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Pests > Pest outbreaks > Invasive pests > What makes a pest invasive? Samoa

Pests Pest outbreaks Invasive pests What makes a pest invasive? Samoa

What makes a pest invasive?

March 2002. A question was asked “what do some organisms suddenly become a pest and, others, sometime, becomes less of a pest?” This was asked in connection with the rapid spread of the African tulip tree, Spathodea campanulata. In the original message, a member asked whether the tree fern (balabala in Fiji), Spathodea, could be considered an invasive species, as it seems to be spreading rapidly these days. Here are the replies from several members, acknowledged.

Furthermore, the literature claims that the cultivated varieties of vetiver will continue growing in the same spot for 100 years without spreading and observations in Fiji seemed to support this claim as vetiver has been there for at least 50 years or so without becoming invasive. However, some members mentions claim that vetiver is naturalising there and has become invasive. Is vetiver undergoing a behavioural change – perhaps induced by climate change or some other factor? It seems that a similar phenomenon happened with African tulip which was a “nice, friendly plant” in Fiji (and Samoa) for quite some time before becoming invasive.

Peter Maddison:

This is indeed an interesting question. There seems to be little to indicate why an organism should suddenly become a pest and again why on occasions it drops out and becomes “benign” again. There are some suggestions that it is due to a genetic change (mutation theory) or to a change in the environment (habitat theory). Perhaps a combination is what happens and that some organisms, after several generations, finally become adapted to a new place and “explode”. If their natural biological control agents catch up then the pest outbreak dies down, but may occasionally outbreak should the natural balance be upset.

One of the most interesting of these effects is such that a species suddenly becomes “migratory” (in the broadest sense of that word). This has happened in recent years in the Pacific with the Leucaena Psyllid, Spiralling and other Whitefly, etc. One of the most interesting is the moth, Thysanoplusia orichalcea, variously known as the Burnished Brass Moth or Soybean Looper. As I understand the story, about 20 years ago this moth spread from its homeland in Africa and Asia to Australia and New Zealand – what caused the change that suddenly allowed this moth to “migrate”?

Returning to your fern and other ‘weeds’, one of our botanists, Alan Esler has calculated that in the North of New Zealand (Auckland), there is a new “weed” appearing every month – from previously apparently benign exotic plants. Plenty of work for weed scientists!

Given the definition that a “weed” is a plant growing in the wrong place – a very human-based definition, local plants can become weeds. One of the issues can become the absolute protection proposed for native species by the Conservationists and the reality of having to deal with invasive species. A recent issue I am dealing with is a waterweed, Tutanawai, Polygonum salicifolium, which is blocking water channels in newly installed stormwater systems. Polygonum salicifolium is a native plant and there are those who advocate that the plant should be allowed to “do its natural thing”. There are also those charged with preventing flooding who say “clear the channels.” This issue will be a contentious one. Perhaps, coming from Europe, where for the most part many weeds of exotic origin are now regarded as native, and native plants that become weedy are regarded as weeds and can be controlled, there will be considerable cultural differences as to how pests and weeds are perceived.

Regarding the Balabala (the original message intimated that Cyathea lunulata was becoming more invasive), I agree with Dick Watling – some local use of the bush knife may solve the problem. If you really think, it has changed and become more aggressive in some areas, then some investigation of spore or sporophyll survival is warranted – remember that in ferns, there is alternation of generations and that the answer may lie in some habitat change that allows better survival of the sporophyte generation.

Joel Miles:

Invasiveness is very difficult to predict. In fact, it is difficult even to define “invasive,” so predicting or explaining it is even more of a challenge. Here in Palau we have a growing problem with Merremia peltata, a native vine. It appears that this is mainly due to changes in lifestyle – more land clearing and road building give the vine more places to get established. In combination with this, communities do not get together and do maintenance in the way they did in the “old days.”

Another problem with predicting invasiveness is the delay between introduction and spread. I have seen a summary of a study from Europe showing an average of over 50 years (I can’t remember exactly) between introduction of plants and their recognition as a problem. I would expect the delay to be shorter in our Pacific Island climates, but who can say? Here in Palau, Chromolaena odorata has spread from one island in 1988 to almost everywhere in 2002, so spread can be very rapid. We are also seeing an explosion of Mikania micrantha, first seen here in 1993 and now in at least 16 locations. On the other hand, Spathodea campanulata – African Tulip Tree – has been here for over 50 years and is still not widespread, while in Hawai’i it is all over the place, and you note that Fiji also has a problem with it. We believe we can eradicate it here, and are in the process of finding and killing trees. The situation is the same here with Schefflera actinophylla, the octopus tree. Just a few isolated specimens in Palau, but a very big problem in Hawai’i and elsewhere. We are trying to eradicate it here before it spreads, but we have no idea why it has not spread yet.

Pat Dale:

I offer at arm’s length, a couple of suggestions as to why weeds may sometimes change their status for no apparent reason. One may be that when they are “benign” it is because they are lacking an effective pollinator, as happened in New Zealand with the Moreton Bay fig. It was just an ornamental for 100 years till its pollinator wasp arrived. Then it produced viable seed, and every fig eating bird could spread it, so now it grows from stumps and fence posts and stone walls wherever a bird may relieve itself. Likewise, I suppose some plants lack a dispersal agent when they first arrive. Neither of these means seem to apply to balabala, but perhaps they have wider application to other plants than is generally appreciated.

Dick Watling:

I am glad that Peter and Joel have provided some interesting perspectives on `invasiveness’, they have encouraged me to “push the envelope a little farther”. At the start, I should say I am generally an ‘invasive-sceptic’ as much of the debate is anthropormorphically driven, generally from the perspective of the profession of the person concerned. So we get a `weed’ defined as ‘a plant growing in the wrong place’, which, of course, is a complete nonsense because the real issue here is not the plant, but the place. So we get agriculturists seeing Spathodea as invasive because it grows in places where crops could or should be grown and botanists may see it as invasive because it is growing in places where native plants should be growing. From my equally prejudiced perspective it is a very useful tree – my perspective is that of an ecologist. I note that in Fiji over 50% of the natural vegetation community has been lost – converted from forest (of a variety of types) to grass-reed-scrubland (apart from agricultural land) – the proximate agent for this is fire, almost exclusively. The native forest vegetation does not have a well developed ecotonal-successional community which is resistant to fire and which could buffer native forest – this is to be expected in a forest community which during its evolution was not subject to regular fire (if ever). So fire continues to make inroads into forest and fire-resistant introduced grasses and shrubs follow, exacerbating the whole process – this has almost run its course in Fiji now and is held at bay on the larger islands only by rainfall – higher altitude in the leeward areas and lower down in the windward areas.

Spathodea is successful in just such situations and by readily forming a closed canopy, it resists the establishment of fire resistant grasses and shrub and so protects the native forest community. Conditions under Spathodea are excellent for true native forest species to regenerate and being poorly adapted to cyclones, Spathodea often allows such species to emerge quicker than might otherwise be expected. Even in the absence of cyclones, Spathodea is not a long-lived tree and true forest species would certainly take over – decades is the timescale, of course, which is not quick enough for some, but in forest community terms, that’s fine.

It is interesting that Spathodea has become the pet hate in recent years, but here in Fiji, its spread pales in significance by comparison with Piper aduncum. I do not hear many complaints about this tree which as far as I am concerned is another useful tree, for exactly the same reasons, but it is relatively inconspicuous, as compared to Spathodea, and so nobody really notices it. And what about the good old Rain Tree Paraserianthes saman, another exotic? It has been here much longer and covers an order of magnitude more land than Spathodea, I have never heard it being described as invasive; why not, because it surely is?

A quick flight over the western division and anyone can clearly see that just about every watercourse supports Rain Trees, almost to the watershed. Very lucky, I say, because under those Rain Trees are native forest remnants just hanging on. In the absence of the rain trees, they would have been burned out decades ago. Personally, I would like to see a more rounded discussion on this issue of invasiveness, it is too readily steered by either the `professional’ perspective of the traditional `productive’ sectors or the `emotion-charged’ perspective of the more recent exotic-native debate.

Rod Randall:

Dick made some interesting comments about ‘invasive’ trees and in the main I would agree with his longer term view point as long as these plants don’t impact in any other way to the detriment of the remaining ecosystems, which I’m sure Dick would recognise anyway. These plants then become ‘unwanted’ which is my definition of a weed, I prefer simple definitions.

So why do plants become weedy after having been in the ‘system’ for some time, often many decades? In my view it has little to do with genetics, or climate change, but much more to do with opportunity and in most cases these opportunities are a result of human activities.

It could be a simple case of a change in management strategies, a new soil cultivation technique, moving to herbicides from handweeding, changing stock from sheep to cattle, using dozers to clear roads rather than handcutting, etc, etc. An example is Emex australis in Australia. It has very spiny achenes (fruit for want of a better word) that always present one spine vertical. After the introduction of rubber tyres on agricultural machinery this weed spread rapidly and became a major agricultural weed in Australia. Prior to rubber tyres, Emex was a minor player in the weed scene.

When you look through the weed literature especially for the UK, you see numerous references for “rare ruderal weeds”. These were species once common in the crops and pastures fields that have suddenly become ‘rare’. At the same time a whole new suite of weeds have emerged in British agriculture. Why? Management of crops and pastures has changed dramatically over the last 50 years with machinery and herbicides replacing horses, mouldboard ploughs and handweeding.

Or it may be something less obvious like the introduction of a pollinator as Pat mentioned or the introduction of fertile males as in the case of Pampas grass (Cortaderia spp.) in several countries where the only plants in cultivation where female, then a male arrives and voila, seedlings everywhere. The root cause in most cases, in my view, is human mediated. You can almost always track back the start of a weed problem to a human intervention.