Flocks of sheep were spreading across the land occupying the open, non-forested area. Major forest clearance came later but during the early sheep-farming phase the spread of domestic grazing animals must have been accompanied by the spread of exOtic pasture grasses and clovers. Our records are not at all clear on exactly when the different pasture species were introduced but it was the clovers that affected the blue butterflies. Today it is these ubiquitous pasture legumes that serve as larval foodplants for both species of blue butterfly, hence accounting for their common status. In the past, before these plants were introduced, it was unlikely that any native foodplant was

suitable for the common blue but we do know that the endemic southern blue larvae can feed on native broom (Carmichaelia spp.). Thus in pre-European New Zealand the southern blue was restricted to open habitats with native brooms whereas the common blue was unlikely to have occurred at all. The present situation is that the common blue (Zizina labradus) is abundant throughout the North Island in exotic grasslands. It prefers mosaics of grass and shingle and its larval foodplants are introduced clovers, trefoils and lucerne. In the South Island it is limited to the Nelson area, northern Marlborough and the West Coast. Our endemic southern blue occurs through the drier regions of Canterbury and Otago, becoming scarce in the far south. Its favourite haunts are stony lakeshores and riverbeds where it is associated with Fescue grassland and matagouri communities. Larvae are normally found on clovers. The two species hybridise extensively where their populations meet in Marlborough and

north Canterbury. To reconstruct the history of these blue butterflies I am suggesting that unprecedented modification of the New Zealand landscape that began in the mid-1800s extended foodplants and habitat of both these butterflies, bringing them into close association for the first time. I am hypothesising that hyridisation may have occurred where they met and that the outcome of their meeting in most places was deleterious to the endemic blue, leaving behind the more successful common blue, or perhaps hybrid populations in which common blue genes predominated. The net effect of this disruption was the shrinking of territory held by the endemic species as the rapidly expanding common blue took over the North Island and much of the South Island. There is evidence that the southern blue was previously more widespread than it is today. For instance G.V. Hudson collected it in the Nelson district prior to 1898 but it has not been found there recently. In my

own experience, I have seen the southern blue disappear from the Waiho Gorge river flats below the Franz Josef Glacier over a five year period in the 1980s and in the North Island there are earlier records of ox-leyi-type individuals from the volcanic plateau and Hawke's Bay. This scenario poses many questions, some of which we cannot answer with any confidence. An interesting one is how the common Australian butterfly came to be on the spot when this new opportunity arose. I have suggested that it was unlikely to have been present before European settlement simply because it had no known foodplant here but it is conceivable that it existed somewhere and fed upon a native legume. Perhaps it was shipped across the Tasman Sea amongst stock food (it is, after all, Australia’s most common grassland butterfly), or perhaps it was windblown. History is unlikely to give us these answers now. The important point is its ability to intergrade with the southern blue and possibly

threaten its very existence — and here the situation resembles the black and pied stilts where again we are hazy about the early stages of the process. Many questions of interest to evolutionists and conservationists arise from this survey of the blue butterflies. Is the species replacement hypothesis an appropriate one? Has it reached a stable equilibrium or will it continue to engulf the southern blue entirely? Is the New Zealand common blue identical with the Australian one or has hybridisation produced a different form? Is the altered habitat the prime cause? Should we humans intervene? Would such a course of action indeed by possible? These questions are capable of being researched with the blue butterflies and may have implications for other wildlife issues in this country. Dr George Gibbs is a senior lecturer in entomology at Victoria University and is the author of a book New Zealand Butterflies.