Forager Populations Research Articles

Discrete Population Competition Model of Poisonous Weed Invasion and Spatial Distribution

Poisonous weeds invade ecosystems and compete with edible forage grassland, changing the population spatial distribution. In this paper, a discrete population competition model for the interaction between poisonous weeds and edible forage grass is established, and a diffusion coefficient to describe the invasion and diffusion effect of poisonous weeds is introduced. By using cellular automata methods, the influence of invasive diffusion of poisonous weeds on the spatial distribution of poisonous weed and edible forage populations is simulated. These findings indicated that the invasion of poisonous weeds affected the ecological distribution of grassland, and that the spatial distribution of poisonous weeds also changed from initial uniform distribution to aggregated distribution.

Static environments with limited resources select for multiple foraging strategies rather than conformity

AbstractSocial learning and conformity, or positive frequency‐biased transmission, are important components of the evolution of culture and group identities in animals and humans. Previous theoretical work on the evolution of social learning and conformity when environmental states change found that conformity evolves for a large range of environmental conditions. However, the foraging environment can affect the adaptive benefits of social learning and conformity. Conformity can cause a population to focus mainly on one resource while ignoring other available resources, which can incite competition if the first resource is limited. We study the evolution of social learning and frequency‐biased transmission in a limited‐resource environment using a replicator model of a forager population feeding on two resources in which foragers set their resource preferences during an early learning stage. In sharp contrast with previous models, anti‐conformity, or negative frequency‐biased transmission, rather than conformity, evolves from a population of non‐conformists under most environmental conditions. Numerical simulations suggest that both social learning and conformity cause the foragers to favor one resource over the other even though the resources provide the same benefit to foragers. Resource limitation favors anti‐conformity because anti‐conformity tends to force both resources to be exploited equally. Similarly, depletion of resources selects against social learning. However, increasing social learning is adaptive when independent discovery of foraging cues is difficult because it increases the rate at which individuals learn to find food. Consequently, in an environment with difficult‐to‐learn food cues, increased social learning is more likely to evolve. However, social learning does not emerge from a population of individual learners. Our results show that competition and consumer‐resource interactions can alter the evolutionary course of social learning and transmission bias. As there are many examples of conformity in the animal kingdom, and especially in humans, future studies should explore what extensions of the model would allow conformity to first evolve.

The Rødhals kitchen midden – marine adaptations at the end of the Mesolithic world

The Rødhals kitchen midden was located on a tiny stretch of land 18 km from the nearest major landmass in present-day Denmark. It dates to the Mesolithic-Neolithic transition, roughly 4300 to 3700 cal BC. Its inhabitants practiced a remarkably broad-scale exploitation of marine resources spanning from the collecting of mollusks on the sea-shore, over open-sea fowling and deep-water angling to the killing of small whales. The sparse traces of terrestrial diet are mainly from cattle, sheep, pig and cereals dating to a late stage of the habitation. Strategic raw materials of bone and antler from large forest game were only occasionally imported from across the sea. In terms of artefact types and production modes, the material culture of Rødhals represents the ultimate stage of the local fisher-hunter-gatherer Ertebølle Culture and an initial stage of the farming-based Funnel Beaker Culture. The extreme marine adaptation seen at this site may reflect a historically extraordinary situation, where an indigenous population of foragers had lost major parts of its territory to immigrant farmers.

A model of resource partitioning between foraging bees based on learning.

Central place foraging pollinators tend to develop multi-destination routes (traplines) to exploit patchily distributed plant resources. While the formation of traplines by individual pollinators has been studied in detail, how populations of foragers use resources in a common area is an open question, difficult to address experimentally. We explored conditions for the emergence of resource partitioning among traplining bees using agent-based models built from experimental data of bumblebees foraging on artificial flowers. In the models, bees learn to develop routes as a consequence of feedback loops that change their probabilities of moving between flowers. While a positive reinforcement of movements leading to rewarding flowers is sufficient for the emergence of resource partitioning when flowers are evenly distributed, the addition of a negative reinforcement of movements leading to unrewarding flowers is necessary when flowers are patchily distributed. In environments with more complex spatial structures, the negative experiences of individual bees on flowers favour spatial segregation and efficient collective foraging. Our study fills a major gap in modelling pollinator behaviour and constitutes a unique tool to guide future experimental programs.

Spatial segregation in a sexually dimorphic central place forager: Competitive exclusion or niche divergence?

Sexual competition is increasingly recognized as an important selective pressure driving species distributions. However, few studies have investigated the relative importance of interpopulation versus intrapopulation competition in relation to habitat availability and selection. To explain spatial segregation between sexes that often occurs in non-territorial and central place foragers, such as seabirds, two hypotheses are commonly used. The 'competitive exclusion' hypothesis states that dominant individuals should exclude subordinate individuals through direct competition, whereas the 'niche divergence' hypothesis states that segregation occurs due to past competition and habitat specialization. We tested these hypotheses in two populations of an extreme wide-ranging and sexually dimorphic seabird, investigating the relative role of intrapopulation and interpopulation competition in influencing sex-specific distribution and habitat preferences. Using GPS loggers, we tracked 192 wandering albatrosses Diomedea exulans during four consecutive years (2016-2019), from two neighbouring populations in the Southern Ocean (Prince Edward and Crozet archipelagos). We simulated pseudo-tracks to create a null spatial distribution and used Kernel Density Estimates (KDE) and Resource Selection Functions (RSF) to distinguish the relative importance of within- versus between-population competition. Kernel Density Estimates showed that only intrapopulation sexual segregation was significant for each monitoring year, and that tracks between the two colonies resulted in greater overlap than expected from the null distribution, especially for the females. RSF confirmed these results and highlighted key at-sea foraging areas, even if the estimated of at-sea densities were extremely low. These differences in selected areas between sites and sexes were, however, associated with high interannual variability in habitat preferences, with no clear specific preferences per site and sex. Our results suggest that even with low at-sea population densities, historic intrapopulation competition in wide-ranging seabirds may have led to sexual dimorphism and niche specialization, favouring the 'niche divergence' hypothesis. In this study, we provide a protocol to study competition within as well as between populations of central place foragers. This is relevant for understanding their distribution patterns and population regulation, which could potentially improve management of threatened populations.

Modeling scale up of anthropogenic impacts from individual pollinator behavior to pollination systems.

Understanding how anthropogenic disturbances affect plant–pollinator systems has important implications for the conservation of biodiversity and ecosystem functioning. Previous laboratory studies show that pesticides and pathogens, which have been implicated in the rapid global decline of pollinators over recent years, can impair behavioral processes needed for pollinators to adaptively exploit floral resources and effectively transfer pollen among plants. However, the potential for these sublethal stressor effects on pollinator–plant interactions at the individual level to scale up into changes to the dynamics of wild plant and pollinator populations at the system level remains unclear. We developed an empirically parameterized agent‐based model of a bumblebee pollination system called SimBee to test for effects of stressor‐induced decreases in the memory capacity and information processing speed of individual foragers on bee abundance (scenario 1), plant diversity (scenario 2), and bee–plant system stability (scenario 3) over 20 virtual seasons. Modeling of a simple pollination network of a bumblebee and four co‐flowering bee‐pollinated plant species indicated that bee decline and plant species extinction events could occur when only 25% of the forager population showed cognitive impairment. Higher percentages of impairment caused 50% bee loss in just five virtual seasons and system‐wide extinction events in less than 20 virtual seasons under some conditions. Plant species extinctions occurred regardless of bee population size, indicating that stressor‐induced changes to pollinator behavior alone could drive species loss from plant communities. These findings indicate that sublethal stressor effects on pollinator behavioral mechanisms, although seemingly insignificant at the level of individuals, have the cumulative potential in principle to degrade plant–pollinator species interactions at the system level. Our work highlights the importance of an agent‐based modeling approach for the identification and mitigation of anthropogenic impacts on plant–pollinator systems.

Children are important too: juvenile playgroups and maternal childcare in a foraging population, the Agta.

Non-maternal carers (allomothers) are hypothesized to lighten the mother's workload, allowing for the specialized human life history including relatively short interbirth intervals and multiple dependent offspring. Here, using in-depth observational data on childcare provided to 78 Agta children (a foraging population in the northern Philippines; aged 0-6 years), we explore whether allomaternal childcare substitutes and decreases maternal childcare. We found that allomother caregiving was associated with reduced maternal childcare, but the substitutive effect varied depending on the source and type of care. Children-only playgroups consistently predicted a decrease in maternal childcare. While grandmothers were rarely available, their presence was negatively associated with maternal presence and childcare, and grandmothers performed similar childcare activities to mothers. These results underscore the importance of allomothering in reducing maternal childcare in the Agta. Our findings suggest that flexibility in childcare sources, including children-only playgroups, may have been the key to human life-history evolution. Overall, our results reinforce the necessity of a broad conceptualization of social support in human childcare. This article is part of the theme issue 'Multidisciplinary perspectives on social support and maternal-child health'.

Cultural evolution: Is causal inference the secret of our success?

A new study investigates hunters' causal understandings of bow design and mechanics among the Hadza, one of the last remaining foraging populations. The results suggest that sophisticated technology can evolve without complete causal understanding.

Co-occurrence of Ostensive Communication and Generalizable Knowledge in Forager Storytelling : Cross-Cultural Evidence of Teaching in Forager Societies.

Teaching is hypothesized to be a species-typical behavior in humans that contributed to the emergence of cumulative culture. Several within-culture studies indicate that foragers depend heavily on social learning to acquire practical skills and knowledge, but it is unknown whether teaching is universal across forager populations. Teaching can be defined ethologically as the modification of behavior by an expert in the presence of a novice, such that the expert incurs a cost and the novice acquires skills/knowledge more efficiently or that it would not acquire otherwise. One behavioral modification hypothesized to be an adaptation for teaching is ostensive communication-exaggerations of prosody and gesture that signal intent to transmit generalizable knowledge and indicate the intended receiver. On this view, the use of ostensive communication in conjunction with the transmission of generalizable knowledge constitutes evidence of teaching. Oral storytelling appears to meet these criteria: Indigenous peoples regard their traditions as important sources of ecological and social knowledge, and oral storytelling is widely reported to employ paralinguistic communication. To test this hypothesis, descriptions of performed narrative in forager societies were coded for the use of 14 ostensive-communicative behaviors and the presence of generalizable knowledge. Although biased toward North America, the study sample comprised 53 forager cultures spanning five continents, 34 language families, and diverse biomes. All cultures evinced the predicted behaviors. Results suggest that foragers use storytelling as a mode of instruction, thus providing cross-cultural evidence of teaching in forager populations.

Effects of ambient temperature on dive behavior of East Pacific green turtles before and after a power plant closure

Water temperature plays a critical role in mediating energy budgets and influencing the behavior of marine ectotherms. Previous research has demonstrated that marine ectotherms such as sea turtles conserve energy by decreasing activity levels in colder water—often at or near the latitudinal limits of their range. San Diego Bay is near the northern edge of the range for eastern Pacific green turtles and is home to a year-round foraging population. Turtles in San Diego Bay experienced a significant decrease in ambient water temperature following the closure of a fossil-fuel-based power plant and corresponding loss of the plant’s warm-water effluent. Time-depth recorders were placed on 13 turtles before (n = 5) and after (n = 8) the closure of this plant to determine how changing water temperature influenced dive duration. Deployments lasted 2-25 d, with a mean deployment of 7.5 d. Green turtle behavior in different thermal regimes revealed a strong relationship between dive duration and water temperature; dive duration was significantly longer when water temperatures were colder, especially when water was below 14.4°C. Establishing the inactivity threshold for this population is critical to future management in light of temperature variability in coastal habitats and the impacts this may have on sea turtle energetics. Understanding organismal response to relatively rapid shifts in thermal conditions is relevant to assessments of the direct and indirect anthropogenic effects on aquatic environments.

Abundance, placement and sexual identity of the epizoic barnacle Chelonibia testudinaria relative to the size and species of host turtles in Mabul Island, Malaysia

AbstractSea turtles are partners in commensal relationships with a variety of epibionts, including barnacles. The acorn barnacle Chelonibia testudinaria is one species commonly found associated with sea turtles and other marine fauna throughout temperate to tropical waters including the Indo-Pacific. We conducted a study to assess the occurrence of this barnacle, relative to host life stage and species in a mixed foraging population of green (Chelonia mydas) and hawksbill (Eretmochelys imbricata) turtles (juvenile, sub-adult, adults (female and male)) in Mabul Island, Sabah, Malaysia (Celebes Sea). Expecting similar relative abundance by life stage, we found instead a significant effect between the combined dependent variables (abundance and size of barnacles) and the life stages of sea turtles in Mabul after controlling for the covariates of size and species of sea turtles (F(8, 428) = 5.77, P < 0.001, Pillai = 0.19). Among green turtles with barnacles, though adult males had larger barnacles compared with the female turtles, the mean barnacle abundance on adult females (43.4 individuals ± 5.19 SD) was higher than all other life stages. Most of the barnacles (85.6%; N = 1931) were found on the plastron of the sea turtles. The highest number of barnacle reacquisition was found among the juvenile turtles. In assessing the complemental males of the barnacles, we found they were consistently attached to the shells of the larger of the hermaphrodites from each region of the host's body despite average shell-size differences with each region.

Why are population growth rate estimates of past and present hunter-gatherers so different?

Hunter–gatherer population growth rate estimates extracted from archaeological proxies and ethnographic data show remarkable differences, as archaeological estimates are orders of magnitude smaller than ethnographic and historical estimates. This could imply that prehistoric hunter–gatherers were demographically different from recent hunter–gatherers. However, we show that the resolution of archaeological human population proxies is not sufficiently high to detect actual population dynamics and growth rates that can be observed in the historical and ethnographic data. We argue that archaeological and ethnographic population growth rates measure different things; therefore, they are not directly comparable. While ethnographic growth rate estimates of hunter–gatherer populations are directly linked to underlying demographic parameters, archaeological estimates track changes in the long-term mean population size, which reflects changes in the environmental productivity that provide the ultimate constraint for forager population growth. We further argue that because of this constraining effect, hunter–gatherer populations cannot exhibit long-term growth independently of increasing environmental productivity.This article is part of the theme issue ‘Cross-disciplinary approaches to prehistoric demography’.

Foraging in Czechia: The nation’s precious hobby

ABSTRACT The article fills the gap in the geographical literature on foraging in a Central and Eastern European context. Its goals are to investigate the extent and character of foraging practice and the population of foragers in Czechia, as well as to uncover what motivates the practice of foraging for non-timber forest products. A mixed methods approach was used in the reported study, which combined the triangulation of data from official statistics, three waves of a representative data survey, and a case study of mushroom pickers in Prague, the capital city of Czechia. The results show that foraging is a widespread practice in Czechia with stable but fluctuating harvest volumes. Czechs gather mainly mushrooms and edible berries. The results also show that foraging is a traditional practice of people with different demographic characteristics and from diverse socio-economic backgrounds, with a myriad of diverse motivations and connotations far from mere economic ones. Strolling outside in the fresh air and in forests is clearly a dominant motive. Thus, the author concludes that foraging in Czechia is far from being limited to livelihood strategies to tackle food insecurity.

The effects of animal personality on the ideal free distribution.

The ideal free distribution (IFD) has been used to predict the distribution of foraging animals in a wide variety of systems. However, its predictions do not always match observed distributions of foraging animals. Instead, we often observe that there are more consumers than predicted in low-quality patches and fewer consumers than predicted in high-quality patches (i.e. undermatching). We examine the possibility that animal personality is one explanation for this undermatching. We first conducted a literature search to determine how commonly studies document the personality distribution of populations. Second, we created a simple individual-based model to conceptually demonstrate why knowing the distribution of personalities is important for studies of populations of foragers in context of the IFD. Third, we present a specific example where we calculate the added time to reach the IFD for a population of mud crabs that has a considerable number of individuals with relatively inactive personalities. We suggest that animal personality, particularly the prevalence of inactive personality types, may inhibit the ability of a population to track changes in habitat quality, therefore leading to undermatching of the IFD. This may weaken the IFD as a predictive model moving forward.

Postpartum Maternal Mood Among Hadza Foragers of Tanzania: A Mixed Methods Approach.

Infant and maternal mortality rates are among the highest in the world in low and middle-income countries where postpartum depression impacts at least one in five women. Currently, there is a dearth of data on maternal mood and infant health outcomes in small-scale non-industrial populations from such countries, particularly during the postnatal period. Here, we present the first investigation of postpartum maternal mood among a foraging population, the Hadza of Tanzania. We administered the Edinburgh Postnatal Depression Scale (EPDS) to twenty-three women, all with infants under the age of 12months. Semi-structured interviews on happiness and unhappiness during the post-partum period served as a validity cross-check for the EPDS. The combined results of the EPDS surveys and the interview responses suggest that a high proportion of Hadza women experience significant mood disturbances following birth and that postpartum unhappiness is associated with self-reports of pain, anxiety, and disturbed sleep patterns. These findings suggest that many of the mothers in our sample are experiencing post-partum unhappiness at levels similar to or higher than those reported for low to middle income countries in general, including Tanzania. These data are critical for improving our understanding of the etiologies of postpartum mood disturbances cross-culturally.

Using Programming Languages and Geographic Information System to Determine Spatial and Temporal Variability in a Green Turtle Foraging Population on Liuchiu Island, Taiwan.

Facial photo identification (ID) has proven to be a non-invasive method for identifying individual wild animals, and in recent years it has been effective on megafauna such as sea turtles. However, when processing hundreds of photos over a long period of time, variation in facial scale patterns makes identifying individuals complicated. This means that there is a high possibility that the individual is misidentified, which results in incorrectly determining population sizes. This study used the programming languages Python and SQL to determine green turtle foraging population size in the nearshore waters of a coral island, Liuchiu Island, from 2011 to 2017. The programs determined that the foraging population was 432 turtles, approximately 90% of which resided there one year or less and selected only one foraging site. Those that stayed for more than two years selected two foraging sites. Less than 3% stayed throughout the 7 years. The core residence area was from Beauty Cave to Vase Rock. This study found that the nearshore waters of Liuchiu Island are a temporary development/foraging site for immature green turtles. This is the first study to use Python analysis to determine a foraging sea turtle population in the field.

Identifying forage populations of concern: A new perspective based on predator recruitment considerations

Concerns about forage fish taxa declines caused by fishing or environmental change and their impact on predator populations have become an important issue in fisheries management. However, it has proven difficult to identify critical forage taxa, and elusive to establish clear linkages between forage and predator dynamics. We provide a new avenue for identifying forage taxa of concern by focusing on organisms consumed during predator recruitment, the juvenile phase of the predator lifecycle that is most important in determining predator abundance. Using a case study of four coastal predatory fish species on the West Florida Shelf, Gulf of Mexico, USA, we show that consumption of exploited fish and crustacean species is less common during the phase of predator recruitment than in the predator lifecycle as a whole. Many taxa consumed during this critical phase are not systematically monitored. Among the monitored taxa, many showed relative-abundance declines despite not being targets or bycatch of any known fishery, which points towards environmental factors as drivers. We conclude that environmentally-driven declines in forage resources consumed during the critical phase of predator recruitment is an overlooked, but potentially important, “forage fish” concern.

Improved foraging by switching between diffusion and advection: benefits from movement that depends on spatial context

Animals use different modes of movement at different times, in different locations, and on different scales. Incorporating such context dependence in mathematical models represents a substantial increase in complexity, but creates an opportunity to more fully integrate key biological features. Here, we consider the spatial dynamics of a population of foragers with two subunits. In one subunit, foragers move via diffusion (random search) whereas in the other, foragers move via advection (gradient-following search). Foragers switch back and forth between the subunits as functions of their spatial context (i.e., depending on whether they are inside or outside of a patch, or depending on whether or not they can detect a gradient in resource density). We consider a one-dimensional binary landscape of resource patches and non-habitat and gauge success in terms of how well the mobile foragers overlap with the distribution of resources. Actively switching between dispersal modes can sometimes greatly enhance this spatial overlap relative to the spatial overlap possible when foragers merely blend advection and diffusion modes at all times. Switching between movement modes is most beneficial when organism’s gradient-following abilities are weak compared to its overall capacity for movement, but switching can actually be quite detrimental for organisms that can rapidly follow resource gradients. An organism’s perceptual range plays a critical role in determining the conditions under which switching movement modes benefits versus disadvantages foragers as they seek out resources.

Periodic catastrophes over human evolutionary history are necessary to explain the forager population paradox

The rapid growth of contemporary human foragers and steady decline of chimpanzees represent puzzling population paradoxes, as any species must exhibit near-stationary growth over much of their evolutionary history. We evaluate the conditions favoring zero population growth (ZPG) among 10 small-scale subsistence human populations and five wild chimpanzee groups according to four demographic scenarios: altered mean vital rates (i.e., fertility and mortality), vital rate stochasticity, vital rate covariance, and periodic catastrophes. Among most human populations, changing mean fertility or survivorship alone requires unprecedented alterations. Stochastic variance and covariance would similarly require major adjustment to achieve ZPG in most populations. Crashes could maintain ZPG in slow-growing populations but must be frequent and severe in fast-growing populations-more extreme than observed in the ethnographic record. A combination of vital rate alteration with catastrophes is the most realistic solution to the forager population paradox. ZPG in declining chimpanzees is more readily obtainable through reducing mortality and altering covariance. While some human populations may have hovered near ZPG under harsher conditions (e.g., violence or food shortage), modern Homo sapiens were equipped with the potential to rapidly colonize new habitats and likely experienced population fluctuations and local extinctions over evolutionary history.

Indicators of pelagic forage community shifts in the California Current Large Marine Ecosystem, 1998–2016

Forage fishes are ecologically and economically important in marine ecosystems worldwide and thus are a focal topic for ecosystem-based fisheries management. In the California Current Large Marine Ecosystem (CCLME), the community dynamics of forage populations have been studied at regional spatial scales, but not across regions. To evaluate indicators of the forage community at the ecosystem-wide scale, we examine temporal variability of forage assemblages in Northern (Oregon/Washington), Central (California; Point Reyes to Monterey Bay) and Southern (southern California) regions of the CCLME from 1998 through 2016 which include years with unprecedented climate variability. Forage communities fluctuated greatly between years within each region owing to regionally low abundances of common taxa in at least some years (e.g., rockfishes Sebastes spp. in each region). Comparison of species assemblages among regions indicate that that temporal shifts in assemblage structure were largely synchronous throughout the CCLME. However, dynamics of most individual taxa were out of phase between regions, indicating that different taxa drove the variability in each region. Within regions, taxa with similar adult ecological niches tended to co-vary, suggesting synchronous responses to environmental forcing. Major changes in forage assemblage structure are descriptively linked to large oceanographic perturbations such as the transition from El Niño to La Niña conditions in 1998, anomalous upwelling in 2005, and the onset of a large marine heatwave in fall-winter 2013–2014. Changes in forage assemblage structure are reflected in prey switching in the diet of California sea lions, Zalophus californianus, in the Southern region. The multivariate forage indices that we develop can serve as effective indicators of regional forage community composition shifts in the CCLME and provide managers with context on spatio-temporal changes in the structure of forage fish communities important to top predators in this system.