Ecotones - Energies at the Edge of the Landscape
Fred Horton + Owen Wang + Wenbo Zhang
Ecotones, also known as the edge effect or tension zones, are biodiversity hotspots characterized by abrupt physical and biological transitions. They usually occur at many spatial and time scales, ranging from transitions between ecosystems to local microbial and molecular transactions. The constantly shifting gradients of ecotones allow edge species and the numerous environmental players to host niche milieu and synergies, mutualism between communities, macroevolution through mutation, and possibly extinction. Therefore, how do the diverse forms of interplaying and responsive energies in ecotones adapt and organize the two(+) communities, create sparks for colonizer organisms' speciation, and reconstruct the biotic and abiotic condition(s)?
By comparing sharp, spatial, local, regional ecotones at various scales and boundaries, the research examines the numerous strategies that ecotones and their environmental players adapt, self-organize and decentralize through processes such as bistability and feedback loops. Understanding these complex systems' various energies could lead to new ways of thinking and understanding the human-built environment and its connection with nature.
Total stream system ecotone:
The total stream system can be seen as an ecotone on a large, continental scale, mediating between land and sea. On a landscape scale, streams form river/land ecotone on the overlapping edge between land and water. Fish are highly dependent upon the land/water interface, specially the shallow zones covered with aquatic vegetation, for spawning and for their early fry stages.
Zooming into the river itself, although water is connected and unidirectional, it can be divided into different patches with distinct boundaries depending on hydraulic forces and biotic resources. The boundary perspective recognizes lateral linkages of energy exchange (e.g. edge-channel exchanges) as well as longitudinal ones(upstream-downstream linkages). At those edges, many species assemblages overlap, and many invertebrate species live near the limits of their ecological tolerance. The boundaries can be viewed as semi-permeable membranes, allowing some ecological flows yet impeding others.
Hardwood Hammock and Mangroves ecotone:
Influenced by microtopography (distance from the ocean + elevation relative to sea level)
Saltmarshes are coastal wetlands that are flooded and drained by salt water brought in by the tides. The saltmarsh ecotone acts as a biological filter between the marine and terrestrial environment. They regulate fluxes of water and organisms while storing and removing nutrients and pollutants from the environment.
The soft marsh substrate, referred to as "pluff mud," prevents large grazing animals from consuming halophyte stalks. Instead, the grass dies back each fall and bacteria decompose it into a rich soup known as detritus that along with algae serves as the basis of the productive salt marsh food web. The interaction between plant growth and the pluff mud is characterized by positive, facilitative interactions. This in turn beneﬁts the growth of the vegetation because salt stress and tidal currents are reduced by the increased elevation of the sediment relative to the average water level. Therefore positive feedback allows the ecosystem to buffer the effects of variation in physical conditions, and they underlie its ability to function as a complex adaptive system.
However, as the system develops, it approaches a critical threshold. The closer the marsh gets to this threshold, the more vulnerable it becomes to disturbances, which may result in cascading vegetation losses and collapse of the salt marsh ecosystem. Salt marshes provide a clear mechanistic example, that self-organization in ecosystems may lead to ecosystem collapse and induce avalanche-like dynamics that may in part determine the spatial organization of natural systems.
Salt marshes share this property with disturbance-dominated systems.
Further ecotone research:
Mire’s energy: Low in nutrient Strong wind pushes fallen tree boles (nutrient) away from the mire
Forest’s energy: fallen logs = raised surface above the mire = fortress
Sphagnum’s energy: feedback on soil conditions (lowers the pH)
Vascular’s energy: resisting the spread of the inhibiting soil by locally recycle the nutrients from the dead plants
Energy of the community above treeline
Wind as a negative factor on seedling and tree survival
Frequent fires that prevents pines from going up
Energy of trees
Sheltering each other (positive feedback loop) from high winds
Improve conditions in their favor
Increasing fine soil component and therefore water holding capacity
Increasing organic matter and nutrients
Increasing soil moisture by trapping snow. (resists fire)
Takes advantage of the sharp abiotic discontinuity to prevent fire
Tree islands’s energy:
Higher evapotranspiration than the surrounding mash, pull in water and phosphorus from mash
Tree islands as nesting places for colonial wading birds, which deposit nutrients
Projecting above the surrounding marsh = capture airborne nutrients & dry deposition
Marsh’s energy: frequent disturbances