Download FOOD NICHE OVERLAP IN CO-EXISTING BARN OWL TYTO ALBA

POLISH JOURNAL OF ECOLOGY
(Pol. J. Ecol.)
61
1
179–181
2012
Research note
Grzegorz KOPIJ
Department of Vertebrate Ecology, Wroclaw University of Environmental & Life Sciences,
Kożuchowska 5b, 51-631 Wroclaw, Poland, e-mail: [email protected]
Department of Wildlife Management & Research, University of Namibia, Katima Mulilo Campus,
Private bag 1096, Winela Rd., Katima Mulilo, Namibia, e-mail: [email protected]
FOOD NICHE OVERLAP IN CO-EXISTING BARN OWL
TYTO ALBA (SCOPOLI 1769) AND GREAT HORNED OWL
BUBO VIRGINIANUS GMELIN 1788 IN INTESIVELY USED
FARMLAND
ABSTRACT: The study was conducted at a
site where the Great Horned Owl and the Barn
Owl nested in a close proximity to each other in
an intesivelly managed farmland in north-eastern
Oregon. By the means of pellet content analysis it
has been shown that food niches of both species
almost totally overlapped. Their main diet was
composed of Microtus voles (numerically: 79% in
Great Horned Owl and 86% in Barn Owl) which
were probably superabundant in this site.
KEY WORDS: diet, Tyto alba, Bubo virginianus, intensively used farmland, food niche,
Oregon
One of the central concept in ecology is
the competitive exclusion principle, which
explain species coexistence. In general this
concept forbids the coexistence of two species with identical or similar food niches.
However, some species have been shown to
coexist on the same resources with no recorded evidence of niche differentiation (e.g.
Kopij 2006). The quest for mechanisms permitting such coexistence may be of crucial
importance in conservation biology, as competitive exclusion can influence the range,
distribution and densities of species (B e gon
et al. 1990). This coexistence may be possible through a combination of factors such as
journal 33.indb 179
non-limiting food resources and a high rate
of predation (Wo o dw ard and Hi l d re w
2002, Kopij 2006). It has been shown that
food niche overlap between two competing
species decreases as their body size increases
(He re r a and Hi r a l d o 1976; Woodward
and Hi l d re w 2002). It increases in summer,
when prey are abundant and small, and decresases in winter when the prey is larger but
scarcer (Wo o dw ard and Hi l d re w 2002).
It also shows some geographical variations
(He r rer a and Hi ra l d o 1976). Niche diffrerentiation is also not the only mechanism
by which two competing species can coexist
(B egon et al. 1990).
In the Nearctic Region, the Barn Owl Tyto
alba (Scopoli 1769) nests often co-occurs sympatrically with the Great Horned Owl Bubo
virginianus Gmelin 1788 (Johnsgard 2002,
Konig and Weick 2008). Under such condition, their diet was analysed under seminatural conditions, namely in a short-grass prerie
mixed with farmlands in Colorado (Mar t i
1974), and semidesert vegetation mixed with
a farmland in California (Rudoph 1978).
Since both owl species differ significantly in
body size (Great Horned Owl: mean weight
– 1400 g; Barn Owl: mean weight – 500 g;
Johnsgard 2002), significant differences
2013-04-30 10:05:52
180
Grzegorz Kopij
in prey composition, mean biomass of prey
consumed and proportions of particular prey
species have been evidenced in both studies.
This presented note contributes further to
the issue of food niche segregation between
the the Great Horned Owl and Barn Owl. In
this case, however, studies were conducted
in highly modified habitat, i.e. in intesivellymanaged farmland. The study aims to show
to what extend the food niche of both owl species may overlap under such condition.
The study was conducted at a site where
the Great Horned Owl and the Barn Owl
nested in a close proximity to each other (ca.
1 km apart), on a farm yard at the East Oregon Agricultural Research Station at Union,
Union County, NE Oregon (45°12’36’’N,
117°51’53’’W). The farmstead comprised a
wet pastureland grazed by horses, cattle, and
deers, and intesivelly cultivated fields (mainly
corn and wheat) further afield on one side,
and well-timbered suburbs on another side.
The diet composition was determined
through pellet content analysis. Pellets were
collected on 29 September 2010; 91 pellets from
the Great Horned Owl and 189 pellets form the
Barn Owl were collected (Table 1). Only compact, i.e. entire pelltes were taken into analysis.
The pellets were analysed with universally accepted procedure (cf. Ya lden 2009). Identification of rodents and calculation of their biomass were based on Mas er and Stor m (1970)
and Ver ts and C ar raway (1998). The other
prey were identified with the aid of reference
collection hold by the Department of Forestry
of the Oregon State University.
The following trophic estimators were
used to characterize diet of both owl species: H’ – Shannon and Weaver’s prey species diversity index, FNB – Simpson’s food
niche breadth index, O – food niche overlap
Pianka’s index, and M – mean prey biomass
(Shan non and We ave r 1949):
H’ = - Σpilogepi
FNB = 1/∑pi2
O = ∑ pi ∑qi / ∑ (pi2∑ qi2)1/2
where pi is the proportion of the total number of individulas which belong to the i-th species in one owl’s diet, and qi is the proportion of
the total number of individulas which belong
to the i-th species in the other owl’s diet. Both
FNB and O were calculated on the fine resolution using prey species or genera.
Rodents constituted the bulk of the summer diet both of the Barn Owl (98.4% by
weight) and Great Horned Owl (91.8% by
weight). The remaining prey were represented by birds (by weight 3.2% in the Barn Owl
and 8.2% in Great Horned Owl diet). In the
diet of the Barn Owl the medium-sized birds
were represented exclusively by the Rock Pigeon Columba livia, which commonly nested
in the barn occupied by the owls.
Among rodent prey, voles Microtus spp.
were by far the commonest prey in the diet
of both owl species compared. The Northern
Pocket Gopher Thomomys talpoides (R ichards on 1828) supplemented the rodent diet.
Other rodents, such as Deer Mouse Pero-
Table 1. Summer diet of the Great Horned Owl and Barn Owl at Union, NE Oregon.
F – percentage of the frequency of occurrence, N – the percentage of prey items, W – the percentage of
the approximate biomass (g).
F
Bubo virginianus
N
W
F
69
19
9
1
79
13
6
1
64
21
3
4
73
22
2
1
-
86
10
1
1
-
78
18
1
<1
-
2
-
1
-
8
-
2
1
1
<1
3
1
91
141
7290
189
328
15312
Prey taxa
Mammalia
Microtus spp.
Thomomys talpoides
Peromyscus maniculatus
Mus musculus
Sciurus spp.
Aves
Medium-sized birds
Small birds
Number of pellets/prey items/biomass
(g)
journal 33.indb 180
Tyto alba
N
W
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Food niche overlap Tyto alba and Bubo virginianus
myscus maniculatus (Wag ner 1845), House
Mouse Mus musculus L., 1758 and squirells
Sciurus sp. were preyed upon occassionally
(Table 1).
Although the mean prey biomass was
higher in the Great Horned (59.3 g, N = 123)
than in the Barn Owl (46.7 g, N = 328), the
index of prey species diversity was very similar, i.e. H’ = 0.67 for the Great Horned Owl
and H’ = 0.52 for the Barn Owl. Food niche
breadth index was much the same for both
owl species (FNB = 1.54 and 1.33 for the Great
Horned and the Barn Owl, respectively). The
index of food niche overlap on fine resolution was O = 0.93, which means that the food
niches almost totally ovelapped.
The high similarity of the Barn Owl and
Great Horned Owl diet nesting in the same
site was rather unexpected as the owls differ both in size and in behaviour (Jonsg ard
2002). This high-degree overlap in the diet
could result from the fact that their main Microtus prey was probably super-abundant and
easily available around nesting sites of these
owls. It is well documented that under such
condition, where one prey species become
super-abundant different owl species prey at
the same site mostly on this super-abundat
prey species (Mi k kol a 1983, Tay l or 1994,
Houston et al. 1998). It is invisigated that in
years when the main prey species is much less
numerous, the degree of food niche overlap
is much lower. The sample size was, however,
not representative enough to give space for
further generalisations on feeding relationships between the two owl species.
ACKNOWLEDGMENTS: I greatly appreciate help of the following persons in collecting pellts: Patricia Kennedy and Bruce Park form
the East Oregon Agricultural Research Station of
the Oregon State University. Dr. Eric Forsman has
confirmed identification of some prey items.
181
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Received after revision August 2012
journal 33.indb 181
2013-04-30 10:05:52