HNA Summary Report
Habitat

A habitat is an organism's "home." Defining the characteristics of the "home" for a host of river species is challenging. Many species may also have different habitat needs at different life stages and times of year (see sidebar). Habitat can be described in different levels of detail to narrow down the potential areas that may be occupied by an organism of interest. First, larger geographic areas and land cover types can be used. Next, other relevant attributes of habitat, such as current velocity, water depth, forest community type, etc. can be applied. For this HNA, habitats have been characterized broadly at the first level using floodplain land cover and aquatic area types. The "habitats" thus defined may be quite large, of low resolution, and only generally identify where species are likely to occur. Future refinements of this HNA will include additional physical and chemical habitat attributes and will define habitat for individual species in greater detail.

River floodplain ecosystems support a wide variety of species, which are distributed along flood frequency gradients (see Fig. 1 above). Low elevation floodplain areas, which are usually inundated, support aquatic and wetland plants. Areas subject to frequent flooding support flood tolerant species. The least flood tolerant plant species occur on well-drained, high elevation areas. Flooding is the major disturbance on low elevation floodplains. Fire was once an influence on high elevation floodplains, but fires have been suppressed and agriculture is currently the major influence.

Habitat classification systems can be quite complex, and so can the analytical tools used to investigate the charac-teristics of habitat. Three important habitat characteristics used in the relatively young sciences of landscape and conservation ecology were incorporated into the HNA. Habitat fragmentation is a measure of the size of continuous blocks, or patches, of plant species or communities (see image at left). Habitat connectivity is the consideration of organisms’ ability to move through a landscape to fulfill its normal life cycle (see image below). Some organisms have limited mobility, and rely on wind, water, or other animals for dispersal of seeds or young. Other more mobile species, particularly fish, are restricted in their movements within the river system by dams and levees. At the other end of the spectrum, birds are generally highly mobile and can traverse obstacles that present barriers to other species.

Habitat diversity is a measure of the mix of species or communities present in a given area. Low diversity habitats have large expanses of a single species or community type (e.g., sedge meadow). High diversity habitats support many species or communities. The classification system used to characterize habitat and the size of the area under investigation can greatly influence these types of analyses.

Human activity often fragments, isolates , and simplifies river habitats. Habitat diversity in the Mississippi River (left), its backwaters (foreground), and its tributaries (top) exists in contrast to the crop fields protected by levees (center).