Persistence of insect biodiversity within a timber production landscape mosaic

Van der Mescht, Aileen (2020-04)

Thesis (PhDConsEcol)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Habitat transformation, through the expansion of agricultural activities, leads to the formation of novel landscapes, which drives global declines in insect diversity. This leads to high contrast between the structural characteristics of neighbouring biotopes, changing the local distribution of species across the landscape. Contrast between natural and anthropogenic biotopes determines the degree to which species are able to spillover between adjacent biotopes. Agroecosystems are complex landscapes, where natural and transformed biotopes are adjacent. Across the South African timber production landscape, ecological networks (ENs) of conservation corridors are employed to mitigate the adverse effects of plantation forestry. Boundaries between the natural and planted areas are sharp, and provide the opportunity to investigate how contrast between multiple biotopes drives insect assemblages. The aim here is to determine how ecological contrast in a forestry landscape affects local distribution of insect diversity, and to investigate its significance for insect conservation. I used various sampling methods, specifically pitfall traps, active searches, sweep netting, and vacuum sampling to assess how multiple epigaeic and foliage-dwelling insect taxa and their assemblages respond to changing contrast. I also use a newly-developed ecoacoustics approach, where male bush cricket calls were recorded to determine their response to landscape contrast. Across all sampled assemblages (epigaeic, foliage-dwelling, and calling bush crickets), contrast played an important role in driving the distribution of insect diversity. Epigaeic and foliage-dwelling assemblages were highly distinct in grasslands due to high contrast between these and all other biotopes. Contrast was lowest between Eucalyptus plantations of different ages, resulting in insect assemblages that were indistinguishable from each other. Plantations maintain their own unique insect assemblage, with some spillover from indigenous forest into mature plantations, which is indicative of lower contrast at the plantation-forest interface. Bush crickets and their soundscape, responded to changing contrasts across this landscape. Their diversity and activity were highest at the natural forest-grassland ecotones. For all ecotone types, there was spillover. Bush cricket species are highly adapted to their specific acoustic environments, and there is acoustic niche partitioning between them. Thoracistus thyraeus, a threatened species, was recorded in both natural and transformed biotopes, representative of local bush cricket diversity, as well as promoting insect conservation and the high value of ENs in this transformed landscape. Natural biotopes, such as forest, grasslands and ecotone between them, are essential for maintenance of all insect species in this transformed landscape, and require active management. Owing to low contrast between plantation age classes, transformed plantations maintain their own unique insect assemblage no matter plantation age. Forest species spillover into plantations, supporting the value of natural forests in this production landscape. Furthermore, low contrast promotes higher insect diversity and is beneficial at the plantation edges. Acoustic sampling provides a rapid and easy sampling technique showing the underlying spatial patterning of biodiversity, with very low sampling effort. Overall, the South African forestry landscape conserves insect biodiversity, provided patches of grasslands, natural forests, and their edges are maintained in their natural state.

AFRIKAANSE OPSOMMING: Habitattransformasie, veroorsaak deur die uitbreiding van landboubedrywighede, lei tot die vorming van nuwe landskappe, wat die wêreldwye afname in insekdiversiteit dryf. Dit lei tot 'n hoë kontras tussen die strukturele eienskappe van naburige biotope, wat ‘n verandering veroorsaak in die plaaslike verspreiding van spesies oor die landskap. Die kontras tussen natuurlike en antropogeniese biotope bepaal die mate waartoe spesies tussen aangrensende biotope kan versprei. Agro-ekosisteme is komplekse landskappe waar natuurlike en getransformeerde biotope aanliggend is. Regoor die SuidAfrikaanse houtproduksie landskap word ekologiese netwerke (EN's) van bewaringskorridors gebruik om die nadelige gevolge van plantasiebosbou te verlig. Grense tussen natuurlike en aangeplante gebiede is skerp, en bied die geleentheid om te ondersoek hoe kontras tussen verskeie biotope insek gemeenskappe dryf. Die doel hier is om te bepaal hoe ekologiese kontras in 'n bosboulandskap die plaaslike verspreiding van insekdiversiteit beïnvloed, en om die belangrikheid daarvan vir insekbewaring te ondersoek. Ek het verskillende steekproefmetodes gebruik, spesifiek pitval strikke, aktiewe soektogte, vee-net en lugsuier monsterneming om te bepaal hoe verskeie grond en plant-wonende insektaksa en hul gemeenskappe reageer op veranderende kontras. Ek gebruik ook 'n nuut ontwikkelde eko-akoestiese benadering, waar manlike boskriek roepgeluide opgeneem is om hul reaksie op landskap kontras te bepaal. Kontras het 'n belangrike rol gespeel as dryfkrag in die verspreiding van insek-diversiteit oor alle gemeenskappe (grond, plant-wonende insekte en roepende boskrieke). Grond en plant-wonende insek gemeenskappe was hoogs onderskeidend in grasvelde vanweë die hoë kontras tussen hierdie en alle ander biotope. Die kontras was die laagste tussen Eucalyptus plantasies van verskillende ouderdomme, wat gelei het tot insek gemeenskappe wat nie van mekaar onderskei kon word nie. Plantasies handhaaf hul eie unieke insek gemeenskappe, met 'n bietjie verspreiding van inheemse woud na volwasse plantasies, wat 'n aanduiding is van 'n laer kontras by die plantasie-woud-ekotoon. Boskrieke en hul klankbeeld het gereageer op veranderende kontraste oor die landskap. Hul diversiteit en aktiwiteit was die hoogste by natuurlike woud-grasveld ekotone. Daar was verspreiding tussen alle ekotoontipes. Boskriekspesies is hoogs aangepas tot hul spesifieke akoestiese omgewings, en daar is akoestiese nisverdeling tussen hulle. Thoracistus thyraeus, 'n bedreigde spesie, is opgeneem in natuurlike en getransformeerde biotope. Die spesie is verteenwoordigend van die plaaslike boskriek diversiteit, en as ʼn vlagskip spesie bevorder dit die bewaring van insekte en die hoë waarde van EN's in hierdie getransformeerde landskap. Natuurlike biotope, soos woud, grasvelde en die ekotoon tussen hulle, is noodsaaklik vir die instandhouding van alle insek spesies in hierdie getransformeerde landskap en benodig aktiewe bestuur. Vanweë die lae kontras tussen plantasie-ouderdomsklasse, handhaaf getransformeerde plantasies hul eie unieke insek gemeenskap, ongeag planttyd. Woud spesies versprei na plantasies, wat die waarde van natuurlike woude in hierdie produksie-landskap ondersteun. Verder bevorder lae kontras groter insek diversiteit en is dit voordelig vir plantasie grense. Akoestiese monsterneming bied 'n vinnige en maklike tegniek wat die onderliggende ruimtelike patroon van biodiversiteit toon, met 'n baie lae steekproefneming inspanning. In die algemeen bewaar die Suid-Afrikaanse bosboulandskap insekte se biodiversiteit, mits kolle grasveld, natuurlike woude en hul grense in hul natuurlike toestand behou word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108205
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