COS 240-5 - Interaction between climate change and invasion of a freshwater macroalga suggest potential for reduced impacts and at least limited resilience of native communities under warmer conditions

Abstract: Climate change and biological invasions are two key drivers of global change and two of the most important potential risks to biodiversity and ecosystem functioning. However, relatively little empirical evidence exists to assess their potential interactions. Changing climate conditions could influence the spread and distribution of invasive species, the impacts of invasive species, and the effectiveness of different strategies for management of invasions. Nitellopsis obtusa is a green macroalga that is a relatively recent invader to lakes of the upper Midwestern United States. N. obtusa grows in dense aggregations that can impact both native species and lake ecosystem functioning. Limited evidence from species distribution models suggest that starry stonewort impacts may be reduced under future climate conditions, but these predictions are highly uncertain. These predictions are also based on the extent of suitable habitat, and do not account for either changes in the abundance of the N, which is generally the most important predictor of invasion impact, or the role of interactions between N. obtusa and native species.

To more directly assess the potential influence of climate change on N. obtusa invasions we established a series of permanent monitoring transects in lakes across a latitudinal gradient from Indiana to northern Minnesota. We used this gradient as proxy for future climate change and tracked N. obtusa and native species abundances across multiple years. We observed significant variability between years and across lakes that indicated N. obtusa invasions did not follow the expected pattern of constant expansion. Rather, in different years we observed both increases and decreases of N. obtusa and native species correlated with climate parameters and dependent on community composition. Most notably we observed a dramatic shift in N. obtusa phenology, with earlier growth and peak abundance in warmer years as well as more rapid loss of biomass as compared to cooler years. We also observed lower N. obtusa abundance with warming summer conditions, while overall native species abundance was positively correlated with warming winter conditions. But this pattern varied between different native species. These patterns are generally aligned with an invasive species near the warm edge of its climate niche that may be negatively impacted by climate change. These results also suggest there may be potential for resilience of native communities to N. obtusa invasions and the ability of native communities to recover when N. obtusa abundances are reduced offers support for active management for invader removal.