Connectivity is defined as the maintenance of lateral, longitudinal, and vertical pathways for biological, hydrological, and physical processes (Annear, 2004). It refers to the flow, exchange, and pathways that move organisms, energy, and matter throughout the watershed. The most obvious example of connectivity may be the free flow of water downstream in a river and the passage of fish upstream. The construction of a high dam across a stream is a vivid and obvious illustration of fragmentation or the loss of connectivity.
This exchange of energy, nutrients and material does not stop at the water's edge, it can be observed at many scales throughout the surrounding landscape. Complex, interdependent processes are continuously present throughout the watershed landscape and are required to maintain the ecological health of the system as a whole.
There are four dimensions of connectivity between a river and its contributing watershed. These are longitudinal, lateral, vertical and temporal.
For the river system, this continuum of hydrologic, biological, and chemical interactions and connections is described along the same four dimensions used to describe the hydrologic system.
Explore the Connectivity Health Scores to see a series of index values that show health trends in the connectivity of ecological systems in Minnesota.
The river continuum concept emphasizes the longitudinal dimension of the stream ecosystem. The RCC proposes a progressive shift, from headwaters to mouth, of physical gradients and energy inputs and accompanying shift in trophic organization and biological communities (Vannote et al, 1980; graphic - Stream Corrico
Connectivity refers to the flow, exchange and pathways that move organisms, energy and matter throughout the watershed system. These interactions create complex, interdependent processes that vary over time.
As with hydrology, stream connectivity can be described in four dimensions:
Additionally, the concept of landscape connectivity expands on this idea to include the entire watershed ecosystem as connected by the flow of organisms, energy and nutrients.
Within the stream system, longitudinal connectivity refers to the pathways along the entire length of a stream. As the physical gradient changes from source to mouth, chemical systems and biological communities shift and change in response. The River Continuum Concept (RCC) can be applied to this linear cycling of nutrients, continuum of habitats, influx of organic materials, and dissipation of energy.
In the mid-reaches,
The river grows and the gradient lessens with few riffles and rapids.
Lateral connectivity allows the stream access to its floodplain during high water events. This access is critical for the healthy ecosystem function. Nutrients and organic matter are transported to the stream from the floodplain, plant and wildlife species flourish in the diverse successional stages of inundated areas, and aquatic species gain access to seasonal habitats essential to their life cycles.
Lateral connectivity refers to the periodic inundation of the floodplain and the resulting exchange of water, sediment, organic matter, nutrients, and organisms. Lateral connectivity becomes especially important in large rivers with broad floodplains.
Periodic floods refill oxbow lakes and recharge wetlands. Inundated areas may be used as spawning areas by species such as northern pike. Floodwaters carry nutrients and organic matter from the land to the stream's aquatic plants, plankton, stream invertebrates, and fish. Seasonal flooding produces a variety of streamside vegetation and habitat for a diversity of birds and mammals (Minnesota DNR, Healthy Rivers).
Access to floodplain is also important for small streams that can experience dramatic episodic flooding. Heavy, localized rains can cause small streams to rise several feet in a few hours. This flashiness is largely a result of more overland flow and less infiltration following the conversion of native land cover to row crops and human communities. Large amounts of sediment are mobilized by these events, impacting all trophic levels and altering biological communities in the stream and the adjacent floodplain.
Mixing of surface water and ground water occur in the hyporheic zone. This biologically active zone contains water percolating through the permeable soils adjacent to the open streambed. Important microbial activity and chemical transformations are enhanced in this area (Stream Corridor, FISRWG).
Vertical connectivity is represented by the connection between the atmosphere and groundwater. The ability of water to cycle through soil, river, and air as liquid, vapor, or ice is important in storing and replenishing water. This exchange is usually visualized as unidirectional–precipitation falling onto land and then flowing over land or percolating through the ground to the stream.
An equally important transfer of water occurs from the streambed itself to surrounding aquifers. Groundwater can contribute to flows in the river at certain times in the year and at certain locations on the same stream. Streams may either gain or lose water to the surrounding aquifer depending on their relative elevations. Lowering the water table through groundwater withdrawals may change this dynamic exchange in unanticipated ways (Stream Corridor, FISRWG).
The slow movement of water through sediments to the river produces several ecological benefits.
This is particularly important in smaller, cooler streams for the maintenance of critical habitat for fish, wildlife and invertebrate species.
A stream exhibits temporal connectivity of continuous physical, chemical, and biological interactions over time, according to a rather predictable pattern. These patterns and continuity are important to the functioning of the ecosystem. Over time, sediment shifts, meanders form, bends erode, oxbows break off from the main channel, channels shift and braid. A stream rises and falls according to seasonal patterns, depending on rain and snowmelt. Throughout most of Minnesota, free-flowing rivers experience high water in spring, falling flows in summer, moderate flows in fall, and base flows in winter. The watershed has adjusted to these normal fluctuations, and many organisms have evolved to depend on them (Minnesota DNR, Healthy Rivers).
Landscape connectivity is 'the degree to which the landscape facilitates or impedes movement among resource patches' (Taylor et al, 1993). Biological components - both plant and animal – must have access to all the habitats necessary for all stages of their life cycle. This includes both physical and temporal access to habitats. For example, the need for seasonal timing is acute for many wildlife species to accommodate breeding, reproduction, and migration. For plant species it is equally important for dispersal, growth and competition.
Landscape connectivity has two components:
Habitat does not need to be structurally connected in order to be functionally connected. Some organisms have the ability to bridge the gaps between habitat patches and can link resources by crossing over uninhabitable or partially inhabitable locations (Taylor, 2006). For example, a neotropical migrant bird will perceive a landscape as connected across a greater range than would a salamander restricted to moist forest floors (With). “These movements… of individuals, materials, nutrients, energy or disturbances… are affected by how (habitat) patches are arrayed in the mosaic…. Although landscape connectivity is often thought of in terms of corridors - roughly linear strips of habitat connecting otherwise isolated habitat patches – connectivity is in fact a complex product of:
As people use the land, the natural landscape is divided into ever-smaller pieces by elements like railways, utility lines, roads, houses, and parking lots. The remaining natural areas, or fragments, are reduced in size and degraded in quality, resulting in a decline in plant and animal populations, and the disappearance of some sensitive animal species and plant communities.
How does fragmentation impact the environment?