SHORT LETTERS
A Natural Short-term Experiment on the Effect of Habitat Alteration on Birds
I performed a transect count of birdlife in Trexlertown Pines in Trexlertown, east-central Pennsylvania
(~41degreesNX75.5degreesW) once before the area was developed into Rodale Park by Lehigh County. I also performed a comprehensive
trail census after the park was built. Both counts occured in June, the TP one in 1992, and the RP one in 2004. The Tp census
produced 29 species, while the Rp one only produced 19 species.
I used Thiollay's percent difference equation to analyze the extent of the change (1992):
PD= Ei Ixi-yiI
_______
= 25%, where:
E (xi+yi)
xi= no. of sitings of species i at site x
yi= no. of sitings of species i in site y, and
E (xi+yi)= x+y, in this case
Exi=x, and Eyi= y...IxI= the absolute value of x
An increase in area theoretically doubles the abundance of species.
As a general rule, the larger the area, the greater the diversity (Furness and Greenwood 1993). This is consistant with behavioral
ecological theory regarding dispersal of single species (Krebs and Davies 1984). My findings demonstrated a 25% decrease
in avian species richness since the park was put in (95% CI 3%<p<31%, p=0.05), which does not account for a subjective
description of total elimination and fragmentation of about 50% of the original unfragmented and unpeopled landscape of TP. However,
many species counted at RP were migrant flyovers, open habitat birds, and edge habitat species. There were even Eastern Bluebird (Sialia
sialis) boxes on a grass plot outside of the woods cared for br county park employees. Since we saw evidence of breeding,
and they were on park property, we counted this species.
Many studies have been conducted on species in woodlots and habitat
fragments (Roth and Johnson 1993; Galli, Leck, and Forman 1976; MacArthur 1964; Haila et al 1993; Peters and Grubb 1983).
Island biogeographic theory (Higgs 1981; MacArthur 1964) had bits roots oftentimes in the studies of woodlot dynamics and
recruitment. Studies of the insular syndrome (Thiollay 1993, for eg.) have benefited from these studies, and ecological theory
regarding habitat patches in general: endemic bird islands have thus been afforded special status and sometimes protection.
However, few studies have related the size of habitat fragments
to species numbers (Grubb 1986) and various possible thresholds and points of diminishing increases. (At what point does
a habitat become saturated?) As forests in Pennsylvania increase in size due to deemphasis on family farming in some areas
(and succumb to sprawl in others) hopefully species richness will follow, as populations of Pileated Woodpeckers (Dryocopus
pileatus) have indeed rebounded, including near small urban areas (Brauning and Stevenson pers. obs.).
According to Harris (1984), S (no. of species) = 16.3AExp0.16, where A= area. Thus in theory the number of species in
RP should be 23.3. However, demographic coupling of migrant and temporate populations does not occur because their life tables
are different (Ricklefs 1992). Turnover in small fragments of summer territories may be a function of change in location year-to-year
and ignorant od population dynamics (Haila et at 1993).
Ornithologists such as Lack have used islands or woodlots for
island biogeographical studies; more studies need to address landlocked habitats in fields, suburbs, and cultivated areas
in order to study species isolation, population recruitment, and source/sink dynamics, especially involving endangered species,
such as the Florida Scrub Jay (Aphelocoma coerulescens). Also, more summer breeding season studies should be done (egs. Roth
and Johnson 1993; Bollinger and Linder 1994).
In conclusion, my study was corroborated by Galli et al in winter
(1976), Bryant (1986), and by Thiollay across the tropics (1989, 1992, 1993, and 1997)- all seeing a relationship between
habitat disturbance and negative impacts on species structure. Bryant (1986) showed a change in species composition,
from predominantly Red-shouldered Hawks to predominantly Red-tailed Hawks, in the Waterloo Region of Ontario due to selective
logging during 1953-1978. But isolation studies enable us to control certain variables, while albeit not others, to isolate dynamics
which to study. This has not been taken advantage to the scale that perhaps it should be.
My study is potentially misleading because it is a single-sample
one (Wiens 1981). Logistics called for such a study only, and unless birders did controlled birding of each stage and journaled
it, the natural experiment for TP/RP is done, incomplete, and inconclusive for sure. But it sets up a model for the observant bird
monitor keeping an eye on development patterns in his or her region. If they do, they may find a natural experiment with
four or so years on each end of the development cycle to monitor the island's bird populations in either season and study
a variable affecting the birdlife there that has not been studied in depth so far, like changes in foraging exploitation of
the fragment before and after development, and contribute to the knowlege of avian island biogeography.
ACKNOWLEDGEMENTS
I thank B. Bolle and F. Stevenson for their kind field assistance.
I also thak D. Klem for introducing me to the idea of a natural experiment and first proposing TP/RP as a good site for one.
LITERATURE CITED: Appologies-This Is Incomplete
Galli, A.E., et al. 1976. Avian distributional patterns in forest islands of different size in central New Jersey. Auk
93: 356-364.
MacArthur, R.H. 1964. Environmental factors affecting bird species diversity. Ecology 42: 594-598.
Krebs, J.R. 1984. Behavioural ecology: an introduction. 2nd Ed. Blackwell Scientific Publications, Oxford.
Peters, W. D., and T.C. Grubb, Jr. 1983. An experimental analysis of sex-specific foraging in the Downy Woodpecker, Picoides
pubescens. Ecology 64(6): 1437-1443.
Thiollay, J-M. 1993. Habitat segregation and the insular syndrome in two congenic raptors in New Caledonia, the White-bellied
Goshawk ACCIPITER HAPLOCHROUS and the Brown Goshawk A. FASCIATUS. Ibis 135: 237-246.
Thiollay, J-M. 1997. Disturbance, selective logging and bird diversity: a Neotropical forest study. Biodiversity and
Conservation 6:1155-1173.
Thiollay, J-M. 1993. Resonse of a raptor community to a shrinking area and degradation of tropical rain forest in the
south western Ghats (India). Ecography 16: 97-110.
Thiollay, J-M. 1992. Influence of selective logging on bird species diversity in a Guianan rain forest. Conservation
Biology 6(1): 47-63.
Bryant, A.A. 1986. Influence of selective logging on Red-shouldered Hawks, BUTEO LINEATUS, in Waterloo Region, Ontario,
1953-1978. Canadian Field-Naturalist 100: 520-525.
Grubb, Jr., T.C. 1986. Beyond birding: field projects for inquisitive birders. The Boxwood Press, Pacific Grove, CA.
Higgs, A.J. 1981. Island biogeographical theory and nature reserve design. J. of Biogeography 8: 117-124.
Roth, R.R., and R.K. Johnson. 1993. Long-term dynamics of a Wood Thrush population breeding in a forest fragment. Auk
110(1): 37-48.
Haila, Y., et al. 1993. Turnover of breeding birds in small forest fragments: the "sampling" colonization hypothesis
corroborated. Ecology 74(3): 714-725.
Ricklefs, R.E. 1992. The megapopulation: a model of demographic coupling between migrant and resident landbird populations.
In Ecology and Conservation of Neotropic Migrant Landbirds. Smithsonian Institution Press, Washington.
Wiens, J.A. 1981. Single-sample surveys of communities: are the revealed patterns real? Am. Nat. 117: 90-98.
Note: This was posted Mon. 3 Oct. 05. I will try to pst a new short piece every week, or more.
