Biology subjects

Iritani, R.

Publications and source records attributed to Iritani, R..

14 recordsLinked to original sources

bioRxiv2026

How do variable patch sizes hold higher beta diversity?

Beta diversity is a fundamental component of regional biodiversity that accumulates from compositional differences between local species assemblages. Although beta diversity is increasingly applied across spatial scales, its scale dependence remains incompletely synthesized, particularly concerning variation in spatial scale used to define alpha diversity (grain size). Using tree community data from Barro Colorado Island, we find that both additive and multiplicative beta diversity are consistently higher for unequal than equal grain divisions across different spatial extents and numbers of grains. These results can be explained by Jensen's inequality based on the concave-down relationship of alpha diversity with area that in turn increases beta diversity through the scaling relationship between alpha and gamma diversity. Combined with our additional analysis showing that this prediction is robust to more general models of species richness, including species--area and species--sampling-effort curves, these results reveal a previously unrecognized, predictable range of beta diversity variation.

ecology↗

bioRxiv2026

Evolutionary invasion analysis for structured populations: a synthesis

Natural populations exhibit complex class structures that fundamentally shape evolutionary dynamics. Yet predicting adaptation in such systems remains a central challenge in ecology because ecological realism inherently generates high-dimensional models that obscure biological interpretation and hinder general evolutionary prediction. Here, we overcome this long-standing obstacle by developing structural evolutionary invasion analysis, a unifying theoretical framework integrating invasion determinants with the Projected Next-Generation Matrix (PNGM). The invasion determinant provides a general, closed-form algebraic condition for invasion, while the PNGM structurally compresses life-cycle graphs by eliminating non-focal classes. Crucially, this compression formally derives timescale separation, rigorously preserves Fishers reproductive values, and guarantees that the location, convergence, and evolutionary stability of equilibria remain identical to those of the full ecological system. By applying the framework to examples spanning evolutionary epidemiology, stage structure, and inclusive fitness, we show how to capture the direct impact of ecological complexity on evolutionary outcomes while retaining analytical tractability. Our results establish a general principle linking ecological structure to evolutionary prediction, enabling ecologists to analyze adaptation and eco-evolutionary dynamics in heterogeneous populations.

evolutionary biology↗

bioRxiv2025

Revealing Long-Term Multi-Factor Climate Impacts on Antarctic Phytoplankton: A Trend-Based Approach Using STL and Piecewise SEM

Climate change imposes multiple interacting stressors on ocean ecosystems. The Southern Ocean is among the regions where these impacts are particularly pronounced. Phytoplankton, which perform the bulk of primary production, both maintain the Antarctic marine food web and contribute to the role of the Southern Ocean as the largest oceanic sink for atmospheric CO2. Previous observational and modeling studies have revealed certain drivers-phytoplankton relationships and highlighted the important confounding factors. In this study, we quantified how climate and environmental drivers interact and influence phytoplankton biomass over a 20-year period, across the entire Southern Ocean, within a multi-factor framework. Specifically, we examined the relationships among atmospheric CO2, surface water pH, sea ice concentration, sea surface temperature, mixed layer depth, dissolved iron concentration, and chlorophyll a concentration. To perform this analysis, we introduced and applied two novel methodological approaches. Firstly, to address seasonal autocorrelation, we combined seasonal trend decomposition using LOESS (STL) with piecewise structural equation modeling (SEM). This approach allowed for robust estimation of long-term driver magnitudes and causal pathways, even in the presence of limited sample sizes and complex data structures. Secondly, given the highly interconnected nature of environmental drivers, we employed a data-driven SEM approach to objectively identify causal structures and avoid biases associated with a priori pathway selection. Our results show a clear signal of ocean acidification driven by increasing CO2, iron limitation of phytoplankton biomass during spring, and seasonal differences in both the magnitude and structure of the causal pathways, demonstrating the dynamic nature of seasonal processes in the Southern Ocean. This study presents the first Southern Ocean-wide, multi-factor assessment of climate and environmental impacts on phytoplankton biomass over a two-decade period. It introduces a methodological framework mitigates seasonal autocorrelation to accurately capture long-term seasonal trends, and demonstrates the utility of data-driven causal discovery in resolving complex ecological interactions.

ecology↗

bioRxiv2025

The evolution of host utilization strategy: At what timing parasitoids attack and consume their host

Parasitoid wasps exhibit remarkable diversity in life-history traits and are categorized into two major groups based on the timing of emergence: those that begin consuming host tissues immediately after hatching ("idiobiont") and those that can delay the emergence depending on host maturation ("koinobiont"). Although delayed emergence allows parasitoids to exploit unparasitized, young hosts, it incurs a mortality cost during the waiting period. While numerous empirical studies have examined the adaptive significance of this trait, how host stage-structure and ecological factors jointly shape the evolution of emergence timing remains poorly understood in a formalized context. Here, we develop a stage-structured mathematical model to analyze the evolutionary dynamics of delayed emergence and its consequences for host exploitation. By explicitly incorporating host developmental stages, our model explores the joint evolution of two traits: the preference for young versus old host, and the timing of emergence. Our results reveal that the optimal strategy is determined by the relative reproductive value of host stages, which depends on the balance between host growth and mortality. We demonstrate that delayed emergence evolves when the future value of a growing host outweighs the immediate cost of mortality, thereby driving the divergence between idiobiont and koinobiont strategies. These findings provide a theoretical framework that unifies the diversity of parasitoid life histories. Significance StatementParasitoids exhibit striking diversity in life-history traits such as body size, fecundity, and clutch size, reflecting adaptations to host exploitation. A key behavioral distinction is whether parasitoids consume the host immediately (idiobionts) or can delay emergence while the host continues to grow (koinobionts). Many empirical studies have suggested and tested some hypothesis. On the other hand, the adaptive significance of delayed emergence have not been formulated yet, so the quantitative relationship between selection pressure and life history traits is yet to be better understood. Here, we develop a mathematical model that integrates host growth with parasitoid emergence timing. Our analysis supports the adaptive significance of delayed emergence and shows how delayed emergence can promote diversification of parasitoid life-history strategies. These results suggest that delayed emergence is a key driver of idiobiont-koinobiont dichotomy, providing new insights into the evolutionary basis of parasitoid-host interactions and aligning directly with the central aims of behavioral ecology.

ecology↗

bioRxiv2024

How can interspecific pollen transfer affect the coevolution and coexistence of two closely related plant species?

Interspecific pollen transfer (IPT), the movement of pollen grains between different plant species by sharing pollinators, incurs costs (fitness reduction) for seed production. IPT thereby reduces the reproductive success of co-flowering plants sharing pollinators, thus preventing their coexistence. However, the impact of IPT on the evolutionary dynamics and evolution-mediated ecological dynamics of sex allocation resource investment to pollen versus ovules) is poorly understood. Here, we investigate the consequences of the female costs incurred by IPT for the co-evolution and coexistence of two plants, by using a mathematical model where two plant species interact with each other via resource competition, pollen movements within and between species, and reduced fertilization due to IPT. The ecological situation we consider here is that an invasive species with female-biased sex allocation immigrates into a habitat of a resident species whose sex allocation is evolutionarily maintained at Fisherian sex allocation (FSA). By using adaptive dynamics theory, we found that regardless of the strength of IPT, natural selection favours the equal allocation to pollen grains and ovules (FSA) for both species. If the mutual impact of IPT on two species is similar in magnitude, we find that the eco-evolutionary dynamics can lead to their stable coexistence. In contrast, when only the invasive species negatively impacts the resident species through IPT, the evolution in invasive species from female-biased sex allocation to FSA causes the extinction of the resident species. Given that local mate competition in small populations is expected to result in female-based sex allocation, our finding suggests that if invasive species are relaxed from local mate competition, they may drive the resident species to extinction. Our study highlights the importance and complexity of the evolution of biased sex allocation driven by IPT to understand the coexistence of closely related plant species.

ecology↗

bioRxiv2023

Estimation of the expectation of species richness difference of two-subcommunities from site-suitability difference in a species-independent stochastic model

Understanding how community assembly during the initial faunization phases determines the difference in species richness is a fundamental question in ecology. However, there are few predictions for when and to what degree the differences in species richness between subcommunities will emerge. Here, we investigate the expectation of richness difference in a pair of subcommunities, assuming that species may have different and independent presence probabilities. We then introduce several indices and examine how expected richness difference is determined by the indices. We found that (i) species differences (the average of species presence probabilities in two subcommunities divided by the total presence probability) have inconsistent effects on richness difference; (ii) the degree of spatial heterogeneity (average of differences in species presence probabilities in two subcommunities across species) may, but not always, have a good predictive ability for richness difference; and (iii) the absolute difference in average presence probabilities (site-suitability difference) has robust predictive ability for richness difference unless richness difference is very small. This work provides a theoretical framework for predicting and analyzing species richness difference from presence-absence data based on null models.

ecology↗

bioRxiv2022

Jaccard dissimilarity in stochastic community models based on the species-independence assumption

A fundamental problem in ecology is understanding the causes of change in species composition among sites (i.e. beta diversity). At present it is unclear how spatial heterogeneity in species occupancy across sites shapes these patterns. To address this question, we develop probabilistic models that consider two spatial or temporal sites, where presence probabilities vary both among species and between the sites. We derive analytical and approximate formulae for the expectation of pairwise beta-diversity. Using a novel graphical tool, Stochastic Incidence Plots (SIPs), which depict the presence probabilities in two sites across species labels, we develop a means to conceptualize the heterogeneity in presence probabilities: the steepness or unevenness of SIPs reflects species-level heterogeneity, while the degree of overlap between SIPs indicates site-level heterogeneity. Utilizing SIPs and a combinatorial approach in a two-species scenario, we demonstrate that beta-diversity is lower when SIPs are parallel compared to when they are anti-parallel. We also find that this prediction is testable with the well-known checkerboard pattern in incidence matrices. Finally, we applied the method to the species distribution models for five woodpecker species in Switzerland, showing that their spatial distributions will change significantly in the future. Overall, this work improves our understanding of how pairwise beta-diversity responds to occupancy heterogeneity.

ecology↗

bioRxiv2022

Species interactions and diversity: a unified framework using Hill numbers

Biodiversity describes the variety of organisms on planet earth. Ecologists have long hoped for a synthesis between analyses of biodiversity and analyses of biotic interactions among species, such as predation, competition, and mutualism. However, it is often unclear how to connect details of these interactions with complex modern analyses of biodiversity. To resolve this gap, we propose a unification of models of biotic interactions and measurements of diversity. We show that analyses of biodiversity obscure details about biotic interactions. For example, identical changes in biodiversity can arise from predation, competition or mutualism. Our approach indicates that traditional models of community assembly miss key facets of diversity change. Instead, we suggest that analyses of diversity change should focus on partitions, which measure mechanisms that directly shape changes in diversity, notably species level selection and immigration, rather than traditional analyses of biotic interactions. SpeculationsOur paper proposes that observations of biodiversity cannot be used to distinguish different types of biotic interactions. For generations ecologists have been fascinated with the links between biodiversity and biotic interactions (i.e. competition, mutualism and predation). Many of us expect that observations of biodiversity provide vital clues about how biotic interactions operate in nature, but it is hard to tell when these clues are reliable. Our work integrates models of biotic interactions and measurements of biodiversity diversity change. This highlights how an observed change in diversity can be compatible with any type of biotic interaction (i.e. competition, mutualism, predation etc.). So for example, the same increase in Shannon diversity could indicate the superior ability of a competitor the success of mutualists or a predators tendency to harvest dominant prey species. This is so because diversity measures are designed to be concerned with changes in relative abundances but not account for absolute abundance changes. Observations of biodiversity change are unlikely to yield insights about biotic interactions per se because biodiversity itself obscures species absolute abundances. Therefore, models of diversity change should focus on mechanisms that are less influenced by changes in absolute abundances such as species-level selection.

ecology↗

bioRxiv2021

Partitioning the temporal changes in abundance-based beta diversity into loss and gain components

O_LIEcologists have long recognized that the losses and gains in local species abundances can either decrease or increase spatial beta diversity, phenomena often referred to as biotic homogenization and differentiation, respectively. However, quantifying such dynamic impacts of species abundances on beta diversity has remained a methodological challenge. C_LIO_LIHere, we develop a numerical method to additively partition the temporal changes in beta diversity into distinct components that reflect the losses and gains in local species abundances. Our method is based on Ru[z]i[c]ka and Bray-Curtis indices and the normalized abundance-based Whittakers beta diversity. The temporal changes in these measures are partitioned into components that represent biotic homogenization and differentiation driven by abundance losses and gains at both species and community levels. C_LIO_LIApplication of the method to a Swedish fish community dataset revealed decreases in beta diversity between 1990 and 2018. The homogenization of fish communities was explained by gains, but not losses, in species abundances across sites. Species-level partitioning further showed that the homogenization was largely caused by the increased population sizes of a particular species in sites where it was already present. C_LIO_LIThe results highlight that our partitioning method effectively identifies local population and community processes embedded in regional biodiversity patterns. We believe that explicit analyses of the losses and gains in species abundances should bring deeper insights into the dynamics of beta diversity. C_LI

ecology↗

bioRxiv2020

Solving the sex ratio scandal in Melittobia wasps

The scandalous sex ratio behaviour of Melittobia wasps has long posed one of the greatest problems for the field of sex allocation. In contrast to the predictions of theory, and the behaviour of numerous other organisms, laboratory experiments have found that Melittobia females do not produce less female-biased offspring sex ratios when more females lay eggs on a patch. We resolve this scandal, by showing that, in nature, females of M. australica have sophisticated sex ratio behaviour, where their strategy also depends upon whether they have dispersed from the patch where they emerged. When females have not dispersed, they will be laying eggs with close relatives, which keeps local mate competition high, even with multiple females, and so they are selected to produce consistently female-biased sex ratios. Laboratory experiments mimic these conditions. In contrast, when females disperse, they will be interacting with non-relatives, and so they adjust their sex ratio depending upon the number of females laying eggs. Consequently, females appear to use dispersal status as an indirect cue of relatedness, and whether they should adjust their sex ratio in response to the number of females laying eggs on the patch.

evolutionary biology↗

bioRxiv2020

Cooperative interactions among females can lead to even more extraordinary sex ratios

Hamiltons local mate competition theory provided an explanation for extraordinary female biased sex ratios in a range of organisms. When mating takes place locally, in structured populations, a female biased sex ratio is favoured to reduce competition between related males, and to provide more mates for males. However, there are a number of wasp species where the sex ratios appear to more female biased than predicted by Hamiltons theory. We investigated theoretically the extent to which cooperative interactions between related females can interact with local mate competition to favour even more female biased sex ratios. We found that: (i) cooperative interactions between females can lead to sex ratios that are more female biased than predicted by local competition theory alone; (ii) sex ratios can be more female biased when the cooperative interactions are offspring helping parents before dispersal, rather than cooperation between siblings after dispersal. Our results can be applied to a range of organisms, and provide an explanation for the extreme sex ratio biases that have been observed in Sclerodermus and Melittobia wasps.

ecology↗

bioRxiv2020

Eco-evolutionary dynamics of prior selfing rate promotes the coexistence without niche partitioning under reproductive interference

O_LIWhen the two or more plants species share the same pollinators, pollinator-mediated reproductive interference make coexistence difficult. Recent studies suggested prior autonomous selfing mitigate reproductive interference, could enabling coexistence without pollination niche partitioning (pre-emptive selfing hypothesis). However, there are no studies to test whether evolution of prior selfing promote the coexistence, considering eco-evolutionary dynamics of population size, selfing rate and inbreeding depression. C_LIO_LITo examine conditions that the evolution of prior selfing promote coexistence under mutual reproductive interference especially in the point of view for pollinator availability and dynamics of inbreeding depression, we constructed individual-based model in which two plant species compete against each other in the form of mutual reproductive interference and can evolve prior autonomous selfing rate. We expected that purging of deleterious mutations could cause evolutionary rescue because inferior species could rescue population density through the evolution of prior selfing if the strength of inbreeding depression decreases with an increase of populations selfing rate. C_LIO_LIOur simulation demonstrated that the evolution of prior selfing could promote the coexistence while reproductive interference caused competitive exclusion without evolution. We found that lower pollinator availability tended to prefer rapid evolutionary shift to higher prior selfing rate, it neutralizes the negative effect of reproductive interference, and population dynamics exhibit neutral random walk in both species. When the strength of inbreeding depression decreased with an increase in populations selfing rate, moderate pollinator availability resulted in long-term coexistence in which relative-abundance-dependent selection on the prior selfing rate rescue population density of inferior species intermittently. C_LIO_LISynthesis. We showed that the evolution of prior selfing could increase population growth rate of inferior species and consequently enable the long-term coexistence with evolutionary rescue. This is the new mechanisms explaining co-evolutionary coexistence of closely related plant species without niche partitioning and consistent with recent studies reported that closely related mixed-mating species are sympatrically growing even under the mutual reproductive interference. C_LI

ecology↗

bioRxiv2018

When parasites are selected to kill the young

The impact of infectious disease is often very different in juveniles and adults, but theory has focused on the drivers of stage-dependent defense in hosts rather than the potential for stage-dependent virulence evolution. Stage-structure has the potential to be important to the evolution of pathogens because it exposes parasites to heterogeneous environments in terms of both host characteristics and transmission routes. We develop a stage-structured (juvenile-adult) epidemiological model and examine the evolutionary outcomes of stage-specific virulence under the classic assumption of a transmission-virulence trade-off. We show that selection on virulence against adults remains consistent with the classic theory. However, the evolution of juvenile virulence is sensitive to both demography and transmission pathway with higher virulence against juveniles being favored either when the transmission pathway is assortative (juveniles preferentially interact together) and the juvenile stage is short, or in contrast when the transmission pathway is disassortative and the juvenile stage is long. These results highlight the potentially profound effects of host stage-structure on determining parasite virulence in nature. This new perspective may have broad implications for both understanding and managing disease severity. Impact summaryUnderstanding the evolution of parasite virulence remains one of the most important questions in evolutionary ecology. Virulence is often very different in young and old hosts, but previous theory has presumed that these differences are attributed to adaptation in host defense rather than parasite adaptation. However, stage-structure within host populations can expose parasites to heterogeneous environments, which may lead to differential selection on parasite virulence (stage-specific virulence). Surprisingly, no study has investigated the effects of hosts stage-structure on the evolution of stage-specific virulence. We present a theoretical analysis to examine when selection can favor higher virulence against juveniles (juvenile-virulence) versus adults (adult-virulence). Our key result is that higher juvenile-virulence is selected for either when the transmission is assortative within age classes and maturation is slow, or when the transmission is disassortative (occurring predominantly between-classes) and maturation is relatively fast. These at first sight contrasting outcomes can be understood as adaptation to the exploitation of the more available host stage. Although the data on assortativity in infectious disease systems is limited, empirical studies for the virulence of Great Island Virus in guillemots (Uria aalge) and for salmon louse in pink salmon (Oncorhynchus gorbuscha) are consistent with our predictions. Our work provides testable predictions for stage-specific virulence and presents a novel mechanism that may explain variation in virulence in nature. There are also management implications for conservation, public health, vaccination programs, and farming to understanding the drivers of stage dependent virulence.

ecology↗

bioRxiv2018

Reproductive interference hampers species coexistence despite conspecific sperm precedence

Negative interspecific mating interactions, known as reproductive interference, can hamper species coexistence in a local patch and promote niche partitioning or geographical segregation of closely related species. Conspecific sperm precedence (CSP), which occurs when females that have mated with both conspecific and heterospecific males preferentially use conspecific sperm for fertilization, might contribute to species coexistence by mitigating the costs of interspecific mating and hybridization. We examined whether two closely related species exhibiting CSP can coexist in a local environment in the presence of reproductive interference. First, using a behaviourally explicit mathematical model, we demonstrated that two species characterized by negative mating interactions are unlikely to coexist because the costs of reproductive interference, such as loss of mating opportunity with conspecific partners, are inevitably incurred when individuals of both species are present. Second, we experimentally demonstrated differences in mating activity and preference in two Harmonia ladybird species known to exhibit CSP. According to the developed mathematical model of reproductive interference, these behavioural differences should lead to local extinction of H. yedoensis because of reproductive interference by H. axyridis. This prediction is consistent with field observations that H. axyridis uses various food sources and habitats whereas H. yedoensis is confined to a less preferred prey item and a pine tree habitat. Finally, by a comparative approach, we showed that niche partitioning or parapatric distribution, but not sympatric coexistence in the same habitat, is maintained between species with CSP belonging to a wide range of taxa, including vertebrates and invertebrates living in aquatic or terrestrial environments. Taken together, these results lead us to conclude that reproductive interference generally destabilizes local coexistence even in closely related species that exhibit CSP.

ecology↗