During some recent fieldwork doing forest biogeomorphology with colleagues in the Czech Republic, the idea of biogeomorphic equivalents came up. A biogeomorphic ecosystem engineer organism has a biogeomorphic equivalent if another species can potentially do the same biogeomorphic job. For example, bacteria that consume iron are important agents of weathering. There exist numerous species of iron-eating microbes, so if one is eliminated for whatever reason, another takes its place. Thus these Fe-processing bacteria have biogeomorphic equivalents.

Acidophilous iron-oxidizing bacteria (USGS photo).

On the other hand, there exists no biogeomorphic equivalent for the stream-damming effects of beavers. The disappearance of Castor canadensis from a landscape means the loss of their biogeomorphic effects, as no other organism (save humans, of course), dams up streams.

Wyoming beaver dam (photo: Wildlife Conservation Society).

Some have argued that in geomorphology and physical geography the term "tipping point" does not describe any concepts or phenomena not long recognized by the fields. The tipping point concept does not (it is argued) have any conceptual or analytical value added. I agree. Here is a previous post on tipping point metaphors.

Blanco River, Texas.

Notwithstanding that, tipping point terminology is au courantin both public discourse and science, particularly as it relates to global and broad scale environmental change. Thus--perhaps analogously to buzzwords such as "sustainability" and "resilience"-- if you want to be a part of broader scientific conversations, it pays to employ the trendy term.

Where forests grow on thin soils over bedrock, the effects of individual trees (as well as the effects of forest cover and litter) may work to deepen or thicken the soil. This occurs due to root penetration of bedrock joints and fractures. This in turn facilitates weathering and funnels moisture into the rock. Uprooting of trees may “mine” bedrock encircled by roots, and leave a locally thicker mound as rootwads deteriorate. If trees do not uproot, as stumps rot away the depressions—often extending deeper than surrounding soil—fill with soil, sediment, and organic matter. This thickening of the soil is a form of direct, positive ecosystem engineering in that it increases habitat suitability for the engineer organisms (the trees). 

Chinquapin Oak growing in limestone, Mercer County, Kentucky.

Eventually, however, the average soil or regolith thickness may increase such that tree roots no longer contact bedrock. Then the biogeomorphic ecosystem engineering effects of the trees on soil thickness ceases. In effect, you have self-limited biogeomorphic ecosystem engineering. 

As sea level rises--and it is rising!--it is causing geomorphological, hydrological, and ecological changes in coastal environments. Though "bathtub" models, which show where the shoreline would be with sea level increased by a certain amount, are useful in showing areas of potential impact, that's not how actual responses to sea level work. Not only does the ocean level change, but also the base level for rivers and terrestrial processes, salinity, ecological habitats, hydroperiods, and any number of other factors. 

Sand and mud flats along the eroding Neuse River estuary shoreline, NC. 

This post continues a series exploring the idea of evolutionary creativity in Earth Surface systems (ESS). Previous posts introduce the evolutionary creativityconcept, explore the possibility of algorithmic evolution modelsof ESS, and discuss the appearance over time of new varieties of landforms. 

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Biologists have put a great deal of effort into identifying the variety of life forms. While there is only one extant species of the genus Homoand 11 of Canis, there are more than 600 species of Quercus (oak trees) and >350,000 of beetles (and counting).  

This continues a discussion that started by considering the possibility of evolutionary creativity in landscapes and continued by exploring the possibility of algorithmic evolution models of Earth surface systems (ESS). 

In biological evolution by natural selection, new varieties arise via genetic mutations. Occasionally one of these turns out to be advantageous--the fitness of the organism is increased, and the variation is "selected" (recall that Darwin used natural selection to contrast with selective breeding of animals and plants by humans). The selected organism can reproduce, and pass the selected trait to its progeny.