Breamhead is a high priority mainland island reserve at Whangarei Heads

This is paper written by a second year student in the Conservation and Environmental Management Programme (anonymous).

Explain why V.M is important in managing a possum control program in a forest like Bream Head. How can results of a study change management directions?

Vegetation monitoring is a relatively new concept, by which indicator trees are graded according to the amount of, foliage cover, browse level, and trunk use, to name a few. Indicator trees are trees which are favoured by possums, therefore are the first to show signs of browse. Vegetation monitoring is an important new tool in the on going fight with the possum in New Zealand's remaining forests.

In Northland we have many areas of fragmented bush that are being monitored this way such as Trounson, Waipoua, Katui, Kauri mountain and Bream Head.

Up until the 90s the success of a poison operation was calculated solely by a method called trap catch rates. This method involves setting traps and counting the number caught and converting the figures to number of possums caught per 100 trap nights.

This was done before and after a poison drop to estimate the percentage kill. This method is still used but is now being combined with vegetation monitoring(V.M) .

The goal of any possum control operation is to increase vegetation cover and improve forest health, not to decrease possum numbers(Patton el al). This is where V.M becomes important. It allows us to observe scientifically the changes in the forests health before and after control.

By using V.M as well as trap catch rates we can now determine a level of possum abundance that is required if the forest is to be healthy and able to survive. By monitoring forest health we learn what works, and that way, we continue to improve our management (Edwards 96). This is the case at Bream Head at the present time. Here DoC are planning on reducing the level of management by filling bait stations only twice a year instead of once every two months. V.M will continue and if possum damage becomes significant they will return to the present system.

Results so far from V.M have indicated certain strengths and weaknesses of the method of assessment. A forest is dynamic with more than one contributing factor, for example, foliage loss is not only the result of possum browse but can also be caused by other environmental variants such as frosts and droughts.

It was found in Waipoua that a heavy frost on the night of the30 of June 94 caused the death of much exposed foliage in a wide range of tree species. This influenced foliage loss scores(Payton et al). This makes it hard to come up with an indicator that is sensitive to only one change.

From analysing the results It has been found that the best indicator of forest health is to look at the amount of possum browse on leaves(Rachel). This can decreased significantly within a year of control and may prove to be a more sensitive short term indicator.(Brmhd93-97). The problem found with this test is the amount of variation between surveyors, and at times it is very hard to distinguish browse, from insect damage.

Vegetation monitoring may also prove useful in uncovering tree species which are being heavily effected by possums, and are in danger of becoming locally extinct. This would intensify focus on this species and it may induce further assistance, i.e. begin a planting Programme.

I think the vegetation monitoring programs in place are good and enable us to see how the trees respond to possum control, however as stated by DoC officer Keith Broome, looking at the trees is only one measurement . Ideally, invertebrates, birds and other wild life should be monitored so the health of the total forest ecosystem can be assessed(Edwards 96).

By comparing results from previous data, it appears that although possum numbers have decreased, vegetation cover remained the same or had increased only slightly over the first few years.

This indicates to me that there is a level of damage to which trees can endure, but once the level of brows increases above this, the trees deteriorate rapidly and are slow to respond to management. In some cases management has come too late and the trees have died. For example, 20% of the trees with less than 1/3 of foliage remaining died at Waipoua(Payton et al). At Bream Head a total of 24 trees out of the original 174 tagged trees died or could not be found.

This is disturbing ecologically and even more so when it is considered that it is often the same tree species that are dying. For example at Bream Head, 23% of Houpara and 15 % of Pohutukawa surveyed in 1993 were dead by 1997 (Brmhd 93-97).

This can ultimately change the forest structure by removing trees which were once dominant (i.e. Rata and Pohutukawa) and replacing them with more resilient species such as Manuka. This large scale disturbance also opens the forest up to invasion by weed species, such as, Ginger and Lantana.

This change in forest structure can also have effects on the native forest fauna, especially keystone species such as the Kukupa, which relies heavily on trees such as Puriri that are being targeted by the possums. The disappearance of several key species, such as Puriri, could lead to the demise of a whole forest.

From this information it appears that the impacts of possums go further than first thought. It is also believed that by exerting pressure on trees by browsing, possums have reduced the trees ability to cope with external variants such as extreme El Nino summers, such as the one past. It has been observed through V.M at Bream Head, Waipoua and Trounson that forest cover has decreased by as much as 20% in some tree species this summer(Rachel)

Some limited leaf loss may have occurred any way, but it is likely that added pressure of browsing was a major contributing factor.

Through our own observations at Bream Head it was noted that although trees appeared healthy, with good foliage cover, there are low levels of fruit and flowers being produced. This too can jeopardise the forests ability to renew itself. Rachel McCellan of DoC made the comment that these levels of fruit were the same as at Trounson. There may be several reasons for this low abundance of fruit, one being possibly the fact that flowers and fruit are preferred over leaves by possums, therefore, small residual populations of possums can still have an effect on the phonology of the trees.

Another explanation may be that the trees may take some time before they are able to produce fruit again after being under so much pressure.

Since the 1840s possums have been breeding prolifically, spreading all over mainland New Zealand. Breeding success is very high and with no predators and no competition for resources, their numbers have soared.

Possums were first spotted in Waipoua in the 60s(Payton et al) and by this time had probably colonised Bream Head and most other areas. In 1982 a report was written stating that possums were in good health and that there was room for further population expansion. This conclusion was drawn by looking at fur condition for any signs of fighting which would indicate a population near its limits. The suggestion was made that possum numbers should be watched to make sure they did not reach a level which may cause damage. In 1990 a similar study was carried out with similar results but the numbers had increased by 22% in just ten years(Payton et al). This huge increase in numbers is believed to be due to the favourable conditions found in the north and the lush variety in vegetation. Unfortunately it was not until the damage became visible to the wider public in 1990 with scenes of dead and dying crowns of emergent Rata which were visible from the State highway which traverses Waipoua forest(Payton et al) that possum control was started at Waipoua, 30 years after the first sighting. In 1993 possum control began at Bream Head.

This lack of fore sight by the Department in the early-mid 80s has ended up costing them more by having to have extremely intensive management and the cost to the environment has been significantly more.

In July of 1990 the anticoagulant pesticide sodium monofluoroacetate (1080) was aerially spread over 17000 ha of reserve and private land at Waipoua.

Since that time ground trapping has been used to limit the increase in the possum populations.(Payton et al). This was the first of many intense attempts around the north to reduce possum damage, the most significant being at Trounson. By poisoning the surrounding area they have decreased the amount of re-invasion. This is a good practise as young males move quickly into unmarked territory, soon re-establishing a population.

Trounson, Bream Head and Waipoua are examples of three different management levels. Trounson is an example of intense management. It covers a total of 445 hectares and is divided into a grid system of 100 metres. This allows for access to the 500 bait stations. Four different baits are used, 1080 was initially used for quick kill, however possums have been known to develop a shyness to it. Alternative anticoagulant poisons are used such as Talon and Venom B. Bait stations are filled seven times a year

(Annual report96).

At Trounson they have had to cut 45 kilometres of tracks through the forest to enable them to service the stations.(Trounson annual report 96) By doing this are we further degrading the forest, leaving disturbed areas where weeds can easily become established, especially through accidental introduction with such regular use. However one must not lose sight of the ultimate goal, which is to protect and enhance the forest, this means the pro's and con's of management systems must be weighed up and looked at sensibly. In the case of Trounson I feel that the positive effects far out weigh any negatives.

Management at Bream Head is much like Trounson, however stations are 150 metres apart. 1080 and Talon are used alternately every two months(Bream head possum control 93-97).

In comparison, Waipoua is a low management forest. In 1990 an aerial poison operation with 1080, covered 17,000 hectares of forest and surrounding areas. Ever since ground trapping with Victor leg holds has been the only form of control(Payton et al)

Although the trapping program at Waipoua must be commended to a certain level for providing continuos employment, and reducing the amount of toxins used, the effectiveness of the trapping program must be questioned.

From experience I know that some trappers are not working whole heatedly towards decreasing possum numbers. Traps are sometimes left sprung for long periods of time, set irresponsibly or data sheets used incorrectly. I believe that the money spent employing these people may be better spent on bait stations surrounding the forest. It may also be necessary to once again look at spreading 1080 over the forest, in a lower concentration than was initially done to stop the steady increase in possum numbers.

As mentioned earlier all levels of management have good and bad points, but it is important that management techniques are viable. For example it would be impossible to have 100 metre grids on Bream Head due to the contour of the land. It would also be unrealistic to try without adequate resources to control possums at Waipoua to the same level as Trounson.

It has been suggested that the continual use of anticoagulant baits could cause problems for non target biota, namely invertebrate. Some bugs are known to eat baits and subsequently poisons become available to high animals such as birds(Trounson 97). It is most important that research is done to establish the effects. The possibility of using indicator species that are sensitive to these poisons and using their presence/absence could provide a clue to toxin presence. It is important that such effects are found out but at the same time we must keep up with current strategies until alternative methods are found. Other such methods include such things as biological controls inhibiting reproduction as discussed in the paper by S.E. Jolly and E.B. Spur. (Spur and Jolly 96). In the meantime management strategies that can be employed to minimise toxic residues could be to reduce the amount of poisons used in stations. If a significant rise in vegetation degradation occurs, the level of management could be intensified once again.

In a discussion with Conservancy Officer Rachel McClellan, the probability of a possum exclusion fence was mentioned. It was, however her opinion, that a fence was unlikely in the near future, as the Northland Conservancy already has one mainland island and it could not afford to support another. I believe the only way a fence could become a reality would be if it was supported by an outside company, one such as the Port Corp. Costs would be around 1million dollars to erect the fence and another million dollars to totally eradicate all the pests and predators. Unfortunately it seems as though DoC are waiting for the community to initiate the project, but the community are unaware of the situation or the possibilities. If a fence was erected Bream Head would have the opportunity to become New Zealand's best onshore island as it is sufficiently close off shore islands, such as the Hen and Chick group,that the establishment of many rare bird populations such as the Kaka would naturally occur.

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