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2009 SOUTHEASTERN NATURALIST 8(2):347–354
Impacts of Nutria Removal on Food Habits of American
Alligators in Louisiana
Steven W. Gabrey1,*, Noel Kinler2, and Ruth M. Elsey3
Abstract - In southern Louisiana, high population densities of exotic Myocastor
coypus (Nutria) have been implicated in causing significant coastal marsh damage
through extensive herbivory. Wildlife officials instituted a Nutria removal program
in 2002 to reduce this marsh loss. Because Alligator mississippiensis (American
Alligator) frequently consume Nutria, concern arose regarding the program’s impacts
on alligator food habits. Therefore, we conducted our study to determine if
the Nutria removal program affected the frequency of occurrence of Nutria remains
in alligator stomachs collected from five parishes in southern Louisiana. Three parishes
had high Nutria densities and removal programs; two parishes had low Nutria
densities and no Nutria removal. We collected >550 alligator stomachs during three
September trapping seasons and examined the contents of each. We used logistic
regression to model effects of year (1 year prior to the removal program compared
to two years during removal) and parish (three with Nutria removal programs compared
to two without) on the probability that an alligator stomach contained Nutria
remains. Overall, about one-third of the alligator stomachs contained Nutria remains.
Nutria removal appeared to have no effect on the probability of a stomach containing
Nutria remains even after two years of Nutria removal. In addition, the probability
that an alligator stomach contained Nutria remains was similar among all parishes
regardless of whether Nutria removal occurred or not. We recommend that continuance
of the Nutria removal program be based on its effectiveness in reducing marsh
damage and not on perceived impacts to alligator food habits.
Myocastor coypus Molina (Nutria) was introduced from South America
into the marshes of southwest Louisiana in the 1930s (Kinler et al. 1987).
In the decades since, this large (>5 kg) and highly fecund aquatic rodent
has dispersed throughout the marshes of the entire Gulf Coast region into
the Atlantic coast as far north as Maryland and Delaware. In some regions,
notably the Barataria Basin of southeastern Louisiana and the Chesapeake
Bay area, populations of Nutria have become sufficiently dense that their
foraging activities have resulted in “eat-outs” of coastal marsh vegetation
(Baroch and Hafner 2002, Marx et al. 2004), areas of marsh that have been
completely devegetated and consequently significantly contribute to the
high rate of marsh deterioration.
To reduce the extent of these eat-outs, the Fur and Refuge Division of the
Louisiana Department of Wildlife and Fisheries (LDWF) instituted a Nutria
1Department of Biology, Northwestern State University, Natchitoches, LA 71497.
2Louisiana Department of Wildlife and Fisheries, 2415 Darnall Road, New Iberia,
LA, 70560. 3Louisiana Department of Wildlife and Fisheries, Rockefeller Wildlife
Refuge, 5476 Grand Chenier Highway, Grand Chenier, LA 70643. *Corresponding
author - firstname.lastname@example.org.
348 Southeastern Naturalist Vol. 8, No. 2
control program in 2002 with the goal of reducing Nutria populations in
those marshes with the highest densities and most extensive marsh damage
(Marx et al. 2004). Although the general public readily accepted this program,
there has been some concern regarding the impact of Nutria removal
on Alligator mississippiensis Daudin (American Alligator) populations because
alligators frequently consume Nutria (Elsey et al. 1992, McNease and
Joanen 1977, Taylor 1986, Wolfe et al. 1987).
In this paper, we compare frequency of occurrence of Nutria in alligator
stomachs collected from two parishes with high Nutria densities before and
during the first two years of the removal program. We also compare frequency
of occurrence of Nutria in alligator stomachs collected from five parishes
with differing levels of Nutria density and Nutria removal. Valentine et al.
(1972) attributed a decline in the frequency of occurrence of Nutria remains in
alligator stomachs from 56% to 5% in four years on Sabine National Wildlife
Refuge because a Nutria harvest program reduced the Nutria population by
an estimated 30%. Similarly, we expected the occurrence of Nutria remains
in our stomach samples to decline over the duration of the Nutria removal
program in those parishes with Nutria removal. In addition, we expected
the frequency of occurrence of Nutria remains to be lower in parishes with
Nutria removal programs than in parishes without removal.
Alligator stomach collection and content analysis
We collected alligator stomachs from cooperative trappers in Terrebonne
and Lafourche parishes (Fig. 1) during the September 2002, 2003, and 2004
alligator-trapping seasons. These two parishes comprise part of the Barataria
Basin, the region of the state with the greatest density of Nutria and the most
severe marsh damage problems (Marx et al. 2004). Nutria removal coincided
with the November to March furbearer season and began in November 2002.
Thus, alligator stomachs collected in September 2002 came from marshes
with high Nutria density before the start of the removal program; alligator
stomachs collected in subsequent years came from the same marshes as in
2002 after one (2003) and two (2004) Nutria removal periods. We expanded
the geographic range from which we collected stomachs to include locations
in Cameron, Vermilion, and St. Charles parishes (Fig. 1) during the 2003 and
2004 September alligator trapping seasons. Marshes in these three parishes
had either small Nutria populations and no Nutria removal (Cameron, Vermilion)
or limited Nutria removal (St. Charles). We were unable to collect
alligator stomachs from these three parishes in 2002. All marshes from which
alligators and Nutria were harvested are classified as fresh marsh except those
in Lafourche Parish, which are classified as intermediate (Chabreck 1970).
In Louisiana, alligator trappers harvest animals from specific marsh locations
(leases) through arrangements made with the appropriate landowner
and LDWF. Our cooperating trappers harvested alligators from marshes that
were owned privately (Terrebonne, Lafourche, Vermilion parishes), federally
(Cameron Parish, Lacassine National Wildlife Refuge), or by LDWF
2009 S.W. Gabrey, N. Kinler, and R.M. Elsey 349
(St. Charles Parish, Salvador Wildlife Management Area). All trappers maintained
these same leases throughout the duration of our study. In addition, the
Nutria removal protocol required that Nutria hunters identify the locations
from which they harvested the Nutria. Consequently, we were able to obtain
from records maintained by LDWF (Marx et al. 2004) yearly Nutria removal
totals for those leases from which alligator stomachs were collected. Thus,
the number of Nutria removed reported here is the number removed from
only those leases from which we also collected alligator stomachs.
Stomachs were removed and length and sex of alligators recorded as trappers
brought them to any of several processing sheds. We collected stomachs
only from adult alligators (>1.8 m total length) because trappers do not target
juveniles (<1.8 m) and because juveniles rarely consume vertebrates (Elsey
et al. 1992, Giles and Childs 1949, Wolfe et al. 1987). Stomachs were frozen
until analyzed. We identified prey items to the lowest taxon possible based
on state of digestion and recorded percent frequency of all food items.
We used backwards stepwise logistic regression (Agresti 1996) to determine
which explanatory variables infl uenced the probability that an alligator
stomach contained Nutria remains. All analyses were conducted using Proc
Figure 1. Louisiana parishes from which American Alligator stomachs were collected
during the September 2002, 2003, and 2004 alligator trapping seasons. Nutria
removal programs were in effect in Lafourche, Terrebonne, and St. Charles parishes
but not in Cameron or Vermilion parishes.
350 Southeastern Naturalist Vol. 8, No. 2
Logistic (probit link, binary distribution, backwards selection) in SAS/STAT
9.1.3 (SAS Institute, Inc., Cary, NC). The initial model included year, parish,
and sex, all possible interactions, and an intercept; non-significant effects
(P > 0.05) were removed iteratively until only significant effects remained.
Although larger alligators consume more mammals than do smaller alligators
(McNease and Joanen 1977), we did not include alligator length
in our models for the following reasons: (1) Platt et al. (1990) indicated
that alligators change from invertebrate to vertebrate prey once they reach
the 1.2–1.8-m size class and only five of the alligators from which we collected
stomachs were <1.8 m in length) (range = 1.7–1.8 m); (2) stomachs
from three of thirteen 1.8-m long alligators in our sample contained Nutria
remains; and (3) Elsey et al. (1992) found Nutria remains in stomachs of several
alligators that were between 1.2 and 1.8 m in length. Thus, all alligators
in our sample were of sufficient size to capture and ingest Nutria. In addition,
Proc Logistic and other maximum-likelihood estimators may fail to find a
solution when a continuous explanatory variable with many values (such as
length) is included in the model; the appropriateness of the resulting model
is then questionable (Agresti 1996).
We included sex as an explanatory variable because, except for the brief
spring mating season, male and female alligators typically use different habitats—
males in deep open water or canals compared to females in vegetated
marshes and swamps (Goodwin and Marion 1979, Newsom et al. 1987 and
references therein). Thus, any between-sex differences in Nutria consumption
could refl ect differences in foraging habitat.
Pre-Nutria removal vs. post-Nutria removal. We first compared the probability
that an alligator stomach contained Nutria remains among the three
years (2002, before Nutria removal; 2003, after one year of Nutria removal;
and 2004, after two years of Nutria removal) for stomachs collected from
the two parishes (Terrebonne and Lafourche) for which we had pre-removal
data. In this model, the parameters of interest were the Year main effect or
the parish by year interaction, as they indicated a change in the occurrence
of Nutria as a function of Nutria removal.
Nutria removal vs. no Nutria removal. We also compared the probability
that an alligator stomach contained Nutria remains among parishes with low
Nutria densities and no removal (Cameron and Vermilion) and parishes with
high Nutria densities and removal programs (Terrebonne, Lafourche, and
St. Charles) for the two years (2003 and 2004) for which we have data from
all five parishes. We used the same approach and initial model as described
above. In this second analysis, the parameters of interest are the Parish main
effect or the parish by year interaction as they indicate that Nutria occurrence
differed among parishes with or without Nutria removal.
Characteristics of alligators harvested
We collected stomachs from 553 alligators harvested during the September
trapping seasons of 2002, 2003, and 2004. Ten stomachs lacking data for
2009 S.W. Gabrey, N. Kinler, and R.M. Elsey 351
sex, length, or parish, and 72 stomachs that contained no food items were
excluded from analyses.
Of the remaining 471 stomachs, 73% were collected from male alligators
and 27% were from females. The mean length of harvested alligators used
in analyses was 231.0 cm ± 36.5 SD. Males (mean = 238.5 cm ± 38.3 SD)
were larger than females (mean = 209.4 cm ± 17.7 SD; t = 11.2, df = 432.8,
P < 0.001). Four percent (n = 5) of female alligators were longer than 2.4 m
compared to 42% (n = 143) of males.
Occurrence of Nutria in alligator stomachs
In Terrebonne Parish, the proportion of alligator stomachs containing
Nutria remains ranged from 14% in 2002 to 37% in 2004; during this time,
more than 13,000 Nutria were removed from the same leases from which the
alligators were harvested (Table 1). In Lafourche Parish, the proportion of
alligator stomachs containing Nutria remains ranged from 44% in 2002 to
36% in 2004; during this time, more than 8000 Nutria were removed from
the same leases from which the alligators were harvested (Table 1). The proportion
of alligator stomachs that contained Nutria remains was relatively
constant within each of the other three parishes from 2003 to 2004. During
that time, >3300 Nutria were removed from alligator leases in St. Charles
Parish; no Nutria were removed from leases in Cameron and Vermilion parishes
Pre-Nutria removal vs. post-Nutria removal. Backwards removal of
non-significant effects resulted in a logistic regression model that retained
the parish by sex interaction and its main effects (Table 2). The -2 log L
goodness-of-fit test was not significant (X2 = 12.93, df = 8, P = 0.11), indicating
that the model was appropriate. The predicted probability that a
stomach from a female alligator contained Nutria remains was higher in
Lafourche compared to Terrebonne parishes (0.46 and 0.15, respectively).
The predicted probability that a stomach from a male alligator contained
Nutria remains was similar between the two parishes (0.36 and 0.32 in
Lafourche and Terrebonne parishes, respectively). Because the final regression
model did not include the variable of interest (year), we do not discuss
this model further.
Table 1. Percentage of American Alligator stomachs containing Nutria remains (n = number of
stomachs examined) and number of Nutria removed from five parishes in southern Louisiana
from 2002 through 2004. Nutria were removed from the same trapping leases from which alligator
stomachs were collected.
Nov. 2002– Nov. 2003–
Sept. 2002 Mar. 2003 Sept. 2003 Mar. 2004 Sept. 2004
% of stomachs No. of Nutria % of stomachs No. of Nutria % of stomachs
Parish w/Nutria (n) removed w/Nutria (n) removed w/Nutria (n)
Terrebonne 14 (57) 8826 27 (77) 5120 37 (86)
Lafourche 44 (27) 3842 37 (46) 4665 36 (49)
St. Charles 621 31 (35) 3336 34 (44)
Cameron 0 33 (15) 0 35 (20)
Vermilion 0 27 (11) 0 21 (14)
352 Southeastern Naturalist Vol. 8, No. 2
Nutria removal vs. no Nutria removal. Backwards removal of nonsignificant effects resulted in a logistic regression model that retained the
intercept only, indicating that no interactions or main effects infl uenced
the probability that an alligator stomach contained Nutria remains (intercept
estimate = -0.44, standard error = 0.07; Wald X2 = 44.04, df = 1, P < 0.01).
The intercept-only model predicts the probability that an alligator stomach
contains Nutria remains is 0.33. The -2 log L goodness-of-fit test was not
significant (X2 = 27.25, df = 19, P = 0.15), indicating that the model was appropriate.
Because the final regression model did not include the variable of
interest (parish), we do not discuss this model further.
In the early part of the twentieth century, mammals—primarily Ondatra
zibethicus L. (Muskrat)—were a frequent but variable component of
adult alligator food habits in Louisiana (Arthur 1931, Giles and Childs
1949, Kellogg 1929, O’Neil 1949). Following their introduction into the
marshes of southwest Louisiana in the 1930s, Nutria became an increasingly
important component, while the significance of muskrats declined
(Elsey et al. 1992, McNease and Joanen 1977, Valentine et al. 1972, Wolfe
et al. 1987). The changes in occurrence of Nutria in alligator food habits
studies parallel changes in abundance of Nutria and muskrats in Louisiana
marshes (Baroch and Hafner 2002, Kinler et al. 1987), suggesting that alligators
readily adapt to variation in prey availability.
Valentine et al. (1972) reported that on Sabine National Wildlife Refuge
in southwestern Louisiana, the frequency of occurrence of Nutria in alligator
stomachs decreased dramatically as a result of an intensive Nutria-trapping
program in which about 30% of the Nutria population was removed during
1961–1965. Thus, there is evidence for possible impacts of Nutria removal
programs on alligator food habits, although effects of the Nutria removal on
alligator demographics or population size were not considered.
We expected the current Nutria removal program in Lafourche and Terrebonne
parishes to have similar effects on alligator food habits. However,
our analysis shows no corresponding decline in the proportion of alligator
stomachs containing Nutria remains despite two Nutria removal seasons
Table 2. Maximum likelihood estimates, standard errors, and significance tests of logistic regression
parameters predicting the probability that an American Alligator stomach contained
Nutria remains following backwards stepwise elimination of non-significant effects. The initial
model included sex, two parishes (both of which had Nutria removal), and three years (2002,
prior to Nutria removal; 2003, after one Nutria removal season; 2004, after two Nutria removal
seasons). See Methods for details.
Parameter Value df Estimate Std. Error chi-square P > chi-square
Intercept 1 -0.49 0.09 29.30 <0.01
Parish Lafourche 1 0.26 0.09 8.36 <0.01
Sex Female 1 -0.07 0.09 0.53 0.47
Parish x sex Lafourche x female 1 0.20 0.09 5.10 0.02
2009 S.W. Gabrey, N. Kinler, and R.M. Elsey 353
during which more than 8000 and 13,000 Nutria were removed from the alligator
leases in Lafourche and Terrebonne parishes, respectively. We also
expected to detect differences in the proportion of alligator stomachs containing
Nutria remains between parishes with low Nutria densities and no
removal (Cameron and Vermilion) and parishes with high Nutria densities
and removal (Lafourche, Terrebonne, and St. Charles); however, we found
no such difference.
One possible explanation for the differences between our results and
those of Valentine et al. (1972) is that in our study, two years of Nutria
removal in Terrebonne and Lafourche parishes were insufficient to reduce
Nutria populations to such low densities that they became unavailable to
alligators. The amount of time necessary to see such a decrease in Nutria
density and a resultant effect on alligator food habits likely depends on the
density of Nutria prior to the removal and the proportion of the Nutria population
removed, as well as alligator density prior to and during the removal.
We examined alligator stomach contents over two years of Nutria removal
compared to four years by Valentine et al. (1972). Thus, it is possible that
additional Nutria removal is necessary in our study area for a change in alligator
food habits to become apparent. Although the current Nutria removal
program has continued every year since the end of our study, we were unable
to continue collecting stomachs past 2004 to determine any such long-term
impacts of Nutria removal.
An annual average of 296,000 Nutria have been removed from throughout
Louisiana over the five years of the Nutria removal program (Scarborough
and Mouton 2007). During the same period, Louisiana’s coastal alligator
population increased from an estimated 34,635 nests in July 2002 to 42,150
nests in July 2007 (LDWF, New Iberia, LA, unpubl. data). This population
increase, combined with our data, strongly suggest that the current Nutria
removal program has no negative impacts on alligator food habits or on population
density. Thus, we recommend that decisions regarding the continuance
of the Nutria removal program be based on the efficacy of achieving the objective
of reducing the extent of “eat-outs” to reduce marsh damage.
This study was funded by the Louisiana Department of Wildlife and Fisheries
as part of the Nutria Damage Control and Vegetative Restoration Research program,
Award # NA16F22601 Authority NOAA, OMB Circular A-87 and A-133, 15 CFR
Part 24. We thank W. Parke Moore III for administrative assistance in support of this
research and L. Morris for statistical guidance. LDWF field personnel, particularly G.
Linscombe, E. Mouton, and J. Marx, helped with stomach collections, and provided
Nutria harvest data and other guidance. J. Jenkins, K. Franklin, and K. Capshaw
separated and identified stomach contents. Numerous undergraduate students from
Northwestern State University helped with stomach collections. Three anonymous
reviewers provided valuable input on earlier drafts of this manuscript. We are grateful
for the cooperation of the many licensed alligator trappers and processors who
provided us with logistical help and access to their facilities.
354 Southeastern Naturalist Vol. 8, No. 2
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