Changes of "arthropod island" along with the age of Caragana shrub in desertified land

"Arthropod island" contributed by Caragana shrub was found to play the key role in the structure and function recovery process of desertifided ecosystems. Taking the Caragana shrubs of different ages in desertified land of Yanchi, Ningxia as study models, the response ratio (RR) and relative interaction intensity (RII) were used to discuss the changes of "arthropod island" along with the age of the shrub at the community and population levels. The results showed that at the community level, when the "arthropod island" was compared between shrubs of different ages, the RR and RII based on total abundance and richness were the same but the RR and RII on Shannon index and evenness index were completely different. At the population level, RII was found to indicate 17% more arthropod richness than RR. In particular, for the shrubland of 24 years, 38% more arthropod richness was indicated by RII in comparison to RR. It was suggested that RII was a good indicator of interactions between shrub canopy and taxonomical distribution. Based on RR value, only the total abundance indicated an "arthropod island" in 6-year-old shrubland; in a 15-year-old shrubland, the Shannon index and evenness index indicated a marked (P < 0.05) increase of "arthropod island", while the total abundance showed no differences between shrub-canopy and intershrub values. However, in 24- and 36-year-old shrubland the RR values based on the Shannon index and evenness index declined strongly while that based on total abundance increased markedly (P < 0.05). Based on RII value, 50% more arthropod taxa indicated an "arthropod island", which was characterized by assemblage effect contributed by shrub canopy. In conclusion, RR and RII can be used to quantify "arthropod island" for community level and population level, respectively. Along with the shrub ages, the "arthropod island" contributed by the shrub canopy can facilitate the recovery of arthropod diversity outside shrub canopy, thus beneficial for the recovery process of desertified grassland ecosystems.