Estimating Allee Dynamics before They Can Be Observed: Polar Bears as a Case Study

Allee effects are an important component in the population dynamics of numerous species. Accounting for these Allee effects in population viability analyses generally requires estimates of low-density population growth rates, but such data are unavailable for most species and particularly difficult to obtain for large mammals. Here, we present a mechanistic modeling framework that allows estimating the expected low-density growth rates under a mate-finding Allee effect before the Allee effect occurs or can be observed. The approach relies on representing the mechanisms causing the Allee effect in a process-based model, which can be parameterized and validated from data on the mechanisms rather than data on population growth. We illustrate the approach using polar bears (Ursus maritimus), and estimate their expected low-density growth by linking a mating dynamics model to a matrix projection model. The Allee threshold, defined as the population density below which growth becomes negative, is shown to depend on age-structure, sex ratio, and the life history parameters determining reproduction and survival. The Allee threshold is thus both density- and frequency-dependent. Sensitivity analyses of the Allee threshold show that different combinations of the parameters determining reproduction and survival can lead to differing Allee thresholds, even if these differing combinations imply the same stable-stage population growth rate. The approach further shows how mate-limitation can induce long transient dynamics, even in populations that eventually grow to carrying capacity. Applying the models to the overharvested low-density polar bear population of Viscount Melville Sound, Canada, shows that a mate-finding Allee effect is a plausible mechanism for slow recovery of this population. Our approach is generalizable to any mating system and life cycle, and could aid proactive management and conservation strategies, for example, by providing a priori estimates of minimum conservation targets for rare species or minimum eradication targets for pests and invasive species.     

Divergent density feedback control of migratory predator recovery following sex‐biased perturbations

Uncertainty in risks posed by emerging stressors such as synthetic hormones impedes conservation efforts for threatened vertebrate populations. Synthetic hormones often induce sex‐biased perturbations in exposed animals by disrupting gonad development and early life‐history stage transitions, potentially diminishing per capita reproductive output of depleted populations and, in turn, being manifest as Allee effects. We use a spatially explicit biophysical model to evaluate how sex‐biased perturbation in life‐history traits of individuals (maternal investment in egg production and male‐skewed sex allocation in offspring) modulates density feedback control of year‐class strength and recovery trajectories of a long‐lived, migratory fish—shovelnose sturgeon (Scaphirhynchus platorynchus)—under spatially and temporally dynamic synthetic androgen exposure and habitat conditions. Simulations show that reduced efficiency of maternal investment in gonad development prolonged maturation time, increased the probability of skipped spawning, and, in turn, shrunk spawner abundance, weakening year‐class strength. However, positive density feedback disappeared (no Allee effect) once the exposure ceased. By contrast, responses to the demographic perturbation manifested as strong positive density feedback; an abrupt shift in year‐class strength and spawner abundance followed after more than two decades owing to persistent negative population growth (a strong Allee effect), reaching an alternative state without any sign of recovery. When combined with the energetic perturbation, positive density feedback of the demographic perturbation was dampened as extended maturation time reduced the frequency of producing male‐biased offspring, allowing the population to maintain positive growth rate (a weak Allee effect) and gradually recover. The emergent patterns in long‐term population projections illustrate that sex‐biased perturbation in life‐history traits can interactively regulate the strength of density feedback in depleted populations such as Scaphirhynchus sturgeon to further diminish reproductive capacity and abundance, posing increasingly greater conservation challenges in chemically altered riverscapes.     

Shifting from Right to Left: The Combined Effect of Elevated CO2 and Temperature on Behavioural Lateralization in a Coral Reef Fish

Recent studies have shown that elevated CO₂ can affect the behaviour of larval and juvenile fishes. In particular, behavioural lateralization, an expression of brain functional asymmetries, is affected by elevated CO₂ in both coral reef and temperate fishes. However, the potentially interacting effects of rising temperatures and CO₂ on lateralization are unknown. Here, we tested the combined effect of near-future elevated-CO₂ concentrations (930 µatm) and temperature variation on behavioural lateralization of a marine damselfish, Pomacentrus wardi. Individuals exposed to one of four treatments (two CO₂ levels and two temperatures) were observed in a detour test where they made repeated decisions about turning left or right. Individuals exposed to current CO₂ and ambient temperature levels showed a significant right-turning bias at the population level. This biased was reversed (i.e. to the left side) in fish exposed to the elevated-CO₂ treatment. Increased temperature attenuated this effect, resulting in lower values of relative lateralization. Consequently, rising temperature and elevated CO₂ may have different and interactive effects on behavioural lateralization and therefore future studies on the effect of climate change on brain functions need to consider both these critical variables in order to assess the potential consequences for the ecological interactions of marine fishes.     

Potential Component Allee Effects and Their Impact on Wetland Management in the Conservation of Endangered Anurans

Effective management of wetland quantity and quality is crucial for effective conservation of declining amphibian populations. In particular, frogs and toads that employ aggregative breeding strategies may suffer negative population impacts in response to changes in availability of aquatic breeding habitat, including overabundance of suitable habitat, if density of conspecifics attending aggregations is positively correlated with reproductive success. Here we document such a positive relationship, potentially the first example of a component Allee effect in an anuran, in the critically endangered Houston toad (Bufo houstonensis). We assessed the relationship between mean yearly chorus size and reproductive success of males at the pond level using an information theoretic model selection approach and a two-sample t-test. The chosen model contained the single variable of mean yearly chorus size to predict probability of reproduction, as selected using the Akaike Information Criterion corrected for small sample size and Akaike weight. Mean chorus sizes were significantly higher among ponds exhibiting evidence of reproduction than in those that showed no evidence of reproduction. Our results suggest that chorusing alone is a poor proxy for inference of population stability and highlight a need for reassessment of widely-used amphibian monitoring protocols. Further, amphibian conservation efforts should account for potential Allee effects in order to optimize benefits and avoid underestimating critical population thresholds, particularly in species exhibiting rapid population declines.     

Harvesting Effects,Recovery Mechanisms,and Management Strategies for a Long-Lived and Structural Precious Coral

Overexploitation is a major threat for the integrity of marine ecosystems. Understanding the ecological consequences of different extractive practices and the mechanisms underlying the recovery of populations is essential to ensure sustainable management plans. Precious corals are long-lived structural invertebrates, historically overfished, and their conservation is currently a worldwide concern. However, the processes underlying their recovery are poorly known. Here, we examined harvesting effects and recovery mechanisms of red coral Corallium rubrum by analyzing long-term photographic series taken on two populations that were harvested. We compared the relative importance of reproduction and re-growth as drivers of resilience. Harvesting heavily impacted coral populations causing large decreases in biomass and strong size-class distribution shifts towards populations dominated by small colonies. At the end of the study (after 4 and 7 years) only partial recovery was observed. The observed general pattern of low recruitment and high mortality of new recruits demonstrated limited effects of reproduction on population recovery. Adversely, low mortality of partially harvested adults and a large proportion of colonies showing new branches highlighted the importance of re-growth in the recovery process. The demographic projections obtained through stochastic models confirmed that the recovery rates of C. rubrum can be strongly modulated depending on harvesting procedures. Thus, leaving the basal section of the colonies when harvesting to avoid total mortality largely enhances the resilience of C. rubrum populations and quickens their recovery. On the other hand, the high survival of harvested colonies and the significant biomass reduction indicated that abundance may not be an adequate metric to assess the conservation status of clonal organisms because it can underestimate harvesting effects. This study highlights the unsustainability of current harvesting practices of C. rubrum and provides urgently needed data to improve management practices that are still largely based on untested assumptions.     

Demographic and Component Allee Effects in Southern Lake Superior Gray Wolves

Recovering populations of carnivores suffering Allee effects risk extinction because positive population growth requires a minimum number of cooperating individuals. Conservationists seldom consider these issues in planning for carnivore recovery because of data limitations, but ignoring Allee effects could lead to overly optimistic predictions for growth and underestimates of extinction risk. We used Bayesian splines to document a demographic Allee effect in the time series of gray wolf (Canis lupus) population counts (1980–2011) in the southern Lake Superior region (SLS, Wisconsin and the upper peninsula of Michigan, USA) in each of four measures of population growth. We estimated that the population crossed the Allee threshold at roughly 20 wolves in four to five packs. Maximum per-capita population growth occurred in the mid-1990s when there were approximately 135 wolves in the SLS population. To infer mechanisms behind the demographic Allee effect, we evaluated a potential component Allee effect using an individual-based spatially explicit model for gray wolves in the SLS region. Our simulations varied the perception neighborhoods for mate-finding and the mean dispersal distances of wolves. Simulation of wolves with long-distance dispersals and reduced perception neighborhoods were most likely to go extinct or experience Allee effects. These phenomena likely restricted population growth in early years of SLS wolf population recovery.     

Temperature and CO2 additively regulate physiology,morphology and genomic responses of larval sea urchins,Strongylocentrotus purpuratus

Ocean warming and ocean acidification, both consequences of anthropogenic production of CO₂, will combine to influence the physiological performance of many species in the marine environment. In this study, we used an integrative approach to forecast the impact of future ocean conditions on larval purple sea urchins (Strongylocentrotus purpuratus) from the northeast Pacific Ocean. In laboratory experiments that simulated ocean warming and ocean acidification, we examined larval development, skeletal growth, metabolism and patterns of gene expression using an orthogonal comparison of two temperature (13°C and 18°C) and pCO₂ (400 and 1100 μatm) conditions. Simultaneous exposure to increased temperature and pCO₂ significantly reduced larval metabolism and triggered a widespread downregulation of histone encoding genes. pCO₂ but not temperature impaired skeletal growth and reduced the expression of a major spicule matrix protein, suggesting that skeletal growth will not be further inhibited by ocean warming. Importantly, shifts in skeletal growth were not associated with developmental delay. Collectively, our results indicate that global change variables will have additive effects that exceed thresholds for optimized physiological performance in this keystone marine species.     

Identifying key biodiversity areas as marine conservation priorities in the greater Caribbean

Increasing rates of Anthropocene biodiversity extinctions suggest a possible sixth mass extinction event. Conservation planners are seeking effective ways to protect species, hotspots of biodiversity, and dynamic ecosystems to reduce and eventually eliminate the degradation and loss of diversity at the scale of genes, species, and ecosystems. While well-established, adequately enforced protected areas (PAs) increase the likelihood of preserving species and habitats, traditional placement methods are frequently inadequate in protecting biodiversity most at risk. Consequently, the Key Biodiversity Area (KBA) Partnership developed a set of science-based criteria and thresholds that iteratively identify sites where biodiversity is most in need of protection. KBA methodology has been rarely applied in the marine realm, where data are often extremely limited. We tested the feasibility of KBA population metrics in the Greater Caribbean marine region using occurrence and population data and threat statuses for 1669 marine vertebrates. These data identified areas where site-specific conservation measures can effectively protect biodiversity. Using KBA criteria pertaining to threatened and irreplaceable biodiversity, we identified 90 geographically unique potential KBAs, 34 outside and 56 within existing PAs. These provide starting points for local conservation managers to verify that KBA thresholds are met and to delineate site boundaries. Significant data gaps, such as population sizes, life history characteristics, and extent of habitats, prevent the full application of the KBA criteria to data-poor marine species. Increasing the rate and scope of marine sampling programs and digital availability of occurrence datasets will improve identification and delineation of KBAs in the marine environment.

     

Patterns and prediction of population recovery in marine reserves

Marine reserves (no-take zones) are widely recommended asconservation and fishery management tools. One potential benefitof marine reserves is that they can reduce fishing mortality.This can lead to increases in the abundance of spawners,providing insurance against recruitment failure and maintainingor enhancing yields in fished areas. This paper considers thefactors that influence recovery following marine reserveprotection, describes patterns of recovery in numbers andbiomass, and suggests how recovery rates can be predicted.Population recovery is determined by initial population size, theintrinsic rate of population increase r, and the degree ofcompensation (increases in recruits per spawner as spawnerabundance falls) or depensation (lower than expected recruitmentat low abundance, Allee effect) in the spawner-recruitrelationship. Within a reserve, theoretical recovery rates arefurther modified by metapopulation structure and the success ofindividual recruitment events. Recovery also depends on theextent of reductions in fishing mortality (F) as determined bythe relationship between patterns of movement, migration, anddensity-dependent habitat use (buffer effect) in relation to thesize, shape and location of the reserve. The effects ofreductions in F on population abundance have been calculatedusing a variety of models that incorporate transfer rates betweenthe reserve and fished areas, fishing mortality outside thereserve and life history parameters of the population. Thesemodels give useful indications of increases in production andbiomass (as yield per recruit and spawners per recruitrespectively) due to protection, but do not address recruitment.Many reserves are very small in relation to the geographicalrange of fish or invertebrate populations. In these reserves itmay be impossible to distinguish recovery due to populationgrowth from that due to redistribution. Mean rates of recoverycan be predicted from r, but the methods are data intensive. Thisis ironic when marine reserves are often favoured for managementor conservation in data-poor situations where conventional stockassessment is impossible. In these data-poor situations, it maybe possible to predict recovery rates from very low populationsizes by using maximum body size or age at maturity as simplecorrelates of the intrinsic rate of natural increase.     

Genetic diversity and population structure of the ark shell Scapharca broughtonii along the coast of China based on microsatellites

As a commercially important species in East Asia, the natural resources of Scapharca broughtonii have been suffering from severe population decline across its main habitats. In China, recovery efforts for S. broughtonii are in progress. To provide scientific bases for fisheries management and conservation program, genetic diversity and population structure of seven wild populations of S. broughtonii from the northern China coast was assessed using seven microsatellite loci in this study. High genetic diversity was present in all the seven populations, as observed in mean allelic richness per locus (11.3–12.5), and average expected heterozygosity (0.835–0.867). No significant difference in allelic richness or expected heterozygosity was observed among the seven populations. Pairwise F_ST estimates and NJ tree topologies based on D_C distances indicated that the seven populations fell into two groups, showing a clear division between the populations from the south and north of the Shandong Peninsula. Genetic differentiation was further analyzed using AMOVA and assignation tests. Genetic barrier analysis using Monmonier algorithm also confirmed that the Shandong Peninsula was the putative barrier separating the northern and southern populations. In addition, marine currents probably play an important role in high gene flow among three populations from the same marine gyre.     

Generating spatial data for marine conservation and management

Do fishers know best when it comes to identifying areas with rare and depleted fish species? The global conservation crisis demands that managers marshal all available datasets to inform conservation management plans for depleted species, yet the level of trust placed in local knowledge remains uncertain. This study compares four methods for inferring species distributions of an internationally traded, rare and depleted genus of marine fishes (Hippocampus spp.): the use of (i) fisher interviews; (ii) government research trawls, (iii) scientific diving surveys, and (iv) citizen science contributions. We analyzed these four datasets at the genus and individual species levels to evaluate our conclusions about seahorse spatial occurrence, diversity of species present and the cost effectiveness of sampling effort. We found that fisher knowledge provided more information on our data-poor fish genus at larger spatial scales, with less effort, and for a cheaper price than all other datasets. One drawback was that fishers were unable to provide data down to the species level. People embarking on conservation endeavors for data-poor species may wish to begin with fisher interviews and use these to inform the application of government research, scientific diving, or citizen science programs.     

Neglecting uncertainty behind Allee effect estimation may generate false predictions of population extinction risk

Estimation of extinction thresholds arising from Allee effects (Allee thresholds) and related probabilities of population extinction is notoriously difficult. One way is to analyze adequately parameterized population models. Traditionally, a point estimate is substituted for the Allee effect strength in such models. However, each point estimate entails an underlying uncertainty. We explore how accounting for this uncertainty affects the probability of population extinction, and show that this probability decreases sigmoidally with increasing population density, even in the absence of any stochasticity. Deviations from when only a point estimate of the Allee effect strength is used can be significant, unless stochasticity is added and the stochastic noise intensity is high. Significant deviations from when only a point estimate is used also occur when the Allee threshold and the environmental carrying capacity of the species are close enough one to another. We also show that the impact of the uncertainty in the Allee effect strength estimate increases as the Allee effect strength itself increases and decreases as the species recovery potential increases. This is not a good news, since we would like to preferentially and efficiently manage slowly recovering populations prone to strong Allee effects. Still, there is a way to come up with relatively good Allee threshold estimates. Besides an obvious option of collecting as many data as possible, the impact of the uncertainty can be mitigated by diversifying Allee effect experiments such that we put more emphasis on larger size groups. This is somewhat surprising, given that frequent complaints on the (im)possibility of detecting Allee effects concern difficulties in locating, observing and experimenting on rare populations. Our results extend current theory surrounding Allee effects and have broad ramifications for applied ecology.     

Mitochondrial and Nuclear DNA Based Genetic Assessment Indicated Distinct Variation and Low Genetic Exchange Among the Three Subspecies of Swamp Deer (<Emphasis Type="Italic">Rucervus</Emphasis> <Emphasis Type="Italic">duvaucelii</Emphasis>)

The swamp deer (Rucervus duvaucelii) occurs, primarily, in the wet grasslands of the Himalayan foothills as well as the dry grasslands of central India. Three subspecies have been identified, namely R. duvaucelii duvaucelii, R. duvaucelii branderi and R. duvaucelii ranjitsinhi. Degradation of grassland habitats led to a drastic decline in the total swamp deer population in the early 19th century. Even though the species has recently shown signs of recovery, it is still vulnerable to the small-population paradigm. Effective management plans need to be put in place to increase the population through scientific intervention. The current genetic variation within the three subspecies of R. duvaucelii is unclear, and this is hindering effective conservation planning. We examined the genetic variability, population structure and demography of the three subspecies of swamp deer using the mtDNA control region and microsatellite analysis. Despite the spatial isolation of the populations, we found a high level of variation and weak divergence among the subspecies. The genetic differentiation (F _ST ) between the subspecies and the mismatch distribution of haplotypes indicated recent colonization by these subspecies. Population bottleneck analysis indicated that the existing subspecies and their populations are at demographic equilibrium and are stable. The study highlights the need for effective conservation management intervention to maintain the population size and genetic diversity. It also indicates that all the subspecies need to be managed as separate conservation units.     

Conserving marine biodiversity: insights from life-history trait candidate genes in Atlantic cod (Gadus morhua)

Recent technological developments have facilitated an increased focus on identifying genomic regions underlying adaptive trait variation in natural populations, and it has been advocated that this information should be important for designating population units for conservation. In marine fishes, phenotypic studies have suggested adaptation through divergence of life-history traits among natural populations, but the distribution of adaptive genetic variation in these species is still relatively poorly known. In this study, we extract information about the geographical distribution of genetic variation for 33 single nucleotide polymorphisms (SNPs) associated with life-history trait candidate genes, and compare this to variation in 70 putatively neutral SNPs in Atlantic cod (Gadus morhua). We analyse samples covering the major population complexes in the eastern Atlantic and find strong evidence for non-neutral levels and patterns of population structuring for several of the candidate gene-associated markers, including two SNPs in the growth hormone 1 gene. Thus, this study aligns with findings from phenotypic studies, providing molecular data strongly suggesting that these or closely linked genes are under selection in natural populations of Atlantic cod. Furthermore, we find that patterns of variation in outlier markers do not align with those observed at selectively neutral markers, and that outlier markers identify conservation units on finer geographical scales than those revealed when analysing only neutral markers. Accordingly, results also suggest that information about adaptive genetic variation will be useful for targeted conservation and management in this and other marine species.     

Consequences of experimental cortisol manipulations on the thermal biology of the checkered puffer (Sphoeroides testudineus) in laboratory and field environments

Anthropogenic climate change is altering temperature regimes for coastal marine fishes. However, given that temperature changes will not occur in isolation of other stressors, it is necessary to explore the potential consequences of stress on the thermal tolerances and preferences of tropical marine fish in order to understand the thresholds for survival, and predict the associated coastal ecological consequences. In this study, we used exogenous cortisol injections to investigate the effects of a thermal challenge on checkered puffers (Sphoeroides testudineus) as a secondary stressor. There were no significant differences between control and cortisol-treated fish 48 h following cortisol treatment for swimming ability (using a chase to exhaustion protocol), blood glucose concentrations or standard metabolic rate. In the lab, control and cortisol-treated puffers were exposed to ambient (29.1±1.5 °C), ambient +5 °C (heat shock) and ambient −5 °C (cold shock) for 4 h and to evaluate the consequences of abrupt temperature change on puff performance and blood physiology. Following cold shock, control fish exhibited increases in cortisol levels and weak ‘puff’ performance. Conversely, fish dosed with cortisol exhibited consistently high cortisol levels independent of thermal treatment, although there was a trend for an attenuated cortisol response in the cortisol-treated fish to the cold shock treatment. A 20-day complementary field study conducted in the puffer’s natural habitat, a tidal creek in Eleuthera, The Bahamas, revealed that cortisol-injected fish selected significantly cooler temperatures, measured using accumulated thermal units, when compared to controls. These results, and particularly the discrepancies between consequences documented in the laboratory and the ecological trends observed in the field, highlight the need to establish the link between laboratory and field data to successfully develop management policies and conservation initiatives with regards to anthropogenic climate change.     

Backward bifurcations and strong Allee effects in matrix models for the dynamics of structured populations

In nonlinear matrix models, strong Allee effects typically arise when the fundamental bifurcation of positive equilibria from the extinction equilibrium at r=1 (or R₀=1) is backward. This occurs when positive feedback (component Allee) effects are dominant at low densities and negative feedback effects are dominant at high densities. This scenario allows population survival when r (or equivalently R₀) is less than 1, provided population densities are sufficiently high. For r>1 (or equivalently R₀>1) the extinction equilibrium is unstable and a strong Allee effect cannot occur. We give criteria sufficient for a strong Allee effect to occur in a general nonlinear matrix model. A juvenile–adult example model illustrates the criteria as well as some other possible phenomena concerning strong Allee effects (such as positive cycles instead of equilibria).     

Measuring marine fishes biodiversity: temporal changes in abundance, life history and demography

Patterns in marine fishes biodiversity can be assessed by quantifying temporal variation in rate of population change, abundance, life history and demography concomitant with long-term reductions in abundance. Based on data for 178 populations (62 species) from four north-temperate oceanic regions (Northeast Atlantic and Pacific, Northwest Atlantic, North mid-Atlantic), 81% of the populations in decline prior to 1992 experienced reductions in their rate of loss thereafter; species whose rate of population decline accelerated after 1992 were predominantly top predators such as Atlantic cod (Gadus morhua), sole (Solea solea) and pelagic sharks. Combining population data across regions and species, marine fishes have declined 35% since 1978 and are currently less than 70% of recorded maxima; demersal species are generally at historic lows, pelagic species are generally stable or increasing in abundance. Declines by demersal species have been associated with substantive increases in pelagic species, a pattern consistent with the hypothesis that increases in the latter may be largely attributable to reduced predation mortality. There is a need to determine the consequences to population growth effected by the reductions in age and size at maturity, and in mean age and size of spawners, concomitant with population decline. We conclude that reductions in the rate of population decline, in the absence of targets for population increase, will be insufficient to effect a recovery of marine fishes biodiversity, and that great care must be exercised when interpreting multi-species patterns in abundance. Of fundamental importance is the need to explain the geographical, species-specific and habitat biases that pervade patterns of marine fishes recovery and biodiversity.     

Dangerously few liaisons: a review of mate-finding Allee effects

In this paper, we review mate-finding Allee effects from ecological and evolutionary points of view. We define ‘mate-finding’ as mate searching in mobile animals, and also as the meeting of gametes for sessile animals and plants (pollination). We consider related issues such as mate quality and choice, sperm limitation and physiological stimulation of reproduction by conspecifics, as well as discussing the role of demographic stochasticity in generating mate-finding Allee effects. We consider the role of component Allee effects due to mate-finding in generating demographic Allee effects (at the population level). Compelling evidence for demographic Allee effects due to mate-finding (as well as via other mechanisms) is still limited, due to difficulties in censusing rare populations or a failure to identify underlying mechanisms, but also because of fitness trade-offs, population spatial structure and metapopulation dynamics, and because the strength of component Allee effects may vary in time and space. Mate-finding Allee effects act on individual fitness and are thus susceptible to change via natural selection. We believe it is useful to distinguish two routes by which evolution can act to mitigate mate-finding Allee effects. The first is evolution of characteristics such as calls, pheromones, hermaphroditism, etc. which make mate-finding more efficient at low density, thus eliminating the Allee effect. Such adaptations are very abundant in the natural world, and may have arisen to avoid Allee effects, although other hypotheses are also possible. The second route is to avoid low density via adaptations such as permanent or periodic aggregation. In this case, the Allee effect is still present, but its effects are avoided. These two strategies may have different consequences in a world where many populations are being artificially reduced to low density: in the first case, population growth rate can be maintained, while in the second case, the mechanism to avoid Allee effects has been destroyed. It is therefore in these latter populations that we predict the greatest evidence for mate-finding Allee effects and associated demographic consequences. This idea is supported by the existing empirical evidence for demographic Allee effects. Given a strong effect that mate-finding appears to have on individual fitness, we support the continuing quest to find connections between component mate-finding Allee effects (individual reproductive fitness) and the demographic consequences. There are many reasons why such studies are difficult, but it is important, particularly given the increasing number of populations and species of conservation concern, that the ecological community understands more about how widespread demographic Allee effects really are, and why.     

Effect of retentive markers on the dynamics of settlement: The case of arthropod silk

During a settlement decision, the presence of conspecifics is crucial to species subject to Allee effects, for which the number of founders affects the subsequent growth of the colony. Marking the area (physically or chemically) conveys information about the number of conspecifics present in a new patch. Here, we study how an individual affinity for the marker affects the dynamics of a foundation process. A generic population model is presented, in which marking and affinity for the marker are at stake. Our results show that population size thresholds can appear, below which settlement is not possible. This model is then used to study the dynamics of migration and aggregation in a set of interconnected populations. We show that affinity for the marker can induce asymmetries in the population distribution. Anelosimus eximius is a social spider subject to Allee effects, for which silk potentially acts as a marker. We test our predictions with field experiments involving two populations of A. eximius in a Y-shaped setup. The agreement between our experimental and theoretical results strongly supports the validity of the model. This allows us to use the model to estimate a realistic set of parameters of biological significance to this social spider.     

The presence of heterospecific males causes an Allee effect

The Allee effect is a positive causal relationship between any component of fitness and population density or size. Allee effects strongly affect the persistence of small or sparse populations. Predicting Allee effects remains a challenge, possibly because not all causal mechanisms are known. We hypothesized that reproductive interference (an interspecific reproductive interaction that reduces the fitness of the species involved) can generate an Allee effect. If the density of the interfering species is constant, an increase in the population of the species receiving interference may dilute the per capita effect of reproductive interference and may generate an Allee effect. To test this hypothesis, we examined the effect of heterospecific males on the relationship between per capita fecundity and conspecific density in Callosobruchus chinensis and C. maculatus. Of the two species, only C. maculatus females suffer reproductive interference from heterospecific males. Only C. maculatus, the species susceptible to reproductive interference, demonstrated an Allee effect, and only when heterospecific males were present. In contrast, C. chinensis, the species not susceptible to reproductive interference, demonstrated no Allee effect regardless of the presence of heterospecific males. Our results show that reproductive interference in fact generated an Allee effect, suggesting the potential importance of interspecific sexual interactions especially in small or sparse populations, even in the absence of a shared resource. It may be possible to predict Allee effects produced by this mechanism a priori by testing reproductive interference between closely related species.