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High prices for rare species can drive large populations extinct: the anthropogenic Allee effect revisited

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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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... For simplicity, it is assumed that the revenue of per-unit-effort to attack species i, r i , and the cost of per-unit-effort to attack species i, c i , are both constant. Strong evidence shows that the price is a function of species abundance and market supply (Holden and McDonald-Madden, 2017). However, the assumption about constant unit revenue is reasonable, because a specific time duration for running the model is considered. ...
... Note that, like many related works (Holden and McDonald-Madden, 2017), the population size is assumed to be continuous; however, rescaling to discrete measures is relatively straightforward. ...
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... Additionally, an analysis of CITES species up-listed from Appendix II to Appendix I between 1980 and 2003 reveals that while trade volumes decreased sometime after the commercial ban on trade had been enforced, trade volumes spiked one year before the ban came into effect, with an average trade volume increase of 135% compared to the previous year (Rivalan et al., 2007). This phenomenon can be characterized by what is known as the anthropogenic Allee effect, wherein an increased rarity of species causes the demand to increase at the same time, in spite of the high cost of procurement (Holden & McDonald-Madden, 2017). Along these lines, some experts argue that the CITES ivory trade ban encouraged poaching and smuggling of African elephant ivory in order to meet the demand for ivory in the absence of legal trade (Stiles, n.d.). ...
... Australian desire for alien pets is substantially biased towards threatened species (Toomes et al. 2020), and increased trade demand for threatened species is a known component of international wildlife trade (Courchamp et al. 2006;Holden and McDonald-Madden 2017). ...
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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. 2.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. 3.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. 4.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, focussed on enhancing their chances of recovery. This article is protected by copyright. All rights reserved.