Allee Effects at the Individual Level

The simplest definition of an Allee effect is an increasing relationship between some component of fitness (e.g., reproductive success, growth, or survival) and some measure of the number of conspecifics (e.g., group size, density) with which an individual interacts (Figure 2). As the number of interactors increases, the benefits arising from interactions level off. Ecologists expect that fitness also declines as a function of the number of interactors as a consequence of competition and other costs, again leveling off at high interaction levels. The net fitness (the product of the two curves) results in a unimodal or humped relationship between fitness and the number of interactors. Although this unimodal pattern has also been termed an Allee effect, it is the beneficial effects of interactions upon fitness that truly define Allee effects. Table 1 provides a number of examples of Allee effects exhibited by a wide range of species. This wide variety of benefits indicates that Allee effects are as fundamental to organismal biology as competition. Consider just one example - the ability to locate a mate. Success at mate

Number of interactors

Figure 2 Fitness as a function of the number of conspecifics with which an individual interacts. Both Allee effects (bold line), and competition (dashed line) can occur, resulting in a net unimodal fitness relationship (solid line) as a function of the number of interactors.

Table 1 Examples of different types of Allee effects that accrue as functions of different measures of conspecific interactions

Fitness measure

Best measure of interactions

Examples11

Reproduction

Mate finding

Density

Butterflies

Fertilization

Density

Sea urchins,

success

flowering plants

Offspring care

Group size

Mongoose

Growth

Prey detection

Group size

Shoaling fish

Prey capture

Group/

Wild dogs

neighborhood size

Survival

Predator

Group/

Flocking birds

detection

neighborhood size

Predator

Group/

Schooling fish

avoidance

neighborhood size

Thermoregulation

Group/

Bobwhite quail

neighborhood size

aCited in Liermann M and Hilborn R (2001) Depensation: evidence, models, and implications. Fish and Fisheries 2: 33-58.

aCited in Liermann M and Hilborn R (2001) Depensation: evidence, models, and implications. Fish and Fisheries 2: 33-58.

finding will determine whether an individual contributes genes to future generations. Therefore, the only species not subject to this particular type of Allee effect are clonal or parthenogenetic organisms.

The above definition stresses the number of interactors as the measure by which Allee effects are reckoned. Many ecologists have focused on population size or population density (the number of individuals per unit area) as the best measure, but this may not always be the case (Table 1). For example, suppose an Allee effect existed in which prospecting females preferred areas with multiple male territories, each 1 ha in area. Consider two forests with territories: the first is 1 ha, and the second is 10 ha. Both have the same population density of males, but only the second has a functioning neighborhood. Likewise,

suppose an Allee effect occurred in an insect species such that associating with conspecifics reduced the likelihood of predation. Total population size might be a misleading measure because the scale at which the Allee effect operates (the scale at which the insect interacts with its conspecifics and predator) occurs at a much smaller scale than the population measure. These examples illustrate that the best measure to examine Allee effects will depend both on the nature of the fitness component and how individuals interact to change that component. In some cases (e.g., interactions of plants via their pollinators), conspecific density may be the best measure, but in others group or neighborhood size may be a much better measure of Allee effects than density.

If net fitness benefits follow the unimodal pattern created by Allee effects, individuals choosing an area to live would maximize their fitness by settling in locations with a medium number of interactors. In fact, individuals might choose even higher, suboptimal levels as long as their net fitness surpassed the fitness resulting from settling alone. Evidence of this behavior, called conspecific attraction, has been found in many animals across most phyla. The consequence is an aggregated (or contagious) dispersion pattern and subdivision of interactions among individuals in the population.

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