A Rare plant that relies on others

Amyema linophylla is a mistletoe which is parasitic principally on two species of leafless tree; Buloke (Allocasuarina luehmannii) and Belah (Casuarina pauper).  It has narrow, grey-green leaves, clusters of erect, red tubular flowers and white, fleshy fruit which are produced in late summer and autumn.  Amyema is widespread in woodland vegetation of north-western Victoria where the rainfall is generally less than 500 mm a year.  Despite its broad geographic range Amyema is never found in large numbers (often less than 10 in a population) and there are usually substantial distances between host trees.

Amyema linophylla is classified as vulnerable in Victoria because its numbers are significantly below what they would have been at the time of European settlement and more that 80% of the records are from roadsides, paddocks, other private land and public land not managed for conservation.  The host species themselves were once much more abundant than they are today.  With the development of agriculture in the low rainfall areas of Victoria Buloke and Belah were cut down for timber or firewood, as they were often the largest trees in the area. They were also removed because they occupied the most fertile of the soils in the drier ecosystems.  Today more than 90% of the stands of Buloke and 60% of Belah are on unprotected public or private land.  In most cases 25% or more of the understory plant species are non-natives.

If potential host trees are less numerous and more widely spaced than normal the transfer of seeds (contained in the sticky fruit) to a new host is made more difficult and hence less likely.  As the Amyema numbers are reduced in any area a situation may arise where there is not enough fruit to attract frugivorous birds (often Mistletoe Birds) to act as vectors.  Consequently the presence of a healthy fruiting plant of Amyema linophylla does not mean that it will reproduce. 

The difficulties faced by Amyema linophylla are not as pronounced for most other Victorian mistletoes as few are as specific when it comes to suitable host trees.  For example, the commonest Victorian mistletoe, Amyema pendula, is parasitic on a wide range of eucalypts and wattles so the loss of a single species is not necessarily a major problem.  At the other end of the scale the problem of host availability is compounded for Notothixos subaureus (Golden Mistletoe) because it only parasitises another mistletoe, Dendropthoe vittelina (Long-flower Mistletoe), which itself is parasitic on a single, uncommon tree species, Angophora floribunda (Rough-barked Apple).

The loss of mistletoes from native vegetation may seem to some observers to be a good thing because parasites are, by their nature, an extra burden for their host plants.  Nevertheless, mistletoes seldom kill their hosts - it would be fatal to them as well - and there are many animals that take advantage of the nectar from mistletoe flowers - which are usually in bloom at a different time of year to the host - and their fleshy fruit.  Indeed one group of animals is very much dependant upon mistletoes for their survival - the butterflies.  The leaves of nearly every Victorian mistletoe are the preferred food of caterpillars of at least one of two groups of butterfly, the Azures (Ogyris spp.) and the Jezebels (Delias spp.).  Why mistletoe leaves provide suitable food for the caterpillars when those of the host plants (eucalypts, acacia and casuarinas) do not isn't clear, but what is clear is that the long-term survival of some of the most attractive Victorian butterflies is in serious jeopardy if there aren't enough of these parasitic plants.