Abstract
Grassland fertilisation drives non-random plant loss resulting in areas dominated by perennial grass species. How these changes cascade through linked trophic levels, however, is not well understood. We studied how grassland fertilisation propagates change through the plant assemblage into the plant-flower-visitor, plant-leaf miner and leaf miner-parasitoid networks using a year's data collection from a long-term grassland fertiliser application experiment. Our experiment had three fertiliser treatments each applied to replicate plots 15m2 in size: mineral fertiliser, farmyard manure, and mineral fertiliser and farmyard manure combined, along with a control of no fertiliser. The combined treatment had the most significant impact, and both plant species richness and floral abundance decreased with the addition of fertiliser. While insect species richness was unaffected by fertiliser treatment, fertilised plots had a significantly higher abundance of leaf miners and parasitoids and a significantly lower abundance of bumblebees. The plant-flower-visitor and plant-herbivore networks showed higher values of vulnerability and lower modularity with fertiliser addition, while leaf miner-parasitoid networks showed a rise in generality. The different groups of insects were impacted by fertilisers to varying degrees: while the effect on abundance was the highest for leaf miners, the vulnerability and modularity of flower-visitor networks was the most affected. The impact on the abundance of leaf miners was positive and three times higher than the impact on parasitoids, and the impact on bumblebee abundance was negative and double the magnitude of impact on flower abundance. Overall, our results show that while insect species richness was unaffected by fertilisers, network structure changed significantly as the replacement of forbs by grasses resulted in changes in relative abundance across trophic levels, with the direction of change depending on the type of network. Synthesis. By studying multiple networks simultaneously, we were able to rank the relative impact of habitat change on the different groups of species within the community. This provided a more holistic picture of the impact of agricultural intensification and provides useful information when deciding on priorities for mitigation.
Highlights
Nutrient enrichment is one of the most common causes of plant species decline worldwide (Hautier et al, 2015; Tilman et al, 2001)
We studied how grassland fertilisation propagates change through the plant assemblage into the plant–flower-visitor, plant–leaf miner and leaf miner–parasitoid networks using a year's data collection from a long-term grassland fertiliser application experiment
Our results show that while insect species richness was unaffected by fertilisers, network structure changed significantly as the replacement of forbs by grasses resulted in changes in relative abundance across trophic levels, with the direction of change depending on the type of network
Summary
Nutrient enrichment is one of the most common causes of plant species decline worldwide (Hautier et al, 2015; Tilman et al, 2001). The studies which consider the impact of fertiliser addition using a network of species interactions approach do not consider the cumulative effects of the nutrient inputs that occur in the productive systems where fertilisation occurs every year. Parasitoids may respond to increased biomass in the herbivore community by choosing bigger species, decoupling species-level responses (de Sassi, Staniczenko, et al, 2012; de Sassi & Tylianakis, 2012; Stiling & Moon, 2005) These effects, the loss of species richness, may reduce the stability of the ecological network, making the system more sensitive to species extinction (Solé & Montoya, 2001). This allows us to rank the impact of fertiliser loads on these three different components of the community Using this long-term experimental system and studying three different insect networks simultaneously, we asked four questions: Q1: Does grassland fertilisation affect the species richness and abundance of plants, flower visitors, leaf miners and parasitoids? Using this long-term experimental system and studying three different insect networks simultaneously, we asked four questions: Q1: Does grassland fertilisation affect the species richness and abundance of plants, flower visitors, leaf miners and parasitoids? Q2: Does grassland fertilisation alter the network of species interactions, that is, the structure of the interactions among species? Q3: What is the relative magnitude of change in the four components of the system; plants, flower visitors, leaf miners and parasitoids, in response to fertilisation? Q4: Are any changes in the network which are attributable to grassland fertilisation, mediated by changes in plant species dominance?
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