(Science news release Web site: http://www.unl.edu/pr/science.html)
Lincoln (Neb.) - Oct. 8, 1998 - Moths that are the prey of blue jays may not appreciate the fact, but without the bird's insatiable appetite for their kind, at least some of their close relations might become extinct.
University of Nebraska-Lincoln behavioral biologists Alan B. Bond and Alan Kamil report in this week's issue of the international science magazine, Nature, that blue jays' ability to detect their prey is important to population equilibrium among different types of the same moth species, and to polymorphism, which is the continued existence of the same moth in different forms.
Bond and Kamil said in their article, "Apostatic Selection by Blue Jays Produces Balanced Polymorphism in Virtual Prey," that they conducted a series of experiments aimed at displaying the effects of predation on a "virtual population" of moths displayed as images on computer screens.
In nature, blue jays commonly prey on cryptic moths, insects with muted colors and complex patterns that camouflage them on tree bark. In the experiments, the jays were real, but their prey was digital, produced by generating random modifications of photographs of closely related moth types.
The digitized images were - at least to human eyes - disparate in appearance. When presented to the birds via a computer monitor, the digitized moths were rendered difficult to detect by presenting them on complex, randomized backgrounds with similar pixel intensities and pattern irregularities. (A pixel is any of the small discrete elements that together constitute an image). When the jays correctly pecked at a "moth," they were rewarded with a worm.
The researchers conducted a series of experiments involving a virtual population of these digital moths. In this cybernetic ecosystem, moths were exposed to predation by displaying them one at a time on computer screens. Those that the jays detected were considered "killed." Those that survived each day's encounter with the birds were allowed to "breed," bringing the population back up to its original numbers for the following day. In each experiment, the researchers varied the initial proportions of different moth types, but did not otherwise manipulate the system. The resulting dynamics were, therefore, a realistic reflection of how predators may interact with prey populations.
Bond and Kamil started with a population consisting of equal numbers of three moth types, one of which had markings more cryptic than the others' and was therefore more difficult to detect. After 50 generation days, the most cryptic type had increased to about 75 percent of the population, while the other two moths dropped to around 12 percent apiece. In two subsequent replications, the number of individuals of one of the less cryptic moth types started out at 75 percent of the population, with the other two making up the remainder. Each time, the most cryptic moth rapidly returned to 75 percent of the population.
The researchers determined that the stabilization effect came about because the birds were much more accurate at detecting common moth types, while they tended to overlook rare ones. The result was overselection of moths with high abundances, allowing those of lesser numbers to increase and thereby become targets themselves.
Bond and Kamil concluded from their experiments that changes in prey detection in and of itself can produce stability and also maintain prey polymorphism - the continued existence of the same species of moth in different forms.
The advantage for the predator in concentrating on the most abundant moth type is that it temporarily improves its ability to detect that moth, maximizing its short-term success. When the population reaches equilibrium, however, each moth type runs an equal risk of detection. Thus, the long-term consequence is to generate a distribution of prey populations that yields a significantly reduced rate of return.
The introduction of novel prey items led to the researchers finding that some prey might face extinction as the result of the predators' inability to detect them.
In their first experiment involving novel prey, Bond and Kamil introduced prey items that were dissimilar to the original population. In one case, this produced a delayed response to detection: the new prey was overlooked for a number of generations and increased until it comprised a substantial portion of the population. When the novel prey finally drew the birds' attention, however, the jays came to concentrate on it and drove its numbers down to the levels of the two less cryptic types in earlier experiments.
However, the researchers found that this was not always the outcome of introducing novel prey. A highly cryptic new moth type found few blue jays able to detect it even after 30 generations. This novel moth increased to dominate the population and was maintained at maximum levels throughout the experiment. In the real world, the researchers noted, "this situation would have driven the other moths to extinction."
Those with World Wide Web access can see if they're better at detecting moths than Bond and Kamil's blue jays at (http://www.abcnews.com/sections/science).
The article in Nature is the second this year in which Kamil
is listed as a co-author. Last November, he and Juli Jones, a
graduate student, published an article about their experiments
with Clark's nutcrackers in which they found that the bird used
geometry and spatial memory to search for buried seeds.
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