Projection of Interspecific Competition (PIC) Matrices: A Conceptual Framework for Inclusion in Population Risk Assessments

Accounting for intraspecific and interspecific competition when assessing the effects of chemical and nonchemical stressors is an important uncertainty in ecological risk assessments. We developed novel projection of interspecific competition (PIC) matrices that allow for analysis of population dynamics of two or more species exposed to a given stressor(s) that compete for shared resources within a landscape. We demonstrate the application of PIC matrices to investigate the population dynamics of two hypothetical fish species that compete with one another and have differences in net reproductive rate and intrinsic rate of population increase. Population status predictions were made under scenarios that included exposure to a chemical stressor that reduced fecundity for one or both species. The results of our simulations demonstrated that measures obtained from the life table and Leslie matrix of an organism, including net reproductive rate and intrinsic rate of increase, can result in erroneous conclusions of population status and viability in the absence of a consideration of resource limitation and interspecific competition. This modeling approach can be used in conjunction with field monitoring efforts and/or laboratory testing to link effects due to stressors to possible outcomes within an ecosystem. In addition, PIC matrices could be combined with adverse outcome pathways to allow for ecosystem projection based on taxonomic conservation of molecular targets of chemicals to predict the likelihood of relative cross-species susceptibility. Overall, the present study shows how PIC matrices can integrate effects across the life cycles of multiple species, provide a linkage between endpoints observed in individual and population-level responses, and project outcomes at the community level for multiple generations for multiple species that compete for limited resources. Environ Toxicol Chem 2024;43:1406-1422. Published 2024. This article is a U.S. Government work and is in the public domain in the USA.

A multidimensional density dependent matrix population model for assessing risk of stressors to fish populations

Modeling exposure and recovery of fish and wildlife populations after stressor mitigation serves as a basis for evaluating remediation success. Herein, we develop a novel multidimensional density dependent matrix population model that analyzes both size-structure and age class-structure simultaneously. This modeling approach emphasizes application in conjunction with field monitoring efforts (e.g., through effects-based monitoring programs) and/or laboratory analysis to link effects due to stressors to outcomes in populations. We applied the model to investigate Atlantic killifish (Fundulus heteroclitus) exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin with effects on fertility and survival rates. The Atlantic killifish is an important and well-studied model organism for understanding the effects of pollutants and other stressors in estuarine and marine ecosystems. For each exposure concentration, the corresponding plots of total population size, population size structure, and age structure over time were generated. The present study serves as an example of how a multidimensional matrix population model can integrate effects across the life cycle, provide a linkage between endpoints observed in the individual and ecological risk to the population as a whole, and project outcomes for multiple generations.

Niche-processes induced differences in plant growth, carbon balance, stress resistance, and regeneration affect community assembly over succession

The relationship between plant traits and species relative abundance along environmental gradients can provide important insights on the determinants of community structure. Here we bring extensive data on six key traits (specific leaf area (SLA), seed mass, seed germination rate, height, leaf proline content and photosynthesis rate) to test trait-abundance relationships in a successional chronosequence of subalpine meadow plant communities. Our results show that in late-successional meadows, abundant species had higher values for seed mass, seed germination rate, and SLA, but had lower values for height, photosynthesis rate, and leaf proline content than rarer species. The opposite patterns of trait-abundance relationships were observed for early-successional meadows. Observations of strong trait convergence and divergence in these successional communities lend greater support for niche processes compared to neutral community assembly. We conclude that species' niches that determine plant growth (plant height and photosynthesis rate), carbon balance (SLA, photosynthesis rate), regeneration (seed mass and seed germination rate), and abiotic stress resistance (leaf proline content) under different environmental conditions have strong influence on species relative abundance in these sub-alpine meadow communities during succession.

Detrimental effects of a novel flow regime on the functional trajectory of an aquatic invertebrate metacommunity

Novel flow regimes resulting from dam operations and overallocation of freshwater resources are an emerging consequence of global change. Yet, anticipating how freshwater biodiversity will respond to surging flow regime alteration requires overcoming two challenges in environmental flow science: shifting from local to riverscape-level understanding of biodiversity dynamics, and from static to time-varying characterizations of the flow regime. Here, we used time-series methods (wavelets and multivariate autoregressive models) to quantify flow-regime alteration and to link time-varying flow regimes to the dynamics of multiple local communities potentially connected by dispersal (i.e., a metacommunity). We studied the Chattahoochee River below Buford dam (Georgia, U.S.A.), and asked how flow regime alteration by a large hydropower dam may control the long-term functional trajectory of the downstream invertebrate metacommunity. We found that seasonal variation in hydropeaking synchronized temporal fluctuations in trait abundance among the flow-altered sites. Three biological trait states describing adaptation to fast flows benefitted from flow management for hydropower, but did not compensate for declines in 16 "loser" traits. Accordingly, metacommunity-wide functional diversity responded negatively to hydropeaking intensity, and stochastic simulations showed that the risk of functional diversity collapse within the next 4 years would decrease by 17% if hydropeaking was ameliorated, or by 9% if it was applied every other season. Finally, an analysis of 97 reference and 23 dam-affected river sites across the U.S. Southeast suggested that flow variation at extraneous, human-relevant scales (12-hr, 24-hr, 1-week) is relatively common in rivers affected by hydropower dams. This study advances the notion that novel flow regimes are widespread, and simplify the functional structure of riverine communities by filtering out taxa with nonadaptive traits and by spatially synchronizing their dynamics. This is relevant in the light of ongoing and future hydrologic alteration due to climate non-stationarity and the new wave of dams planned globally.

Functional traits associated with plant colonizing and competitive ability influence species abundance during secondary succession: Evidence from subalpine meadows of the Qinghai-Tibetan Plateau

It is widely recognized that colonists and competitors dominate early and late succession, respectively, with selected species having different colonizing and competitive abilities. However, it remains unknown whether colonizing and competitive ability can determine species abundance directly over succession. The data for five key functional traits were collected (photosynthesis rate, leaf turgor loss point, leaf proline content, seed mass, and seed germination rate), which are direct indicators of plant competitive and colonizing abilities including growth, drought and cold stress resistance, dispersal, and seed dormancy. Here, we tested the effects of colonizing and competitive abilities on species abundance, by employing a linear mixed-effects model to examine the shifts in the relationship between species abundance and these five colonization and competition-related traits in species-rich subalpine secondary successional meadows (at 4, 6, 10, 13 years of age, and undisturbed, respectively) of the Qinghai-Tibetan Plateau. The abundant species at the early-successional meadows tend to have high photosynthetic rate, high leaf proline content, low seed mass, and seed germination rate for having high colonizing ability, but low competitive ability. By contrast, late-successional communities tend to be dominated by species with high competitive ability, but low colonizing ability, indicated by large seeds, high seed germination rate, low photosynthetic rate, and leaf proline content. The observed directional shifts in the relationships between traits (photosynthetic rate, leaf proline content, seed mass, and seed germination rate) and abundance with successional age, bring two new understandings of community assembly during succession of subalpine meadows in the Qinghai-Tibetan Plateau. First, it discloses that the differences in species abundance over succession can be directly attributed to differences in colonizing and competitive abilities of different species. Second, it expands the effects of multiple life historical differences including growth, resource competitive ability, cold stress resistance, dispersal, and seed germination strategy, represented by functional traits on community assembly along succession, that is, from the species to the community level.

Remembrance of things past: modelling the relationship between species' abundances in living communities and death assemblages

Accumulations of dead skeletal material are a valuable archive of past ecological conditions. However, such assemblages are not equivalent to living communities because they mix the remains of multiple generations and are altered by post-mortem processes. The abundance of a species in a death assemblage can be quantitatively modelled by successively integrating the product of an influx time series and a post-mortem loss function (a decay function with a constant half-life). In such a model, temporal mixing increases expected absolute dead abundance relative to average influx as a linear function of half-life and increases variation in absolute dead abundance values as a square-root function of half-life. Because typical abundance distributions of ecological communities are logarithmically distributed, species' differences in preservational half-life would have to be very large to substantially alter species' abundance ranks (i.e. make rare species common or vice-versa). In addition, expected dead abundances increase at a faster rate than their range of variation with increased time averaging, predicting greater consistency in the relative abundance structure of death assemblages than their parent living community.

Detecting larval export from marine reserves

Marine reserve theory suggests that where large, productive populations are protected within no-take marine reserves, fished areas outside reserves will benefit through the spillover of larvae produced in the reserves. However, empirical evidence for larval export has been sparse. Here we use a simple idealized coastline model to estimate the expected magnitude and spatial scale of larval export from no-take marine reserves across a range of reserve sizes and larval dispersal scales. Results suggest that, given the magnitude of increased production typically found in marine reserves, benefits from larval export are nearly always large enough to offset increased mortality outside marine reserves due to displaced fishing effort. However, the proportional increase in recruitment at sites outside reserves is typically small, particularly for species with long-distance (on the order of hundreds of kilometers) larval dispersal distances, making it very difficult to detect in field studies. Enhanced recruitment due to export may be detected by sampling several sites at an appropriate range of distances from reserves or at sites downcurrent of reserves in systems with directional dispersal. A review of existing empirical evidence confirms the model's suggestion that detecting export may be difficult without an exceptionally large differential in production, short-distance larval dispersal relative to reserve size, directional dispersal, or a sampling scheme that encompasses a broad range of distances from the reserves.

Modeling impacts on populations: fathead minnow (Pimephales promelas) exposure to the endocrine disruptor 17beta-trenbolone as a case study

Evaluation of population-level impacts is critical to credible ecological risk assessments. In this study, a predictive model was developed to translate changes in fecundity of the fathead minnow (Pimephales promelas) in a short-term laboratory toxicity test to alterations in population growth rate. The model uniquely combines a Leslie population projection matrix and the logistic equation. Application of the model requires only a life table for the organism of interest, a measure of carrying capacity for the given population, and an estimation of the effect of a stressor on vital rates. The model was applied to investigate population dynamics for fathead minnow exposed to the androgen receptor agonist 17beta-trenbolone. Organismal-level responses for fathead minnows exposed to varying levels of 17beta-trenbolone were used to determine projected alterations in a population existing in a small body of water containing varying concentrations of the androgen. Fathead minnow populations occurring at carrying capacity and subsequently exposed to 0.027 microg/L of 17beta-trenbolone exhibited a 51% projected decrease in average population size after 2 years of exposure. Populations at carrying capacity exposed to concentrations of 17beta-trenbolone > or = 0.266 microg/L exhibited a 93% projected decrease in average population size after 2 years of exposure. Overall, fathead minnow populations exposed to continued concentrations of 17beta-trenbolone equal to or greater than 0.027 microg/L were projected to have average equilibrium population sizes that approached zero.

Fecundity and life-history strategies in marine invertebrates

The reproductive strategies of an organism play a major role in the dynamics of the population and the biogeography and continuity of the species. Numerous processes are involved in reproduction leading to the production of offspring. Although diverse processes are involved in oogenesis (the production of eggs) and spermatogenesis (the production of sperm), the basic patterns of gametogenesis are similar amongst invertebrates, with the proliferation and differentiation of germ cells leading to the final production of mature gametes. The production of gametes, especially eggs, is energetically expensive, and therefore strongly sensitive to selective pressures. An organism can ingest and assimilate a limited amount of energy from the environment. The different ways by which energy is allocated to growth and reproduction in order to maximize fitness forms the basis of the differing life-history strategies that have developed in marine invertebrates. Fecundity is defined as the number of offspring produced by a female in a determined time period. The term fecundity needs to be explicitly defined in each study in order to obtain the maximum information from the data analysed. Because of the variety of egg production patterns found among marine invertebrates, a wide range of methodologies has been developed to quantify fecundity. These include direct egg counts in brooding species, spawning induction in live individuals and histological studies of preserved material. Specific considerations need to be taken into account for colonial organisms, because of their modular organization. The production of eggs requires an optimal allocation of energy into growth and reproduction for the maximization of parental fitness. Fecundity is central in studies of life-history theory and in the development of life-history models because it is directly related to energy allocation and partitioning. There are important relationships and trade-offs between fecundity and other life-history traits, such as egg size, female size and age, age at first reproduction, reproductive effort and residual reproductive value. These trade-offs, together with morpho-functional constraints and genetic variation determine the evolution of life histories through natural selection. Fecundity is a highly plastic character within the limits defined by the bioenergetics and life-history strategy of the organism. Egg production is affected mainly by environmental factors such as food quantity and quality, temperature or presence of toxic elements in the habitat. The differences in fecundity found among closely related species from different biogeographical locations reflect, at least in part, the differing environmental conditions of their habitat.

The temporal variability of animal abundances: measures, methods and patterns

From first principles, the temporal variability of a time series of abundances can be defined as the average deviation of values from a mean value on a proportional scale. In this paper we review: (i) the different kinds of temporal variability; (ii) the different ways in which it can be measured; (iii) the design of appropriate sampling schemes; (iv) methods of analysing variability; and (v) patterns in temporal variability. We emphasize that some commonly applied measures are not appropriate, that several do not measure the desired feature of time series, and the importance of considerations of trend and sampling error. A number of suggestions are made for the improvement of the basis for comparative analyses of levels of variability, and some of the potential pitfalls are identified. Given the serious faults in many previous analyses of ecological patterns in the temporal variability of animal abundances, emphasis is laid on the theoretical basis for different patterns, and hence a set of hypotheses for testing is generated.

The equilibrium concept and density dependence tests What does it all mean?

Tests of density dependent regulation of population size depend on the concept of equilibrium population size. Such an equilibrium is a purely theoretical construct whose existence in the field is debatable and whose value cannot be measured. An equilibrium is supposed to fluctuate in time, but the extent of the fluctuations relative to those of the population size is unknowable. It is impossible to separate a fluctuating population size from a fluctuating equilibrium value and from fluctuating deviations from an equilibrium value. Because it cannot be determined whether a given population size is above, at, or below equilibrium, the course of population size in unpredictable and density dependence tests cannot be expected to produce useful results. Stabilization tests may provide a more useful alternative.

The effect of temporal environmental heterogeneity on community structure: a replicated experimental study

Periodicity, predictability and stochasticity of environmental perturbations are shown to influence the community structure that develops in microcosms. Sets of replicate, microalgal communities were subjected to different temporal patterns of rarefaction and resource resupply and their species-abundance patterns after 120 days of such manipulations were determined. Perturbations having, 1, 7, and 28 day periodicities differentially effected community structure. The predictability of these perturbations had a less profound influence on the communities which developed than the average perturbation periodicity.

Survivorship patterns in an annual plant community

Age-specific survivorship was studied in a number of plant species which occur in a distinct, predominantly annual plant community of Texas, in association with the activities of pocket gophers. Of eleven species analyzed in detail, all functioned as annuals. Contrary to theoretical expectations, the species had low rates of juvenile mortality, and did not exhibit the demographic specializations of weedy or colonizing species. It is suggested that the annual habit in this case may be adaptive in sites with continual disturbance, such as that due to pocket gopher activity.

The meanings of r- and K-selection

This paper catalogues several different dichotomies that have all been termed r- and K-selection. The status of the concept of r- and K-selection is discussed and a more restricted usage of the terms is recommended.