In 1958, Carl B. Huffaker, an ecologist and agricultural entomologist at the University of California, Berkeley, did a series of experiments with predatory and herbivorous mite species to investigate predator–prey population dynamics. In these experiments, he created model universes with arrays of rubber balls and oranges (food for the herbivorous mites) on trays and then introduced the predator and prey mite species in various permutations. Specifically, Huffaker was seeking to understand how spatial heterogeneity and the varying dispersal ability of each species affected long-term population dynamics and survival. Contrary to previous experiments on this topic (especially those by Georgii Gause), he found that long-term coexistence was possible under select environmental conditions. He published his findings in the paper, "Experimental Studies on Predation: Dispersion Factors and Predator–Prey Oscillations".
Experimental design The aim of Huffaker’s 1958 experiment was to “shed light upon the fundamental nature of predator–prey interaction” and to “establish an ecosystem in which a predatory and a prey species could continue living together so that the phenomena associated with their interactions could be studied in detail”. He used two mite species, the six-spotted mite Eotetranychus sexmaculatus as the prey species and Typhlodromus occidentalis as the predatory species. Oranges provided a background environment and a food source for the herbivorous mites. The amount of available food on each orange was controlled by sealing off portions of each orange using damp paper and paraffin wax. Huffaker introduced patchiness into the system by replacing oranges with rubber balls of a similar size. He referred to the resultant systems as "universes." Huffaker created a series of 12 universes in his experiment, trying different arrangements to reach a universe in which the predator population would not annihilate the prey population, and in which, instead, the two species could coexist.
Part 1: Control universes (No predators present) Huffaker set up three different "universes" to examine the effect of patchiness on the prey species' population dynamics. 20 prey species mites were initially placed on one orange; their population was observed and recorded over several weeks. Huffaker noted that one source of error was the difference in nutritional value between oranges. Oranges were replaced every 11 days. The mites had the chance to deplete the nutritional value of the oranges completely before replacement. Temperature and humidity were kept constant at 83 °F (28 °C) and above 55%. Universe A: 4 half exposed oranges clumped together. Universe B: 4 half exposed oranges spread between 36 orange balls. Universe C: 20 oranges with 1/10 exposed alternated with 20 rubber balls. Huffaker found that mites migrated to new oranges only when the original orange habitat and food source had been depleted or overpopulated. Each universe produced a fluctuating mite population due to complete exploitation of orange resources leading to population spikes followed by rapid declines. Universe B had more stable oscillations, but a lower average population. Huffaker attributed this to the combination of dispersal difficulty and food supply; the other two universes only dealt with food supply because the food sources were so close together that dispersal did not play a major role in population dynamics.
Part II: Universes with prey and predator species Huffaker created nine different universes to examine the effect of predator–prey interaction and spatial heterogeneity on populations of predator and prey species. Prey species were always added several days before predator species were added. Predator species were placed on oranges colonized by prey species. Additionally, Huffaker spread petroleum jelly on the trays between the oranges and rubber balls to serve as an "impediment but not an exclusion to movement" of the mites, creating heterogeneity. These universes fell into three groups.
Group 1: Clumped food source spacing In this group the orange food sources were close together, requiring very little migration effort by either prey or predator species. Changes between the universes were only in amount of food available for the prey species. The effect of food source abundance on population dynamics was tested here. Universe A: 4 half exposed oranges close together, the same as universe A of part 1. Universe B: (increased food source) 8 half exposed oranges grouped together and joined by wires. 40 mites were released initially, 20 mites on 2 oranges. Universe C: (increased food source again) 6 wholly exposed oranges grouped together. Prey introduced on 2 of the oranges. Predators introduced on only one orange.
Group 2: Interrupted food source spacing In this group the food sources were not continuous, but were dispersed among rubber balls. This added an element of difficulty for dispersal of prey and predator species. The effects of spatial heterogeneity and mites species' searching ability on predator and prey population dynamics were tested here. Universe D: (food source dispersal introduced) 4 half exposed oranges dispersed randomly among 36 rubber balls. (difficulty reaching other oranges) Universe E: (increase food source) 8 half exposed oranges dispersed among 32 rubber balls. Universe F: 20 oranges with 1/10 exposed alternated with 20 rubber balls
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