Non-native species, size distributions, and nutrient recycling in southwestern stream communities

Abstract

Non-native species introductions are a ubiquitous form of environmental change. However, the role of introductions in ecosystem functioning is still poorly understood, especially in highly invaded systems with multiple non-native species. This thesis employs a functional trait framework— in which the net ecosystem effect of changing community structure is evaluated by quantifying change in the community-level distribution of biological traits— to assess the potential effects of multi-species introductions on ecosystem functioning in the Verde River, Arizona, USA. Specifically, I assess changes in body size distributions associated with non-native fish and crayfish introductions to estimate change in consumer-mediated nutrient recycling— an important size-scaling function in which aquatic consumers can control the rates and ratios of inorganic nutrient availability. In chapter 1, I compare the central moments of individual size distributions of basin-wide native and non-native fish species pools. Native and non-native species pools were characterized by significantly different mean, coefficient of variation, and skewness moments of individual size distributions. These differences were more pronounced within trophic guilds than across the whole community, and notably differences in higher order moments (CV, skewness) were relatively greater than differences in the mean of size distributions. In Chapter 2 I evaluated whether such changes in the variance of size distributions affected nutrient recycling independent of the mean. I coupled nutrient recycling incubations and field sampling of habitat-specific size distributions of a non-native crayfish (Orconectes virilis) to scale up nutrient recycling from individuals to aggregate ecosystem functioning using mean-only or variance-incorporating approaches. A mean-only approach overestimated true rates of aggregate nutrient recycling by as much as 20% in habitats with low mean body size, but the bias induced by ignoring variance declined with the mean of the distribution. Given that the relationship between individual body size and nutrient recycling is a general representation of many size-functioning relationships, these qualitative results likely hold for other consumer-mediated functions. In Chapter 3 I determined whether body size provided a common functional currency to predict individual nutrient recycling across multiple fish species, and in turn whether native to non-native fish turnover was expected to generate large differences in aggregate nutrient recycling in the Verde River. Body size-recycling models with species-specific parameters were more parsimonious than global models with parameters shared across species. Using species-specific models I found that non-native dominated communities excreted ammonium at similar rates as native-dominated communities, but phosphate at significantly lower rates. The resultant difference in the N:P ratio was surprisingly large, generally independent of body size, and potentially important for aquatic microbial communities in this system. By contrast, the global model incorporating body size but not taxonomy did not capture this significant N:P difference. Together these chapters suggest that quantifying change in body size distributions yields important ecological insights, but taxonomic identity or additional traits are still necessary for a full evaluation of the ecosystem-level effects of multi-species introductions.