I used a direct unconstrained gradient analysis to test if variation in water boatman species composition was associated with consolidation index while accounting for salinity. The environmental data, consolidation index and mean sample salinity (salinity), were scaled and converted to Euclidean distances and ordinated using non-metric multidimensional scaling, then vectors for the species data were overlaid. Seven water boatman species showed significant (p < 0.1) associations with environmental ordination (consolidation index and salinity), and four of these showed that their variance was strongly explained (R-squared > 0.7) by the environmental variables (Table 2). The ordination indicates that Callicorixa audeni and Sigara decoratella were strongly negatively associated with consolidation index and weakly associated with salinity (Fig. 14). Trichocorixa borealis appears to be strongly positively associated with consolidation index and weakly associated with salinity (Fig. 14). Cenocorixa expleta, Cenocorixa utahensis, Sigara bicoloripennis, and Trichocorixa verticalis appear to be strongly positively associated with salinity and weakly associated with consolidation index (Fig. 14).  However, Cenocorixa utahensis was collected from only one quarter section, so its relationship to the environmental variables cannot be safely interpreted.

Figure 15: Scatter plots of total counts (abundance) in quarter sections of four migratory water boatman species, with trend lines based on linear models. 

We tested if pond consolidation at the quarter section-level, as measured with our area-based consolidation index, is associated with changes in water boatman species composition and a decrease in migratory species abundances in the northwest Prairie Pothole Region. We found that two species, Callicorixa audeni and Sigara decoratella, which dominate migratory swarms (Srayko et al. 2022; personal observations by myself, John Acorn, and Stephen Srayko), showed a significant negative association with pond consolidation. We consider these species key drivers of water boatman seasonal mass-migrations between wetlands and large rivers in the northwest Prairie Pothole Region. Therefore, we suggest that the migration potential of water boatman communities from prairie pothole ponds and, consequently, the migration-related ecosystem services that they provide (dietary subsidy and wetland-river nutrient transfer) are reduced by the practice of consolidating numerous, non-permanent ponds into fewer, permanent water bodies.  

Our finding that small, shallow ponds are valuable for producing migratory water boatmen aligns with previous observations in the Prairie Pothole Region (Hungerford, 1948; Hilsenhoff, 1984; Srayko et al. 2022). The effect of decreasing pond density combined with increasing pond size (pond consolidation) within landscape units in the Prairie Pothole Region on water boatmen and their fall mass-migration potential has not previously been investigated.  Decreasing pond density may affect water boatman populations through decreased metapopultation rescue effects, where neighbouring populations become more distant from each other and, therefore, less connected (Semlitsch & Bodie, 1998).

We only identified and counted adult water boatmen for our study because the majority of immature water boatmen are not identifiable to species. Also, due to varying life cycles across species, our summer sampling may have biased counts towards species that are mature at this time of year, as the results in Table 1 might suggest. However, wetland-river migrants are known to mature by late summer before their fall migration, so we can be confident of the usefulness of our samples collected in late July/early August. Future water boatman surveys may benefit from using DNA bardcoding to identify nymphs. Our study area was limited to the Central Parkland Natural Subregion of Alberta due to drought in 2024. Therefore, this northwest portion of the Prairie Pothole Region is our domain of inference; however, it is from the Central Parkland and a similar region in Saskatchewan that the impact of the water boatman migration is known (Srayko et al. 2022), so our conclusions are valuable to the management of wetlands in this region. 

Our findings have implications for wetland loss mitigation in the Prairie Pothole Region. We provide evidence that bank mitigation, where single wetlands equalling the total area of numerous lost wetlands are created, misses important habitat qualities for certain aquatic invertebrate communities. The Alberta Wetland Policy recommends that land users conserve a variety of pond sizes (Environment and Sustainable Resource Development, 2013). We support this recommendation and enhance it with the evidence presented here that conserving groups of neighbouring ponds with varying permanencies is associated with a higher potential for their water boatman communities to contribute to seasonal mass-migrations. We suggest that it is important that wetland conservationists make an effort to counteract the preferential loss of groups of smaller, shallower ponds in the Prairie Pothole Region by searching for opportunities to preserve areas with higher pond densities. Additionally, we recommend that agricultural producers avoid draining areas that contain high pond densities since the cost of doing so may be outweighed by the benefits that these dense pond groups provide. The productivity loss associated with pond consolidation is likely exasperated by climate change: productive smaller pond habitats that are susceptible to drying  may dry earlier and more frequently as the climate in the Priaire Pothole Region becomes warmer and more volatile. This will increase the importance of maintaining higher pond densities to provide refugia for migratory water boatmen.