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Intra-annual variation of arthropod-plant interactions and arthropod trophic structure in an endangered grassland in the Free State province, South Africa

Arthropods are valuable biological indicators owing to strong relationships with primary producers. The supposition that arthropod-plant interactions are constant over seasons was tested using Mantel tests on correlations between these groups. A total of 78 plant species and 108 arthropod families were sampled monthly from the endangered Bloemfontein Dry Grassland at the Free State National Botanical Gardens between April 2009 and March 2010. Interaction strengths between plants and arthropods varied within a calendar year; being strongest in winter months (July and August), initiation of the growth season (November) and peak growth season (February and March). It was hypothesised that variation was caused by changing primary productivity. The trophic composition of the arthropod community supported this hypothesis as the ratio between primary consumers (herbivores) and secondary consumers (predators and parasitoids) showed patterns that coincided with periods of strong arthropod-plant interactions. Patterns were explained by multiple theories linking plant and arthropod diversity. It was concluded that arthropod assemblages are closely linked to primary producers, the abiotic environment and top-down trophic forces at different times of the year. Future arthropod-based studies should be performed with clear objectives and seasonal benchmark conditions to overcome these variations.