Disturbance
• Definition: any process or condition external to the natural physiology of living organisms that results in the sudden mortality of biomass in a community on a time scale significantly shorter than the accumulation of the biomass.
Disturbance
• Examples:– Fires– Windstorms– Floods– Extreme cold temperatures– Treefalls– Epidemics– Bulldozers
Disturbance
• Might kill a few, many, or all of the organisms in a community, or may kill a portion of a single individual (as often happens with plants)
• Not all mortality results from disturbances (e.g., natural death of individuals from aging)
Disturbance
1) Intensity: proportion of total biomass killed; often inversely related to frequency
2) Frequency: number of disturbances in a given time interval; determines how far succession can proceed; annual patterns (e.g., storms, temperatures, precip) can give highly predictable frequencies for certain types of disturbances such as fires or floods
Disturbance
3) Timing: coincidence of the disturbance with important cycles or events in the ecosystem affected by disturbance; e.g., prairie fire in spring has different effects on species composition and nutrient cycling than a fire in the fall
4) Disturbance area: absolute and relative size of disturbance area and the shape have important effects on recolonization/succession
Disturbance
5) Resource Availability: immediate mortality is the most dramatic effect, but most important usually are the longer-term consequences for resource availability (e.g., nutrients, light)
Disturbance
• Some level of disturbance is normal• Diversity tends to increase at
intermediate levels of disturbance• Disturbance in ecological time
(succession) creates a mosaic called patchiness that promotes heterogeneity on community/landscape scales
• Heterogeneity also results from variation in topography, geology, etc. but these are mechanistically independent
Disturbance: Heterogeneity
• From a conservation perspective, heterogeneity is desirable because the greater it is, the more niches are available
• Therefore important to understand the natural disturbance regime
• Also important to understand effect of disturbance on different species
Fragmentation
• Species-Area Curve in reverse• Reduction in total amount of a
habitat, also quality of habitat changes (smaller, more isolated patches)
• End result is often a patchwork of small, isolated natural areas in a sea of developed landscape
Fragmentation
• Results in biotic impoverishment:– Local extinctions or extirpations– Smaller populations– Loss of genetic diversity– Shifts in composition and abundance
patterns
• As populations become more isolated, dispersal ability becomes critical
Fragmentation
1) Naturally patchy landscape had rich internal patch structure vs. simplified patches in a fragmented landscape
2) Less contrast between patches in a natural landscape vs. greater contrast in a fragmented landscape
3) Certain features of fragmented landscapes pose specific threats to populations and species
Fragmentation
If patchiness is good, why is human-generated fragmentation bad?
Less continuityLess complexityHigher contrast
in artificial/fragmented landscapes
Biological Consequences of Fragmentation
1) Extirpation/Extinction
2) Barriers and Isolation
3) Exclusion
1) Extirpation/Extinction
• Extirpation = elimination of a species in a given area but with populations of that species remaining elsewhere
• Extinction = total elimination of a species
1) Extirpation/Extinction
• Direct elimination of populations or species in the areas where habitat is destroyed
• Rare endemic species with narrow distributions are most vulnerable, but even common species may not be immune
• Species exploited or persecuted by humans (for food, medicine, fear)
1) Extirpation/Extinction
• What if a keystone species is eliminated?
• Direct losses may result if destroyed habitat contained specialized resources (e.g., destruction of wetlands resulting in population crashes of wading birds, amphibians)
2) Barriers and Isolation
• Insufficient fragment size to sustain populations (e.g., grazers)
• Restriction of movement often leads to reduced genetic variability, especially in species with poor dispersal ability
• Restriction of movement also affects species that need a mix of different habitats at different life stages
2) Barriers and Isolation
• Not just a problem in terrestrial habitats (dams may block access by migratory fishes)
• The smaller and more isolated the population, the greater the risk of extinction
3) Exclusion
• Change in quality of habitat may exclude species
• Fragmentation produces greater contrast between patches leading to intensified edge effects
• Some species require “interior” habitat
• Other species may be attracted to edges (for food) but then suffer higher rates of nest predation
3) Exclusion
• Some species are adapted to edge conditions; management for these species may negatively affect total biodiversity (e.g., increase in deer populations)
Fragmentation—Summary
• Fragmentation results from habitat destruction and is a reality of the modern world
• Species losses associated with fragmentation occur for a variety of reasons
• Loss of genetic diversity is a common result
Fragmentation—Summary
• Diversity may increase through the introduction of exotic species, which tend to invade disturbed or fragmented habitats more easily, but competition from exotics may eventually reduce overall biodiversity
• Threat of global warming and climate change is especially ominous
Invasive Species
• Defined as species introduced from somewhere else (non-native or exotic) that compete aggressively with natives
• Continuing introduction of non-natives has great implications for native communities and conservation
• Long-distance dispersal can happen naturally but is not all that common
Invasive Species
• Deliberate or accidental introduction of species has been happening since humans have been traveling extensively
• Many examples of disastrous invasions but not all introductions result in disaster
• Effects of invasions depend a great deal on which species and which communities are involved
Invasive Species
• Successful invaders tend to be opportunistic, have broad diet, high genetic variability, good dispersal ability, among other things
• Invadable communities tend to have a low diversity of native species, be disturbed, lack species similar to the invader, among other things
Brown Tree SnakeOn Guam
Evidence showsthat it drove10 species toextinction on theisland; 2 of thesewere endemic.
Global warming/climate change
• Global warming—increase in temperatures due to the build-up of greenhouse gases (CO2 emissions) and other carbon sources
• Effects—melting of glaciers and ice caps, rise in sea levels, changes in weather patterns, changes in weather intensity, higher levels of disturbance, loss of biodiversity, etc.
Global warming/climate change
• Recent Intergovernmental Panel on Climate Change report indicates at least 90% chance that most warming since 1950 from continuing emissions of CO2
• CO2 is the #1 contributor (40%), but black carbon is now recognized as a significant contributor, responsible for ca. 18% of current global warming
Global warming/climate change
• Examples of effects of global warming/climate change on biodiversity:– Coral bleaching– Sky islands in the southwestern
U.S.– Sundarbans in India and
Bangladesh
Coral bleaching
• Generally attributed to higher surface water temperatures
• Heat and increased UV cause the corals to expel their symbiotic algae (zooxanthellae) leaving the white coral skeleton
• Can recover after short periods of bleaching but longer periods may cause death or higher susceptibility to disease
Sky Islands
• Sky islands are forested mountain ranges separated by deserts/grasslands
• Perhaps the best known system is in the southwestern U.S.
• High diversity, fairly high endemism
• Fire-maintained ecosystem• Provided much early inspiration
for Aldo Leopold
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