Lexicon terms: Ecological Memory, Ecosystem Restoration and Novel Ecosystems

Ecological Memory

Ecological memory is a term from ecological restoration, which contributes to a site’s ability to regenerate after disturbance, “After a community or ecosystem is lost, it may leave behind an ecological memory.  The site history, soil properties, spores, seeds, stem fragments, mycorrizae, species, populations, and other remnants may influence the composition of the replacement community or ecosystem to varying degrees.”  (Schaefer 171)  Ecological memory may be located on site or off – in urban areas, ecological memory can be stored in parks, golf courses, cemeteries, etc.  In order for “memory” to be transferred from one site to another, corridors between the patches must exist.  (Schaefer, Andersson)   External ecological memory – sites outside a city, parks, etc, can help smaller patches inside the city “remember”… “The adaptive cycles that represent the various balanced states within an ecological system are interconnected where higher later levels can help lower levels to “remember” their earlier successional states as they recover from disturbance.  Ecological memory helps younger stages “remember” what they are to become and establishes the trajectory for ecological respiration that was historically present on the site.”  (172-173)

Ecological memory consists of both biological and genetic legacies.  Biological legacies refer to seeds, plant materials, animal populations, etc.  Genetic legacies refer to the adaptations of a species to a site.  In ecosystem restoration, it is preferable to use plants from a similar climate, soil type etc to the site.

“There is an internal component to ecological memory consisting of biological legacies represented by remnants of species in the immediate area.  An external component encompasses more mature surrounding support areas.  In cities, native ecosystems usually occur as patches of habitat connected by corridors in a matrix of streets and buildings.”  (Schaefer 171)

Genetic legacies – “It is important in some cases that ecological restoration match the genetic history of plant stock being used on a project to the site being restored, although in future due to the impacts of climate change this may involve using plant stock from other regions that are better suited to the new climatic conditions at a site.  It has been known for a century that the different growth forms of trees in different parts of their range may be genetically determined as an adaptation to the particular growing conditions in different parts of their range.”  (Schaefer 171)

Schaefer argues that ecological memory is important in restoration projects within cities – by using ecological memory, appropriate plant selection may be made, so that the plantings do not just represent a native plants ornamental landscape, but function as an ecosystem.

“Getting from ornamental to fundamental, to a system rather than a collection of ornaments, is one of the biggest challenges facing restoration projects within cities.  Urban habitats have their own typology that covers a spectrum extending from fully functional ecosystems in remnant fragments to purely decorative plantings in pots and boulevards, expanding the mandate of ecological restoration and increasing the complexity of situations.” (Schaefer 172)

Novel Ecosystems (aka Emergent Ecosystems)

Novel ecosystems are new combinations of species, often including invasive and native species.  (Is spontaneous vegetation a component of an urban novel ecosystem?)

“The key characteristics are (1) novelty: new species combinations, with the potential for changes in ecosystem functioning; and (2) human agency: ecosystems that are the result of deliberate or inadvertent human action, but do not depend on continued human intervention for their maintenance.”  (Hobbs et al 2)

“Invasive species can create new ecosystems and communities that had never occurred before on the planet.  Such novel or emergent ecosystems no longer require human intervention to persist. Such novel ecosystems characteristically occur in urban, cultivated, or otherwise degraded landscapes with dispersal barriers, and they were created by direct or indirect disturbance from humans. The human impacts cause the extinction of local native species (Hobbs et al. 2006).”  (Schaefer 173)

Hobbs et al describe novel ecosystems as a result of human action, environmental change, and invasive species introduction.  They argue that while novel ecosystems are not currently well-researched because of the long-standing assumption that invasive species must be degrading ecosystems (rather than contributing to them).  However, these novel ecosystems may represent low-cost alternatives to ecosystem restoration projects that traditionally would have sought to eradicate non-native species.  Hobbs et al wonder how a management regime might address these ecosystems.  Describing the reception of this idea, they write:

“Key questions for the future are how we develop management schemes that maximize beneficial changes and reduce the less beneficial aspects (which also depend on how and by whom ‘benefit’ is defined). Because novel ecosystems result from human actions, management is required to guide their development. How we manage these new ecosystems effectively is a point for debate: what should the goals be and how should  these systems fit with other systems along the wild–intensively managed gradient? If the system is transient, how do we guide it along a particular trajectory? If it is stable, can we manage it effectively to gain benefit from its current state or devise effective methods of directing it to a new, more preferable state? It is certainly clear that these systems will be very difficult, if not impossible, to return to some ‘more natural’ state in terms of time, effort and money. This is a very important point as it argues simultaneously for (1) conserving less impacted places now so they do not change into some new, possibly less desirable, form; and (2) not wasting precious resources on what may be a hopeless quest to ‘fix’ those systems for which there is little chance of recovery back to some pre-existing condition. Rather, we should perhaps accept them for what they are and what benefits they provide.

This may seem to some to be a defeatist approach which recognizes that some ecosystems are more or less transformed irreversibly and that invasive species are likely to persist in some cases. Indeed,  comments from reviewers of the draft manuscript indicated a lack of willingness to accept such ecosystems as a legitimate target for ecological thought or management action. For instance, one reviewer commented that the examples are ecological disasters, where biodiversity has been decimated and ecosystem functions are in tatters, and that ‘it is hard to make lemonade out of these lemons’.

Our point is, however, that we are heading towards a situation where there are more lemons than lemonade, and we need to recognize this and determine what to do with the lemons. We suggest that the approach is simply pragmatic and provides a way for prioritizing scarce conservation and management resources. As Redford & Richter (1999) discuss, there is a variety of ways in which humans and ecosystems interact, and novel ecosystems are likely to have some useful kinds of functions, while not others. We should perhaps move away from the one-dimensional dichotomy between natural and human dominated towards a more effective depiction of how human beings interact with nature.”  (Hobbs et al, 4-5)

Del Tredici seconds this view point, emphasizing that the site conditions in which the old ecosystem existed simply no longer exist: climatic and hydrologic patterns have changed, etc.

“The notion that every city has a native flora that can be restored is an idea with little credibility in light of the facts that (1) most urban land has been totally transformed from what it once was; (2) the climate conditions that the original flora was adapted t no longer exist; and (3) most urban habitats are strictly human creations with no natural analogs and no indigenous flora.  A native flora once grew where the city now stands, but the idea that this vegetation can somehow be restored to the site is both ecologicall and evolutionarily impossible (Gould 1998).  Certainly we can plant native species in the city and they will grow – but only if we provide them with the right kind of soil and maintain them the way we would any other intentionally cultivated plant.  In an urban context, the concept of restoration is really just gardening dressed up to look like ecology (Janzen 2998; Del Tredici 2007).”  (Del Tredici 16)

Schaefer also argues that ecological restoration should consider broader functions and processes rather than targeting static species/community metrics/historic data (174).  He describes the emergence of novel ecosystems as a change in ecological memory.  He establishes basic formulas for describing this change,

“Changes in ecological memory can be expressed in the following formula as a starting point for more serious analysis. Ecological memory (EM) equals the original species and processes remaining or latency (L), minus disturbance or memory loss (D). In this scenario, ‘‘D’’ includes invasive species and habitat destruction like removing the original vegetation, removal or turning soil, increased impervious surface, pollution, and other sources of degradation that diminish ecological memory. Thus:


where the current ecological memory for a site ‘‘EM’’within the adaptive cycle is equal to the original memory remaining ‘‘L’’ minus the loss of ecological memory or ‘‘D’’ .

At some point, we can imagine that the disturbance has been so great that invasive species ‘‘I’’ are no longer part of the disturbance but are part of the latency of the original ecosystem (Hobbs et al. 2006). So, ‘‘I’’ added to the original ecosystem ‘‘EM’’ creates a novel ecosystem ‘‘NE’’ and a new adaptive cycle. Thus,

NE =EM + I

where ecological memory has crossed a threshold of change to create the new adaptive cycle.”  (173)

Section summary:

Ecologists, geographers and landscape architects are increasingly questioning attitudes surrounding invasive species and weeds.  In the field of ecological restoration, some are arguing that invasive species are not really “invaders”.  Rather, they are part of a novel ecosystem, which has formed in response to environmental change.  Having developed without (intentional) human input or maintenance, these novel ecosystems may provide a new model of ecosystem restoration and maintenance.  This is analogous to efforts to incorporate spontaneous vegetation into the practice of landscape architecture –native and invasive species already present on site may be incorporated into the landscape design, reducing the amount of maintenance required.

This set of definitions provides a framework within which to consider the Pruitt-Igoe site.  The demolition of the housing project created concrete and brick rubble, which effectively altered the soil composition on site.  In response to this disturbance, a community of native and invasive species emerged.  Could the Pruitt-Igoe demolition also be considered an ecological restoration project?  To what extent do these species convey the history of the site as suggested by Kuehn?


Schaefer, Valentin “Alien Invasions, Ecological Restoration in Cities and the Loss of Ecological Memory” Restoration Ecology vol 17, issue 2 March 2009

Andersson, E. 2006. Urban landscapes and sustainable cities. Ecology and Society 11:34 (available from http://www.ecology and society.org/vol11/iss1/art34/) accessed 24 June 2007.

Hobbs, R. J., A. Salvatore, J. Aronson, J. S. Baron, P. Bridgewater, V. A. Cramer, P. R. Epstein, J. J. Ewel, et al. 2006. Novel ecosystems: theoretical and management aspects of the new ecological world order. Global Ecology and Biogeography 15:1–7

Del Tredici, Peter “Wild Urban Plants of the Northeast: a field guide” Ithaca, NY: Cornell University Press, 2010

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