Plant neighbourhood effects on insect herbivores associated with Junipers and Pistacia in Cyrenaica- Libya

Abstract

This research set out to test whether there are neighbourhood effects on insect herbivory among the plants of the Al Jabal Al Akhdar region of Libya. Neighbouring plants in the locale of an individual plant might help or hinder it in the task of defence against insect herbivores, depending on their levels of defence (chemical or physical), and their interactions with potential herbivores. Such ‘neighbourhood effects’ are part of the complex network of species interactions that structure ecological communities. Having chosen to concentrate on the two main species of three study sites, Juniperus phoenicea and Pistacia atlantica, nine plots were mapped in detail and the insect herbivores sampled from focal plants, and then from all plants. Plant leaves of Juniperus and Pistacia were sampled for chemical analysis of their phenol content. Phenol levels were much higher in Pistacia (2308 ± 43 ppm, n = 45), than in Juniperus (1516 ± 43 ppm, n = 45), and there were effects of elevation as well, plants from middle elevation plots had the highest levels, while those from the lowest elevation at the coast had the lowest levels of tannins. The number of individual insects per plant was affected by the neighbourhood in both Pistacia No. individuals on the nearest conspecific (x2 = 5.22, df = 1, P = 0.022) and No. individuals on the nearest allospecific (x2 = 52.12, df = 1, P = 0.00) and Juniperus No. individuals on the nearest conspecific (x2 = 43.89, df = 1, P = <.001) and No. individuals on the nearest allospecific (x2 = 93.89, df = 1, P = <.001). There were clear effects of neighbouring plants on the density of insects of individual plants, in both Pistacia and Juniperus. From analysis result we found that the number of insect herbivores were a strongly affected by the local neighbourhood plant species, which attacked the focal shrubs (Juniperus sp and Pistacia sp) in the study sites, where the number of insect herbivores were not affected by the same neighbourhood (surroundings) plant species, however, the different neighbourhood plant species affected in each insect herbivores. There were no signs of any protective neighbourhood effects of tannin levels. Thus in the plant communities of Al Jabal Al Akhdar, associational avoidance appears to be the major mechanism of neighbourhood effects, rather than associational resistance.