5 Managing populations of invasive woody plants
Having decided which species to control, the next decision is where in the species' range to begin control and how much to remove in each area. Allowance must be made in the costing of control for follow-up control operations if it is thought that they may be needed.
5.1 Where to control
As with the choice of which species to control, the decision about which areas to control needs to be taken after a consideration of the costs and benefits of controlling different areas. There has been a tendency in the past to concentrate research and effort on the methods of eradicating individual plants, without giving enough attention to where the priority of control should be. Much past effort has been directed towards clearing weeds where the invasion is most pronounced, where the greatest impact on native vegetation has already taken place. This tendency has often been reinforced by a reliance on volunteer labour, which means that effort has to be expended in areas that are accessible, safe and where the clearest results can be obtained. This is an overconcentration on tactics rather than strategy. A strategic approach needs to consider the importance of "nascent foci" (Moody & Mack 1988). Moody & Mack modelled the invasion process and clearly showed that, to slow the overall area invaded, in just about all conceivable circumstances, it was more effective to eradicate small founding populations ("nascent foci") first.
5.1.1 Scattered populations of the species
Although clearing isolated populations is the best strategy to control an invasion, the costs per plant can be high. The areas are often remote and sometimes steep, and plants are often difficult to find. However, it is usually much less expensive to clear these areas than more heavily invaded areas, so the policy has a favourable cost:benefit ratio. Areas with a low abundance of the invading species could be sub-optimal for it, and so not vulnerable to heavy invasion and therefore major change, they could act as corridors for the invasion of more vulnerable areas.
5.1.2 Clearing limited areas
With more advanced invasions where the prospect of eradication from the whole of the invaded range is slight the objective could be to keep the weed out of limited areas; control of outlying satellite populations may still take place, unless a decision has been made to rely solely on biological control in the long term. But this decision should be a last resort, as not only are the areas outside likely to become very heavily invaded, but also the seed input may increase so much that the policy of exclusion of the alien becomes unsustainable.
These limited areas should therefore be as large as possible and the presence of buffer zones of reasonably intact and uninvaded vegetation around them will increase their viability.
One type of area is the Special Ecological Area (SEA) that has been developed in the Hawaii Volcanoes National Park (Tunison & Stone 1992). These areas now cover 5,266 hectares of the park (7.5% of the total), ranging in area from 17 to 3,380 hectares. The main reasons for the development of the concept of SEAs were:
· The impossibility of managing widespread alien species.
· The need to protect the most intact, representative and diverse areas, and valuable research and interpretive sites.
· The relatively small alien plant populations within the area and so the relative ease of control.
· The need to integrate feral animal control. Sometimes smaller areas are fenced to keep damaging animals out.
· When the incremental control of weeds is effective. When control of a species is assured the area can be expanded or more areas established.
· Biological control cannot be guaranteed to be successful against all weeds.
Workload requirements inside SEAs in the Hawaii Volcanoes National Park have dropped by 73% as the weed density has decreased, uprooting replaced stump cutting treatments and workers became more efficient at finding the plants (Tunison & Stone 1992). In several other places in Hawaii control is focusing on steep gullies as they are relatively uninvaded, largely because pigs cannot get access to them (Tanimoto & Char 1992).
The use of "Intensive Control Areas" (ICA) is a similar idea, used in the control programme against Cinchona succirubra on Santa Cruz Island in the Galapagos Islands. Control was concentrated in a 1,000 hectare ICA, which was gradually expanded westwards into the prevailing wind from where most of the C. succirubra seeds came from (Macdonald et al. 1988).
5.1.3 Clearing heavily invaded areas
Clearing heavily invaded areas should normally be a lower priority because the costs of clearance and follow up management can be great with little overall impact on the rate of spread. Once the outlying populations are eradicated of course the benefit of starting to eradicate the heavily invaded areas rises. There are two circumstances in which clearing dense populations of the alien could be as high a priority as clearing scattered populations:
· If the value of the alien (its timber for example) is greater than the cost of harvesting it. This may be the case with Pittosporum undulatum in Jamaica.
· If the soil seed bank of dense infestations of the alien plant were to start to rise and/or the soil seed bank of native species were to start to fall after say 20 years, so that after this the chances of re-establishing native vegetation is much reduced, it might make more sense to remove 20 year old populations than younger more scattered ones.
5.2 How much to remove
In sexually reproducing species, removing seed producing individuals is obviously necessary to stop an invasion. In the case of dioecious species the removal of male individuals is not necessary. The relationship between the number of seeds dispersed by birds and size of tree is often not a simple one, larger trees sometimes attracting a disproportionately large number of birds (potential dispersers) than smaller trees (Denslow 1987). With wind-dispersed trees dispersal distance can be much greater if an individual emerges above the canopy, but dispersal distance is usually more restricted than for bird disseminated species. In case the invasive species spreads by vegetative means only, special attention must be paid to the removal of all individuals in riverine habitats as propagules may be spread to distant sites during floods.
Where the alien is scattered and the individuals are generally of a small size (and so the soil seed bank, if present, is likely to be small) it will probably be best policy to remove all reproductive or near reproductive individuals at least.
Removal of an alien tree from heavily invaded forests leads to a greater opening up of the canopy than its removal from lightly invaded areas, increasing the chance of establishment of other alien weeds. This risk is often added to by the usually greater proximity of alien seed sources to areas already heavily invaded, most weed propagules usually coming from the same source (such as a botanic garden) or from the same direction. In regions where the invading woody plants form monotypic stands on a previously shrub-/tree-free landscapes, their removal will result in large bare areas where non-native grasses and forbs often become dominant. However, removing all individuals of the alien from a heavily invaded area does have advantages:
· It will stop, at least temporarily, the species extending its range by seed produced in that area.
· It is relatively straightforward, easy to organise, and more economic than selective treatment of scattered alien plant individuals.
· Follow up management can be concentrated in relatively small, well-defined areas.
But it also has disadvantages:
· Dense recruitment of the same species may be triggered, as with Pittosporum undulatum in Jamaica.
· The invasion of other weed species may be promoted. These can be more difficult to control than the original alien (as with Acacia longifolia replacing Hakea sericea in South Africa (Pieterse & Cairns 1986)) or less desirable (for example, a persistent scrambling herb replacing an alien tree).
· The sudden opening of the canopy can lead to soil erosion, the risk of fire and forest clearance for agricultural crops.
Ways of reducing the probability that the cleared areas will remain dominated by alien plants include:
· Reduce the population of the alien gradually. The rate at which the alien can be removed depends on the amount of disturbance necessary for the establishment of juveniles of the species and other alien plant species (in comparison with native species).
· Girdle trees instead of cutting. In Hawaii the girdling of Myrica faya trees lead to the dense regeneration of a native tree fern, Cibotium glaucum, whilst cutting and removing all M. faya trees lead to the sporadic recruitment of M. faya and the native tree Metrosideros polymorpha but dense growth of mostly alien herbaceous and woody species (Aplet et al. unpublished).
· Use biological control. However, biological control that kills a large proportion of the population suddenly (rather than slowly or, more slowly still, by preventing seed production) leaves the area open to the same sorts of dangers. The danger of serious damage from fire could actually be higher because of all the dead material left standing.
· Replant the areas with native plants (preferably) or "non-invasive, non-persistent" alien plants. This could be expensive, and has been tried very infrequently and nowhere, to our knowledge, with success. There does seem to be potential though for the development of this technique. Similar techniques are used by foresters who aim to replant an area with timber trees as soon after harvest as possible. Direct seeding of native pioneer grass and/or shrub species is an alternative option.
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