ANTHROPOGENIC ENVIRONMENTS & NON-EQUILIBRIUM ECOLOGY

Topics for this lecture:

       1) Basic concepts of ecological dynamics & succession

       2) Equilibrium vs. non-equilibrium paradigms

       3) Anthropogenic environmental change:

Ecological dynamics:  basic concepts

Ecological community:  set of species (flora & fauna) inhabiting a given place

Succession: change in community composition over time (e.g., following a disturbance)

Ecological disturbance:  major perturbation (in species composition, ecosystem function, etc.)

–        Disturbance agents = fire, wind, volcanism, etc. (see below for more)

–        Disturbances vary in magnitude and frequency

–        Intermediate-level disturbance regime continually "resets" the succession cycle in small areas ("patches")

–        This results in a shifting “patch mosaic” at the landscape scale

Patch mosaic = set of habitat patches differing in species composition, age since disturbance, etc.

–        Mosaics classified by patch size & degree of heterogeneity (“coarse-grained, fine-grained”)

Ecological succession

Ecological succession refers to a fairly regular sequence of changes in plant (and to a lesser extent animal) species composition following a disturbance

Initially, pioneering plant species ("weeds," generalists with a high reproductive rate) colonize disturbed habitats (e.g., Mt. St. Helens after eruption)

These are gradually replaced (out-competed) by more specialized species (lower reproductive rate, more shade-tolerant, etc.)

Classic view is that late-succession habitats (“climax”) have the highest species diversity and stability

–         In many cases, it turns out that diversity actually highest in intermediate period (between early and late succession stages)

–        Evidence suggests that many late-succession habitats are less stable than earlier phases

Ecological stability = likelihood that a system (e.g., habitat, set of interacting species) will remain unchanged over time

–        Stability ultimately depends on frequency and magnitude of disturbances

Ecological resilience = likelihood that a system will return to roughly same state (species diversity, ecological function) following disturbance

–        Disturbances = physical (e.g., fires, floods, hurricanes, volcanic eruptions) or biological (e.g., predation, disease, overgrazing)

–        Biological causes include anthropogenic (human-caused) ones:  fire, clearing, harvesting, etc.

Equilibrium paradigm

Equilibrium = stable state reached after succession ends

This can sometimes be a dynamic or cyclical equilibrium (e.g., predator-prey cycles)

Equilibrium paradigm dominated ecology for many decades, but is now very much out of favor among professional ecologists (though the prevailing view among environmentalists, in folk theories of "the balance of nature," etc.) (see Botkin 1990 for an accessible review)

Postulates of the equilibrium paradigm:

–         Ecological communities = highly integrated, consistent species composition

–        Succession = predictable, transient process tending towards “climax” state

–         Climax = stable, “natural” state of ecosystems

–         Disturbances = harmful, “unnatural”

Non-equilibrium paradigm

Postulates of the non-equilibrium paradigm:

–         Ecological communities are weakly integrated, with variable & historically contingent composition

–         Succession is a recurrent process, not just a short trip to climax or stability

–        In many ecosystems, succession is irregular or unpredictable, and system can "flip" into a new state unpredictably

–         Disturbance plays key role in ecological dynamics & community composition; many species and ecological communities would be unable to persist without a regular pattern of disturbances ("disturbance regime")

Deciding whether equilibrium or disequilibrium dynamics best describes any particular case can be difficult, but ecologists mostly agree that ecosystems are far more complicated & unpredictable than portrayed in the old equilibrium paradigm

ANTHROPOGENIC ENVIRONMENTAL CHANGE

Anthropogenic means created by human activity

Only built environments are truly anthropogenic, but many (some would argue most) terrestrial habitats, and increasingly marine ones as well, are strongly modified by human activity over time, and these frequently labeled anthropogenic (Balée 2006; Denevan 1992)

Many different forms of anthropogenic environmental change, but in pre-industrial societies the most obvious ones include regular burning (of grasslands or forest understory), forest clearing, irrigation, terracing, and fertilizing

Anthropogenic effects can be intentional (e.g., clearing a patch of forest to plant a garden) or unintentional (e.g., digging up edible plants aerates the soil, increasing future growth)

From an ecological point of view, such anthropogenic impacts are just another form of disturbance

The pervasiveness of anthropogenic habitat modification is such that humans (even pre-industrial ones) can be considered "ecosystem engineers" that have had a dominant role in shaping many environments for thousands of years

Anthropogenic disturbances are often the result of attempts to maintain or increase productivity of key ecological resources (i.e., a form of resource management)

Peacock & Turner (2000:139) argue that this occurs at several scales:

–  Population management = enhance local populations of a resource species

–  Community management = create or maintain diversity of resource-rich locales or habitats

–  Landscape management = multi-habitat (large-scale)

Enhancement techniques

Many different methods designed to enhance or manage resources (plus others that may lead to enhancement as byproduct)

Interior Salish methods documented by Peacock & Turner (2000) include:

– burning (season- & habitat-specific)

– weeding & tilling

– replanting

– pruning & coppicing

– partial removal (e.g., tree bark)

– pest removal

– rotating of harvesting areas (multi-annual cycle)

This list is quite surprising, since it applies to a non-agricultural (hunting-gathering) society (native to southern interior of what is now British Columbia, Canada), and focuses almost completely on plants; agricultural and pastoral societies have additional practices

Many enhancement methods = disturbance regimes of moderate intensity & frequency

Ecologists have documented that such "intermediate disturbance regimes" result in high levels of biodiversity, both at the species level within patches (by keeping succession going) and at the landscape level (by creating a patch mosaic of different successional stages)

As Peacock & Turner (2000:164-5) note:

“The key to use of disturbance regimes in promoting biodiversity is regulating the intensity and frequency of the disturbance events…. Traditional landscape management…resulted in intermediate levels of disturbance.”

Anthropogenic fire regimes

Classic (equilibrium) view sees disturbances (including fire) as harmful & disruptive

If anthropogenic, then also viewed as “unnatural”

As discussed above, ecologists have mostly given up the first postulate (that disturbance is harmful & disruptive), but they've had a harder time letting go of the "anthropogenic = unnatural” assumption

European/colonial forestry & land use has long attempted to suppress fires (in order to maximize timber production, prevent property damage, etc.)

Thus, indigenous burning regimes (which were nearly universal -- see Pyne 1998 for a review) were actively suppressed by colonial and post-colonial regimes worldwide

Results of this policy include disappearance of open habitats (succession), and the build-up of fuel (unburned underbrush, etc.) which periodically resulted in catastrophic fires (caused by lightning or human accidents)

Purposes & effects of anthropogenic burning:

•         Create new clearings

–        Swidden cultivation

•         Improve productivity of favored plants

–        Browse or graze for hunting & herding

–        Growth of berries, tubers, etc.

•         Remove underbrush or grass

–        Expose animal tracks & burrows for hunting

–        Improve ease of travel

–        Reduce fire hazards near camps/villages

–        Reduce pests or insects

•         Other

–        Create dry firewood

–        Heat soil to lengthen growing season

–        Create/maintain successional mosaic (Mardu example--see Bird et al. 2005)

Many fire ecologists are now realizing not only that recurrent burning is a good thing for ecosystem resilience and biodiversity, but that historic anthropogenic burning regimes have shaped many ecosystems and should be brought back in some form or other (Pyne 1998)

References

Balée, William (2006)  The research program of historical ecology. Annual Review of Anthropology 30:75-98.

Bird, Douglas, Rebecca Bird, and Christopher Parker (2005) Aboriginal burning regimes and hunting strategies in Australia's Western Desert. Human Ecology 33(4):443-464.

Botkin, Daniel B. (1990) Discordant Harmonies: A New Ecology for the Twenty‑first Century. NY: Oxford University Press.

Denevan, William M. (1992) The pristine myth: the landscape of the Americas in 1492. Annals of the Association of American Geographers 82:369‑385.

Peacock, Sandra L. and Nancy J. Turner (2000) "Just like a garden": traditional resource management and biodiversity conservation in the interior plateau of British Columbia.  In Biodiversity and native America, ed. Paul E. Minnis and Wayne J. Elisens, pp. 133‑179.  Norman, OK: U of Oklahoma Press.

Pyne, Stephen J. (1998) Forged in fire:  history, land, and anthropogenic fire.  In Advances in historical ecology, ed. William Balée, pp. 64‑103.  NY: Columbia U. Press.

Smith, Eric Alden and Mark Wishnie (2000) Conservation in small-scale societies. Annual Review of Anthropology 29:493-524.