Download SEASONAL VARIATION IN AVIAN COMMUNITY STRUCTURE

SEASONAL
VARIATION
STRUCTURE:
MECHANISMS
IN AVIAN
COMMUNITY
DIFFERENCES
IN
REGULATING
DIVERSITY
J. T. ROTENBERRY,
1 R. E. FITZNER,2 AND W. H. RICKARD
2
•ShrubsteppeHabitat Investigation Team, Department of Biology, University of New Mexico,
Albuquerque,New Mexico 87131 USA and
2PacificNorthwestLaboratory, Richland, Washington99352 USA
ABSTRACT.--Thirty avian censuseswere conducted over 2 yr along a wooded streamside in
southcentralWashington. Avian speciesdiversity and richnessshow temporal periodicity (summer
maxima and winter minima), but evennessdoesnot. Comparison with contemporaneouscensuses
in adjoining shrubsteppeshowssubstantially greater diversity and richnessalong the streamside,
while community evennessis greater in the shrubsteppe.We infer these conditionsto be a result
of the arboreal physiognomyof the streamsideplant community.
Comparing winter censusesto spring censuses,we observedstatisticallysignificantdifferences
in avian community structure: in winter, speciesdiversity is correlated with evennessbut not
richness,while in the breeding season,diversity varies as a function of richness,not evenness.
Following a model proposedby Tramer (1969), we suggestthat community organization during
the spring breeding seasonmay reflect elementsof resource-basedinterspecificcompetition, while
winter assemblagesof birds are regulated by a harsh and variable climate. Received 3 April 1978,
accepted21 February 1979.
THE relationship between species diversity and its components, richness and
evenness, has been receiving increased attention (Peet 1974). Tramer (1969) has
proposed that changesin diversity may be mediated by changesin one or the other
component and that these changes reflect alternative environmental conditions. He
suggeststhat diversity changes with richness in relatively stable, benign environments and varies with evenness under unstable, rigorous conditions. The former
occurs when most populations are near equilibrium, which is possiblebecause the
physical environment and resource levels remain stable over time. Fluctuations in
relative abundance are minimized, and variation depends on the addition or subtraction of species. Populations in variable, rigorous environments, on the other
hand, are often held below equilibria by unfavorable conditions and may expand
when conditions are alleviated. Such growth may be achieved by a variety of mechanisms (e.g. increased survival, immigration) and is likely to vary according to
species.Tramer expects this variability in population numbers to influence consistently the distribution of relative abundances and, hence, community evenness.
Tramer's hypothesis has been tested by Rotenberry (1978) and has been found to
hold for a large set of bird census data ordered along a multidimensional gradient
of climatic variability and harshness.
Tramer's hypothesisexplainslong-term yearly and/or large-scalegeographicvariation in avian community structure. Therefore, it seems reasonable to try to extend
the same principles to account for observed seasonalvariations in local community
organization. It is in this framework that we report the results of a series of bird
censusestaken over a period of 2 yr in contrasting vegetation.
STUDY AREA AND METHODS
Avifaunal censuseswere conducted in Snively Canyon, located on the Department of Energy's Arid
Land Ecology (ALE) Reserve, Benton County, Washington, about 25 km northwest of Richland, Wash499
The Auk 96: 499-505. July 1979
500
ROTENBERRY,
FITZNER,
AND
RICI,•RD
[Auk,
Vol.96
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Cot•mln
kmI
i
i
Fig.
1. Map
ofsoutheastern
Washington
showing
the
location
ofthe
Snively
Canyon
survey
area
and
the
extent
ofshrubsteppe
vegetation
(stippled)
throughout
the
region
(modified
from
Daubenmire
1970).
ington
(Fig.
1).The
gulch
isapproximately
2kmlong,
varying
from
10-75
mwide,
and
upto100
m
deep.
The
small
stream
within
the
canyon
ispermanent
and
isclosely
bordered
bytrees,
Populus
sp.
and
Salix
sp.,
and
deciduous
shrubs,
Prunus,
Sambucus,
Amelanchier,
Rhus,
Philadelphus,
and
others.
Snively
isisolated
from
similar
arboreal
vegetation
byasurrounding
matrix
ofshrubsteppe
vegetation
(Artemisia-Agropyron)
(Daubenmire
1970)
(Fig.
1).The
nearest
similar
vegetation
isfound
along
the
Yakima River, -25 km distant.
Acensus
route
was
run
inthe
morning
(0700-0800
start)
atintervals
of2-6weeks
from
19July
1974
to4August
1976.
Itconsisted
of12census
stops
about
200
mapart
along
the
stream.
Ateach
stop
all
birds
identified
orheard
ina3-rain
period
were
recorded.
Because
oftheshort
distances
between
stops,
carewastakentoavoid
anydouble-counting
ofindividuals.
Acompanion
route
was
set
upintypical
shrubsteppe
vegetation
inApril
1976
inconjunction
with
another
study.
This
route
was
consused
asatypical
cooperative
breeding
bird
survey
(Robbins
and
Van
Velzen
1968):
3-rain
stops,
0.8km(0.5
mi)
apart,
each
sampled
within
a0.4-kin
(0.25
mi)
radius.
Unlike
the
cooperative
surveys,
however,
our
route
contained
25instead
ofthe
usual
50stops.
Four
such
surveys
were
made
inspring
1976
and
three
inspring
1977.
Although
these
samples
obviously
counted
more
individuals
than
Snively
surveys
because
oftheir
substantially
greater
area
ofcoverage,
reduction
ofthe
datatorelative
abundance
allows
a comparison
between
thetwotypes.
Species
diversity
was
calculated
according
toSimpson
(1949),
then
converted
tothe
equivalent
number
ofequally
common
species
byapplying
areciprocal
transformation.
Thus,
Diversity
= 1/• pi2,
where
p,isthe
relative
abundance
ofthe
ith
species
inthe
sample.
This
represents
diversity
asthe
number
ofequally
abundant
species
occurring
inasample
that
would
produce
the
untransformed
index
previously
calculated
forthat
sample.
This
has
the
advantages
ofretaining
"number
ofspecies"
asitsbasic
unit,
whileincluding
theevenness
component
(Peet1974).
July 1979]
AvianCommunity
Dynamics
501
The richnesscomponentis simply the numberof speciesin the sample. Evenness(the distributionof
relative abundances
amongthe speciessampled)was calculatedusinga ratio proposedby Hill (1973):
Evenness
= Diversity/exp(• Pilog
pi).
The denominator is essentiallythe equivalent number of equally common speciesderived from the
information theory measurefor speciesdiversity (Hill 1973).
Equality of meanswas testedusingthe t'-test (Sokal and Rohlf 1969:374), which makesno assumptions
concerningthe homogeneityof samplevariancesnor requiresequal samplesizes.
Partial coefficientsof determination(ggpart)were usedto assessthe separatecontributionsof richness
and evennessto "explaining" variation in diversity (Steel and Torrie 1960). Becausethe calculationof
diversity is mathematically not independent of values of richnessand evenness,correlation analysis of
variation in diversity with respectto those two variables is inappropriate.
RESULTS
The results of 21 of the 30 Snively Canyon surveys are summarized in Table 1
and include only those speciesthat averaged at least one individual per count on
either 11 breeding-season(May-August) or 10 wintering-season(November-March)
counts. These speciescomprise 91.4% of all individuals seen during the breeding
seasonand 82.7% of all seen during the winter. Breeding-seasonsurveysaveraged
148 individuals of 19.5 speciesper count, while winter surveys averaged 59 individuals of 11.3 species. Altogether, 81 species were recorded from all 30 counts
combined. Similarly, the resultsof 7 breeding seasonshrubsteppetransectsare given
in Table 1. An averageof 10.3 speciesand 385 individualswas seen,with 29 species
recorded altogether.
Community statisticsfor the Snively Canyon surveys (separated by season)and
the shrubsteppetransectsare summarizedand comparedin Table 2. Substantially
more individuals were seen per shrubsteppetransect compared to Snively, most
likely a function of the substantially larger area covered. Nevertheless, the average
number of speciesseenwas only - 10, significantlylessthan that observedon spring
season canyon surveys. Likewise, diversity was significantly lower in the shrubsteppe than in the canyon, although evennesswas significantly higher. Within the
canyon, the winter bird assemblageconsistsof significantly fewer individuals and
speciesat a lower diversity than does the breeding-seasonassemblage,although
evennessdoes not appear to differ between the two samples.
Figure 2 summarizesthe descriptivestatisticsof the Snively Canyon bird community as sampled through time. The periodicity of both richnessand diversity is
quite apparent (Fig. 2A); both achieve maximum values during the breeding season
and minima during mid-winter. Community evennessshowsa substantially different
pattern (Fig. 2B); periodicity is undetectable, with maxima being attained between
mid-November and early March 1975-76. Minimum values, however, occurred in
the same time period the previous winter.
DISCUSSION
Arboreal/shrubsteppe
comparisons.--Althoughit is often difficult to make new
usesof data collectedfor other purposes, comparisonof the Snively Canyon censuses
with thoseconductedin the nearby shrubsteppedoesallow us to make severalpoints
concerningthe mechanismswe think are important in structuringthe Snively bird
community. For example, Table 2 clearly shows the greater richnessand diversity
of Snively Canyon breeding avifauna compared to that of the adjacent shrubsteppe.
502
ROTENBERRY,
FITZNER,AND R•ICKARD
[Auk, Vol. 96
TABLE1. Dominantspecies
of birdsrecordedon SnivelyCanyonand shrubsteppe
surveys.Valuesare
averagenumberof individualsseenfor 11 breeding-season
(May-August)and 10 wintering-season
(November-March)countsin the canyon,and 7 breeding-season
countsin the shrubsteppe.
Actual
number of countson which each specieswas observedis in parentheses.
SnivelyCanyon,breeding
SnivelyCanyon,wintering
Kestrel
California Quail
(Falcosparverius)
SageGrouse
(Centrocercus
urophasianus)
1.0
(7)
1.4
(2)
California Quail
(Lophortyxcalifornicus)
Chukar
(Alectorischukar)
MourningDove
(Zenaidamacroura)
EasternKingbird
(Tyrannustyrannus)
(Picapica)
Rock Wren
(Salpinctes
obsoletus)
Starling
(Sturnusvulgaris)
1.8
(6)
20.7
(9)
28.5
(11)
1.9
(5)
8.9
(11)
5.2
(10)
1.0
(7)
5.6
(5)
( Wilsonia pusilla)
1.1
(6)
3.7
(4)
(Molothrus
ater)
I. 1
(3)
(Icterus
galbula)
7.6
(10)
Western
Tanager
(Piranga
ludoviciana)
LazuliBunting
(Passerina
amoena)
Vesper
Sparrow
(Pooecetes
gramineus)
White-crowned
Sparrow
(Zonotrichia
leucophrys)
Song Sparrow
(Melospizamelodia)
38 other species
(5)
1.1
(4)
2.2
(8)
2.6
(6)
1.2
(6)
(Dendroicacoronata)
WesternMeadowlark
4.7
(2)
(Sturnellaneglecta)
3.7
(5)
26.5
(9)
1.8
(5)
1.6
(7)
(Eremophilaalpestris)
Black-billed Magpie
(Pica pica)
AmericanRobin
(Turdusmigratorius)
Ruby-crownedKinglet
(Reguluscalendula)
Dark-eyed Junco
(Juncohyemalis)
White-crowned
Sparrow
(Zonotrichialeucophrys)
(Melospizamelodia)
20 other species
(Sturnella
neglecta)
19.4 (11)
Brewer's
Blackbird
(Euphagus
cyanocephalus) 2.8 (6)
Brown-headed
Cowbird
Northern
Oriole
1.7
Song Sparrow
Wilson's Warbler
Western
Meadowlark
(1)
Yellow-rumped Warbler
Yellow-breated Chat
(Icteria virens)
1.5
Horned Lark
Barn Swallow
(Hirundorustica)
Black-billedMagpie
(Lophortyxcalifornicus)
Ring-necked
Pheasant
(Phasianuscolchicus)
1.5 (5)
5.5 (10)
7.4 (7)
1.0
(1)
1.4
(7)
10.2
Shrubsteppe,
breeding
GreyPartridge
(Perdix
perdix)
Mourning
Dove
(Zenaida
macroura)
HornedLark
(Eremophila
alpestris)
Western
Meadowlark
1.0 (2)
2.1 (7)
150.1 (7)
(Sturnella
neglecta)
144.1 (7)
Vester
Sparrow
(Pooecetes
gramineus)
9.6 (7)
Sage
Sparrow
(Amphispiza
belli)
65.5 (7)
White-crowned
Sparrow
(Zonotrichia
leucophrys) 5.6 (2)
22other
species
7.0
12.1
This is mostlikely a reflectionof the well establishedrelationshipbetweenvegetation
structureand bird speciesdiversity.Studiesencompassing
a wide rangeof habitat
types(e.g.MacArthur1964,Karr and Roth 1971)havedemonstrated
anddiscussed
this correlation,and Willson(1974)hassuggested
that it reflectsthe additionof new
species
throughthe accumulation
of new guildsin a stepwise
fashionasvegetation
complexity increases.
Althoughit seemsintuitivelylikely that the woodedstreamside
permitsthe existenceof a greaternumberof structurally-oriented
guildsthan the physiognomically
simplershrubsteppe,
that the mechanism
suggested
by Willson(1974)is apparently
operatinghereis substantiated
by the observation
that whilediversityand richness
are greaterin arborealvegetation,evennessis not (Table 2), and that increased
diversityis associated
with increasedrichness(i.e. the additionof new species),not
increased
evenness.
July 1979]
AvianCommunity
Dynamics
A
--
Richness
/xe ---Diversity
25
•
503
•0
o
=
10
5
B
•o
0,90
k-
0.50
d
d A
S
1974
Fig. •.
0
ND
F M
A
MU
1975
d
A S
0
N
D
F M A
M d
d
A
1976
5ea•onal variation in bird communtt• attr•bute• of •nive}• Can•on; A•dchne•
and diversity;
B•venness.
Seasonaldynamics.--Thelack of periodicityin the evenness
data (Fig. 2B), particularlywhen considered
in conjunctionwith the distinctseasonality
evidencedby
richnessand diversity, leads to sometentative conclusions
regardingfactorsthat
may regulate community diversity. We observedthat while the difference in evennessbetween both setsof winter counts(November-March 1974-75 and 1975-76)
is highly significant(P < 0.001), it is not reflectedin diversity(P > 0.05). Neither
is there a significantdifferencein richnessbetweenthe two years.
Examination of the partial coefficientsof determinationshowsthat during the
winter, diversityclearlyvariesas a functionof evenness
(g2par!= .91) and is quite
independentof changesin richness(R2p•,.t= .17). A distinctlydifferent pattern,
however, is evidencedduring the breedingseason:diversity becomesmuch more
highlyassociated
with richness(g2part= .87) and is lessso with evenness
(g2part=
ß72). This stronglysuggests
that we are observingdifferent methodsof "regulation"
of communitystructureat the samesite, which may vary accordingto season.
Comparing the two seasons,resourcesduring the breedingseasonare likely to be
morestablethan theyare in winter, the vegetationstructuremorecomplex,and the
climate more benign. Winter weather in the arid interior regionsof the northwest
504
ROTENBERRY,
FITZ•ER, ANDRICKARD
[Auk, Vol. 96
TABLE 2. Comparisonof mean avian community attributes of Snively Canyon breedingand wintering
seasonsurveysand adjacent shrubsteppebreedingseasonsurveys.Asterisksdenotethe probability
that two adjacentmeansin a columnare different(* = P • 0.95; ** = P • 0.99; *** = P • 0.999).
Number
of indi-
Area
Snively Canyon
Shrubsteppe
Season
viduals
Richness
Evenness
Diversity
59
! 1.3
0.70
4.4
Breeding (n = 11)
148
19.5
0.67
6.8
Breeding (n = 7)
385
10.3
0.82
2.9
Winter (n = 10)
can vary considerablyand is often rigorous. For example, long-term weather data
collectedwithin 15 km of SnivelyCanyonshowthat winter minimum temperatures
are more variable than summer maxima (a range of 27øCversus8øC)and have never
failed to drop to at least-6øC duringthe 60+ yr of record(Thorp and Hinds 1977).
Under summerconditionsit is not unreasonable
to expectthat populationsare nearer
their localequilibriumvalues(determinedby resourcestates,vegetationcomplexity,
etc.) than in winter and that the communityorganizationmight then be more likely
to reflect elements of resource-basedinterspecific competition. Indeed, such com-
petition and subsequenthabitat partitioningis generallyassumedto be the driving
force behind the relationshipbetween bird speciesdiversity and vegetationcomplexity(MacArthur 1964,Karr and Roth 1971).Again, we suggestthat thisdiversity/
complexityrelationshipaccountsfor the greater variety of breedingbirds in Snively
Canyon compared to the shrubsteppe.
Kricher (1975) found in the simple systemhe observedthat winter diversity [as
well as summer diversity (Kricher 1972)] varied with richnessrather than evenness.
He concludedfrom this that severeweather, rather than exerting direct effectson
bird species,worked indirectlyby reducingfood supplies.Conversely,the diversity
patterns we observedcertainly suggestthat bird populationshave the potential for
regulation of their numbers outside those determined by their resourcebase.
The difference between Kricher's observations and ours is likely related to differencesin the scaleof sampling. His data were gathered in two separatesuccessional
stagesthat differed in vegetation structure as well as microclimate. Additionally, it
seemsprobable that winter weather variation between his plots (separated by -1
km) was much lessextreme than the difference between summer and winter in our
samples.
In any event, the patterns we have observed are statistically valid and logically
consistentwith Tramer's hypothesis.They point out the likelihood, often ignored,
that examination of a community through its seasonalchangesmight easily suggest
variation in the causal mechanisms that structure that community.
ACKNOWLEDGMENTS
This work was done by Pacific Northwest Laboratories for the U.S. Department of Energy under
Contract AT(45-1)-1830. Rotenberry was supported by USDOE Contract E(45-1)-2042 while on a fellowship from the Northwest Collegeand University Associationfor Science.
LITERATURE
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Bull. 62.
CITED
1970. Steppe vegetation of Washington. Washington Agr. Exp. Station Tech.
July1979]
AvianCommunity
Dynamics
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HILL, M. O. 1973. Diversity and evenness:a unifying notation and its consequences.Ecology 54: 427432.
KARR, J. R., & R. R. ROTH. 1971. Vegetation structure and arian diversity in several New World
areas. Amer.
Natur.
105: 423-435.
KRICHER,J. C. 1972. Bird speciesdiversity: the effect of speciesrichnessand equitability on the diversity
index. Ecology 53: 278-282.
1975. Diversity in two wintering bird communities: possibleweather effects. Auk 92: 766-777.
MACARTHUR, R. H. 1964. Environmental factors affecting bird speciesdiversity. Amer. Natur. 98:
387-397.
PEET, R. K. 1974. The measurementof speciesdiversity. Ann. Rev. Ecol. Syst. 5: 285-307.
ROBBINS,C. S., & W. T. VAN VELZEN. 1967. The breedingbird survey, 1966. U.S. Dept. Interior,
Fish. Wildl. Serv., Spec. Sci. Rep., Wildl. No. 102.
ROTENBERRY,J. T. 1978. Components of avian diversity along a multifactorial climatic gradient.
Ecology 59: 693-699.
SIMPSON, E. H. 1949. Measurement of diversity. Nature 163: 688.
SOKAL,R. R., & F. J. ROHLF. 1969. Biometry. San Francisco, W. H. Freeman and Co.
STEEL, R. G. D., & J. H. TORRIE. 1960. Principlesand proceduresof statistics.New York, McGrawHill Book Co., Inc.
THORP, J. M., & W. T. HINDS. 1977. Microclimates of the Arid Lands Ecology Reserve, 1968-1975.
Richland, Washington, Battelle, Pacific Northwest Laboratories. BNWL-SA-6231.
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