Abstract
IntroductionThe effects of nutrients on stream conditions within individual streams or small areas have been studied extensively, but the same effects over a large region have rarely been examined due to the difficulty of applying large-scale manipulative experiments. In this study, we estimated the causal effects of nutrients within the Western United States on invertebrate richness, an important biological indicator of stream conditions, by using observational data.MethodsWe used the generalized propensity score method to avoid the common problem of statistical inference using observational data, i.e., correlation established based on observational data does not imply a causal relationship because the effects of confounding factors are not properly separated.ResultsOur analysis showed a subsidy-stress relationship between nutrients and invertebrate taxon richness in the whole Western United States and in its sub-ecoregions. The magnitude of the relationship varies among these sub-ecoregions, suggesting a varying nitrogen effect on macroinvertebrates due, in large part, to the varying natural and anthropogenic conditions from ecoregion to ecoregion. Furthermore, our analysis confirmed that causal estimation results using regression can be sensitive to the imbalance of confounding factors.ConclusionsStratifying data into ecoregions with relatively homogeneous environmental conditions or adjusting data by generalized propensity score can improve the balance of confounding factors, thereby allowing more reliable causal inference of nutrient effects. Invertebrates respond to the same nutrient levels differently across different site conditions.
Highlights
The effects of nutrients on stream conditions within individual streams or small areas have been studied extensively, but the same effects over a large region have rarely been examined due to the difficulty of applying large-scale manipulative experiments
Data for the Great Plains stratum and North American Deserts are roughly balanced without generalized propensity score adjustments (Tables 5 and 6)
Dose-response relationship A subsidy-stress relationship was observed between log(TN) and invertebrate richness (IR) in the Western United States (Fig. 3a)
Summary
The effects of nutrients on stream conditions within individual streams or small areas have been studied extensively, but the same effects over a large region have rarely been examined due to the difficulty of applying large-scale manipulative experiments. We estimated the causal effects of nutrients within the Western United States on invertebrate richness, an important biological indicator of stream conditions, by using observational data. In streams, increased nutrient concentrations have altered biological structures and functions such as species richness, composition, abundance, and decomposition rate (Dodson et al 2000; Freeman et al 2009; Smith et al 1999; Gulis and Suberkropp 2004; Rosemond et al 1993). Excessive nutrients can reduce water quality causing problems for drinking water and can deplete dissolved oxygen, Invertebrates occupy an important ecological niche in streams, and their aggregated measures such as total taxon richness are widely used for stream condition assessments (Fore et al 1996; Moss et al 1987). Our current understanding of the causal effects between increased nutrients and invertebrate richness (IR) is still limited
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