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The Effect of Urbanization and Agriculture on Predacious Arthropod Diversity in the Highveld Grasslands

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The Effect of Urbanization and Agriculture on Predacious Arthropod Diversity in the Highveld Grasslands The effect of urbanization and agriculture on predacious arthropod diversity in the Highveld grasslands B M Greyvenstein 22303642 Dissertation submitted in fulfillment of the requirements for the degree Magister Scientiae in Environmental Science at the Potchefstroom Campus of the North-West University Supervisor: Prof S. J. Siebert Co-supervisor: Prof J. van den Berg November 2015 i Acknowledgements I would like to acknowledge all the people involved in this dissertation in all forms from research support, to helping with field work as well as moral support. In particular, I would like to thank my amazing supervisors for their continued help and support in this endeavour. The skills and experience that I have been taught would not have been at all as effective or enriching without their help. I would also like to thank the following incredible people who assisted me with my fieldwork: Dennis Komape, Arnold Frisby, Este Matthew, Ruhan Verster, Melissa Andriessen, Nanette van Staden, Helga van Coller, François Clapton, Anton Botha, Melanie Schoeman and my parents Michaela and Piet Greyvenstein. Lastly, but not at all the least, I give thanks to God, the creator of all that I was studying and in no small part for his continued strength and motivation that I could have only received from his amerce grace and direct divine hand in my life and thoughts. ii Abstract Biodiversity provides vital ecosystem services and more diverse ecosystems are known to be more stable and resilient in the face of disturbance. Predacious arthropods provide a valuable ecosystem service to control pest numbers. We hypothesised that anthropological activities, which result in land-use change and habitat fragmentation, deplete this species pool. However, current knowledge is lacking regarding the diversity and abundance of arthropod predators in crop fields within the agricultural and urban environments of South Africa. We studied the diversity, abundance and species assemblages of predaceous arthropods of the Chrysopidae, Mantodea, Araneae and Coccinellidiae at different intensity levels of disturbances within urban and agricultural sites in the grasslands of South Africa. Study sites included agricultural (maize field-field margin-untransformed grassland) and urban (ruderal-fragmented-untransformed grassland) gradients. The optimal sweep net sampling time was also determined within these agricultural ecosystems. Sweep nets were used to sample fields and field margins during the following daily times (07:00, 12:00, 15:00 and 17:00). Sampling was replicated four times. Shannon diversity index values did not differ between the different sweep net sampling times. Predacious arthropods were however more abundant within maize fields in the mornings (07:00) and adjacent fields around midday (12:00). The results indicated there were no statistical differences despite the higher abundance of predators at the previously mentioned times. The study done on the agro-ecosystem zones did however indicate that the field margins were the most diverse and abundant in terms of predacious arthropod diversity along a maize field-field margin-untransformed grassland gradient. Despite this our results indicated that urban areas had a similar species richness of arthropods as agro-ecosystems, but predator species were more abundant in urban areas. With an increase in distance away from disturbance, in both the agricultural and urban environments, an increase was noticed in diversity and abundance. The maize fields were the least diverse and abundant for these predacious arthropods. Surrounding agricultural landscapes could therefore play an important role in the maintenance of predacious arthropod diversity as well as future integrated pest management strategies. This study generated baseline data to monitor the effects of anthropological activities on predator diversity and abundance in urban- and agro- ecosystems. Keywords: Biodiversity; predacious arthropods; diversity; agriculture; urbanization; ruderal, fragmented grasslands; maize; field margin; untransformed grasslands. iii Table of Contents Acknowledgements .............................................................................................. ii Abstract ................................................................................................................ iii Chapter 1: Introduction ....................................................................................... 1 1.1 Importance of biodiversity ............................................................................. 1 1.1.2 Predators within the food web ....................................................................... 2 1.1.3 Resilience ...................................................................................................... 4 1.2 Threats to biodiversity ................................................................................... 4 1.3 Agriculture as a disturbance ......................................................................... 5 1.3.1 Pesticides ...................................................................................................... 6 1.3.2 Genetically modified crops ............................................................................. 7 1.3.3 Biological control ........................................................................................... 7 1.4 Urbanization as a disturbance ....................................................................... 8 1.4.1 Island biogeography theory ........................................................................... 9 1.5 Conservation ................................................................................................. 10 1.5.1 Conservation in agriculture .......................................................................... 10 1.5.2 Conservation in urban areas ........................................................................ 11 1.5.2.1 Corridors and linkages .............................................................................. 12 1.5.2.2 Sink and source theory ............................................................................. 13 1.6 Aim of the study .............................................................................................. 14 1.7 Selecting representative arthropod groups ............................................... 14 1.7.1 Lacewings (Insecta: Neuroptera: Chrysopidae) ........................................... 14 1.7.2 Praying mantids (Insecta: Mantodea) .......................................................... 15 1.7.3 Spiders (Arachnida: Araneae) ..................................................................... 16 1.7.4 Ladybirds (Insecta: Coleoptera: Coccinellidae) ........................................... 16 1.8 Reference ...................................................................................................... 18 Chapter 2: Optimal sweep net sampling time .................................................. 26 2.1 Abstract ........................................................................................................ 26 2.2 Introduction ................................................................................................... 26 2.2.1 Aim, objective and hypothesis ..................................................................... 29 2.3 Materials and Methods ................................................................................. 30 2.3.1 Study sites ................................................................................................. 30 2.3.2 Sweep net sampling .................................................................................... 33 2.3.3 Data analyses .............................................................................................. 34 2.4 Results ........................................................................................................... 35 2.4.1 Time of day and predator groups ............................................................. 35 2.4.1.1 Lacewings ................................................................................................. 37 2.4.1.2 Spiders .................................................................................................... 38 2.4.1.3 Coccinellids ............................................................................................. 39 2.4.1.4 Praying Mantids ....................................................................................... 39 2.4.6 Time of day with regards to zones ........................................................... 40 2.4.6.1 Maize field .............................................................................................. 40 iv 2.4.6.1.1 Spiders .................................................................................................. 41 2.4.6.1.2 Lacewings .............................................................................................. 42 2.4.6.1.3 Coccinellids ........................................................................................... 43 2.4.6.1.4 Praying mantids ..................................................................................... 44 2.4.6.2 Field Margin ........................................................................................... 45 2.4.6.2.1 Spiders .................................................................................................. 47 2.4.6.2.2 Lacewings .............................................................................................. 48 2.4.6.2.3 Coccinellids
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