Landmarking: An extremely efficient strategy for avoiding demographic Allee effects

Background/Question/Methods

Difficulty in mate-finding at low densities can be a stumbling block for the establishment of non-native species. We examine the impact of one little-studied mechanism of mate-finding, “landmarking”, on demographic Allee thresholds, with an eye to predicting invasion risk. Rather than being attracted by signalling cues from the opposite sex, or being driven to aggregate at a high density of conspecifics, individuals may simply move towards particular locations in the landscape. These locations may have little or no habitat value, but can be detected from long distances. Butterflies employ this strategy (i.e., hill-topping), as do many other groups (bees, flies, fish, birds). The encounter probability of a male and female at a mating location can described using the birthday paradox: the probability that at least two people in a group of individuals will share a birthday. This probability is of import when calculating the chance that airplanes will collide, or that there are matches between random length clone fragments in DNA mapping. We use more recent work on a variant of the birthday problem that considers two different kinds of objects in order to calculate the probability that a male and female will meet at a particular location

Results/Conclusions

Species that use a limited number of sites with high detection probability to locate mates can have very high mating success, and reduced strong Allee thresholds. For a 1:1 sex ratio, the probability of at least one matched male and female pair in a mate-finding location increases as the population size increases. However, the highest probabilities for any given population size are found where there are the fewest possible mate-finding sites. Therefore, the expected number of mated females is greatest, and the strong Allee threshold is smallest, for the fewest number of mate-finding sites. This finding may seem counter intuitive, but it is based on the not unreasonable assumption that individuals have equal probability of locating any potential mate-finding site. Therefore, it is unlikely that landmarking species will have a strong Allee threshold created solely by mate-finding difficulties. If such a threshold does exist, it is likely to be very low, and hard to reach via management actions. Moreover, control actions that focus on reducing suitable mating habitat in an effort to reduce reproduction may have the opposite of the intended effect. We conclude that such "landmarking" is a strategy to be reckoned with when predicting invasive risk.