Summary
When we think about connections in and among aquatic systems, we typically envision clear headwater streams flowing into downstream rivers, river floodwaters spilling out onto adjacent floodplains, or groundwater connecting wetlands to lakes and streams. However, there is another layer of connectivity moving materials among freshwater systems, one with connections that are not always tied to down-gradient flows of surface waters and groundwater. These movements are those of organisms, key components of virtually every freshwater system on the planet. In their movements across the landscape, biota connect aquatic systems in often-overlooked ways.
Graphical Abstract
Salmon move against the flow of water to reach a headwater stream where they spawn. Tadpoles that you see in a wetland will later metamorphose into adults that can move overland to another water body. Adult dragonflies seen flying along a river bank may have emerged from distant wetlands or streams where they began life as fully aquatic larvae. A duck shot and prepared for a dinner table in Texas may have started its life in a wetland in the Dakotas. Movement is a key feature of life; a feature that forms often-intricate patterns of connectivity among aquatic systems, many of which might otherwise appear to be isolated habitats embedded within a surrounding terrestrial matrix.
Hydrologists often consider the flow of water. Similarly, ecologists study flows of organisms or propagules and the genetic material they contain. Connections in biological communities allow for three important ecological functions. First, the cumulative effects of individual movements maintain population dynamics through immigration or emigration. Second, connected pathways allow for refuge seeking during hard times and the recolonizing of habitat after local extinctions. Lastly, flows of genetic materials contribute to long-term sustainability of populations and species by providing the genetic variability needed to adapt to changes in environmental conditions. In a world becoming increasingly influenced by landscape alterations and a changing climate, having the ability to adapt to change may become increasingly important.
In the prairie-pothole region of the mid-continent, the northern leopard frog (Rana pipiens) provides an example of a species that moves across the landscape to connect diverse habitats. This frog overwinters in deep-water or flowing-water systems where it avoids the below-freezing winter temperatures common in the northern Great Plains. In spring, it leaves these over-wintering sites and migrates to small wetlands with temporary ponds to reproduce. The temporary nature of these habitats limits use by fish and other predators that feed on northern leopard frog eggs and tadpoles. Following breeding, northern leopard frog adults move into surrounding upland habitats where they feed on insects, often visiting other wetlands in the process. In the fall, they return to over-wintering habitats. In addition to these migratory movements, juvenile northern leopard frogs will make long distance, dispersal movements that populate new habitats. The movement of adults to wetlands used for reproduction and the long-distance dispersal movements of juveniles serve to provide the genetic mixing that has led to this species’ ability to maintain high diversity in a highly dynamic landscape where genetic diversity would otherwise be expected to be low. The movement of frogs across the landscape also provides for the flow of nutrients and energy from highly-productive wetlands to uplands and other habitats, e.g., streams.
The northern leopard frog is just one of a multitude of species that connect aquatic habitats as they move across the landscape. If we could visualize the pathways used by all organisms connecting the wetlands, lakes, streams, and rivers of a landscape in a manner similar to the way that we can see stream and river networks from aerial and satellite imagery, we would be amazed at their number and complexity. These are the patterns of connections that John Muir was likely referring to when he referred to everything in the natural world being “hitched to everything else in the universe.”
Limited biological connectivity also has important ecological consequences. This is seen especially in dryland landscapes, where the rare aquatic habitats are often isolated, but is also apparent in other relatively isolated springs and headwaters in the humid southeastern United States. Since isolation limiting the movement of biota also limits gene flow, local biodiversity in these habitats is characterized by high endemism (i.e., occurrence of species unique to that area) and increased regional biodiversity. Human alterations that create new pathways for dispersal can have detrimental effects to systems in which species composition has been structured by their relative isolation. Not only is gene flow increased for these isolated communities but well-connected landscapes can facilitate the spread of disease and invasive or harmful species.
When thinking of biotic connections among aquatic ecosystems, ecologists often think in terms of nodes, links, stepping stones and hubs. These are the terms of graph theory, a field of mathematics that allows for the exploration of network connections. In ecology, we can view wetlands on a landscape as nodes. These wetland nodes can be linked to other wetland nodes, or to a lake, stream or river (other types of nodes). Longer-distance movements are facilitated by stepping stones or intervening nodes, allowing for dispersal that wouldn’t be possible in their absence. Hubs are nodes that include multiple routes between nodes and if lost would have a greater effect on overall landscape connectivity than loss of other, less-connected, nodes. By thinking in terms of nodes, links, stepping stones, and hubs, ecologists gain a greater understanding of landscape connectivity when movements are no longer limited by the down-gradient flow of water.
Given the importance of biotic connections to aquatic systems, the authors are working with other partners to map the United States into regions based on the occurrence patterns of wetlands, lakes, streams, and rivers, in addition to climate and other factors influencing movement patterns of biota. Identifying the underlying patterns of nodes, links, stepping stones and hubs that influence animal movements and thus, connectivity, of particular regions will be important as we seek to gain a greater understanding of biotic connectivity among aquatic habitats and how our actions, or lack thereof, influence this connectivity across the Nation.
Figure 1.
The northern leopard frog (Rana pipiens) provides an example of a species that moves across the landscape to connect diverse habitats.
Figure 2.
A prairie pothole region with streams and wetlands. Nodes (colored circles) identify wetlands and yellow links connect wetlands to other wetlands or streams within a distance of 150 m or less. The size of the node indicates the number of links. Purple nodes are examples of hubs (>=4 links) while red nodes are examples of stepping stones to streams.
Figure 3.
Distribution of wetlands as defined by the National Land Cover Database 2011 summarized at 12 digit HUCs (Hydrologic Unit Codes) for the contiguous United States via data from the US EPA EnviroAtlas.