Impacts of Slow Continuous Forcing and Sudden Disturbances

Because of their age and complexity, the responses of ecosystems to slow continuous forcing by the environment—such as global warming, the spread and establishment of invasive plants or the consequences of the presence or absence of ecosystem engineers such as beavers and elephants, but also moles and earthworms (Chap. 20)—are difficult to predict (Part V).

Often the underlying mechanisms are not fully understood and thus their consequences at the ecosystem level are difficult to model. Similarly, although at first ecosystem responses to sudden disturbances are obvious—for example, damage after flooding or eradication of entire ecosystems after a volcanic eruption—the long-term responses and the recovery of the affected ecosystems are less clear. Both types of event—slow continuous forcing and sudden disturbances—have impacts on individuals, species, populations and ecosystems, and thus they change the boundary conditions for all actors and components present in ecosystems, as well as all pro cesses taking place. Such impacts range from small adjustments—for example, the physiological acclimation to changing environmental conditions (Part II)—to changes in species composition and species mortality, or loss of entire ecosystems (Chap. 17), with impacts at different levels of organisation:

- Individuals: transient damage (e.g. by late frosts), weakening by pests and parasites, failing reproduction because of weather events, death by windthrow

- Populations: changes in the structure of populations and gene pools (e.g. by reduction of the number of individual plants or by the presence of new spatial barriers to pollination), extinction by events occurring over large areas (e.g. volcanic eruption)

- Ecosystems: changes in species composition by arrival of non-native species (which might become invasive), loss of ecosystem components (e.g. by land use), changes or loss of nutrient cycling (e.g. after fire or due to leaching)

Slow continuous forcing, as well as disturbances, are thus events changing the species composition, as well as the continuous element or substance turnover, either slowly or suddenly, often in unexpected directions. At the level of the ecosystem, it is important whether the system loses resources or whether these resources are only relocated within the system but stay in the system. This can be nicely shown for carbon (C) cycling within ecosystems, where the following drivers can be distinguished (Schulze et al. 1999):

- Drivers exerting a continuous forcing (e.g. temperature, precipitation, radiation, CO₂)

- Drivers reallocating pools, but organic matter and therefore nutrient resources bound to it stay in the ecosystem (e.g. herbivory by insects, browsing by animals, grazing in extensive land use systems, C inputs into soils after windthrow)

- Drivers removing species and pools such as organic matter and thus resources from the system (e.g. pathogens, pests, fire, harvest)

Slow Continuous Forcing. Factors such as temperature and precipitation directly control many processes of the C cycle in an ecosystem. Similar controls are also present for biomass production by plants, thus affecting the entire carbon budget of the ecosystem. These factors typically show a large temporal variability, resulting in years with large and small carbon gains and losses and highly variable process rates of substance turnover. Recently, new factors driving the C cycle in ecosystems have gained relevance—that is, the increase in the atmospheric CO₂ concentration, atmospheric N deposition, fertilisation and global warming (Part V).

These slow but continuous changes in the growth conditions, particularly of plants as the primary producers in ecosystems, exert a strong forcing on many C-related processes, from leaf gas exchange to organic matter decomposition. In the long-term, this forcing disturbs the delicate balance of interacting ecophysiological and biogeochemical processes, often with pronounced consequences for ecosystem functions and the original species composition. For example, continuously increasing CO₂ concentrations affect growth of plants but also increase their nutrient requirements. Since nutrient availability stays constant (or even decreases because of drier soils), these nutrient requirements, particularly for nitrogen (N), are not easily met, leading to larger C to N ratios in plant tissues, which in turn decompose much more slowly than tissues produced under lower CO₂ concentrations. Thus, the feedback to the atmosphere in terms of respired CO₂ from decomposition is also influenced, not only the assimilation of atmospheric CO₂.

Sudden Disturbances and Reallocation of Pools. These continuously changing drivers and their impacts are quite different from sudden disturbances which, for example, interrupt production and decomposition of organic matter. “Catastrophes” such as hurricanes, snow damage and herbivory by insects are examples of these sudden disturbances, as are smaller disturbances such as those caused by moles or wild boars, as well as grazing in extensive (land use) systems, as long as the organic matter is only relocated among different pools but not exported out of the system. Such disturbances can directly accelerate decomposition of plant and soil organic matter by increased litter production in response to the disturbance, or they can change the aggregate structure of the soil— one of the factors controlling the activities of soil biology.

This type of disturbance may lead to temporary accumulation of organic matter in certain compartments (e.g. as litter or woody debris) and to reallocation of pools and thus resources. However, these disturbances generally do not change the ecosystem, since most ecosystems are quite resilient to natural disturbances—that is, they can adapt and buffer their impacts. Sometimes, combined disturbances happen—for example, drought plus the occurrence of a pathogen. Then, the second disturbance, often only occurring after the first one, can have devastating effects, even leading to species loss and ecosystem change.