The Effect of Forest Fragmentation on Forest Interior Species
Preston, Darlington, and others have shown a clear relationship between the area (A) of oceanic islands and the number of species (S) they contain. The relationship is expressed as S=CAz with studies involving birds showing the value of z to approximate 0.3. This has been clearly demonstrated in research studies; as islands size increases, the number of species increases. Darlington showed that as Island size increases tenfold, the number of species doubles. Wilson and MacArthur's Island Biogeography Theory further predicts that the actual species on an island can change from year to year due to colonization and local extinction (turnover), but the number of species remains at equilibrium.
Forest fragmentation creates forest islands of different sizes on the mainland. In examining this effect of area on species diversity, we examined how key individual forest species were affected by changing forest size by informally surveying 55 forests.
I found that the most area sensitive species were two of the rarer warblers, Black and White Warbler and Worm-Eating Warbler. This is consistent with the background information. These two warblers were listed as having the highest minimum area needed at 304 ha. In this study the Black and White was found no forest less than 227 ha, and the Worm-eating Warbler was found in only one forest less than 391 ha. Other area sensitive birds identified in this study included the Ovenbird, Acadian Flycatcher, Kentucky Warbler, and Red-Shouldered Hawk.
The least area sensitive species were mostly species that were not truly interior birds and were not listed as interior species in my background information. These species included such edge birds as the Carolina Wren and Great Crested Flycatcher.
The fact that smaller forests will not contain certain species raises questions about Wilson and MacArthur's theorizing turnover leading to equilibrium. This fact clearly would impact the immigration needed to maintain equilibrium. This species' data suggest that the immigration that Wilson and MacArthur predict might be limited by the fact that certain species may not colonize a smaller forest island. Area sensitive species tend to drop out as forest size decreases, and the smaller forests are populated predominantly by species that require a more generalized habitat. Finally, our data suggest that management of land in coastal watersheds should focus on preserving large forest blocks. One large forest seems to be clearly better than several small in maintaining the highest species diversity.
How big does a forest have to be to contain certain forest interior species?
- Calculating Forest Sizes in Queen Anne's County To determine mean and median forest sizes in Queen Anne's County, we will use ArcView GIS mapping to map and calculate areas. We will randomly select 3 forests in each Bird Breeding Atlas Block to measure. We will use Excel to select a series of random numbers from 1 to 90, which correspond to a numbered grid that is placed over the block. We will measure the forests under the grid number randomly selected.
- Informal Species-Area Study The team will also research optimum forest sizes for particular species and look for these patterns in our data. In other words, which species "disappear" first as forest size decreases? We will sample 30 - 40 forest areas on the Eastern Shore. Step 1: Fill out forest data sheet, which includes, date, forest number (number forest in order their observed), name and number of block the forest is located in, forest area in hectares (calculated on Arc View GIS maps), and a location of the forest such as a street name. Step 2: For 5-10 minutes listen and observe the species in the forest. Then record any species seen or heard within the forest boundaries on the forest data sheet. Then record the total species heard or seen at the top of the sheet. Step 3: Also record any vegetation marks that characterize the forest (such as dense undergrowth). Step 4: Analyze. Compare the forest interior species to the forest size and how species change as the forest sizes change.