CAS Workshop on Ecosystem Succession Theory and Practice of Ecological Restoration 

Succession, restoration, and optimal ecosystem management

 Guo Qinfeng

 (U.S. Geological Survey, 8711 37th St. SE, Jamestown, ND 58401, USA)

 Abstract: Disturbance frequency and intensity have been dramatically increased by human activities.  Many ecosystems have been highly fragmented and degraded by decades of overuse, and abundant evidence shows that increased disturbances and global warming have increased the chances of biological invasions.  Some natural disturbances such as wildfires have been intentionally prevented or suppressed in some parts of the world to protect properties, lives, and natural resources.  At the same time, more prescribed burning and mechanical thinning have been initiated to manage a wide range of ecosystems worldwide.  Because landscapes are being constantly re-shaped by changing frequency and intensity of disturbances (both natural and anthropic), restoration techniques and optimal management for biodiversity rely on a clear understanding of the nature of disturbance, the biological responses of ecological communities, and spatial-temporal environmental changes.  Here I will discuss these issues, particularly the following six aspects.

Disturbance vs. succession.  Disturbance is normally followed by succession during which biodiversity and productivity increase first, reach the peak in the transitional stage, and then decline in late stages, whereas biomass increases dramatically at the beginning, continues to increase thereafter and sustains at a certain level.  Successional theory will doubtlessly continue to play a critical role in decision making for ecosystem management and restoration efforts.  Remarkable progress has been made in recent studies of biodiversity and ecosystem function over space, it is time to examine whether related theories on spatial patterns also apply to temporal patterns.  Future studies that simultaneously monitor diversity, biomass, productivity, and many associated biotic and abiotic variables from the same localities in succession would improve our understanding of biodiversity-biomass-productivity relationships in ecosystem development.

Life history.  It is highly predictable that the short-lived plant species including many exotics might have been dramatically increased in terms of both richness and abundance at local and regional scales while many long-lived species might have been lost due to increased disturbances.  Usually herbaceous species have large numbers of small seeds while woody species have fewer but larger seeds.  Large seeds have shorter viability retention in the soil seed banks than small seeds.  Using controlled burns to eliminate harmful exotic species can have the opposite effects, because frequent fires prevent large-seeded species from germinating, establishing, and reproducing to build up another viable seed bank.  As a result, the burned habitats will lose these species.  In contrast, if a habitat is protected from disturbances for extensive time, small-seeded species, which need disturbance and open space to germinate, would be lost.  Species genetic diversity can also offer insightful information for setting up conservation or protection priorities.  Increasing evidence shows that the species with low genetic diversity are especially vulnerable to disturbances.  

Balancing disturbance frequency and intensity.  In general, small disturbances are numerous and large ones are few with fire regimes tending to be synchronized across whole regions (following the Gutenberg-Richter law).  The critical issue is how we balance frequency and intensity of disturbances for optimal biodiversity maintenance under such circumstances.  The great challenge now becomes how to determine the “intermediate” level of disturbance or the “optimal” biomass level in any particular habitat so that high diversity and habitat productivity (as equivalent to carbon sequestration rate) can be achieved and maintained.  The increasing support for the intermediate disturbance hypothesis (IDH) and the commonly observed unimodal diversity-biomass relationship should serve as guidance for optimal grazing and fire management.  Management strategies must be habitat-sensitive, because disturbance regimes and succession rates are highly variable among systems.

            Local vs. regional considerations.  Optimal management at one locality or conservation unit is of little help without incorporating with surrounding areas or habitats.  Therefore, management strategies at one unit have to consider the status of neighbor units across landscape, and the proximity factor (e.g., metapopulation or metacommunity) must be taken into account.  For example, smaller burns and the localities closer to undisturbed sites and the species with greater dispersal power or closer to their “source” populations would recover faster than the otherwise.  On the other hand, only one management practice, no matter how carefully and elegantly conducted, will not satisfy our conservation goals, because such management favors only one subset of the total species pool.  Diversifying management can increase landscape heterogeneity and maintain a mixture of habitat types, which may cover a broad range of successional states, so that the chance of local extinction of rare species will be minimized. 

            Short-term vs. long term effects.  Any designed management practice may have very different short-term and long-term consequences.  The practices beneficial in a short-term may become detrimental over long-term.  This means that we may have to switch the management plan once short-term goals are reached. Similarly, the functional roles of the same species can shift as well.  For example, exotic species in California chaparral can often be regarded beneficial in a short term right after fires because they germinate and establish very fast so that severe soil erosions can be reduced or prevented.  However, because these species adapt frequent fires so well, they often outcompete and eliminate native species.  Long-term studies on succession cycles and associated biotic changes would help us to understand both short-term and long-term ecological effects of various disturbance regimes.  

            Biological invasion vs. climate change.  Global climate warming as a prolonged large-scale disturbance affects ecosystems in dramatic and profound ways.  Global warming can affect biodiversity by causing or enhancing disturbances and instability in ecosystems.  Moreover, by favoring exotic species, global warming threatens native species.  Mathematical models that can better predict future frequency, intensity, and distribution of disturbances under projected climatic scenarios are critically needed.  While our understanding of nature continues to grow and our management policies continue to be significantly improved, future research must address and evaluate the relative roles of all the factors listed above in our conservation efforts.  Experimental and simulation studies that examine the responses of different functional groups to various disturbance and climate regimes would be very helpful.

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