Arctic Shrub Expansion: Implications for Stream Benthic Invertebrate Community Structure & Function

Abstract

The Arctic is rapidly warming and this increase in temperature has been found to drive range expansion of tall shrubs in the Arctic tundra ecosystems. Although the consequences of shrub expansion have been studied in the terrestrial tundra, effects on tundra stream ecosystems remain understudied. Riparian tall shrubs, such as willows, can provide stream ecosystems with leaf litter and shade, enhancing food availability and habitat complexity. This can alter the abundance, diversity, and community composition of benthic invertebrates. These aquatic species provide important ecological functions such as organic matter processing and oxygenation of the sediment, and reflect the health and resilience of streams. To assess the effects of the tall shrub expansion on Arctic tundra stream functioning, benthic invertebrates were sampled in ten tributaries of the Komag River. I compared the abundance, diversity, community composition, and diet of the benthic invertebrates between streams with and without tall shrubs. The abundance and diversity of the invertebrates showed no relationships with tall shrub presence, but species composition shifted in the presence of tall shrubs. Despite the observed change in species composition, functional composition was similar in all streams. Further, stable isotope analysis revealed that many benthic invertebrates, regardless of tall shrub presence, were feeding on food sources more depleted in 13C than willow or periphyton. My results indicate that the leaf litter from tall shrubs in the stream may not be as extensively used by benthic invertebrates as in boreal and temperate streams. Rather, energy pathways in Arctic tundra streams seems dependent on autochthonous sources, such as filamentous algae, moss, or possibly methanogenic bacteria. The shift in species composition might be induced by tall shrub-associated factors besides leaf litter input. As climate warming continues, the Arctic tundra is predicted to more closely resemble boreal ecosystems, likely increasing allochthonous matter supply to streams. The results of this study increase understanding of the responses of Arctic stream benthic invertebrate communities to tall shrub expansion.