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The complex formed by phytophagous insects, their host plants and their predators is one of the most important component of ecosystems biodiversity. In this work the role of this complex on the biodiversity of the arid region of Chile was studied. It is postulated that a higher biodiversiyty of phytophagous insects would be associated to more favorable climatic conditions, higher host plant diversity, and higher nitrogen, ash and water content of the vegetation. Also, a higher biodiversity of predators and parasitoids will be sustained by a higher phytophagous insect biodiversity. The study was conducted in the II Region of Chile through an altitudinal transect crossing the following bioclimatic zones from the coast (0 m of altitude) to the Andes (4,000 m of altitude): Coastal Desert (or Desierto Literal, DL), Continental Desert (or Desierto Interior,.DI), Pre-Andean Tropical (or Tropical Marginal, TM), and Andean Tropical (or Tropical de Altura, TA). Canopy arthropods were collected by beating plant foliage. Meanwhile defoliating, mining and cecidia forming insects were sampled by collecting them directly from plant organs. A total of 97 plant species distributed in 28 families were collected and identified. The DL had the lowest plant biodiversity, meanwhile DI and TM ecological zones were the richest. Most of DL plant species were unique, meanwhile most of DI vegetation was also included in TM At family level, Asteraceae was the most diverse, followed by Chenopodiaceae, Solanaceae, Nolanaceae, Fabaceae and Portulacaceae in a decreasing order. Asteraceae and Chenopodiaceae were distributed over all ecological zones. Both, water and ash leaf and stems content varied considerably among ecological zones, according to the following decreasing gradient: DL > DI > TM > TA. On the contrary, nitrogen leaf content was less variable, with the exception of TA zone species having very low values. Nitrogen stems content was more homogeneous than leaf content. From a global point of view, a decreasing gradient for all these nutritional elements was found from DL to TA zones, closely related to altitudinal gradient. At family level, Asteraceae showed a decreasing gradient of water content from coast to Andes. Meanwhile, their leaf ash content was significantly different among DL and TM species only. Stems ash content was not significantly different. A total amount of 12,893 canopy arthropod specimens were collected, distributed among 464 morphospecies and 19 zoological groups, considering Homoptera as a group. Homoptera, Hemiptera and Lepidoptera larvae showed a higher species richness among phytophagous insects, as well as Hymenoptera among parasitoids and Araneae among predators. Both, at species and number of individual level, DL and TM presented a higher concentration of arthropods, as well as number of plant species where they were collected. From a species point of view there were minimal similarities among ecological zones. Nevertheless, when individual abundance is considered, a close association between DI and TM, and the isolation of DI were detected. The same was also detected for vegetation species. Considering global diversity values (H’) of arthropods, the following decreasing gradient was found: DL, TM, TA and DI, closely related to plant richness.

As a group, Homoptera was the most broadly distributed over plant species. Hymenoptera, Hemiptera, Psocoptera and Thysanoptera showed a narrower distribution. Nolana divaricata, Baccharis petiolata, Heliotropium taltalense. Nicotiana solanifolia, A triplex atacamensis, Fabiana densa, Baccharis incarum, Nolana crassulifolia, Haplopappus rigidus, Cortaderia atacamensis, Eremocharis fruticosa, Atriplex leuca, standed out by their arthropods species richness. In relation to plant damage produced by phytophagy, 25 plant species were affected by defoliation, mining or cecidia forming insects, the latest being the most relevant. Sucking phytophagy, the most important of phytophagy, was evaluated through the high abundance of insects belonging to this functional class (Homoptera, Hemiptera. Thysanoptera) presented in all ecological zones, and mainly in DL and TM.