© 1988 WHC WILDLIFE HABITAT COUNCILSM Importance of Disturbance in Habitat Management October 2006 Fish and Wildlife Habitat Management Leaflet Number 37 Introduction Any event that disrupts an ecosystem or plant or wild- life population structure and changes its resources, substrates, or physical environment is called a distur- bance. Disturbances are natural components of vir- tually all of the world’s ecosystems and can include fires, floods, droughts, storms, herbivory, and disease outbreaks. Disturbances are critically important for maintaining healthy and productive ecosystem func- tions. For example, periodic disturbances are essen- tial for early successional plants and animals, overall biotic diversity, enhancing the capacity of ecosystems to produce clean air and water, and allowing nutrient cycling to occur. Wes Burger, Mississippi State University Oak savannahs maintained by periodic fire Disturbances interact in a complex manner with cli- mate and soils to produce and maintain a plant com- munity that is unique to that site. In a healthy eco- amples of disturbance-adapted systems are used to system, the plant community is in a state of dynamic demonstrate the importance of disturbance on the equilibrium, and there is variability in its species com- landscape. Disturbance management options are dis- position and successional stages following distur- cussed, with suggestions for incorporating distur- bance. This variability is desirable because such habi- bance into management plans. tats can accommodate a diverse wildlife community adapted to different plant species and successional Fire-adapted systems stages. Fire has historically been an integral factor in main- Fully functioning ecosystems have a natural resis- taining native prairies, shrublands, and forests across tance and resilience to disturbances. Resistance re- North America. While fire frequency and severity vary fers to the ecosystem’s ability to retain its plant and by region and ecosystem type, all fires are influenced animal communities during and after a disturbance. by the plant community and climate of a site. In turn, Resilience refers to the magnitude of disturbance an the severity and frequency of a fire influences the ecosystem can withstand and regain its original func- plant community that will recolonize a site and the tion after the disturbance. As an ecosystem is de- wildlife species that will inhabit it during the succes- graded, its resistance and resilience to disturbance sional stages following the fire. weaken. In these cases, a disturbance can push an ecosystem past a certain threshold. Once that thresh- The flora and fauna in native prairies, shrublands, and old is reached, ecosystem processes change, resulting forests have lived with periodic fire for thousands of in changes in the plant and animal communities. As years. Many plant species have specific adaptations these changes occur, the ecosystem is in a transition that allow them to continue to survive in a post-fire from its original state to a new state. environment. Cottonwoods and oaks exhibit epicor- mic sprouting, which means that new branches re- The purpose of this leaflet is to provide landown- sprout if old ones are burned. Other trees, such as ers with an improved understanding of natural dis- willows, have the ability to resprout from their roots turbances and their ecological importance. Three ex- after a fire. Some trees, such as the ponderosa pine, Importance of Disturbance in Habitat Management have thick bark, which protects them from fire. Many Underbrush in various successional stages after fire at plants actually depend on fire to complete their life Tall Timbers Research Station in Florida cycles. For example, many pine trees have cones that will not open until heated by fire; blueberries exhibit fire-enhanced flowering and fruit production. Fire releases nutrients and uncovers bare soil. The blackened, bare soil warms quickly, which stimu- lates soil microbial activity, nutrient cycling, and plant growth. In forests, fire opens up part of the canopy to sunlight, which allows sun-loving plant species to recolonize the site. In prairies, fire can remove dead vegetation that hinders new growth, reduce invasive plants, encourage native species, and create wildlife habitat. Following fires, plant communities go through suc- 1 year cessional changes. Many native wildlife species and Steve Dinsmore popular game species, such as bobwhite quail, white- tailed deer, and wild turkey, are dependent on period- ic fire to create and maintain suitable habitat. Surface fires can stimulate the growth of herbaceous foods for deer, elk, moose, and hares, and can enhance ber- ry production for black bears and other wildlife. Small mammal populations generally increase in response to new vegetation growth, providing a food source for carnivores. Fire can also reduce internal and external parasites on wildlife. Fire suppression has been widespread throughout North America since European settlement. Fire sup- pression causes fuels to accumulate and can result in high-intensity, more destructive fires. Many native years plant and animal species find it difficult to adapt to Steve Dinsmore fire suppression. Fire intolerant species are able to in- vade and displace native species in areas protected from fire. In the Pacific Northwest, increased disease and insect outbreaks appear to be related to fire sup- pression. However, the benefits of wildfire are becom- ing better known and fire suppression has given rise to managed fires in many areas. Herbivory-adapted systems Herbivory is an important disturbance in many range- land ecosystems. Before European settlement, bi- son, elk, prairie dogs, and other herbivorous wildlife grazed the North American rangelands. Today, live- stock have replaced native herbivores in many range- 3 years land ecosystems. So, past and present herbivores ex- Steve Dinsmore ert a strong influence on a site’s plant community. Importance of Disturbance in Habitat Management Woody plants, for example, cannot establish them- Flood-adapted systems selves in areas that are moderately or heavily grazed. Instead of woody plants, grasses and forbs make up The interface between water and land can be sub- most of the plant community in rangeland ecosys- ject to daily, seasonal, and long-term changes. Plants tems. Herbivores influence ecosystem properties such found within this dynamic zone must be tolerant of as nutrient cycling and productivity by compacting short- to long-term inundation, have sufficient struc- soils and adding organic material to them. ture to withstand the physical force of moving water, or be capable of rapidly colonizing flood-prone areas Native rangeland plants have adapted to grazing by between events. developing extensive root systems or an ability to re- sprout quickly. In turn, many wildlife species have In river and stream ecosystems, floods move water adapted to grassland plant communities and are high- and sediment through the channel and onto the flood ly dependent on rangeland ecosystems for their hab- plain. High water flows maintain ecosystem produc- itat needs. Rangeland plant communities provide tivity and diversity by removing fine sediments that food, escape, nesting, and brood-rearing cover for would otherwise fill the interstitial spaces in produc- many mammals, birds, and reptiles. For example, the tive gravel habitats. Floods bring leafy and woody ma- bunching nature of native grasses provides excellent terial into the channel, which creates structure and nesting habitat for ground nesting birds, such as the provides detrital foods for aquatic species. Many tem- northern bobwhite quail. Rangeland grasses and forbs porary habitats, such as river bars and riffle-pool se- support a wide variety of insects, which serve as food quences, are formed and maintained by high flows. for many grassland bird species. Animals associated with areas subject to flooding Rangeland ecosystems are adapted to a certain level have adapted to varying flow regimes, including sea- of grazing; however, overgrazing can be severely det- sonal flooding and droughts, and long-term, more in- rimental to these ecosystems. If a site is overgrazed, tense flood events. The timing of floods is important a threshold is reached and ecosystem processes may because the life cycles of many aquatic and ripari- change. The subsequent loss of vegetation, redistri- an species are timed to either avoid or exploit floods bution of nutrients, and dispersal of exotic plant spe- and/or droughts. For example, the seed release of ri- cies can increase erosion, degrade water quality, and parian trees such as willows and cottonwoods is syn- alter the hydrology, fire regime, and plant and ani- chronized with the timing of spring-flood recession mal community of a site. Conversely, if grazing were to maximize dispersal efficiency. Native fish in desert completely removed from rangeland ecosystems, the streams avoid being affected by flash floods by sens- plant community would go through a number of suc- ing higher flow speeds and moving to sheltered areas cessional stages, leading to a dominance of woody within the stream. Stoneflies and other aquatic insects plants. Woody encroachment in rangelands reduces enter their diapause stage (a period during which their attractiveness for grassland-dependent wildlife growth or development is suspended and physiologi- species. cal activity is diminished) during the drought season. Several species of fish exhibit seasonal movement