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High prices for rare species can drive large populations extinct: the anthropogenic Allee effect revisited
Abstract and Figures
Consumer demand for plant and animal products threatens many populations with extinction. The anthropogenic Allee effect (AAE) proposes that such extinctions can be caused by prices for wildlife products increasing with species rarity. This price-rarity relationship creates financial incentives to extract the last remaining individuals of a population, despite higher search and harvest costs. The AAE has become a standard approach for conceptualizing the threat of economic markets on endangered species. Despite its potential importance for conservation, AAE theory is based on a simple graphical model with limited analysis of possible population trajectories. By specifying a general class of functions for price-rarity relationships, we provide the first theoretical analysis of AAE models, and show that the classic theory can understate the risk of species extinction. AAE theory proposes that only populations below a critical Allee threshold will go extinct due to increasing price-rarity relationships. Our analysis shows that this threshold can be much higher than the original theory suggests, depending on initial harvest effort. More alarmingly, even species with population sizes above this Allee threshold, for which AAE predicts persistence, can be destined to extinction. Introducing even a minimum price for harvested individuals, close to zero, can cause large populations to cross the classic anthropogenic Allee threshold on a trajectory towards extinction. These results suggest that traditional AAE theory may give a false sense of security when managing large harvested populations.
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... The AAE has gained traction in the literature since its introduction by Courchamp et al. (2006) (e.g., Almeida, Bonachela, and Lockwood 2023;Angulo and Courchamp 2009;Angulo, Deves, et al. 2009;Fraser, Roberts, and Brock 2023;Gault, Meinard, and Courchamp 2008;Hausmann, Cortés-Capano, and Di Minin 2023;Holden and McDonald-Madden 2017;R. B. Harris, Cooney, and Leader-Williams 2013), in part due to the growing global demand by wealthy international consumers for luxury wildlife products and goods through legal and illegal trade channels ('t Sas-Rolfes et al. 2019;Heinrich et al. 2016). ...
... The AAE posits that if consumers disproportionately value goods they perceive to be rare, they will be willing to pay higher prices to obtain individuals of these species even when the cost of their acquisition is exorbitant (Courchamp et al. 2006;Holden and McDonald-Madden 2017). If this rarity-based premium results in a selling price rising faster than harvest cost as the population is depleted, a tipping point exists at a population size below which harvesting to extinction will always be the most profitable strategy for harvesters, since the marginal benefit of harvest outweighs the marginal cost of harvest ( Figure 1, open circle). ...
... The AAE is fundamentally driven by how the price-abundance relationship, a depensatory price dynamic typically represented as the rate at which the per individual price rises as the overall population is depleted to zero, relates to the marginal cost of harvest (Courchamp et al. 2006;Holden and McDonald-Madden 2017). Modeling frameworks that incorporate these dynamics depict price as a nonlinear function of abundance (or market supply, e.g., Burgess et al. 2017;Hausmann, Cortés-Capano, and Di Minin 2023;Krishna et al. 2019), with parameters that designate the strength of this relationship (e.g., Almeida, Bonachela, and Lockwood 2023;Courchamp et al. 2006;Dao et al. 2023;Holden and McDonald-Madden 2017;Thurner, Converse, and Branch 2021;Verma 2016). ...
The harvest and sale of wildlife can drive species to extinction when consumers are willing to pay high prices for the last harvested individuals of a very rare species, a phenomenon known as the anthropogenic Allee effect (AAE). Because demand for rarity is an inherent human desire, the AAE has the potential to affect a wide range of exploited species across several geographic regions. Here, we assess the current extent of empirical evidence for the AAE, how such evidence has been measured, and how this evidence interfaces with existing models of the AAE. We find substantial gaps in the empirical evidence base for the AAE and suggest that this deficit prevents assessment of the AAE in species extinctions. We provide a framework for generating empirical evidence that can identify when the AAE is likely occurring or has the potential to occur in the future, and recommend directions for both empirical and theoretical modeling research designed to strengthen our ability to forecast the ecological and market conditions that result in an AAE.
... Recent bioeconomic theory explains how each of these conditions and others can allow fishing to cause extinction by preventing profits from dissipating as a species is depleted. For example, in single-species fisheries, fishing profits can be maintained as the target species' abundance declines if prices increase faster than costs, either due to supply and demand or rarity effects (Burgess, Costello, et al., 2017;Courchamp et al., 2006;Dao et al., 2023;Holden & McDonald-Madden, 2017). ...
... Under open-access conditions, we assume that the dynamics of effort are given by Equation (2), which mirrors the Gordon-Schaefer model, with the addition of catch non-linearity ( i ) and the assumption that price can be non-constant. Specifically, we assume that there is a constant per-unit-effort fishing cost, c, and that the price of catch is a function of the stock size [p i (N i )], either directly (Courchamp et al., 2006;Holden & McDonald-Madden, 2017) or indirectly as a function of catch (Burgess, Costello, et al., 2017). ...
There have been few documented extinctions of fished species, but many bioeconomic models predict that open‐access incentives make extinction possible. Open‐access multi‐species fisheries can cause species' extinction if other, faster‐growing species maintain profits at fatal effort levels. Even target species can be profitably harvested to extinction if their prices rise sufficiently as they are depleted. Here, we explore interactions between these potential extinction mechanisms by modelling an open‐access multi‐species fishery with one or multiple fleets exploiting two species, each with different growth rates, ex‐vessel prices, and price dynamics. Increases in the strong stock's (the stock with higher productivity relative to fishing susceptibility) price as it is depleted increase the range of conditions under which the weak stock can be driven extinct and shrinks the range of bioeconomic parameters in which both species can coexist under open‐access. Catch hyperstability – whereby species become easier to catch as they are depleted – makes the weak stock weaker as it is depleted and further narrows the scope for coexistence. Fleet diversity in targeting ability can prevent weak stock extinction, as competition or switching balances species abundances. With few documented global fished‐species extinctions, our results raise important questions, which we discuss. Is the apparent lack of extinctions largely due to management? Are more species in lightly‐managed fisheries threatened with extinction than previously acknowledged? Have more extinctions than we realize already happened in data‐ and management‐poor fisheries? Or have fishes' high fecundity and the oceans' vastness provided protection against extinction that is uncaptured by existing theoretical models?
... Understanding the value of wildlife is essential for biodiversity conservation. In conservation practice, it is important to recognize that a high price often indicates rarity and high demand, necessitating greater concern for high-priced species and individuals 12,42 . In our study, we found that C. brevimanus commands a higher price than other species and is the rarest species in the entire online land hermit crab selling database ( Fig. 2). ...
Wildlife trade poses a significant challenge to biodiversity conservation, yet invertebrate species often receive less attention. In this study, we focused on land hermit crabs to analyze 10 years of data from Japan's largest online selling platform, alongside public interest trends. Our findings reveal that a total of five species and 6,862 individuals were sold over the past decade. Notably, the number of individuals sold annually and the total revenue significantly increased, correlating with Google Trends data, which suggests rising popularity and demand. The price of these crabs may reflect consumer preferences and potential impacts on species conservation. Our analysis shows that body size, species, and the number of individuals per sale influence price. Additionally, while price is also affected by coloration in certain species and body sizes, no clear pattern was observed. A significant trend we identified is the increase in the proportion of small-sized crabs sold, alongside a decrease in large and extra-large sizes, which could indicate over-fishing. The selling season peaked during the summer, with a small number of sellers accounting for a large share of sales, most of whom were based in Okinawa. This study offers valuable insights for policymakers, including the need to manage catch volumes and sizes, collaborate with local stakeholders, and re-evaluate selling certifications. Importantly, it provides the first evidence of long-term trade in land hermit crabs, offering critical information for the conservation of these popular exotic invertebrates globally.
... Our study was limited to fitting simple, stationary, one-dimensional difference equation models with only two equilibria describing a population growing towards carrying capacity in the absence of harvest. When fisheries are better described by more complicated dynamics such as, multi-species interactions , ecologically or economically induced Allee effects (Perälä et al. 2022;Holden and McDonald-Madden 2017), non-stationary parameters (e.g. due to time-dependent environmental regime shifts, Johnson et al. 2015), time delays (e.g. ...
Density-dependent population dynamic models strongly influence many of the world's most important harvest policies. Nearly all classic models (e.g. Beverton-Holt and Ricker) recommend that managers maintain a population size of roughly 40-50 percent of carrying capacity to maximize sustainable harvest, no matter the species' population growth rate. Such insights are the foundational logic behind most sustainability targets and biomass reference points for fisheries. However, a simple, less-commonly used model, called the Hockey-Stick model, yields very different recommendations. We show that the optimal population size to maintain in this model, as a proportion of carrying capacity, is one over the population growth rate. This leads to more conservative optimal harvest policies for slow-growing species, compared to other models, if all models use the same growth rate and carrying capacity values. However , parameters typically are not fixed; they are estimated after model-fitting. If the Hockey-Stick model leads to lower estimates of carrying capacity than other models, then the Hockey-Stick policy could yield lower absolute population size targets in practice. Therefore, to better understand the population size targets that may be recommended across real fisheries, we fit the Hockey-Stick, Ricker and Beverton-Holt models to population time series data across 284 fished species from the RAM Stock Assessment database. We found that the Hockey-Stick model usually recommended fisheries maintain population sizes higher than all other models (in 69-81% of the data sets). Furthermore, in 77% of the datasets, the Hockey-Stick model recommended an optimal population target even higher than 60% of carrying capacity (a widely used 0123456789().: V,-vol 123 127 Page 2 of 19 V. Cattoni et al. target, thought to be conservative). However, there was considerable uncertainty in the model fitting. While Beverton-Holt fit several of the data sets best, Hockey-Stick also frequently fit similarly well. In general, the best-fitting model rarely had overwhelming support (a model probability of greater than 95% was achieved in less than five percent of the datasets). A computational experiment, where time series data were simulated from all three models, revealed that Beverton-Holt often fit best even when it was not the true model, suggesting that fisheries data are likely too small and too noisy to resolve uncertainties in the functional forms of density-dependent growth. Therefore, sustainability targets may warrant revisiting, especially for slow-growing species.
... If the anti-poaching efforts would have been effective in reducing the number of rhino poaching incidents, then it should have caused an additive decline in the number of rhino poaching incidents on top of the effect caused by the dwindling rhino density, which it did not (Fig. 2B). Moreover, this poaching pressure is not likely to decrease in the near future, because of the high price that consumers are willing to pay for rare animal products, which can drive the species to global extinction (12). The recent changes in South Africa's anti-poaching strategy to prioritize situational awareness, access control, and dehorning of all rhinos in Kruger NP (11) may very well lower the poaching pressure to some extent (13). ...
The decrease in African rhino poaching incidents since 2015 has prompted many to praise the effectiveness of anti-poaching efforts. To test the validity of this statement, we calculated how far poachers moved on average from 2007 to 2022 to find a rhino in the context of the dwindling rhino densities. These calculations demonstrate that the total poaching pressure has remained persistently high since 2013. Given the concurrently declining arrest rates, our results show that the rhino protection practices of the past decade have been insufficient. Instead, we propose that rhinos can best be protected in small and well-monitored “safe havens” while focusing on long-term rhino horn demand reduction.
... In addition, we know that the quantity of products greatly affects the price of products. Holden and McDonald-Madden [2017] considered the relationship between the price of products and its rarity as follows: ...
The rarity of species increases its market price, consequently leading to the overexploitation of the species and even the extinction of the species. We study how the harvest intensity and the additive Allee effect impact on the Gordon–Schaefer model. In addition, by Sotomayor’s theorem and Poincaré–Andronov theorem, we prove the existence of Hopf bifurcation, saddle-node bifurcation and transcritical bifurcation, respectively. Finally, we illustrate our results by numerical simulations. We find that both the cost per unit of harvest and the additive Allee effect have a significant impact on human exploitation of the population. As the additive Allee effect reduces to the weak Allee effect, the lower harvest cost encourages humans to increase the exploitation of species. This threshold is a switch that controls the strong Allee effect. If it exceeds its threshold, then the motivation of humans to exploit the species increases.
... We found some support for the notion that in Indonesia, at a city level, rarity led to higher asking prices as where more Prevost's squirrels were offered for sale the mean price was lower. This would suggest that rarity, as expressed by the number of individuals that are offered for sale in a particular location, does reflect itself in asking prices (Courchamp et al. 2006;Holden and McDonald-Madden 2017) and may suggest an increase in desirability of customers to purchase these squirrels. Contrary to our expectation, we did not find more individuals offered for sale within the species' range (e.g. ...
Squirrels traded for pets or consumption have the potential to be vectors for zoonotic disease transmission and to establish themselves as invasive species. Callosciurus spp. (Greek for beautiful squirrel) are popular in the pet trade due to their medium size and many colour variants, though several feral populations have established themselves in various parts of the world and these wild-caught individuals can carry and spread infectious diseases. Here, we investigate the live trade in Prevost’s squirrels (C. prevosti) in Indonesia where they are naturally found on the islands of Sumatra, Bangka and Borneo. Between 2016 and 2024, we recorded 284 Prevost’s squirrels for sale, viz., 115 in the physical markets and 169 online. We detected the species for sale in Sumatra (13 individuals), Bangka (7) and Borneo (3), i.e. within their natural range, but also on Java (242) and Bali (4). The mean asking price per individual was US$63, and younger individuals commanded higher prices than older ones. Asking prices were not higher in cities further away from the species’ native range nor in cities with higher purchasing power. All individuals were wild-caught, and with a domestic quota of five individuals allowed to be traded as pets per year (45 over the study period) most of this trade is illegal under Indonesian law. This is distinctly different from trade in Prevost’s squirrels in for instance Europe or the USA where legally captive-bred individuals are offered for sale. In Indonesia the illegal trade happens in the open and despite the risk of the spread of zoonotic diseases wherever it is traded there appears to be little incentive on the side of the authorities to curb this trade. Their widespread availability on Java and Bali risks the accidental or deliberate introduction on these two islands. Better coordination is needed between the Indonesian authorities, online sales sites, pet traders and consumers to stop the sale of illegally obtained Prevost’s squirrels to limit the risk of them becoming invasive species or contributing to the spread of emerging infectious diseases.
Most conservation policies and management primarily focus on vertebrate animals. However, considering the high demand for invertebrate species in the exotic pet markets, it is crucial to give them great consideration. This research explores Coenobita purpureus, a land hermit crab newly recorded in Taiwan in 2017. We noticed that it has gained popularity in the online pet market recently, despite limited studies confirming its population. To mitigate the potential risks associated with this species, our study investigated online wildlife trade markets, conducted field surveys for its distribution, and scrutinized relevant regulations in Taiwan. The median price of the species significantly increased after its description. This suggests a growing demand in the exotic pet market, potentially leading to unsustainable trade. Besides, the online platform and coloration of individuals also influenced the price. Furthermore, we discovered that C. purpureus is more widely distributed in Taiwan than initially described in the literature, confirming its native status, though the population may be small. We also identified limitations in current Taiwanese regulations and policies regarding the risk of unsustainable trade in potentially threatened invertebrate species. Moreover, we found evidence of individuals being smuggled from China through e-commerce channels. Regulatory measures addressing the smuggling of small amounts of wildlife are also insufficient, potentially posing invasion risks from alien species. Finally, we drew upon the conclusions from these aspects to provide integrated and practical management implications for policymakers. Additionally, we aim to offer this valuable case study to spotlight the state of the global invertebrate trade.
Defaunation and extinction undermine the resilience and functioning of ecological communities and ecosystems. Relative to other disturbances, overexploitation for the global wildlife trade presents a unique case of trait‐based selection, as demand for specific individuals is often tied to unique morphological or aesthetic traits desired by consumers (‘market traits’). Because evolutionary history leads to species that share both market and functional traits, we posit that non‐random patterns of exploitation will result in non‐random loss of functional diversity.
We applied a trait‐based framework to the global songbird trade for 4616 species, 148 of which are plausibly threatened by the trade. We quantified select market traits, such as unique coloration and song quality, and ecological traits related to body size, diet, and foraging strategy to assess whether the trade disproportionately threatens particular functional groups. We additionally looked for patterns of association between market traits and functional traits to assess whether selection on certain market traits could drive selection on associated functional traits.
We found that overexploited birds are a distinct functional subset of the global songbird pool, with the trade disproportionately threatening large bodied, frugivorous, and seed eating birds. Across all songbirds, there were multiple non‐random associations between market traits and functional traits, with the strongest associations observed among trade‐threatened birds; this was consistent with our theory that consumer‐driven selection on market traits could theoretically result in selection on functional traits. However, there was mixed evidence for this hypothesis at a global scale, suggesting that disproportionate threat to functional diversity may be more likely in regions where there is heavy demand for associated market traits.
Policy implications. Our results highlight the need for increased focus on the mechanistic drivers of trait‐based selection on the consumer side of wildlife trade, and how patterns of overexploitation can systematically affect ecological communities and ecosystem services.
Read the free Plain Language Summary for this article on the Journal blog.
This chapter examines the Parisian cemetery Père Lachaise by discussing its unique position as a heritage, funerary and recreational nature site, and the narratives that derive from its memorial standing and environmental situation. To that end, the arguments focus on how the cemetery’s memorial and funerary narratives are adapted in response to Anthropocenic disruptions. The chapter thus proposes to survey Père Lachaise through the spectrum of the Anthropocene to examine the paradigm shifts that redefine institutional sites of death and memory in an era of ecological and planetary decay.
Significance
Many threatened species including elephants, sturgeons, and bluefin tunas are harvested for high-value products. Species can be driven extinct if incentives to harvest do not diminish as populations decline; this occurs if harvest prices rise faster than costs with declining stock. Whereas recent conservation attention for these species has largely focused on market demand, we show—using a theoretical model and an empirical review—that contractions in species’ geographic ranges, which stabilize costs and may be especially common among terrestrial species, might often play a larger role in maintaining harvest incentives. Forces impacting ranges—such as patchy and declining habitats, schooling/herding behavior, and climate change—therefore merit greater attention in assessing overharvesting threats.
The threats of old are still the dominant drivers of current species loss, indicates an analysis of IUCN Red List data by Sean Maxwell and colleagues.
The demand for wildlife products drives an illegal trade estimated to be worth up to $10 billion per year, ranking it amongst the top transnational crimes in terms of value. Orchids are one of the best-selling plants in the legal horticultural trade but are also traded illegally and make up 70% of all species listed by the Convention on the International Trade in Endangered Species (CITES). To study consumer preferences for horticultural orchids we use choice experiments to survey 522 orchid buyers online and at large international orchid shows. Using latent class modelling we show that different groups of consumers in our sample have distinct preferences, and that these groups are based on gender, genera grown, online purchasing and type of grower. Over half of our sample, likely to be buyers of mass-produced orchids, prefer white, multi-flowered plants. Of greater conservation interest were a smaller group consisting of male hobbyist growers who buy their orchids online, and who were willing to pay significantly more for species that are rare in trade. This is the first in-depth study of consumer preferences in the international orchid trade and our findings confirm the importance of rarity as a driver of hobbyist trade. We show that market-research methods are a new tool for conservationists that could provide evidence for more effective conservation of species threatened by trade, especially via campaigns that focus on demand reduction or behaviour change.
Evaluations of wildlife population dynamics have the potential to convey valuable information on the type of pressure affecting a population and could help predict future changes in the population's trajectory. Greater understanding of different patterns of population declines could provide a useful mechanism for assessing decline severity in the wild and identifying those populations that are more likely to exhibit severe declines. We identified 93 incidences of decline within 75 populations of mammalian species using a time-series analysis method. These included linear, quadratic convex (accelerating) declines, exponential concave (decelerating) declines and quadratic concave declines (representing recovering populations). Excluding linear declines left a data set of 85 declines to model the relationship between each decline-curve type and a range of biological, anthropogenic and time-series descriptor explanatory variables. None of the decline-curve types were spatially or phylogenetically clustered. The only characteristic that could be consistently associated with any curve type was the time at which they were more likely to occur within a time series. Quadratic convex declines were more likely to occur at the start of the time series, while recovering curve shapes (quadratic concave declines) were more likely at the end of the time series. Synthesis and applications. The ability to link certain factors with specific decline dynamics across a number of mammalian populations is useful for management purposes as it provides decision-makers with potential triggers upon which to base their conservation actions. We propose that the identification of quadratic convex declines could be used as an early-warning signal of potentially severe decline dynamics. For such a population, increased population monitoring effort should be deployed to diagnose the cause of its decline and avert possible extinctions. Conversely, the presence of a quadratic concave decline suggests that the population has already undergone a period of serious decline but is now in the process of recovery. Such populations will require different types of conservation actions, focussing on enhancing their chances of recovery.
All three surviving elephant species are facing dramatic declines. Poaching driven by consumer demand for ivory in Asia is an important driver of their demise especially in Africa, making their plight a global issue, which has to be addressed by international policy. Michael Gross reports.
Understanding the traits that drive the demand for exotic pets is crucial for improving our ability to prevent the introduction of new invasive species. We investigated the factors influencing species availability within the Australian cagebird trade. We predicted that species price should be an informative indicator of the species abundance in the trade, and that physical attributes, origin of the species, and limitations for free trade and for captive breeding should be important drivers for species price. We investigated the quantitative relationship between species price and these factors using phylogenetically informed models.
The overexploitation of wildlife species is a serious problem in the field of biodiversity conservation. The species subjected to natural Allee effects are even more threatened by exploitation. Moreover, for many wildlife species, their rarity can fuel their exploitation by making them disproportionately desirable and consequently increasing their market price. In this paper, a mathematical model is proposed and analyzed to study how the value that consumers place on rarity can threaten the survival of a species subjected to natural Allee effects. It is assumed that the value of a species increases as its density declines. The analysis of model shows that the increase in the consumers' response to rarity can drive the system to admit Hopf-bifurcation and heteroclinic bifurcation. The occurrence of the heteroclinic cycle indicates that the increase in consumers' response to rarity can cause the extinction of the species. It is found that an increase in the Allee threshold causes a decrease in the threshold value of consumers' response below which extinction is inevitable.
Kenya's government delivered a powerful message against elephant poaching and the illegal ivory trade on 30 April by burning 105 tonnes of ivory, worth up to US$220 million. With stockpile destruction on the rise, it is important to evaluate the impact of this strategy on elephant populations.
Since 1989, 21 countries have burned or crushed 263 tonnes of ivory — most of it (86%) in the past 5 years (see go.nature.com/ivory). However, there is no published evidence so far that these events reduce poaching.
Destroying ivory stockpiles risks a perverse outcome: ivory becomes rarer, fetching higher prices and increasing poaching and illegal stockpiling (see M. 't Sas-Rolfes et al. Pachyderm 55, 62–77; 2014). This has prompted calls by some for a highly controlled legal ivory trade to secure elephant populations (J. F. Walker and D. Stiles Science 328, 1633–1634; 2010) — an option that ivory destruction removes.
It is therefore crucial to track the effects of Kenya's largest-ever ivory burn. Time is short and the stakes are high.
Evaluations of wildlife population dynamics have the potential to convey valuable information on the type of pressure affecting a population and could help predict future changes in the population's trajectory. Greater understanding of different patterns of population declines could provide a useful mechanism for assessing decline severity in the wild and identifying those populations that are more likely to exhibit severe declines.
We identified 93 incidences of decline within 75 populations of mammalian species using a time‐series analysis method. These included linear, quadratic convex (accelerating) declines, exponential concave (decelerating) declines and quadratic concave declines (representing recovering populations). Excluding linear declines left a data set of 85 declines to model the relationship between each decline‐curve type and a range of biological, anthropogenic and time‐series descriptor explanatory variables.
None of the decline‐curve types were spatially or phylogenetically clustered. The only characteristic that could be consistently associated with any curve type was the time at which they were more likely to occur within a time series. Quadratic convex declines were more likely to occur at the start of the time series, while recovering curve shapes (quadratic concave declines) were more likely at the end of the time series.
Synthesis and applications . The ability to link certain factors with specific decline dynamics across a number of mammalian populations is useful for management purposes as it provides decision‐makers with potential triggers upon which to base their conservation actions. We propose that the identification of quadratic convex declines could be used as an early‐warning signal of potentially severe decline dynamics. For such a population, increased population monitoring effort should be deployed to diagnose the cause of its decline and avert possible extinctions. Conversely, the presence of a quadratic concave decline suggests that the population has already undergone a period of serious decline but is now in the process of recovery. Such populations will require different types of conservation actions, focussing on enhancing their chances of recovery.