Introduction

Human impacts in the Prairie Pothole Region

Land modification by humans is resulting in a loss of natural variability by preferential removal of certain habitats in the Prairie Pothole Region of North America. This midcontinent region (Fig. 1) is characterized by millions of shallow geological depressions, which form highly diverse mineral wetlands (Fig. 2). Ponded water bodies (ponds) in these wetlands periodically dry up and are highly variable and dynamic habitats that support a high degree of biodiversity and productive aquatic invertebrate communities.

However, in the agriculturally productive Prairie Pothole Region, many ponds have been removed for alternate land use (Baulch et al. 2021). For example, Alberta's white area, which contains the province's portion of the Prairie Pothole Region, has lost approximately 70% of its ponds in the last 100 years, with 85% of this loss caused by agriculture (Aquality Environmental Ltd., 2020). Despite wetland policy developments in recent years (Alberta Wetland Policy, 2013), ponds are still being removed; for example, the Beaverhill subwatershed in Alberta has lost more than 80% of its wetland area without government permits (Clare & Creed, 2014). There has been a preferential loss of smaller and non-permanent ponds in the Prairie Pothole Region leading to homogeneity in spatial and temporal characteristics of these habitats (McLean et al. 2022), with implications for aquatic invertebrate communities.

Figure 1: The Prairie Pothole Region of North America (green area).

Figure 2: An example of the prairie pothole landscape. Photo from Ducks Unlimited Canada.

Water boatmen

Water boatmen (Hemiptera: Corixidae, Fig. 3) are common and typically abundant aquatic invertebrates in ponds of the Prairie Pothole Region, which contains 34 species of these insects (Euliss et al. 1999). Water boatmen in this region are an important food source for wildlife including ducks (Krapu, 1974), fish (Srayko et al. 2022), and migrating bats (Reimer et al. 2010), and they are important to nutrient flows within and across aquatic habitats. Their seasonal mass-migrations are of particular importance (Figs. 4 and 5); for example, in the northwest Prairie Pothole Region, an estimated 1,500 tonnes of water boatmen enter the North and South Saskatchewan Rivers from surrounding ponds every fall and dominate the diet of some fishes in these rivers during migration events (Srayko et al. 2022).

However, it is unknown what pond types and spatial arrangements best support this wetland-river linkage. There is a range of migration tendencies within the water boatman family (Brown, 1951; Srayko et al. 2022). As in other aquatic insects, it is thought there is a trade-off between flight capability and fecundity, with flight being advantageous in non-permanent habitats and flightlessness being advantageous in permanent habitats. It has been shown that water boatman species’ migration rate and tendency are inversely correlated with the permanence of their habitat (Brown, 1951). Because of this, species composition and migration potential may respond to changes in pond permanency.

Figure 3: A water boatman (Dasycorixa hybrida). Photo by John Acorn.

Figure 4: A swarm of water boatman on the bank North Saskatchewan River in Edmonton, AB during the spring migration.

Figure 5: A congregation of water boatmen (dark area) in the North Saskatchewan River in Edmonton, AB during the fall migration.

Pond consolidation

Consolidation drainage (pond consolidation), in which smaller ponds are completely drained into other basins, causes pond groups to consist of fewer, larger, and more permanent water bodies (Fig. 6). This may result in fewer migratory water boatmen, consequently lowering the dietary subsidy they provide for wildlife during migration events. Pond consolidation may also affect communities through altered vegetation and remove habitat for rare species. Consequently, pond consolidation likely affects prairie pothole water boatman communities and the benefits they provide.

Pond consolidation changes pond abundance, size, and permanency characteristics simultaneously, each in a constant direction; therefore, it can be usefully considered a single, compound predictor variable and examined for effects on prairie pothole water boatman communities. The objective of this study was to determine if unconsolidated pond groups composed of numerous, smaller (in area and depth), non-permanent ponds are valuable in supporting migratory water boatman species and seasonal mass-migration in the northwest Prairie Pothole Region in Alberta. Specifically, I tested if the simultaneous reduction in number and increase in area, depth, and permanency (consolidation) of ponds is associated with changes in water boatman species composition, with a focus on migratory species. As water salinity is known to affect water boatman species composition because salt tolerance varies amond species (Scudder, 1976), I treated it as an important covariate to separate its effect from pond consolidation.

Figure 6: A diagram illustrating pond consolidation using photos. Left is unconsolidated, right is consolidated. Photos from Ducks Unlimited Canada.

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