Behavioural changes in the city: The common black garden ant defends aphids more aggressively in urban environments

Abstract

Urbanisation alters biodiversity patterns and threatens to disrupt mutualistic interactions. Aside from pollination, however, little is known about how mutualisms change in cities. Our study aimed to assess how urbanisation affects the protective mutualism between ants and aphids, investigating potential behavioural changes in mutualistic ants and their implications for aphids in urban environments. To do so, we studied the protective mutualism between the pink tansy aphid (Metopeurum fuscoviride) and the black garden ant (Lasius niger) along an urbanisation gradient in Berlin, Germany. In nine locations along this gradient, we measured aphid colony dynamics and proxies for parasitism, quantified the investment of ants in tending aphids and conducted behavioural assays to test the aggressiveness of ant responses to a simulated attack on the aphids. We found that aphid colonies flourished and were equally tended by ants across the urbanisation gradient, with a consistent positive density dependence between aphid and ant numbers. However, ants from more urbanised sites responded more aggressively to the simulated attack. Our findings suggest that this protective mutualism is not only maintained in the city, but that ants might even rely more on it and defend it more aggressively, as other food resources may become scarce and more unpredictable with urbanisation. We thereby provide unique insights into this type of mutualism in the city, further diversifying the growing body of work on mutualisms across urbanisation gradients.

Keywords: Tanacetum vulgare; aphids; behavioural assay; herbivory; parasitoids; protective mutualism; urbanisation.

FIGURE 1

Field assessment of tending and defensive behaviours of the mutualistic ant Lasius niger along an urbanisation gradient in Berlin, Germany. Ant behaviour was first observed undisturbed, at the (1) group level and (2) individual level. In a second phase, we (3) recorded behavioural responses of individual ants targeted by a simulated attack using a needle. Each sampling session on a given plant host determined the unit of observation for the study. Observations were replicated along an urbanisation gradient in Berlin and repeated 4–7 times over a month.

FIGURE 2

Positive correlation between aphid density and the number of attending ants shown for 3 periods of the field survey (start: 1–11.08.2018; mid: 12–21.08.2018; end: 22.08–1.09.2018). Aphids were counted within an area of the host plant (focal zone) to derive aphid densities (number of individuals/vertical length of the focal zone). Ant numbers are the average number of ants in the focal zone (in decimals, based on 5 counts within 1 min each where ant movement in and out of the zone was monitored). Shown here are the real data and three linear regressions that illustrate the positive interaction between date and numbers of ants and aphids.

FIGURE 3

Proportion of ants showing aggressive reactions to a simulated attack along the urbanisation gradient. Dots represent partial residuals and lines correspond to the fit of the binomial GLMM. % sealing is the percentage of sealed surfaces in a 500 m buffer around the study sites. The % sealing variable was standardised to fit the model and back‐transformed to produce the figure.