Disturbance

Biology/Env S 204Spring 2009

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

• Five main factors:– Intensity– Frequency– Timing– Area – Effect on resources

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

If patchiness is good, why is human-generated fragmentation bad?

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

Black carbon is soot, mainly from cooking stoves.

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

Ganges delta

Originally ca. 16,700 km2

Now ca. 4,100 sq km

1 of 3 largest mangrove forestsSundarbans Forest