Humid tropical forests are highly dynamic ecosystems that are affected by a wide array of environmental processes and disturbances (Figure 19.1). Quantifying the magnitude, frequency, and impacts of natural disturbances is essential for designing hydraulic structures, developing water management strategies, and distinguishing between natural variation and man-made influences.
A disturbance can be defined as any discrete event that transfers mass and energy from one part of a system to another in a manner that disrupts ecosystem, community, or population structure and changes resource availability or the physical environment (see White and Pickett 1985 for a detailed discussion). Natural disturbances can be driven by both external factors – for example, hurricanes, meteor impacts – and the biological properties of the system such as senescence and pathogens. The natural disturbances specified by the United Nations in the International Decade of Natural Disaster Reduction were earthquakes, windstorms, tsunamis, floods, landslides, volcanic eruptions, wildfires, grasshopper and locust infestations, drought and desertification (Board on Natural Disasters, 1999). Additional natural disturbances known to affect the hydrology of humid tropical forests are tree falls, pathogens, exotic invasions and meteor impacts.
Quantifying the effects of disturbances on landform morphology and ecosystem development have been major themes in geomorphology and ecology (Wolman and Miller, 1960, Connell, 1978). This approach has led to the paradigm that landscapes are structured by the processes acting upon them (O'Neill et al., 1986; Urban et al., 1987; Scatena, 1995).