Disturbance

1. Disturbance Biology/Env S 204 Spring 2009

2. 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.

3. Disturbance • Examples: • Fires • Windstorms • Floods • Extreme cold temperatures • Treefalls • Epidemics • Bulldozers

4. 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)

5. Disturbance • Five main factors: • Intensity • Frequency • Timing • Area • Effect on resources

6. Disturbance • Intensity: proportion of total biomass killed; often inversely related to frequency • 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

7. 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

8. 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)

9. 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

10. 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

11. 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

12. 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

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

14. Fragmentation • Naturally patchy landscape had rich internal patch structure vs. simplified patches in a fragmented landscape • Less contrast between patches in a natural landscape vs. greater contrast in a fragmented landscape • Certain features of fragmented landscapes pose specific threats to populations and species

15. Fragmentation If patchiness is good, why is human-generated fragmentation bad? Less continuity Less complexity Higher contrast in artificial/fragmented landscapes

16. Biological Consequences of Fragmentation • Extirpation/Extinction • Barriers and Isolation • Exclusion

17. 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

18. 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)

19. 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)

20. 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

21. 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

22. 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

23. 3) Exclusion • Some species are adapted to edge conditions; management for these species may negatively affect total biodiversity (e.g., increase in deer populations)

24. 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

25. 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

26. 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

27. 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

28. 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

29. Brown Tree Snake On Guam Evidence shows that it drove 10 species to extinction on the island; 2 of these were endemic.

30. 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.

31. 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

32. Global warming/climate change Black carbon is soot, mainly from cooking stoves.

33. 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

34. 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

35. 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

36. Ganges delta 1 of 3 largest mangrove forests Sundarbans Forest Originally ca. 16,700 km2 Now ca. 4,100 sq km