Population Recovery of the Eastern Brown Pelican following Its Extirpation in Louisiana

7 Pages·2003·4.8 MB·English

by  G O Holm

Wilson Bull, 115(4), 2003, pp. 431-437
POPULATION RECOVERY OF THE EASTERN BROWN PELICAN
FOLLOWING ITS EXTIRPATION IN LOUISIANA
G. O. HOLM, Jr.,1’4 T J. HESS, Jr.,^ D. JUSTIC,i L. McNEASE,^ R. G. LINSCOMBE,^
AND S. A. NESBITU
—
ABSTRACT. We report on the exponential growth of the Eastern Brown Pelican (Pelecanus occidentalis
carolinensis) in Louisiana, following its extirpation in 1963, and its subsequent reintroduction (1968-1980).
This population growth pattern is remarkable since the Brown Pelican exhibits low fecundity, a long life span,
and is considered largely nonmigratory in the GulfofMexico. To understand the regional changes in breeding
numbers of Brown Pelicans in the northern Gulf ofMexico, we investigated the long term trends ofthe other
subpopulations in Florida and Texas. The Florida subpopulation of Brown Pelicans has exhibited a stable-limit
cycle, but within the last decade the number ofnesting birds has declined while the numberofnesting colonies
hassteadilyincreased. ThenumberofBrownPelicansinTexasnowexceedsestimatespriortothetimepesticides
caused reproductive failure of the subpopulation. The Louisiana subpopulation now equals or exceeds its his-
torical (pre-pesticide) numbers. Although local recruitment can account for the exponential growth of Brown
Pelicans in Louisiana, we believe that dispersal from colonies outside of Louisiana may have augmented the
growth ofits nesting population. Received 11 February 2003, accepted 15 October2003.
Prior to the 1930s, estimates of the Loui- 1985), the extirpation of Louisiana’s subpop-
siana subpopulation ofthe Eastern Brown Pel- ulation was attributed to direct bird mortality
ican {Pelecanus occidentalis carolinensis) (Blus and Joanen 1975) and more recently to
ranged from 12,000-85,000 (Lowery 1955, a reduction in prey abundance resulting from
1974; King et al. 1977; Wilkinson et al. 1994), endrin contamination (Biglane and LaFleur
with a conservative estimate of 50,000 indi- 1967; Cohen 2000, 2001). The rapid disap-
viduals for Texas and Louisiana combined pearance of the Brown Pelican (Norman and
(Schreiber 1980). Extirpation of the Brown Purrington 1970) coincided with endrin use
Pelican in Louisiana had occurred by 1963 and production discharges (1958-1964) and
(Williams and Martin 1968), and the last re- massive fish kills (1963-1964) caused by en-
corded nesting effort of the native population drin contamination (Mount and Putnicki
was observed in 1961 (Van Tets 1965). Dur- 1966). However, it was not until 1975, when
ing this same period, a severe decline in the a severe die-off of pelicans (after reintroduc-
Texas subpopulation occurred, and King et al. tion) in Louisiana occurred, that the extreme
(1977) estimated that by 1961 <50 adults re- sensitivity ofpelicans to endrin contamination
mained in Texas. Despite the extreme losses was documented (Blus et al. 1979).
in Texas and Louisiana, the number ofBrown In 1968, the Louisiana Dept, of Wildlife
Pelicans nesting in Florida remained generally and Fisheries and the Florida Fish and Wild-
stable (Williams and Martin 1968, Kushlan life Conservation Commission began reintro-
and Frohring 1985, Wilkinson et al. 1994). duction of the Brown Pelican into coastal
A
While reproductive failure from organo- Louisiana. total of 1,276 Florida nestlings
chloride pesticide contamination caused the were transplanted from six colonies in Florida
decline ofpelicans in Texas (King et al. 1977, to three sites in Louisiana between 1968 and
1980 (for detailed transplanting information
see Nesbitt et al. 1978, McNease et al. 1984).
' Coastal Ecology Inst, and Dept, of Oceanography The first nesting attempts oftransplanted birds
and Coastal Sciences, Louisiana State Univ., Baton occurred during the spring of 1971, and at that
Rouge, LA 70803, USA. time, the birds were almost 3 years old (Wil-
^Louisiana Dept, ofWildlife and Fisheries, Rocke- liams and Joanen 1974). During the spring of
feller Wildlife Refuge, Grand Chenier, LA 70643,
1975, endrin pollution caused a die-off of
USA.
^Florida Fish and Wildlife Conservation Comm., about 40% of a standing population of 400-
Wildlife Research Lab., 4005 South Main St., Gaines- 450 Brown Pelicans (Winn 1975, Nesbitt et
ville, FL 32601-9099, USA. al. 1978, Blus et al. 1979). Since then, the
'’Corresponding author; e-mail: [email protected] Louisiana population has continued to grow.
431
432 THE WILSON BULLETIN • Vol. 115, No. 4, December2003
The objective of our study was to investi-
gate the long term growth trend ofthe Eastern
Brown Pelican in the GulfofMexico, with an
emphasis on the nesting birds in Louisiana,
and compare these numbers to those of Flor-
ida and Texas. Given that the Louisiana sub-
population has exhibited an exponential in-
crease of nesting birds, we investigated the
potential for local recruitment versus long dis-
tance emigration and dispersal from surround-
ing subpopulations in augmenting Louisiana’s
population growth.
METHODS
We used breeding Brown Pelican census
data from Louisiana, Texas, and Florida, the
states comprising the largest subpopulations
of breeding pelicans along the U.S. Gulf of
Mexico. These data were based on nest counts
(Florida and Louisiana) or breeding pairs
(Texas); hereafter, nest numbers are used as a
common unit of measure. In Louisiana, peli-
can nests and nestlings were counted yearly
(except 1991) with a mean of four flights (ro-
tor or fixed-wing) per year (1993-2001). The
nesting data were collected by the Louisiana
Dept, ofWildlife and Fisheries (Hess and Lin-
scombe 2001). Surveys prior to 1993 usually
involved a combination of aerial and ground
census techniques, as summarized by Mc-
Nease et al. 1992. In Texas, breeding pairs
were estimated as described in the Texas Co- Date
lonial Bird Survey (Lange 1993). Nesting pair
EIG. 1. The Brown Pelican breeding populations
data from Texas were provided by the U.S. in Louisiana and Texas have grown exponentially
Fish and Wildlife Service (Houston, Texas). while declines occurred in Elorida.
The Florida Fish and Wildlife Conservation
Commission collected Florida nesting data.
Nest counts in Florida were conducted during RESULTS
tpewaok nzeosnteisn:g pAetrliaondtsicanadnsdumGmualrfizoefd bMeetxwieceon icaLnosuiisniaLnoau.i—siTahnea phoapsulgatrioownnofexnpeosnteinntgiaplelly-
coasts. from 1971-1999, with slower rates of growth
= WN^ee''u)setdo caanlceuxlpaotneentthiealingtrrionwsitchgerqouwattihonrat{eNs, o2c0c0u1r,ritnhge dpuorpiunlgati1o9n99p-e2a0k0e1d (aFtig.16,1)4.05Dunreisntgs
of nesting subpopulations in Louisiana and (Fig. 1). A best-fit exponential model describ-
Texas (Manugistics, Inc. 1995). In the equa- ing population growth from 1971-2001 sug-
tion above, N, is the estimated numberofnests gested an intrinsic growth rate (r) = 0.25
for a given year, is the initial number of - 0.97).
nests at time zero, r is the intrinsic growth The overall (1971-2001) mean nestling
rate, and t is the time in years. Number of production per active nest was 1.66. Peak
nests and time served as dependent and in- nestling production occurred in 2001 with
dependent variables, respectively. 34,641 young produced; at this time the high-
;
Holm et al. • PELICAN POPULATION RECOVERY 433
Date
FIG. 3. InFlorida,theAtlanticBrownPelicansub-
populationremainedrelativelystablefrom 1990-2001,
but there was a decrease in the Gulf coast subpopu-
lation.
dated on a 12-ha dredge spoil island (Baptiste
Collette or Plover Island, near the mouth of
the Mississippi River). This immense, concen-
trated shift comprised more than one-half of
the total nesting population in Louisiana.
Interannual fluctuations in the Louisiana
subpopulation occurred during the following
three time intervals: 1989-1990, 1995-1996,
and 1999-2000 (Fig. 1). Decreases in nest
numbers during the spring of 1990 and 1996
correspond to years that had protracted freez-
ing temperatures during the preceding winters.
During 1999 and 2000, a 50-100 year local
drought coinciding with low Mississippi River
stages resulted in high estuarine salinity
across the Louisiana coast.
1965 1970 1975 1980 1985 1990 1995 2000 2005 —
Florida Florida nesting birds have exhib-
.
Date ited cyclic changes in nest numbers from 1968
FIG. 2. While the number of Brown Pelican col- onward (Fig. 1). Following a general pattern
onies increased along Florida’s Gulfcoast (a mean of of increasing growth from 1968-1990, state-
one colony every two years since 1968), the number wide pelican nest numbers have declined over
ofnesting birds decreased (see Fig. 1). The numberof the past decade, and the number of nests
nesting colonies in Texas and Louisiana has remained (6,432) in 2001 fell below the 1968-1990
low despite the immense increase in the number of mean of approximately 8,000 nests.
nesting birds.
The decline in the number of nests in Flor-
ida has occurred most noticeably among the
est yearly mean numberofnestlings per active Gulf coast colonies, while colonies located
nest (2.11) was recorded for Louisiana. The along the Atlantic coast have remained rela-
peak number of nesting colonies (11) in Lou- tively stable (Fig. 3). Concomitant with the
isiana occurred during the 2000 breeding sea- decline in nest numbers on Florida’s Gulf
son (Fig. 2). An interannual increase in nest coast, there was a linear increase in the num-
production (2,639) occurred from 2000-200 ber of colonies, and a mean (1968-2001) in-
1
concomitantly, four nesting colonies were crease of—one colony every two years (Fig. 2).
abandoned (Fig. 1, Fig. 2). In 2000, a new Texas. A peak nesting population of3,373
colony ofapproximately 10,000 nests was ini- breeding pairs among a total ofseven colonies
434 THE WILSON BULLETIN • Vol. 115, No. 4, December2003
was recorded for Texas in 2001 (Fig. 1). The does indicate that pelicans in Florida are seek-
mean number of nesting pairs from 1997- ing either new nesting habitat or improved
2000 was 2,839. The Texas population has forage conditions, or they are avoiding distur-
grown exponentially with an intrinsic growth bance. Schreiber (1976) believed that food
rate (r) = 0.22 {R^ = 0.94, Fig. 1). Fluctua- availability was the main reason for move-
tions in the number of nesting pairs in Texas ment of nonbreeding Brown Pelicans. Adja-
are similar to those observed in Louisiana cent to Florida, Alabama’s Gaillard Island
with noticeable interannual peaks anddeclines nesting colony has grown rapidly; there were
occurring during 1989-1990, 1995-1996, and four nests in 1983 (the first account ofnesting
1999-2000 (Fig. 1). pelicans recorded in the state), and the popu-
lation peaked in 1999 with approximately
DISCUSSION
5,200 nests (R. B. Clay pers. comm.). From
Since its extirpation and reintroduction, the early band returns. Wood et al. (1995) con-
Eastern Brown Pelican in Louisiana has pro- cluded that the Alabama colony (Gaillard Is-
vided a remarkable example of exponential land) was initiated by birds from the panhan-
growth of a nonmigratory population (Palmer dle of Florida.
—
1962) of long-lived (>20 years) birds. Newly Dispersal from natal colonies. In a long
introduced species, with rapid maturation and term analysis of Brown Pelican band returns,
high annual fecundity, commonly exhibit ex- Schreiber and Mock (1988) concluded that
ponential population growth. For example, Florida’s west coast (Gulf of Mexico) birds
Bock and Lepthien (1976a, 1976b) found that comprised a distinct subpopulation from that
populations of Cattle Egrets (Bubiilcus ibis; r of Louisiana’s. In their study, 82% of band
= 0.21, 1956-1971) and House Finches {Car- returns were from Florida’s Gulf coast, indi-
=
podacus mexicanus, r 0.23, 1962-1971) cating a largely resident population. Other
grew exponentially after their introductions band returns were from the Keys (8%), Cuba
into regions ofthe United States. Interestingly, (5%), and Alabama (1%), but none were re-
the Louisiana pelican subpopulation has ported from Louisiana. In a later study on pel-
grown at intrinsic rates similar to that of “r- ican dispersal. Wood et al. (1995) found, con-
selected” bird species. This raises the ques- versely, that banded birds from a natal colony
tion of whether local recruitment can account on the Florida panhandle moved both east-
for the rapid increase in the Brown Pelican ward and westward along the Gulf coast, and
nesting population in Louisiana. movement into Louisiana was documented
Based on a simple exponential model, local (though only one band was recovered in Lou-
recruitment could explain the observed isiana).
growth of the Brown Pelican population in Long range dispersal ofpelicans throughout
Louisiana, provided that the estimated growth the Gulf of Mexico has been reported histor-
rate is 0.25/year. However, since this rate is as ically (Mason 1945, Schreiber and Mock
high as those observed in r-selected species, 1988, Sykes and Langridge 1991, Johnsgard
we suggest an alternative explanation, i.e., 1993, Wood et al. 1995). As of 2001, banded
that immigration into Louisiana has augment- pelicans from Louisiana have been reported
ed this growth. This immigration may have from every Gulf coast state, and also from
involved subadult pelicans, produced else- Cuba, the Yucatan Peninsula, Belize, and
where, joining the Louisiana nesting popula- Guatemala (J. Harris pers. comm.). These re-
tion. This hypothesis also would explain the cent Louisiana band returns confirm earlier
protracted decline in the Florida Gulf coast findings (Mason 1945, Schreiber 1976) that
population concurrent with the rapid increase Brown Pelicans, especially juveniles, move
in the Louisiana breeding population. randomly and for long distances. A distinct
The ability to detect emigration into Loui- pattern ofdispersal from Louisiana’s colonies,
siana from banded birds has been constrained however, is not discernable at this time.
by the remoteness of pelican breeding and Whether breeding birds are shifting toward
feeding areas in Louisiana. However, the the northernmost portions ofthe GulfofMex-
steady increase of nesting colonies on Flori- ico remains debatable. Nonetheless, this pat-
da’s Gulfcoast, while nest numbers decreased. tern is conceivable given the northward ex-
Holm et al • PELICAN POPULATION RECOVERY 435
pansion ofbreeding colonies along the Atlan- Louisiana has grown only 4.8%. Thus, habitat
tic coast ofthe U.S. Since 1980, first breeding fragmentation by coastal land development
records for Brown Pelicans have been report- and the probability of nesting colony distur-
ed for Georgia, Virginia, Maryland, Delaware, bance is ofgreaterconcern in Florida andTex-
and New Jersey (see Wilkinson et al. 1994). as than in Louisiana.
—
Warm temperatures may facilitate a northward Habitatstability. The greatestfuture prob-
dispersal pattern, and the warmest decade lem for Brown Pelicans in Louisiana is the
since instrumentation began (1861) occurred stability of natural nesting habitat. Barrier is-
1990-2000 (Houghton et al. 2001). An inter- lands in Louisiana are the aging relics ofMis-
decadal comparison between the 1980s and sissippi River deltas, and the rapid erosion of
mid-1990s showed a notable reduction in pel- these islands has been largely irreversible, as
ican numbers (>70% or 1,300 individuals) in there is little “new” sand or coarse-grained
Puerto Rico and the U.S. Virgin Islands (Col- material entering the system. This has resulted
lazo et al. 1998). The authors did not find ev- in the need to replenish, with dredge material,
idence of reproductive failure or nesting and historically successful natural nesting islands
roosting habitat changes. — in Louisiana. The use of created islands
Limitations on population growth. Nelson (dredge material) by nesting pelicans has been
(1979) proposed that populations of pelicani- encouraging in Louisiana and elsewhere
form species are controlled mostly by envi- (Wood et al. 1995). High rates of island frag-
ronmental factors (climate and food availabil- mentation in Louisiana conceivably could fa-
ity). Fledging success of the California sub- cilitate the establishment ofnew colonies that
species (P. o. californicus) was dependent on have reduced predator activity (Kushlan and
the local abundance of northern anchovy {En-
Frohring 1985). The fact that within one year
graulis sp.; Anderson et al. 1982). Along the more than 60% of the nesting population ini-
Pacific coastlines ofNorth and South America
tiated a colony on a relatively small dredge
the importance of food abundance on seabird
spoil island reflects this species plasticity to
mortality is well understood, especially as it changes in habitat.
is related to El Nino-Southem Oscillation The mean number of nests per colony in
events (Duffy et al. 1988). In the GulfofMex-
ico, relationships between food supply and Louisiana in 2001 (2,344 nests per colony) is
an order of magnitude higher than that of
breeding success have not been widely stud-
Florida’s statewide density (161 nests per col-
ied. Poor food availability, however, has been
ony). The exponential increase in nests we ob-
implicated in reproductive failures in southern
Florida near the Keys (Kushlan and Frohring served in Louisiana contrasted with the dis-
1985). Moreover, there was a strong, positive proportionate number ofnew nesting colonies
association between statewide mullet (Mugil suggests that a sustained amount of nesting
spp.) landings and Brown Pelican nesting ef- habitat, rather than food availability, will be a
fort in Florida during the period 1968-1992 future concern in Louisiana. In Florida, the
(Nesbitt et al. 1996). Whether food supply has decrease in the number of nesting pairs along
influenced decreasing nest numbers along the Gulf coast, with no decline in the number
Florida’s Gulf coast is unknown. of colonies, is the reverse of what is being
Other density independent forces such as seen in Louisiana. In Florida, the food base,
human disturbance (Jehl 1973, Anderson and rather than the availability of suitable nesting
Keith 1980, Anderson 1988) inhibit successful sites, may limit the population.
breeding at local levels and may influence col- In general, the growth ofthe subpopulations
ony site selection or abandonment. There are of Brown Pelicans in Louisiana, Texas, and
significant differences, for example, in human Alabama, during periods when declines were
population growth in the coastal areas among observed along the Gulf coast of Florida and
Louisiana, Florida, and Texas (Barrett 2001). Puerto Rico and in the Virgin Islands, indi-
From 1990 to 2000, the human population on rectly suggests that Brown Pelicans may have
Florida’s Gulf coast has increased 20.8%, and the ability to shift to habitats where suitable
Texas’ population has grown 18.7%. In con- nesting and forage conditions exist outside of
trast, the population of coastal parishes in intense human disturbance.
436 THE WILSON BULLETIN • Vol. 115, No. 4, December2003
ACKNOWLEDGMENTS
Proceedings of the 19th Ornithological Congress
(H. Ouellet, Ed.). Univ. ofOttawa Press, Ottawa,
We thank E. Erfling (USFWS Houston, Texas) for Ontario, Canada.
providing Brown Pelican data from Texas. R. B. Clay Hess,T.J.,Jr. andJ. Linscombe. 2001. BrownPelican
(AlabamaDept, ofConservation andNatural Resourc- coastal survey, site observations, and summary.
es) provided data from the Gaillard Island colony in Louisiana Dept, of Wildlife and Fisheries, Grand
Alabama. J. Harris (USFWS Slidell, Louisiana) dis- Chenier, Louisiana.
cussed banding efforts, returns, andotherobservations Houghton, J. T, Y. Ding, D. J. Griggs, M. Noguer,
olefttBeraonwdnCPhealnidcealneunresctoilnogniacetsiviintyLoautistihaenBaa.ptWiesteapCporle-- P2.00J1..VCalnimadteercLhianndgeen,20a01n:d tDh.e sXciiaenotsifuic(Ebdass.i)s..
ciatethesuggestionsofP. J. Mockandtwoanonymous Cambridge Univ. Press, Cambridge, United King-
reviewers that improved the manuscript. Lastly, we dom.
thank all those who worked hard to bring the Brown Jehl, j. R., Jr. 1973. Studies ofadecliningpopulation
Pelican back to Louisiana. of Brown Pelicans in northwestern Baja, Califor-
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