Restoration ecology of the Seychelles giant millipede

Lawrence, James Mark (2013-12)

Thesis (PhD)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: The loss and degradation of habitat is recognised as the greatest threat to invertebrate biodiversity. Restoration practices have the potential to reduce these impacts. The Seychelles giant millipede (SGM), Sechelleptus seychellarum, is a threatened and functionally important macro-detritivore endemic to the Seychelles granitic islands. The broad objective of this dissertation was to investigate selected aspects of the restoration ecology of the SGM, with the intention of making practical restoration recommendations that can be used to assist in the conservation of this species. This study was carried out on Cousine Island, Seychelles between 1998 and 2009, in the context of the large-scale plant community restoration that has taken place on the island. Large fluctuations in millipede population densities were found between 1998 and 2009. In 2002, 2003, 2005 and 2007 millipede densities were low, while densities were high in 1998 and 2009. Although the SGM is active all year round, its surface activity was positively correlated with rainfall, with density high during the high rainfall period (i.e. October – April) and low during the low rainfall period (i.e. May – September). Female:male:juvenile ratios were ~ 3:1:1. The implications are that translocations should preferably be done in years of high millipede densities and during the wet season. Alien coconut trees did not affect SGM density, but negatively affected its foraging behaviour, whereas bamboo stands negatively affected both its density and foraging behaviour. The SGM showed feeding preferences for Pisonia grandis and Ficus sp. leaf litter types. Alien bamboo and coconut pose a varied threat to the SGM, and their removal and replacement by indigenous forest species (e.g. P. grandis and Ficus sp.) should form part of an island’s restoration programme. SGM density was an order of magnitude lower in the restored area compared to the natural forest. In contrast, SGM physical condition improved significantly in the restored area, as vegetation structure increased. Furthermore, SGM behaviour in the restored area switched from a predominantly walking to a predominantly feeding behaviour over the study period, resulting in the forest restoration programme on Cousine increasing the foraging area of the SGM by 43%. SGM spatial density did not significantly correlate with edaphic and litter properties, but did positively correlate with the toposcape (i.e. elevation and granitic rock cover). Granite rock crevices in forest covered areas were important diurnal refuges for the SGM, as microclimate conditions in non-forest covered rock refuges were unsuitable. SGM physical condition was significantly lower in non-shaded crevices compared to those shaded by forest. Low granite rock cover in the restored forest limited the SGM colonisation of this area in large numbers, despite canopy cover in the restored forest being comparable with that in the reference natural forest. As most restoration practices are primarily vegetation-based, this study demonstrates that such an approach can be inadequate for restoring habitat for target invertebrates, as many species’ habitat requirements extend beyond that of vegetation. For the SGM, selecting restoration sites that already have abundant rock cover would be the most practical way to increase SGM habitat through forest restoration practices. Taking into consideration the habitat requirements of target invertebrates can help in setting or redirecting restoration goals and thus enhance the conservation value of such practices.

AFRIKAANSE OPSOMMING: Die verlies en agteruitgang van habitat word alom beskou as die grootste bedreiging var die biodiversiteit van ongewerweldes. Herstellingspraktyke kan hierdie agteruitgang verminder of stop. Die Seychelle Reuse Duisendpoot (SGM), Sechelleptus seychellarum, is 'n bedreigde en funksioneel belangrike makro-detritusvoeder wat endemies is aan die Seychelle graniet-eilande. Die breë doelstelling van hierdie verhandeling is die doen van navorsing om praktiese aanbevelings te kan maak om die habitat van die SGM tot so ‘n mate te herstel dat die spesie kan bly voortbestaan. Hierdie studie is tussen 1998 en 2009 uitgevoer op Cousine Island, Seychelles, nadat ‘n grootskaalse herstellingsfase van die plantgemeenskap plaasgevind het. Groot skommelings in duisendpootgetalle is waargeneem tussen 1998 en 2009, viz. duisendpootgetalle was laag in 2002, 2003, 2005 en 2007, terwyl dit hoog was in 1998 en 2009. Alhoewel die SGM gedurende die hele jaar aktief is, is hulle tog in groter getalle aanwesig in die tydperke met hoë reënval (Oktober-April) en laag in die droë tydperk (Mei-September). Die verhouding van wyfies, mannetjies en onvolwassenes was deurgaans ~ 3:1:1. Dit bring mee dat hervestiging van SGM verkieslik gedoen moet word wanneer hul populasies hoog is en dan ook in die nat seisoen. Uitheemse klapperbome het geen invloed op SGM getalle gehad nie, alhoewel hul voedingsgedrag negatief beïnvloed is deur dié bome. Bamboesbosse darenteen, beïnvloed beide populasiedigtheid en voedingsgedrag van SGM negatief. Verder is daar gevind dat Pisonia grandis en Ficus sp. blaardetritus voorkeurvoedsel vir SGM is. Die verwydering van die uitheemse bamboes en klapperbome en vervanging daarvan met inheemse woudspesies (P. grandis en Ficus sp.) moet dus deel vorm van die eiland se herstelprogram om sodoende die SGM te bevoordeel. Die SGM-bevolkingsdigtheid was 'n grootte-orde laer in die herstelde gebied in vergelyking met die natuurlike bos, maar hul fisiese toestand het aansienlik verbeter in die herstelde gebied, waarskynlik omdat die plantegroei as geheel verbeter het. Verder het die SGM se gedrag gedurende die studietydperk in die herstelde area oorgegaan vanaf ‘n oorwegend loopgedrag om na kos te soek, na ‘n oorwegend voedende gedrag. Die vervanging van uitheemse- met inheemse boomspesies op Cousine Island het dus die voedingsarea van SGM met tot 43% verhoog. Die ruimtelike SGM populasiedigtheid is nie beduidend beïnvloed deur blaardetritus nie, maar is wel positief beïnvloed deur die topografie (hoogte en granietbedekking). Bebosde graniet rotsskeure bied belangrike toevlugsoorde vir SGM gedurende die dag, terwyl die mikroklimaat wat deur onbebosde rotsskeure veroorsaak word, totaal ongeskik is vir SGM. Die fisiese toestand van SGM was ook aansienlik swakker in die nie bebosde rotsskeure teenoor dié van die bebosde areas. In herstelde bos met min granietskuiling was die herkolonisering van SGM ook getalsgewys laer alhoewel die bosbedekking vergelykbaar was met dié van die inheemse bos. Dit bewys dus dat herstellingspraktyke wat hoofsaaklik plantegroei teiken, nie altyd die teikenspesie bevoordeel nie, maar dat ‘n meer holistiese benadering wat alle habitatvoorkeure in ag neem, toegepas moet word. Om SGM te bevoordeel moet herstel areas vir herbebossing dus gekies word waar daar reeds genoegsame granietskuiling is. Deur die habitatvereistes van ongewerwelde teikenspesies in ag te neem kan die herstellingspraktyke meer oordeelkundig ingestel word en sodoende kan die bewaringswaarde van sulke praktyke verbeter word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/85596
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