Stability lies in flowers: Plant diversification mediating shifts in arthropod food webs

Abstract

Arthropod community composition in agricultural landscapes is dependent on habitat characteristics, such as plant composition, landscape homogeneity and the presence of key resources, which are usually absent in monocultures. Manipulating agroecosystems through the insertion of in-field floral resources is a useful technique to reduce the deleterious effects of habitat simplification. Food web analysis can clarify how the community reacts to the presence of floral resources which favour ecosystem services such as biological control of pest species. Here, we reported quantitative and qualitative alterations in arthropod food web complexity due to the presence of floral resources from the Mexican marigold (Tagetes erecta L.) in a field scale lettuce community network. The presence of marigold flowers in the field successfully increased richness, body size, and the numerical and biomass abundance of natural enemies in the lettuce arthropod community, which affected the number of links, vulnerability, generality, omnivory rate and food chain length in the community, which are key factors for the stability of relationships between species. Our results reinforce the notion that diversification through insertion of floral resources may assist in preventing pest outbreaks in agroecosystems. This community approach to arthropod interactions in agricultural landscapes can be used in the future to predict the effect of different management practices in the food web to contribute with a more sustainable management of arthropod pest species.

Fig 1. Design of the field experiment representing grids of lettuce and the flower resource Mexican marigold (T. erecta).

Fig 2. The arthropod food web associated with lettuce plants during the pre-flowering stage of T. erecta.

The width of the black, gray and white horizontal bars shows log₁₀ body mass (mg), log₁₀ numerical abundance (individuals/m²) and log₁₀ biomass abundance (mg/m²). Solid lines represent direct consumption interaction. Dotted line represents presence, but without direct consumption of living tissues. Species numbers are identified in Table 2. The vertical position indicates trophic level. Horizontal position is arbitrary. Isolated species, cannibalism or loops are ignored.

Fig 3. The arthropod food web associated with lettuce plants during the flowering stage of T. erecta.

The width of the black, gray and white horizontal bars shows log₁₀ body mass (mg), log₁₀ numerical abundance (individuals/m²) and log₁₀ biomass abundance (mg/m²). Solid lines represent direct consumption interaction. Dotted line represents presence, but without direct consumption of living tissues. Species numbers are identified in Table 2. The vertical position indicates trophic level. Horizontal position is arbitrary. Isolated species, cannibalism or loops are ignored.

Fig 4. The arthropod food web associated with lettuce plants during the late flowering stage of T. erecta.

The width of the black, gray and white horizontal bars shows log₁₀ body mass (mg), log₁₀ numerical abundance (individuals/m²) and log₁₀ biomass abundance (mg/m²). Solid lines represent direct consumption interaction. Dotted line represents presence, but without direct consumption of living tissues. Species numbers are identified in Table 2. The vertical position indicates trophic level. Horizontal position is arbitrary. Isolated species, cannibalism or loops are ignored.

Fig 5. Distribution of empirical and simulated numerical abundance (individuals/m²), body mass (mg) and biomass abundance (mg/m²) through trophic height in function of the number of T. erecta flowers in field (number of flowers/spot).

The color-graded scale at the right of each plot represents the level of either abundance (a,b), body size (c,d), or biomass (e,f) distribution of the lettuce-associated arthropods. The lowest to the highest levels are represented by the transition from dark blue to dark red respectively.