Rarity value and species extinction: the anthropogenic Allee effect

Abstract

Standard economic theory predicts that exploitation alone is unlikely to result in species extinction because of the escalating costs of finding the last individuals of a declining species. We argue that the human predisposition to place exaggerated value on rarity fuels disproportionate exploitation of rare species, rendering them even rarer and thus more desirable, ultimately leading them into an extinction vortex. Here we present a simple mathematical model and various empirical examples to show how the value attributed to rarity in some human activities could precipitate the extinction of rare species-a concept that we term the anthropogenic Allee effect. The alarming finding that human perception of rarity can precipitate species extinction has serious implications for the conservation of species that are rare or that may become so, be they charismatic and emblematic or simply likely to become fashionable for certain activities.

Figure 1. Demonstration of an Anthropogenic Allee Effect with a Simple Model of an Exploited Population

The price (red, thick line) and cost (blue, thin line) per unit harvest in unit time as a function of the population density x when (A) the price is independent of x and (B) the price increases with rarity. The system is in equilibrium whenever the red and blue lines meet, and the bold arrows represent how the population responds when perturbed from equilibrium. In (B), an increased price at low population density induces an Allee Effect.

Figure 2. Empirical Examples of Activities where the Price of Species Is Related to Rarity or Perceived Rarity

(A) Prices of collectible butterflies in Papua New Guinea (modified from [9]). (B) Hunting trophy prices of 57 Caprinae taxa. (C) Selling prices of exotic pet species according to CITES status. Species that have a CITES status (open squares) are more expensive than species with no CITES status (solid squares). Prices were standardized by dividing by (A) male wingspan, (B) trophy size, and (C) adult weight (see text for details). Vertical bars: standard error; sample size in parentheses.

Figure 3. California Commercial White Abalone Haliotis sorenseni Landings for 1972–1992

The price exponentially increased as catch decreased. The catch takes the fishing effort into account, so that decreased catch does not come from a diminishing effort. Results are given as weight in shell divided by the number of ships (open squares) and price (filled circles) [27].

Figure 4. Illustration of the Increased Demand Associated with Perceived Rarity

Volumes of illegal trade of four species examples [a bird (A), two plants (B and C), and a reptile (D)], which have undergone a change in CITES status. The year that the change in status took effect is indicated by the red line.