Smolny Island is a specially protected natural area of regional importance aimed to preserve a unique natural ecosystem of typical floodplain landscapes of the latitudinal section of the Ob River. The purpose of this research is to obtain primary data on the species composition and state of algological, polyporale communities and insects-xylophages of the Smolny Island natural reserve. The standard algological, mycological and entomological research methods were used. The original studies results for several components of the ecosystem are presented for the first time. The composition, structure, occurrence of freshwater algae, forest, polyporale communities and forest insects pests were investigated. The freshwater algocenoses of the island have a typical regional composition, in summer there is a tendency to cenosis characteristic of the region's swamps (diatom-euglena viridis type) in autumn there's diatom, with low occurrence of species. An increased saprobity of the studied waters was detected. In total, 12 insects – xylophages species of the Agaricomycetes class were found in forest communities (coniferous, floodplain, small-leaved cenosis), with the richest species community in the test plot of small-leaved forest and the least variety in the coniferous one. No great risk for the development of a mass insects-xylophages area in the test plots was revealed, but the constant monitoring of the forest stock of the Smolny Island natural reserve is necessary to prevent the development of invasions and preserve the biodiversity of the island. The research results can be recommended as special diagnostic methods for tracking the diversity and condition of island communities.