Understanding competition dynamics between organisms vying for limited resources is important for the proper characterization of ecological relationships. Competition in the natural world can be categorized into interspecific and intraspecific competition. Interspecific competition occurs when two different species compete for resources, while intraspecific competition occurs when there is competition within the same species. Intraspecific and interspecific competition can occur via two different mechanisms: exploitation or interference. Exploitation occurs when resource acquisition by one organism differs from another, while interference occurs when one organism directly hinders the ability of the other to access a resource (Denno et al., 1995).
Studies examining the impact of interspecific competition report that two species which inhabit a similar ecological niche and exhibit similar behaviors show increased competition that often deleteriously affects one of the species, this is known as the competitive exclusion principle (Ricklefs and Relyea, 2014). In the case of a new species invading another’s habitat, this effect can potentially result in competitive displacement. That is, the inferior competitor can be displaced by the superior organism (Reitz and Trumble, 2002). One study examining this effect, simultaneously introduced two species of moths, Corcyra cephalonica and Ephestia cautella (both of which are ecologically similar), into a controlled environment with limited food. By the second generation, the researchers reported the near extinction of E. cautella. Even when C. cephalonica and E. cautella were introduced to an environment where food was abundant, C. cephalonica was again the superior competitor. However, in conditions of abundant food, the detrimental impact on the inferior species was not as severe. The researchers concluded that since both species used the same food resource and had similar lifecycles, competition between the two species resulted in the dominance of the better competitor (Allotey, 1985). Features which make one species a better competitor than another include behavioral traits, such as aggressiveness, and physical traits, such as size and rate of food consumption (Crombie, 1947). In a study testing interspecific competition for space and food between parasitoid wasps, Microplitis prodeniae and Campoletis chlorideae, researchers observed that C. chlorideae was the superior competitor. The study was conducted in a tobacco field where the host to these wasps, Spodoptera litura (the oriental leafworm moth), was present. They concluded that the dominance of C. chlorideae could be attributed to its greater efficiency at controlling sites of food acquisition, most likely due to the physically larger size of adult C. chlorideae, which averages around 5.3 mm, versus M. prodeniae, which averages around 3.35 mm (Zhou et al., 2010). In another study examining interspecific competition for food, researchers used the larva of the two mosquito species: Aedes aegypti and Culex quinquefasciatus. The larva of the two species cohabited in a laboratory setting with access to a controlled amount of nutrients, ranging from either 2.83 mg/larva to 0.95 mg/larva. In conditions of 2.83 mg/larva, where equal numbers of larva from each species cohabited (15 larvae per species), the researchers observed no clear dominance of one species over the other. However, in conditions of limited food (0.95 mg/larva), Ae. aegypti out-competed Cx. quinquefasciatus. The authors concluded that these effects were likely due in part to the different methods of food acquisition between the two species and the time each species spends actively searching for food, two characteristics in which Ae. aegypti is more efficient than Cx. Quinquefasciatus (Santana-Martínez et al., 2017).
In a study examining competition in armored scale insects (Fiorinia externa and Tsugaspidiotus tsugae) researchers documented the impact of both interspecific and intraspecific competition. The study was conducted using both a greenhouse setting and field observations in the Darien, Connecticut hemlock forest. Because both species use hemlock as a food source, when population sizes of the two species increase, food becomes a limiting resource, and the levels of inter- and intraspecific competition rise. Results from this study revealed the superior competitor to be F. externa. The researchers concluded that the competitive advantage of F. externa derives from their ability to colonize the hemlock foliage before T. tsugae. By doing so, F. externa monopolizes the food source and decreases the availability of this resource for T. tsugae (McClure, 1980). When analyzing intraspecific competition for food between larvae of the moth Lobesia botrana, one study observed increasing larval density was positively correlated to larval mortality. The researchers tested four different conditions, providing the same amount of food to 1, 5, 10, and 20 larvae. At the largest density (20 larvae), the researchers observed the lowest survival rate of 59%, as compared to the smallest density of 1, which after 72 trials, exhibited the highest survival rate of 93%. The researchers speculated that higher density conditions were associated with increased mortality due to an increase in aggressive interactions between individuals as the result of overcrowding. These interactions may have stressed the larvae, causing them to consume less food and, as a result, be less capable when faced with aggressive competitors (Thiéry et al., 2014).
The purpose of the current experiment is to examine both intra- and interspecific competition for a limited food source by studying the larvae of two species of beetles: Tenebrio molitor and Zophobas morio. While both species are members of the Tenebrionidae family, the species Z. morio is noticeably larger, and thus, colloquially known as superworms. Studying the effects of intraspecific competition was accomplished by housing each species separately at increasing densities with a small amount of available food, while interspecific competition was studied by housing the two species together with a small amount of food. The effect of intra- and interspecific competition for a limited food source was determined by calculating the organisms’ percent change in mass. Because increased population density results in increased levels of competition (McClure, 1980) and potentially decreased levels of feeding (Thiéry et al., 2014), it is predicted that higher levels of intraspecific competition associated with higher densities of mealworms will result in lower average mass for these larvae. Since ecologically similar species inhabiting the same environment compete for resources (Allotey, 1985), it is predicted that T. molitor will compete with Z. morio for the limited food. As some larger organisms have been shown to be more efficient at food acquisition (Zhou et al., 2010), and species with more efficient methods of food acquisition are superior competitors (Santana-Martínez et al., 2017), it is predicted that Z. morio will be the superior competitor via exploitation mechanisms.