Incorporating continuous trait variation into biomonitoring assessments by measuring and assigning trait values to individuals or taxa

Summary Traits-based analyses of insect assemblages support biomonitoring programme objectives. To date, however, few traits-based metrics have demonstrated the degree of sensitivity or discriminatory power required by biomonitoring programmes. Trait information used for analyses is typically based on static descriptions of dynamic communities and is attributed only to taxonomic units. Given that traits can vary even among specimens from the same species, quantifying trait variation and its consequences could be essential for successful traits-based biomonitoring. Here, we study the consequences of measuring trait expression among individual specimens versus assigning trait states from published databases at the taxon level (genus or family) for the interpretation of trait patterns within aquatic insect assemblages. Specifically, do database body size trait states accurately reflect measured body size values of aquatic insects collected in biomonitoring samples and should body size data be aggregated at the taxon level or assessed at the specimen level to detect differences among sites? We assessed body size, a continuous trait linked to fundamental organism properties and ecological function, for four orders of aquatic insects: Ephemeroptera, Plecoptera, Trichoptera and Odonata. Invertebrate samples were collected from the Miramichi River basin (New Brunswick, Canada) according to the Canadian Aquatic Biomonitoring Network method. Concordance between measured specimen sizes and published trait states was poor; 55% of taxa expressed body sizes considerably smaller or larger than assigned database states. Recalibration of size classes based on specimen measurements yielded three size classes that facilitated detection of assemblage-aggregated size differences among reference sites. Measured body size trait values were able to distinguish these differences in community structure, while values derived from databases yielded erroneous patterns in the size structure among sites. Gaining accurate ecological insights from traits-based biomonitoring may require assessing trait properties at the scale of individual specimens. The benefits of this approach, however, should be balanced against additional effort required in the context of specific study or programme objectives.