Spider food webs: influence of cropping cycle and landscape heterogeneity in Philippine rice ecosystems
by Nico Radermacher
Date of Examination:2022-03-24
Date of issue:2022-04-14
Advisor:Prof. Dr. Stefan Scheu
Referee:Prof. Dr. Stefan Scheu
Referee:Prof. Dr. Andreas Schuldt
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EnglishRice is the most important staple crop of the world, providing approximately 19% of the world population’s caloric uptake and being cultivated on 163 million ha. Rice provides food and income to an estimated 200 million rice farmers and their families and is deeply ingrained in many social cultures worldwide. While the green revolution has led to an impressing increase in yields the input of agrochemicals has resulted in new problems, such as reoccurring outbreaks of rice insect pests. To evaluate the potential of biological control by generalist predators, such as spiders, in rice-ecosystems the diets of spiders were analysed over three consecutive sampling dates (chapter 2) and in three rice-ecosystems of different habitat complexity (chapter 3). Spiders are important bio-control agents of rice insect pests such as planthoppers (Delphacidae) and leafhoppers (Cicadellidae). To investigate temporal changes in spider prey and variations in prey due to landscape structure around rice fields, carbon and nitrogen stable isotopes of rice field arthropods were analysed over three consecutive sampling dates during the rice cropping season. Initial isotope composition of gnats and midges emerging from submersed rice fields indicates a larval algae diet, while later values suggest a switch to rice-derived carbon. Initial δ¹³C values of plant- and leafhoppers were higher in fields of rice-heterogeneous landscapes, indicating migration from source populations feeding on C4 grasses into rice fields; later their δ¹³C values approached those of rice. Isotope values of web-building and cursorial spiders in the earliest samples indicate aquatic gnat and midge prey. The later shift toward terrestrial herbivore prey was more pronounced for small than for larger species and in rice paddies near permanent vegetation, indicating use of prey from the surrounding landscape. The results suggest that rice field spiders are supported by three different carbon pools: (1) aquatic carbon originating from algae and (2) legacy carbon from previous growing cycles, both incorporated via between-season predation on gnats and midges, and (3) carbon from the current rice season incorporated via herbivore prey. To investigate regional differences and variation in spider prey with landscape complexity and management intensity, we analysed carbon and nitrogen stable isotope ratios of rice field arthropods from three rice cropping regions in the Philippines. Research was conducted in the intensively managed, hilly lowland region Laguna, marked by additional fruit and vegetable cultivation, the intensively managed, lowland region Muñoz, characterized by high field sizes and less diverse cropping systems, and the low-intensity highland region Ifugao, known for its rice terraces. δ¹³C values of adult midges indicated that they had consumed algae during development, but midge larvae also relied on rice-derived carbon, especially in Laguna. Plant- and leafhoppers across all regions relied primarily on rice-derived carbon, with planthoppers in Ifugao tending to additionally feed on alternative host plants. Isotope values of spiders indicated a mixed diet with carbon derived from both aquatic and terrestrial prey. In general, spiders in Laguna and Ifugao more heavily relied on aquatic-derived carbon from midges while those in Muñoz incorporated more terrestrial-derived carbon. Landscape diversity, number of habitat patches surrounding the field, connectivity of rice bunds and shape of rice fields significantly impacted the carbon sources and diet composition of spiders, but the extent varied among species. Cursorial wolf spiders (Lycosidae) consumed greater amounts of aquatic prey in Laguna and Ifugao with more complex landscapes, and fed more on terrestrial prey in the less complex landscape of Muñoz. The majority of web-building long-jawed spiders (Tetragnathidae) and orb-weaver spiders (Araneidae) relied on terrestrial prey in Muñoz and Ifugao, but primarily on aquatic prey in Laguna, pointing towards resource partitioning between feeding guilds. In conclusion, increasing aquatic midge prey via mulching and provisioning of suitable habitats via vegetation management on bunds, integrating rice fields into heterogeneous landscapes and avoiding pesticides is likely to support high spider populations and thus strengthen biological control of pest species in rice paddies.
Keywords: rice field; stable isotopes; generalist predators; biological control; rice insect pest; landscape complexity