The Effect of Forest Fragmentation on Forest Interior Species

Abstract

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.

Problem

How big does a forest have to be to contain certain forest interior species?

Methods

1. 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.
2. 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.

Discussion

In looking at the Minimum Area Found for each species and the forest areas each species is found in, birds range from most area sensitive to most area tolerant. Each species is discussed below.

Black and White Warbler
The Black and White Warbler was the rarest of the forest interior birds found with only 1 sighting. It seems to be most area sensitive, being found only in a 227 ha forest. This is consistent with the fact that it is a ground nester and subject to predation.

Worm-Eating Warbler
The Worm-Eating Warbler was one of the rarer birds found during the study. It also seems to be most area sensitive, only being found above 80 ha. It is also a ground nester and subject to predation. It was found at a mean area of 237 ha.

Ovenbird
The Ovenbird is an area sensitive species, not found in forests less than 10 ha. In forests below 80 ha, it is uncommon. Also a ground nester, it is also subject to predation. The mean forest area in which the species was found was 109 ha; however, the species was found in a forest as small as 14.8 ha.

Kentucky Warbler
The Kentucky Warbler was only found three times during this study and is considered an area sensitive species, being found in only forest above 50 ha. The Kentucky Warbler is found in only wet forest and in most cases the habitat was what counted. The minimum area the warbler was found was 57 ha.

Acadian Flycatcher
The Acadian Flycatcher is a species that is area sensitive, as the forests get smaller in size the less and less Acadian Flycatcher found. For example, there was 25% in greater than 80 ha and 0% in less than 10 ha. The Acadian Flycatcher was found in forest as small as 10 ha, however the mean area it was found in was 114 ha. This species is an understory nester and is moderately subject to predators.

Red-Shouldered Hawk
The Red-shouldered hawk was only found twice during this study and therefore there is insufficient data for this species. The red-shouldered hawk is an area sensitive species according to the small amount of data collected. It was not found in forest areas less than 50 ha and was found in a mean forest area of 224 ha.

Hairy Woodpecker
The Hairy Woodpecker is neither area tolerant nor area sensitive. There is 50% found in above 80 ha, but 20% found in less than 10 ha. It is not quite area sensitive because it was found in areas as small as 3.3 ha. Yet it is not as area tolerant, because it was found mostly in larger forest areas.

Pileated Woodpecker
The Pileated Woodpecker is neither area tolerant nor area sensitive. The Pileated Woodpecker is not area tolerant because it was not found in forest area less than 10 ha. Yet it is not area sensitive because it was in 33% of forest 10-19 ha, and was found in forest as small as 10 ha.

Red-Eyed Vireo
The Red-Eyed Vireo is also neither area tolerant nor area sensitive. This species was a commonly found bird, and found in forest as small as 3.3 ha. But as the forest sizes become smaller the percentage of forest sizes the species was found in drops. For example, it was found in 63% of forest above 80 ha, but only 20% of forest less than 10 ha.

Scarlet Tanager
The Scarlet Tanager is a high canopy bird which was found more often than predicted. This species is neither area tolerant nor area sensitive. This species was found in forest as small as 4 ha. This bird is not as subject to predators than ground or understory nesters or feeders. But the bird is no that area tolerant. It was found in 75% of forest sizes over 80 ha and it was always found in forest above 100 ha. It was found in only 10% of forest less than 10 ha. It seems to prefer larger forest yet is found in smaller.

Summer Tanager
The Summer Tanager similar to the Scarlet Tanager is also neither area tolerant nor area sensitive. It was not found in forest less than 10 ha and was found in a mean area of 102 ha. Yet it is found in forest as small as 10 ha.

Pine Warbler
The Pine Warbler is a species that is area tolerant. It was found in 44% of forest 10 -19 ha. So it is found in many smaller forests. It was found in forest as small as 7.4 ha. My background information does not consider this a key interior species; the species was commonly found on the edge of forest.

Wood Thrush
The Wood Thrush is an area tolerant species and was a common species found. It was found in forest as small as 3.3 ha, and was found in 40% of forest less than 10 ha. It doesn't seem to mind small forest. It was also found in many larger forests; it was found in 88% of forest above 80 ha. The Wood Thrush is a bird that is found in forest small and large.

Yellow-Billed Cuckoo
The Yellow-Billed Cuckoo is a species that was found to be area tolerant. It was found in forest as small as 2.3 ha and was found in 20% of forest less than 10 ha. It was also found in 25% of forest more than 80 ha. Like the Wood Thrush The Yellow-Billed Cuckoo is found in forest small and large.

White-Breasted Nuthatch
The White-Breasted Nuthatch is an area tolerant species. It was found in forest as small as 3.3 ha. Also it was found in 22% of forest 10-19 ha. So it doesn't seem to mind smaller forest. My background information does not consider this a key interior species; the species was occasionally found on the edge of the forest.

White-Eyed Vireo
The White-Eyed Vireo is more of an edge bird and is not considered an interior forest bird. This species was not found numerous times. Therefore the data may be insufficient. The White-eyed Vireo is an area tolerant species. It was found three times during the study and was found in forest areas as small as 24.8 ha. It was found mostly in forest areas from 20-49 ha. And it was not found in forest above 80 ha. The data shows that this species is area tolerant but more data would be needed to make a further analysis.

Yellow-Throated Warbler
The Yellow-Throated Warbler is one of the rarer birds found during the study. This warbler was only found once in a forest of 56 ha. There is not enough data to form a conclusion.

Most Area Tolerant Species
All of the species found to be most area tolerant are species that are not true forest interior birds. The species found to be most area tolerant are, Carolina Wren, Tufted Titmouse, Eastern-Wood Pewee, Great Crested Flycatcher, Downy Woodpecker, and the Northern Flicker. These species are not area sensitive because they thrive in the interior of the forest. These birds are not listed as key interior species (see table 4).

Conclusion

In this study I was trying to find how big a forest has to be to contain certain forest interior species? To do this I collected data on different interior species in different sized forest on the Eastern Shore of Maryland. I would see which species are absent or present as forest sizes ranged. Then I analyzed and drew conclusions about which interior species are area sensitive and how big a forest should be to contain certain species. 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 (see table 6). 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 (see discussion for more details).