The measure of spatial position within groups that best predicts predation risk depends on group movement

Abstract

Both empirical and theoretical studies show that an individual's spatial position within a group can impact the risk of being targeted by predators. Spatial positions can be quantified in numerous ways, but there are no direct comparisons of different spatial measures in predicting the risk of being targeted by real predators. Here, we assess these spatial measures in groups of stationary and moving virtual prey being attacked by three-spined sticklebacks ( Gasterosteus aculeatus ). In stationary groups, the limited domain of danger best predicted the likelihood of attack. In moving groups, the number of near neighbours was the best predictor but only over a limited range of distances within which other prey were counted. Otherwise, measures of proximity to the group's edge outperformed measures of local crowding in moving groups. There was no evidence that predators preferentially attacked the front or back of the moving groups. Domains of danger without any limit, as originally used in the selfish herd model, were also a poor predictor of risk. These findings reveal that the collective properties of prey can influence how spatial position affects predation risk, via effects on predators' targeting. Selection may therefore act differently on prey positioning behaviour depending on group movement.

Both Prey and Predator Features Determine Predation Risk and Survival of Schooling Prey

“…However, in line with predictions from Hamilton's selfish herd model (Hamilton, 1971), we also found that fish with a larger (limited) domain of danger were at higher risk of being targeted. Although some previous studies have shown individual predation risk is related to their domain of danger (De Vos and O'Riain, 2010;Lambert et al, 2021;Romenskyy et al, 2020), our results highlight its importance within groups, further supported by the finding of considerable unexplained variance between fish's centre-to-edge position and their limited domain of danger. Such spatial heterogeneity within groups and its effects on differential predation is rarely considered (Jolles et al, 2020).…”

“…Bumann et al, 1997; Ioannou et al, 2019; Krause et al, 2017; but see e.g. Handegard et al, 2012; Lambert et al, 2021), we found that pike, after steadily approaching the school, often slowly entered the school and then launched their rapid attack towards the group centre. That predators such as pike can get very close to their prey (see also Coble, 1973; Hoogland et al, 1956; Krause et al, 1998; Nursall, 1973; Webb and Skadsen, 1980) could potentially be explained by their narrow frontal profile (Webb, 1982), which makes it very hard for prey to detect movement changes, especially when attacked head-on.…”

“…For each shiner we quantified their Voronoi polygon area, limited domain of danger (LDOD; following Lambert et al, 2021), local misalignment, and their inter-individual distance (see Table 1 for an explanation how these measures were computed). LDOD and local misalignment require a spatial threshold outside of which fish will be included.…”

“…There has been a lot of focus in the literature on the spatial effects underlying predation risk, especially in terms of centre-to-edge and front-to-back positioning, partly because they are the easiest to measure, and research largely concentrating on a single or a few key features (but see e.g. Lambert et al, 2021;Romenskyy et al, 2020). However, a wide range of factors may be expected to influence predation risk.…”

“…However, a wide range of factors may be expected to influence predation risk. In particular, following Hamilton's "selfish herd" theory (Hamilton, 1971), the spacing of individuals with respect to nearby neighbours, such as individuals with fewer neighbours/a larger domain of danger experiencing higher predation risk (De Vos and O'Riain, 2010;Lambert et al, 2021;Quinn and Cresswell, 2006;Romenskyy et al, 2020). But also individuals' alignment to nearby neighbours (Ioannou et al, 2012), and features related to their visual information, which is known to strongly influence how well individuals respond to others (Davidson et al, 2021;Rosenthal et al, 2015;Sosna et al, 2019;Strandburg-Peshkin et al, 2013).…”