Abstract
Competition is a driving force regulating communities often considered an intermittent phenomenon, difficult to verify and potentially driven by environmental disturbances. Insecticides are agents of environmental disturbance that can potentially change ecological relationships and competitive outcomes, but this subject has seldom been examined. As the co-existing cereal grain beetle species Sitophilus zeamais Motschulsky and Rhyzopertha dominica F. share a common realized niche, directly competing for the same resources, they were used as models in our study. Intraspecific competition experiments were performed with increasing insect densities and insecticide doses in additive and replacement series using various density combinations of both beetle species maintained on insecticide-free or -sprayed grains. Insecticide-mediated release from competitive stress was not observed in our study of intraspecific competition in grain beetles. The insecticide enhanced the effect of insect density, particularly for the maize weevil S. zeamais, further impairing population growth at high densities. Therefore, insecticide susceptibility increased with intraspecific competition favoring insecticide efficacy. However, the effect of insecticide exposure on competitive interaction extends beyond intraspecific competition, affecting interspecific competition as well. Sitophilus zeamais was the dominant species when in interspecific competition prevailing in natural conditions (without insecticide exposure), but the dominance and species prevalence shifted from S. zeamais to R. dominica under insecticide exposure. Therefore, high conspecific densities favored insecticide efficacy, but the strength of the relationship differs with the species. In addition, the insecticide mediated a shift in species dominance and competition outcome indicating that insecticides are relevant mediators of species interaction, potentially influencing community composition and raising management concerns as potential cause of secondary pest outbreaks.
Highlights
Competition, as a mutually negative interaction between two species sharing the same guild or trophic level, results in reduced abundance or in a decrease in fitness components of the competing species, potentially regulating animal communities [1,2,3,4]
Intraspecific competition A multivariate analysis of covariance indicated overall significant effects of grain beetle species, insect density and insecticide dose on the response variables assessed (P,0.05)
The maize weevil S. zeamais prevailed without insecticide application, exhibiting higher population growth than R. dominica, but both species were affected by conspecific density (F1,36 = 2.45, P = 0.13; Fig. 1a)
Summary
Competition, as a mutually negative interaction between two species sharing the same guild or trophic level, results in reduced abundance or in a decrease in fitness components of the competing species, potentially regulating animal communities [1,2,3,4]. Small change(s) in the shared (realized) niche between two species may compromise the optimal development of one of them, altering the outcome of competition and potentially determining a shift of the prevailing or dominant species [8,9,10,11,12,13]. Environmental disturbances, either natural or artificial, can affect ecological interactions and produce changes in the (realized) niche shared by competing species [11,12,13,14]. Pesticides, insecticides, are seasonal and intermittent agents of environmental disturbance that may alter ecological relationships, analogous to the effects of a flood or storm in a natural community [13,14,15,16,17]. The recovery time can depend on the ability of the species to survive and develop in the contaminated environment
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