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Exploring Patterns of Bird Diversity in a Floodplain River System

Is the changing floodplain landscape linked with bird decline?

Under natural conditions, a floodplain river is a dynamic ecosystem – experiencing seasonal fluctuations in water levels that create the ideal habitat for diverse assemblages of resident and migrating birds. During dry summer conditions, river waters recede from the adjacent floodplain, allowing soil to dry out and lush vegetation to thrive. Breeding waterfowl inhabit the lush grasses and sedges through the fall. When river waters rise between late fall and spring, the floodplain is once again inundated, creating the perfect place for ducks, geese and other migratory birds to stop and feed en route during migration.

From the construction of dams to agricultural development, large floodplain river systems have undergone dramatic alterations over the last two centuries. Once a contiguous landscape of river channel, floodplain forest and wet prairies, most floodplain ecosystems are now a discontinuous mosaic of habitats – and scientists are just beginning to uncover how this habitat loss and fragmentation is connected to declines in avian diversity.

To effectively restore floodplain habitats and bird populations, scientists must first understand how factors such as vegetation (or cover type), season and variation in location influence measures of biodiversity. To gain this knowledge, a team of researchers designed a study to examine how the composition of bird assemblages varies across three successional stages of floodplain habitat including wet prairie, early successional forest and mature forest.

Methods and study site

Ten study sites were chosen within the lower Missouri River floodplain, stretching from northwestern to east-central Missouri. The study sites contained three successional stages of floodplain habitat:

• Floodplain wet prairie: Land cover comprised of mostly prairie grasses and sedges; less than 5 percent tree coverage.

• Early successional forest: Densely forested with trees less than 10 years of age.

• Mature forest: Upper canopy of trees greater than 15 meters tall; open understory with few shrubs or smaller trees.

Birds were surveyed during spring migration and summer breeding seasons from 2002 to 2004. Survey data were used to calculate biodiversity indices including species abundance, species diversity and species evenness in each successional stage. The results were analyzed to characterize how location, cover type and season affect patterns of bird diversity within each floodplain habitat.

Key results

In this study of bird diversity in the lower Missouri River floodplain, more than 15 percent of all avian species in North America were recorded. This impressive avian diversity is higher than that found in other Midwestern habitats, highlighting the importance of this river system in supporting bird communities.

Based on the analysis of survey data, the research team identified several important links between floodplain habitat and avian diversity:

• Cover type most influences avian diversity: The statistical analysis revealed that bird diversity was most influenced by cover type, and slightly influenced by regional location and season. The greatest avian diversity occurred during the summer breeding season in mature forest along the eastern portion of the floodplain, while the least diverse bird community occurred during migration in wet prairie in the western portion of the floodplain.

• Community composition responds to ecosystem succession: As expected, biodiversity measures (species richness, species diversity and species evenness) increased with ecosystem succession. The earliest successional stage in the floodplain habitat is wet prairie – which contained the lowest bird diversity. Mature forest is the most advanced successional stage and supported the highest diversity of avian fauna.

• Each successional stage is home to unique indicator species: The study also revealed several important indicator species for each successional stage (see Table 2). An indicator species is a species that is indicative of a habitat and an important tool for monitoring the biotic integrity of an ecosystem over time.

• Despite lower overall diversity, wet prairie habitat is an ecosystem of critical importance: While species diversity was highest in the mature forest (see Table 1), wet prairies were home to 20 species of high conservation concern – more than early successional forest and mature forest combined.

Implications for conservation and management

Understanding how floodplain habitats influence avian biodiversity will help biologists best manage floodplain ecosystems that have been acquired for conservation along the lower Missouri River as result of the abandonment of agricultural lands after the flooding of the 1990s. This study reveals the importance of maintaining a variety of floodplain successional stages to conserve avian biodiversity.

While all successional stages of floodplain habitat are important for supporting bird diversity, this research highlights wet prairies as a habitat of considerable importance for threatened bird species, including the endangered American Bittern. Because wet prairies are largely ephemeral (existing for only a short time each year) and no longer experience annual flooding, these systems may need to be specially maintained to prevent ecosystem succession. Allowing succession of wet prairies to forest conditions could lead to a loss of the 20 percent of the bird diversity unique to wet prairies, including several species of conservation concern.

Table 1. Bird diversity and common species within each stage of floodplain habitat.

Table 2. Indicator species (species representative of a habitat) identified within each stage of floodplain habitat.