Asymmetric interactions and their consequences for vital rates and dynamics: the smaller tea tortrix as a model system

Abstract

Asymmetric interactions among conspecifics can have diverse effects on population dynamics including stabilization, generation of cycles, and induction of chaotic fluctuations. A difficult challenge, however, is establishing the link between the impact of asymmetric interactions on life history and the consequences for population dynamics. The smaller tea tortrix, Adoxophyes honmai, is a good example. Larval instars differ dramatically in size and have a tendency for cannibalism, which suggests the potential for strong asymmetric interactions among instars. Yet whether these asymmetries have any role in generating the distinct single‐generation cycles observed in the field and laboratory is unclear. Here we report on the development of a new experimental approach to characterize the impact of asymmetric interactions on life history that can be directly embedded into stage‐structured population models. The experiments use donor‐replacement protocols in which focal individuals are challenged to complete their life cycles in competitive environments where the instar and density of the competitors are held constant. The experimentally derived interaction surface contains all the information about stage‐specific interactions and provides a straightforward framework for evaluating alternative ways of abstracting the interactions into traditional models of asymmetric competition. Working with the smaller tea tortrix, we found strong evidence of asymmetric interactions and identified critical “tipping points” in the competitive environment that strongly affected survival but not development. We incorporated the experimentally derived interaction surface into a stage‐structured population model and found that despite the strong impact that asymmetric interactions have on tea tortrix life history, they do not scale‐up to impact the predicted asymptotic population dynamics. Comparing these dynamics with two abstracted models of stage‐structured interactions revealed that while the quantitative details of the emergent dynamics depends on the shape of the interaction surface, the qualitative features, such as the emergence of single‐generation cycles and rapid synchronization of development among individuals, are pleasingly robust.

An integrated experimental and mathematical approach to inferring the role of food exploitation and interference interactions in shaping life history

“…Nelson et al. (2019) show empirical evidence for asymmetric interactions in the tea tortrix where the effect of competitor density on life‐history traits depends on the recipient's and its competitors’ size. They show that medium‐size larvae (3rd and 4th instars) were more impacted by the presence of same‐size or larger larvae (5th instars) than by the presence of smaller larvae (1st and 2nd instars).…”

“…Interference interactions have been observed such as larvae securing resources with silk covers, biting each other and engaging in cannibalism (Simpson et al., 2018), but the extent of these behavioural interactions is unknown because they are difficult to observe. The fact that larvae are aggressive and that increasing competitor density impacts mainly mortality compared to development and body size (Nelson et al., 2019) suggests that interference plays a role in shaping life histories and dynamics of the moth. We investigate the relative importance of interference interactions and food exploitation at three temperatures, as temperature is a strong determinant of the moth life histories (Nabeta et al., 2005; Régnière et al., 2012) and warmer temperatures are predicted to increase intraspecific interactions in ectotherms (Amarasekare & Coutinho, 2014; Johnson et al., 2016).…”

Dynamic population stage structure due to juvenile–adult asymmetry stabilizes complex ecological communities

“…occurrence of stunted populations in fish (36,37), shellfish (38), and dragonflies (39) and the asymmetry observed in intraspecific competition experiments (29,40,41). Therefore, q = 0.7 and φ = 1.8 are adopted as default values for the juvenile-adult ingestion and predation asymmetry parameter (but see SI Appendix, Fig.…”

“…Furthermore, per-capita reproduction rates are for most species less limited by food than juvenile maturation rates, in particular because offspring sizes are small compared to adult body sizes ( 32 ). A competitive asymmetry between juveniles and adults is further supported by the occurrence of stunted populations in fish ( 36 , 37 ), shellfish ( 38 ), and dragonflies ( 39 ) and the asymmetry observed in intraspecific competition experiments ( 29 , 40 , 41 ). Therefore, and are adopted as default values for the juvenile–adult ingestion and predation asymmetry parameter (but see SI Appendix , Fig.…”

Investigating the Dynamics of Elk Population Size and Body Mass in a Seasonal Environment Using a Mechanistic Integral Projection Model

“…Thus, environmentally driven changes to body size distributions can alter the strength of ecological interactions, potentially inducing/modifying feedback loops between population abundance, resource biomass and trait dynamics (Bolnick et al 2011;Hempson et al 2015;Griffiths et al 2018). In addition, environmentally driven shifts in body size can alter the balance of asymmetric interactions among individuals, which can have stabilizing or disruptive effects on population dynamics (Nelson et al 2019), weaken the strength of trophic cascades (Detmer and Wahl 2019), or lead to emergent phenomena, such as Allee effects, facilitation, or predator exclusion (De Roos et al 2003).…”