The structure and spatial variation of ecosystems at the regional level are strongly influenced by natural disturbances. These disturbances affect the availability of habitats for plants, animals, and people as well as their distribution over the landscape. “A disturbance is any relatively discreet event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment” (White and Picket, 1985:7). Large infrequent disturbances (LID) are interpreted and presented in varying ways for different ecosystem types. For example, a flood may cover a few square miles but its impact is relatively as profoundly damaging to resident organisms in its context as to a forest fire which may cover thousands of square miles. Therefore, when one is defining what an LID is, the aerial extent of the disturbance must be taken into consideration along with the dispersal abilities of the re-colonizing species (Paine, Tegner, and Johnson: 537). All LID’s vary in their intensity, rate of incidence, and spatial distribution; some LID’s can sterilize a landscape of all life (the Mt. St. Helens blast zone) and others such as hurricanes and forest fires can leave remnant organism assemblages that act as a foundation for the subsequent regeneration of the landscape. An LID’s patterns are affected by the physiography of the landscape, the climate, the season (the ice cover on Mt. St. Helens caused a mudflow that would not have occurred in summer but saved some species of burrowers and hibernators), and even human interference (fire suppression in forests and subsequent fuel buildup from detritus). These variations of disturbance across the landscape then have an effect on the rate and pattern of biological succession following the event. In this paper I will discuss tornadoes and floods which present similar sharp, linear shapes of disturbance, hurricanes and forest fires for their large reaching effects on landscape, and volcanic eruptions, which consist of complex combinations of blowdown, ash and debris fall, toxic chemical and heat waves, and landslides.
The most destructive and deadly tornadoes occur from super cells – which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone. Each mesocyclone is capable of generating a tornado that develops through a five stage process: dust-whirl, organizing, mature, shrinking, and finally decaying stages. (Foster, Knight, and Franklin:500) Tornadoes generally follow the path of the storm that created them and generally move in a north to northeasterly direction in the interiors of continental land masses. They may vary in size (pathway length and width), intensity (wind speed), and temporal duration but due to the fact that tornadoes are unaffected by the physical landscape they produce a disturbance that is linear in shape with sharp edges between damaged ecosystems and surrounding intact ecosystems. The linearity of a tornado disturbance ensures that habitats and species that were damaged or destroyed during the up to 400 kph. winds would have intact old growth areas rife with species ready to disperse and colonize. In the 1985 tornado “outbreak” that cut northeast across the interior of the United States and central Canada, the recorded path widths of the 41 tornadoes spawned ranged from less than 200 m. to 2750 m., underscoring the afore mentioned effect on size and shape this type of disturbance has on subsequent succession. (Foster, Knight, & Franklin:500) Floods are another form of LID that is climactically driven and forms a relatively narrow linear shape affecting the recovery rate of the disturbed area; however, floods differ in that their shape and diffuse edges are driven by the topography of the drainage channels. When the rate of water input on a landscape exceeds its capacity to drain and diffuse said water supply, a flood occurs. The intensity of a flood and the effects of damage versus benefit to an ecosystem make the classification of a flood as an LID purely anthropogenic as damage to man-made structures become the concern. Riverine species of plants and animals can be well adapted to the frequent inundation, and some benefit from the mixing of the nutrient rich sediment. The exceptions are floods that last for llong periods of time such as the floods of the Mississippi drainage system in 1993, which ironically caused the trees to experience drought stress due to the inability of damaged roots to take up water. (Foster, Knight, & Franklin:502)
When one hears the phrase “Large Infrequent Disturbance”, instinctually we think of fire and hurricanes as, in our lifetimes, they are in fact very frequent, generally cover large geographical areas, and affect millions of lives each year, human and non-human. A fully formed hurricane is a huge (7-8 km up & 1000 km Radius) rotating vortex of wind and water vapor fed by warm ocean water in an area likely to form cloud towers. (Foster, Knight, and Franklin:498) The center, or eye, of a hurricane is an area of low pressure and wind speed surrounded by an eye wall, an area with high wind speed, rain, and powerful convective forces that can extend outwards to 100 km at which point wind speed will be reduced exponentially. The rotational velocity, regional wind direction, wind gradients within the storm, downbursts, resulting tornado offshoots, large topographic variations, vegetation structure, and continental energy drain all act on a hurricane to produce a disturbed landscape that is incredibly complex in its resulting form. An area hit by a hurricane is a large linear swath of damage with very diffuse edges that can extend 1000’s of kilometers and leaves an incredibly patchy landscape of legacies. The patches of old growth forest are important in local and regional succession; high winds and high precipitation cause blowing soil, landslides, and flooding that influence spatial distribution of successional plant species and landform pattern. Persistent landscape level patterns of vegetative structure and composition can be produced in areas prone to the seasonal disturbance of hurricanes. Fire is another potentially huge disturbance that is climactically; driven by periods of drought and strong winds fires are generally started by lightning or humans, but in contrast to hurricanes, fires can persist for months, consume much vaster areas, and biological recovery can take decades. In processes known as pyrolysis (drying of fuels and gas production) and visible ignition or flaming combustion, the fire is driven across a landscape leaving behind used up volatiles that burn slowly and can persist for weeks, even in damp weather. Wind speed and direction, some as a result of convection, act to initiate multiple ignition points due to the blowing embers in a process known as spotting. (Foster, Knight, & Franklin:501) Fire behaves in response to landscape features, the fire burns more readily uphill with the rising heat and less intensely on the leeward side; ridges, river, and lakes can act as firebreaks, and the day/night cycles cause variability in the survival rates of organisms on the resultant diffuse border. All of these factors create a landscape composed of patches of surviving stands, a carpet of dead organic matter, standing dead trees, and surviving organisms that generates a distinct pattern of vegetative structure and age.
There is one disturbance type that has complex and unique consequences on an ecosystem related to each event specifically, volcanic eruptions. Volcanic eruptions are short but incredibly intense disturbances that involve the interactions of multiple disturbances in concert such as explosive blasts, waves of intense heat and toxic chemicals, landslides, airborne bombardment of rock, ash, and lava, and flows of debris (lahars). (Foster, Knight, & Franklin:502) Volcanoes have an epicenter and the degree of disturbance tends to grade to less intense the farther you travel from the source and depending on an explosion or lava flow event, the shape can be isodiametric or linear/dendritic. The edge definition can be sharp in the area of a powerful blast and become diffuse as the effects of the explosion diminish with distance and the effects of heat, chemicals, and ash fall take over across a much larger area (1,000,000,000 sq. km. or more as ash is carried by the winds) (Foster, Knight, & Franklin:503) In the case of the Mt. Saint Helens eruption, the blast zone was sterilized, the heat and chemical wave killed stands of trees much farther out, a massive landslide damaged lakes and streams, and the ash affected areas 1000’s of km’s away. (http://www.fs.fed.us/gpnf/mshnvm/research/faq.html) Local topography has an effect on the gravity based disturbances such as landslide, lava flow, and glowing debris flow but does not influence the immediate explosive blast. In lava flow events, of course, the topography plays a very important role in the patterns of disturbance and subsequent succession regimes (leeward ridges and other breaks left behind some surviving plant assemblages in patches). On Mt. Saint Helens the season played an important part in the pattern of disturbance but also in the survival of some plants and animals. The ice cover that was present in May allowed the survival of burrowed, hibernating organisms as well as creating the devastating mud flows that helped to sterilize the above ground landscape. All of these points when viewed as a whole give us the very complex picture of multiple disturbances, topographic influences, seasonality, and the many scattered foci of succession that are involved in the unique disturbance type of volcanic eruption.
As we have seen, LID’s range from the very small in aerial extent, such as localized flooding, to the near globe spanning effect of ash deposition in the atmosphere as a result of a volcanic eruption. The landscape of the Earth and all of the patterns of vegetation and animal populations are, in some part, shaped by the occurrence of large disturbances, whether it is a yearly event or every 1000 years. An LID can be a result of climate, tectonic activity, or human activity. As humans, we can increase the frequency and intensity of an LID through our influence on global climate change and ecosystem mismanagement, such as fire and flood suppression. The shape, intensity, duration, and frequency of an LID directly impact the spatial distribution of vegetation and animal populations. Landscapes that are regularly affected by LID’s tend to form in response to the type of disturbance and have structures and species that have evolved to cope and even thrive on these disturbances, such as riverine species and seeds that require high temperatures to germinate. As long as there is an atmosphere and water on the Earth and the Sun continues to pump energy into the biosphere, natural disturbances will continue to shape the land and the organisms that reside within will respond, shift, and evolve in accordance.
Reference:
Foster, D.R.; Knight D.H.; and J.F. Franklin. 1998. Landscape Patterns and Legacies resulting from large infrequent, forest disturbances. Ecosystems 1. 497-510.
