Identifying critical interactions in complex competition dynamics between bean beetles

Abstract

Identifying behavioural basis of competitive relationship is essential to understand outcome of interspecific competition. However, it remains difficult to investigate demographic effect of competitive behaviour, because various kinds of behaviours may co-occur in the competition and make the dynamics far complicated in nonlinear ways. We report that the behavioural basis of interspecific interaction can be identified, by focusing on the timescale difference from the occurrence of each behaviour to the appearance of its demographic effect. Between two bean beetles, Callosobruchus chinensis and C. maculatus, major interspecific interactions are resource competition (RC) at the larval stage and reproductive interference (RI) at the adult stage. RC has longer time lag than RI, because effect of RC appears in the adult number of the next generation through larval competition while effect of RI appears instantaneously in the adult number through early death of females. If we detect two effects with different time lags from the competition dynamics, an effect with intergenerational time lag and with no time lag would be considered as RC and RI, respectively. We applied empirical dynamic modelling approach, which is a nonlinear time series analysis for detecting causal interactions and the strength, to two published datasets of experimental competition between those beetles. Results showed the significant causality from the winner species to the loser one in both experiments, but the causality time lag differed between experiments: the causality had no time lag in the C. chinensis-win data, while intergenerational time lag in C. maculatus-win data. Furthermore, detection of the causality with intergenerational time lag from C. maculatus to C. chinensis in both experiments suggests interplay of constant RC and variable RI which can reverse the outcome. This study is the first successful case study that links behavioural-level interactions to demographic-level effects in interspecific competition.

Does past evolutionary history under different mating regimes influence the demographic dynamics of interspecific competition?

“…The time lag difference suggests differential timescales at which interspecific interactions exert their demographic effects. Behavioral mechanisms of interspecific interactions vary in the time lag from the behavioral interaction to the occurrence of its demographic effect (Kawatsu & Kishi, ). In seed beetles, reproductive interference shortens adult C. maculatus female longevity.…”

“…This result appears consistent with the initially high relative extinction risk for polygamous C. chinensis (Figure ). The estimated time lag of 3 or 4 weeks for this effect corresponds to their generation time, suggesting larval resource competition as the underlying mechanism (Kawatsu & Kishi, ). Larvae of C. maculatus are more competitive than those of C. chinensis (Kishi et al, ), and abundant oviposition by C. maculatus females in the previous generation may have intensified larval competition and therefore decreased C. chinensis abundance.…”

“…We examined the competitive dynamics of C. chinensis and C. maculatus after manipulating the evolutionary history of C. chinensis under different mating regimes and tested the hypothesis that sexual selection has consequences for competitive dynamics through its effect on the strength of reproductive interference, a dominant mode of interspecific interaction in our system (Kawatsu & Kishi, ; Kishi et al, ). Although our results were inconclusive and did not fully support our hypothesis, we found that past evolutionary history affected some properties of competitive dynamics.…”

“…EDM is an analysis method for deterministic, nonlinear systems (Chang, Ushio, & Hsieh, 2017). For the applicability of EDM to C. chinensis-C. maculatus competitive dynamics, see Kawatsu and Kishi (2018). Following the ordinary procedure, we first performed simplex projection to determine the embedding dimension E, the embedding lag τ and the number of time steps to predict T P .…”

“…Specifically, we performed CCM to determine the causal effect from one species to the other and their time lags. In the Callosobruchus competition system, larval resource competition and adult reproductive interference show different time lags (Kawatsu & Kishi, ), and thus time lag can be used to infer the behavioral mechanisms of the interactions. We then performed S‐map to quantify the intensity of the interspecific interactions.…”