Takefumi Nakazawa

Fishing effects on age and spatial structures undermine population stability of fishes

Overfishing has caused dramatic changes in structures of exploited populations as well as ecosystems. In this article, we focus on fishing effects on age (size) and spatial structures of exploited fishes. Accumulating evidence has shown that large and experienced spawning individuals are able to produce higher quality and quantity of eggs, known as maternal effects, and that individuals of different age classes tend to spawn in different locations and times. These behaviors are associated with a healthy age structure and contribute to bet-hedging capacity that is important in smoothing out short-term environmental variability. Here, we document a widespread phenomenon of age (size)-truncation of exploited populations driven by size-selective fishery removals. Such size-selective fishing may have evolutionary consequence and may be difficult to reverse. In addition, fishing often reduces population spatial heterogeneity that also contributes importantly to bet-hedging. We review studies showing that the effects of age truncation and reduction of spatial heterogeneity have reduced resilience and elevated the fluctuation amplitude of exploited populations facing a changing environment. Recent analyses indicated that fish populations often exhibit nonlinear nature and have potential to shift dramatically in a short time. All the evidence suggests that fishing, by altering age or spatial structures, may make exploited fishes, more prone to catastrophic shifts. Therefore, to achieve sustainable fisheries, management should conserve the age and spatial structure in addition to viable spawning biomass.

Is the relationship between body size and trophic niche position time-invariant in a predatory fish? First stable isotope evidence.

Characterizing relationships between individual body size and trophic niche position is essential for understanding how population and food-web dynamics are mediated by size-dependent trophic interactions. However, whether (and how) intraspecific size-trophic relationships (i.e., trophic ontogeny pattern at the population level) vary with time remains poorly understood. Using archival specimens of a freshwater predatory fish Gymnogobius isaza (Tanaka 1916) from Lake Biwa, Japan, we assembled a long-term (>40 years) time-series of the size-dependence of trophic niche position by examining nitrogen stable isotope ratios (δ 15N) of the fish specimens. The size-dependence of trophic niche position was defined as the slope of the relationship between δ 15N and log body size. Our analyses showed that the slope was significantly positive in about 60% of years and null in other years, changing through time. This is the first quantitative (i.e., stable isotope) evidence of long-term variability in the size-trophic relationship in a predatory fish. This finding had implications for the fish trophic dynamics, despite that about 60% of the yearly values were not statistically different from the long-term average. We proposed hypotheses for the underlying mechanism of the time-varying size-trophic relationship.

Scale Dependence of Predator-Prey Mass Ratio. Determinants and Applications

Abstract Body size exerts a critical influence on predator–prey interactions and is therefore crucial for understanding the structure and dynamics of food webs. Currently, predator–prey mass ratio (PPMR) is regarded as the most promising modelling parameter for capturing the complex patterns of feeding links among species and individuals in a simplified way. While PPMR has been widely used in food-web modelling, its empirical estimation is more difficult, with the methodology remaining controversial. This is because PPMR (i) may be defined at different biological scales, such as from individuals to communities, and (ii) may also vary with biological factors, such as species identity and body mass, both of which conflict with the conventional model assumptions. In this chapter, we analyse recently compiled gut content data of marine food webs to address the two fundamental issues of scale-dependence and determinants of PPMR. We consider four definitions of PPMR: (i) species-averaged PPMR, (ii) link-averaged PPMR, (iii) individual-predator PPMR, and (iv) individual-link PPMR. First, we show that PPMR values have a complicated scale-dependence characterised by data elements, such as body mass and sample counts of predators and prey, due to averaging and sampling effects. We subsequently used AIC to systematically evaluate how the four types of PPMR are related to predator species identity and body mass. The results indicate that the model providing the best explanation for individual-predator and individual-link PPMRs incorporates both species identity and body mass. Meanwhile, the best model for species-averaged and link-averaged PPMRs was unclear, with different models being selected across sampling sites. These results imply that the size-based community-spectrum models describing individual-level interactions should include taxonomic dissimilarities. Based on the present study, we suggest that future research regarding PPMR must account for scale dependence and associated determinants to improve its utility as a widely applicable tool.

Ontogenetic niche shifts matter in community ecology: a review and future perspectives

Almost all organisms on Earth exhibit ontogenetic niche shifts, which causes great phenotypic variation among individuals and is thus considered to critically mediate community structure and dynamics. In contrast, community ecology has traditionally assumed that species are composed of identical individuals with invariant traits and ignored the potentially important ecological roles of ontogenetic niche shifts. To bridge the gap, here I briefly review ecologically relevant examples which show that basic insights of species-based community theories can be revised by including the ontogenetic perspective. Specifically, I focus on the most representative animals in the study of ontogenetic niche shifts, i.e., fish, insects, and amphibians. Notably, their ontogenetic niche shifts create novel views of community structure: (1) ontogenetic diet shifts of predatory fish couple pelagic and benthic food webs in aquatic systems, (2) ontogenetic shifts in interaction types of pollinating insects couple herbivory and pollination networks in terrestrial systems, and (3) ontogenetic habitat shifts of amphibians and aquatic insects couple aquatic and terrestrial metacommunities at interface areas. Dynamic models of such stage-structured communities suggest that their ontogenetic niche shifts may affect the community resilience and disturbance responses. Exploring more complex systems (e.g., where many species undergo ontogenetic niche shifts several times or continuously) is a future direction, for which describing body size relationships between interacting organisms would be a promising approach. I conclude that both theoretical and empirical advances are needed to facilitate the ontogenetic perspective for better understanding mechanisms underlying biodiversity and ecosystem functioning which are increasingly threatened by anthropogenic disturbance.

Analysis of species coexistence co-mediated by resource competition and reproductive interference

Reproductive interference is any interspecific sexual interaction that reduces the reproductive success of females through promiscuous reproductive activities of heterospecific individuals. This phenomenon is ubiquitous in nature in both plants and animals, and is frequently observed in biological invasions. However, its effects on interspecific competition remain incompletely understood despite growing concern. To study the interactive effects of resource competition and reproductive interference on species coexistence and exclusion, we analyzed a unified competition model including both processes in symmetric and asymmetric scenarios. The results of our model showed that resource competition and reproductive interference act synergistically to promote competitive exclusion. We also found that when the two processes are asymmetric, the species that is superior in reproductive interference can coexist with or even exclude the species that is superior in resource competition. Therefore, coexistence is possible via an unbalanced trade-off between resource use and reproduction. Our results suggest that integration of reproductive interference and resource competition will contribute to a better understanding of interspecific competition and to more effective biodiversity conservation against management of biological invasions.

Predator–prey body size relationships when predators can consume prey larger than themselves

As predator–prey interactions are inherently size-dependent, predator and prey body sizes are key to understanding their feeding relationships. To describe predator–prey size relationships (PPSRs) when predators can consume prey larger than themselves, we conducted field observations targeting three aquatic hemipteran bugs, and assessed their body masses and those of their prey for each hunting event. The data revealed that their PPSR varied with predator size and species identity, although the use of the averaged sizes masked these effects. Specifically, two predators had slightly decreased predator–prey mass ratios (PPMRs) during growth, whereas the other predator specialized on particular sizes of prey, thereby showing a clear positive size–PPMR relationship. We discussed how these patterns could be different from fish predators swallowing smaller prey whole.

A minimum model of prey-predator system with dormancy of predators and the paradox of enrichment

In this paper, a mathematical model of a prey-predator system is proposed to resolve the paradox of enrichment in ecosystems. The model is based on the natural strategy that a predator takes, i.e, it produces resting eggs in harsh environment. Our result gives a criterion for a functional response, which ensures that entering dormancy stabilizes the population dynamics. It is also shown that the hatching of resting eggs can stabilize the population dynamics when the switching between non-resting and resting eggs is sharp. Furthermore, the bifurcation structure of our model suggests the simultaneous existence of a stable equilibrium and a large amplitude cycle in natural enriched environments.

Ontogenetic niche shift, food-web coupling, and alternative stable states

Several recent studies have shown that food web coupling by ontogenetic niche shifts can generate alternative stable states (ASS). However, these studies mainly considered cases where juvenile and adult stages are the top level consumers. The conditions under which ASS occur in more structurally diverse food web configurations have not been explored. In this study, I examine the influence of food-chain length and the trophic positions of juveniles and adults on the existence of ASS. Comprehensive model analysis showed that if both juveniles and adults are top predators, ASS are possible irrespective of their trophic level, because of overcompensation in reproduction and maturation due to strong density dependence, as previously predicted. However, the following potential food-web effects were found: ASS potential (1) disappears if either or both the juveniles and adults have a predator and (2) is once again observed if another predator is added on the stage-specific predator. These mechanisms involve (1) top–down control that relaxes intrastage food competition and (2) top–down cascade that intensifies the intrastage competition, respectively. Furthermore, it was illustrated that the environmental conditions under which ASS occurred varied in complex ways with the coupled food-web configurations. My results provide a novel concept that anthropogenic changes in local community structure (e.g., species extinction and invasion) propagate through space and may cause or prevent regime shifts in broad-scale community structure by altering the resilience to environmental perturbations.

The ontogenetic stoichiometric bottleneck stabilizes herbivore–autotroph dynamics

In the ecological stoichiometry theory of population dynamics, ontogenetic changes in nutrient demand have been ignored. Here, I studied a stage-structured Daphnia–algae herbivore–autotroph model, in which the juveniles of the herbivore had a higher nutrient (phosphorous) demand for maturation than the adults for reproduction. The model predicted that while an increase in the juvenile nutrient demand (i.e., ontogenetic stoichiometric bottleneck) affects stage-specific performances in complex ways through nutrient dynamics and resource quality, in general it has stabilizing effects on the population dynamics.

Long-Term Changes in the Diet of Gymnogobius isaza from Lake Biwa, Japan: Effects of Body Size and Environmental Prey Availability

Body size and environmental prey availability are both key factors determining feeding habits of gape-limited fish predators. However, our understanding of their interactive or relative effects is still limited. In this study, we performed quantitative dietary analysis of different body sizes of goby (Gymnogobius isaza) specimens collected from Lake Biwa between 1962 and 2004. First, we report that the diet was composed mainly of zooplankton (cladocerans and copepods) before the 1980s, and thereafter, shifted to zoobenthos (gammarids). This foraging shift coincided with, and thus can be linked to, known historical events in the lake at that time: decrease in zooplankton abundance with the alleviation of eutrophication, increase in fish body size resulting from fish population collapse, and increase in gammarid abundance due to reduced fish predation pressure. Supporting this view, our data analyses revealed how the long-term changes in the diet composition would be co-mediated by changes in fish body size and environmental prey availability. Specifically, while zoobenthos abundance strongly affected the fish diet composition, larger (smaller) fish preferred zoobenthos (zooplankton). Furthermore, the body size effects were stronger than those of prey availability. These results provide the best long-term evidence that fish feeding habits vary over decades with its body size and prey community due to anthropogenic disturbances.