SUPPLEMENTARY INFORMATION

Transcription

1 doi: /nature25504 Contents: Supplementary Text: Supplementary results and discussion: 1. Evaluations of the S-map method, 2. Ecological interpretations of the detected interspecific interactions in the fish community, 3. Sensitivity of the dynamic stability to the inclusion of subdominant species, 4. Sensitivity of the dynamic stability to observation errors, 5. Calculations of coefficient of variation (CV) of fish abundances, 6. Evaluations of the phase-lock twin surrogate method 1

2 RESEARCH SUPPLEMENTARY INFORMATION Supplementary Text 1. Evaluations of the S-map method In a unidirectional two-species model, the multivariate S-map method successfully recovers the interaction strength in the unidirectional two-species model (Extended Data Figure 1a c). The estimated instantaneous interaction strength (i.e., X(t+1)/ Y(t) divided by X(t)) is almost constant through time, and the value is the same as the true interaction strength, 0.31 (Extended Data Figure 1c). In a bidirectional two-species model, the results showed that the multivariate S-map method correctly estimates true interaction strengths (i.e., long-term average interaction strength from X to Y in this case) regardless of interaction strength from Y to X (Extended Data Figure 1d). In a classic predator-prey system (Paramecium-Didinium system) 1, the expected result from the biological background is that the predator negatively influences the prey, and the prey positively influences the predator. The result of the S-map method is consistent with this expectation (Extended Data Figure 1e, f). In the rotifer-chlorella system were from an experimental predator-prey system 2, the expectation is bidirectional interactions between algae r/algae K and the rotifer. The S-map method suggested that both algae positively influence the rotifer, and the rotifer negatively influences the algae, being consistent with the expectation (Extended Data Figure 1h, i). Interestingly, the results showed that the negative influences from the rotifer to algae, which probably reflect predation pressure, are smaller for algae K than for algae r (red lines in Extended Data Figure 1h, i). In addition, bidirectional interaction between the algae was also detected, suggesting competitive interaction between the algal species (black lines in Extended Data Figure 1h, i). 2. Ecological interpretations of the detected interspecific interactions in the fish community Bidirectional positive interactions between Trachurus japonicus and Sphyraena pingius Red barracuda (Sphyraena pingius) and Japanese jack mackerel (Trachurus japonicus) are often observed to school together in the Maizuru Bay. Piscivorous barracuda 3, however, does not prey upon jack mackerel, presumably because of mackerel s high body height. Instead, jack mackerel, a plankton feeder 4, may have their competitors (e.g., other planktivores) eliminated by shoaling together with barracuda, while barracuda can more easily approach its prey fish by shoaling together with abundant jack mackerel. Positive interaction from Trachurus japonicus to Aurelia sp. Jellyfish is a zooplankton feeder and is commensal with jack mackerel (Trachurus japonicus). Although Trachurus juveniles feed on prey collected by jellyfish, they do not feed on jellyfish itself 5 ; jack mackerel may well feed on parasites of jellyfish and thus are likely to have a positive interaction. Indeed, jellyfish can live longer in aquaria when they are kept with Trachurus than when they are kept alone (R. Masuda, personal observation). No interspecific interactions associated with Engraulis japonicus Engraulis japonicus, chase anchovy, is a pelagic plankton feeder, and thus feeds in different environments from many of the other fish species included in our analysis. This spatial separation of chase anchovy may have resulted in the lack of substantial interaction with other fish species. 2

3 RESEARCH Negative interaction from Plotosus japonicus to Tridentiger trigonocephalus Plotosus japonicus, sea catfish, is an epibenthic omnivore or scavenger and is well known for its poisonous spines. Tridentiger trigonocephalus is a small but aggressive goby and is likely to compete with Plotosus japonicus for prey and habitat. Plotosus usually form a tight aggregate. Plotosus may outcompete Tridentiger through predator avoidance (poisonous spines) and/or efficient feeding by shoaling. Positive interaction from Parajulis poecilepterus to Sebastes cheni Sebastes cheni, black rockfish, mainly feeds on shrimp and has a crepuscular activity, whereas wrasses (Pseudolabrus sieboldi, Parajulis poecilepterus and Halichoeres tenuispinis) are omnivores and active only in daytime 6. The presence of wrasse may reduce activity of shrimps in the daytime and provide more feeding opportunity for Sebastes. Negative interaction from Girella punctata to Pterogobius zonoleucus Girella punctata is an herbivore. Because the major habitat of Pterogobius zonoleucus is algal forest (Sargussum bed), the presence of Girella may have a negative effect on Pterogobius. Negative interaction from Pterogobius zonoleucu to Chaenogobius gulosus Chaenogobius gulosus is a goby that utilizes algal forest only at its pelagic juvenile stage and then becomes demersal, whereas Pterogobius zonoleucu typically lives in an algal forest for its whole life history and thus up to a larger size than Chaenogobius. Therefore, Chaenogobius may be expelled by Pterogobius from the algal forest habitat because of the size disadvantage. Positive interaction from Siganus fuscescens to Parajulis poecilepterus Siganus fuscescens, rabbitfish, is an herbivore and lives in an algal forest, whereas Parajulis poecilepterus, a wrasse, is an omnivore mainly feeding on benthic invertebrates in sandy or rocky bottom habitat. Therefore removal of algae by the feeding activity of rabbitfish may open an opportunity for feeding by wrasse, at least for a short time span. Negative interactions associated with Rudaris erocodes Rudaris ercodes, a pigmy filefish, can feed on both benthic and planktonic animals and utilizes algal beds as a habitat. Rudaris are slow swimmers but have a strong dorsal spine so that predators are discouraged from feeding on this species. Rudaris may be out-competed by Pterogobius because the latter is a more efficient feeder in an algal forest. Rudaris may outcompete Trachurus because of the efficiency of defense against a predator. 3. Sensitivity of the dynamic stability to the inclusion of subdominant species Three of the four subdominant species had more than one causal link, which changed the network structure (e.g., increases in the number of links in the interaction network; Extended Data Figure 4a c). Takifugu poecilonotus did not have causal links with the dominant species and the network structure did not change (Extended Data Figure 4d). Although the inclusion of a subdominant species resulted in a different network structure, the calculated dynamic stability remained almost the same (almost on the 1:1 line; Extended Data Figure 4a d), suggesting that the dynamic stability was robust and not dramatically affected by rare species. 4. Sensitivity of the dynamic stability to observation errors 3

4 RESEARCH SUPPLEMENTARY INFORMATION 4. Sensitivity of the dynamic stability to observation errors The result suggested that, in general, the dynamic stability showed a similar pattern in cases of up to 10% observation errors (Extended Data Figure 4e j). In fact, even when 30% observation errors were added, the stable period (i.e., the dynamic stability < 1.0) did not significantly change (Extended Data Figure 4j). 5. Calculations of coefficient of variation (CV) The mean CV was regarded as an alternative index of community level stability. While CV is easily interpretable, CV may be an inappropriate index of stability in some cases.. For example, stochastic noise may not be homogeneous in time, and CV changes depending on the selected window size. Nonetheless, the mean values of CV during the stable period (i.e., the dynamic stability < 1.0) were lower than those during the unstable period (P < , t-test, two-sided, Extended Data Figure 5). This result suggested that the higher the dynamic stability becomes (i.e., dynamic stability value becomes lower), the lower the variation in realized population dynamics becomes in the fish community. 6. Evaluations of the phase-lock twin surrogate method First, we examined the condition of no interaction between X and Y (β xy = β yx = 0). The results showed that the stronger the seasonality was, the higher the terminal ρ according to the cross-mapping from Y to X was (Extended Data Figure 3c). Terminal ρ was, however, always lower than the upper limit of 95% confidence intervals calculated using the phase-lock twin surrogate (Extended Data Figure 3d), suggesting that the surrogate provides a conservative criterion (i.e., the possibility of a false positive is low). Second, we examined the conditions of unidirectional interaction from X to Y, and bidirectional interactions between X and Y (Extended Data Figure 3e h). For most cases, terminal ρ was higher than the upper limit of 95% confidence intervals (Extended Data Figure 3f, h). However, we should note the possibility of a false negative (i.e., terminal ρ is lower than the upper limit of 95% confidence intervals, despite there being interaction from X to Y). Taking these results altogether, we consider that our surrogate method provides a conservative criterion for judging the causality between time series, but one should be careful of the possibility of false negatives. 4

5 RESEARCH References Cited in Supplementary Text 1. Veilleux, B. G. The analysis of a predatory interaction between Didinium and Paramecium. (University of Alberta, USA, 1976). 2. Kasada, M., Yamamichi, M. & Yoshida, T. Form of an evolutionary tradeoff affects eco-evolutionary dynamics in a predator-prey system. Proc. Natl. Acad. Sci. 111, (2014). 3. Baeck, G. W. & Huh, S.-H. Feeding Habits of Brown barracuda (Sphyraena pinguis, Teleostei) in the Coastal Waters of Gadeok-do, Korea. Korean J. Fish. Aquat. Sci. 37, (2004). 4. Hirota, Y., Uehara, S. & Honda, H. Ontogenetic changes of feeding selectivity in juvenile jack mackerel Trachurus japonicus collected off south-east Kyushu, Japan. Fish. Sci. 70, (2004). 5. Masuda, R., Yamashita, Y. & Matsuyama, M. Jack mackerel Trachurus japonicus juveniles use jellyfish for predator avoidance and as a prey collector. Fish. Sci. 74, (2008). 6. Masuda, R., Matsuda, K. & Tanaka, M. Laboratory video recordings and underwater visual observations combined to reveal activity rhythm of red-spotted grouper and banded wrasse, and their natural assemblages. Environ. Biol. Fishes 95, (2012). 5