Assessing the success of a pyramid for saproxylic insects

Last updated : 23 December 2005
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Habitat restoration actions should always include criteria and protocols aimed to assess the success or failure of such restoration. Only in this way it will be possible to learn from the management carried out and, if necessary, modify it and improve it.

To assess the success of a pyramid for saproxilic insects is not an easy task, for several reasons. In the short term, no colonization can be evident despite the resource provided. The lapse between construction and colonization of the pyramid will be dependent not only on the presence of appropriate insects but also on the decaying stage of the wood utilised to build the pyramid. If fresh wastewood has been utilised, several years could pass before decay make it suitable for Stag Beetles or other saproxylic insects. Furthermore, colonizing insects are not easily detected and counted. There are three possibilities to assess whether a pyramid has been successfully occupied bya a diverse community of saproxylic insects and/or given target species.

(1) Window traps.- Trap windows are usual samplind devices in forest entomology. One or a set of such traps can be arranged by the pyramid and the captures produced can be examined. Traps must be utilised during summer, the flight season of imagoes. One advantage of this method is that it can be implemented even right after the building of the pyramid. Its drawbacks are: (a) it does not really say if the pyramid is actually utilised because insects captured could just be attracted to the pyramid, and not raised within it, and attraction does not necessarily entails effective use of the resource; (b) this method is destructive, because trapped insects are killed, and it is not approrpriate to all kind of saproxylic insects (i.e., its efficiency in trapping Stag Beetle is likely to be very low).

(2) Emergence traps.- Emergence traps are also usual sampling devices in forest entomology. One should cover the whole or part of the pyramid with one of such traps for 7-15 days and examine the insects emerged during that period. Consequently, this traps has to be operated in summer, which is the emergence period of imagoes. Depending on the decay stage of the wood utilised in tye pyramid it will be necessary to wait one or several years before the assessment. Among the advantages of this method are that it really estimates pyramid success; in addition, it does not require insect killing. Its drawbacks are that operation of the trap is not easy and that, depending on the location of the pyramid, this trap can be easily vandalised.

(3) Direct sampling.- It consist of direct excavation, by hand or with tools, of the whole or part of the pyramid in order to find the larvae or imagoes of saproxylic insects. This method can be utilised all year round and asesses the real use of the pyramid. Its drawback is that it destroys the pyramid. If utilised, then, some kind of compensation must be planned in advance, such as the addition of new material to the pyramid. Another drawback is that is does not allow to examine every corner of the pyramid and, as a consequence, target species can be overlooked even if present. A less intrusive alternative has been utilised in the Weluve region (The Netherlands) experience.

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