Habitat Degradation and Fragmentation

1. Habitat Degradation and Fragmentation 1. Habitat degradation- the process by which habitat quality for a given species is diminished 2. Habitat fragmentation- the process by which a natural landscape is broken up into small parcels of natural ecosystems, isolated from one another in a matrix of lands dominated by human activities. The net result of fragmentation is the conversion of large, un-broken, tracts of land into many small tracts containing increased numbers of “edge” species

2. Construction of roads dams and other structures Some common examples of habitat degradation include; Contamination- such as air pollution, water pollution, mis-use of pesticides Soil erosion Fire regimes Excessive water use Deforestation Desertification Draining, Dredging, etc

3. Relationship to “island biogeography” theory Fragmentation Generally- islands are defined as land masses, not connected to a continent, separated by non-terrestrial habitats ( better known as water) Key Island Factors Small Often inhospitable

4. Isolated from “source” populations

5. Fragment Size and isolation Why do larger fragments tend to have more species? This phenomenon is known as Species Area Effect 1. Usually larger fragments have a greater variety of environments than small fragments

6. 2. A large fragment is likely to have both common and uncommon species (I.e., species that occur at low densities... Whereas a small fragment will only have common species

7. 3. Populations of an individual species tend to be smaller on islands Thus, all of the problems of small populations seem to be magnified in island situations

8. Declining distribution of the Sumatran Tiger, which is restricted to undisturbed forest tracts

9. 100 ha Edge= 3545 m 50 ha 50 ha 100 ha Total Edge = 5013 m Edge = 4243 m Effects of Shape on Total Edge of a Habitat Patch The optimal shape for a habitat patch is a circle because it reduces the amount of edge per unit of area.

10. 68 ha Original area = 100 ha 62 ha 28 ha 28 ha Original area = 2 plots of 50 ha each Original area= 100 ha Interior Zones: given an edge width of 100 m If the edge effect is 100m, then the amount of core decreases with patch size.

11. Two concepts important to our understanding of fragmentation and island theory 1. Source populations- groups of interacting individuals of the same species that exhibit positive growth in numbers such that excess individuals can disperse to adjacent areas and supplement initial numbers at the new location. 2. Sink populations- opposite of source populations, death or emigration rates exceed birth rates and population relies on the influx of individuals from source populations. Without new immigrants, sink populations will eventually go extinct locally.

12. How do sources and sinks relate to fragmentation? • Think of the eastern deciduous forest. • Once an unbroken stand of trees extending from the east coast to the Great Plains. • As the forest became broken up into patches isolated from each other by inhospitable habitat (islands), dispersal between patches decreased. • Patches that could act as sources for other patches became extremely valuable. • As the valuable patches were lost, populations in patches (sinks) that required individuals from the sources began to decline in numbers. • Many populations became locally extinct.