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.
- Longitudinal (upstream and downstream)
- Lateral (midchannel to floodplain)
- Vertical (underground, in the sediment surrounding the channel)
- Temporal (continuity over time) (Annear, 2004).
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:
- longitudinal – linear connectivity
- lateral – floodplain connectivity
- vertical – hyporheic (below the stream bed)
- temporal (time) – many scales; seasonal, multiyear, generational
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.
- A headwater woodland stream has steep gradient with riffles, rapids and falls.
- Sunlight is limited by overhanging trees, so photosynthesis is limited.
- Energy comes instead from leaves and woody material falling into the stream
- Aquatic insects break down and digest the terrestrial organic matter.
- Water is cooled by springs and often supports trout.
In the mid-reaches,
- the gradient decreases and there are fewer rapids and falls.
- The stream is wider, sunlight reaches the water allowing growth of aquatic plants.
- Insects feed on algae and living plants.
- Proportion of groundwater to runoff is lower so stream temperatures are warmer.
- The larger stream supports a greater diversity of invertebrates and fish.
The river grows and the gradient lessens with few riffles and rapids.
- Terrestrial organic matter is insignificant in comparison to the volume of water
- Energy is supplied by dissolved organic material from upstream reaches.
- Drifting phytoplankton and zooplankton contribute to the food base as does organic matter from the floodplain during flood pulses.
- Increasing turbidity reduces sunlight to the streambed causing a reduction in rooted aquatic plants.
- Backwaters may exist where turbidity has settled and aquatic plants are abundant.
- Fish species are omnivores and plankton feeders such as carp, buffalo, suckers, and paddlefish.
- Sight feeders are limited due to the turbidity (Minnesota DNR, Healthy Rivers).
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.
- The water is filtered of many impurities.
- It usually picks up dissolved minerals.
- The water is cooled.
- The water is metered out slowly over time.
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:
- Structural connectivity – the spatial structure of a landscape that can be described from map elements
- Functional connectivity – the response of individuals to landscape features (Brooks).
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:
- patch quality (e.g. resistance to movement or patch-residence time)
- boundary properties ...
- the movement characteristics of the features of interest” (Weins).
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?
- Fragmentation results in a dramatic increase in 'edge' habitat, which provides increased access to the more protected interior habitats by predators, including domestic animals.
- Fragmentation creates barriers to wildlife movement, and is especially harmful to reptiles and amphibians that depend on the ability to move between their aquatic habitats and upland areas.
- Fragmentation creates opportunities for harmful exotic plant species to invade. Many exotic species can out-compete native plant communities and often provide little or no habitat value.
- Because it's associated with human activity, fragmentation often brings pesticides, noise, lights, and other pollutants and disturbances that can profoundly impact a species' ability to function (Minnesota DNR Natural Resource Guide, "Changes to the Land").