Effects of species richness and nutrient availability on the invasibility of experimental microalgal microcosms

Abstract

The relationship between biological diversity and community invasibility is a central theme in ecological research. It is often assumed that such relationship is negative at small scales but positive at large scales, with a shift of the major driving factors from biotic to abiotic ones such as resource availability in spatial scale. However, the relative importance of factors driving diversity-invasibility relationships does not necessarily depend on spatial scale per se, and existing studies often test the effects of biological diversity and resource availability on community invasibility in isolation, but rarely consider their simultaneous effects on community susceptibility to invasions. The present study assembled freshwater microalgal communities (including Closterium libellula , Cosmarium sportella , Selenastrum capricornutum , Scenedesmus quadricauda and Actinastrum hantzschii) , and tested how community invasibility was affected by the individual and interactive effects of microalgal richness and nutrient availability. The results showed that the invasion success of the model invader, Pediastrum integrum , decreased with an increase in resident richness, but increased with an increase in nutrient availability. Also, there was an interactive effect of microalgal richness and nutrient availability on community invasibility. Nutrient-enriched microcosms with the lowest microalgal richness were most invasible, whereas non-enriched microcosms, regardless of whether they received nutrient enrichment or not, were most resistant to invasion challenge. A track of nitrogen concentrations in growth medium further showed that high-richness communities were associated with higher nitrogen removal efficiency than low-richness communities. Therefore, strong biotic resistance of high-diversity communities to invasions can be a general ecological pattern found in both macro- and microorganisms, and a resource-based approach can help explain diversity-invasibility relationships and elucidate the mechanisms that give rise to community invasibility.