Rate of species descriptions in Odonata

5 Pages·1997·1.3 MB·

by  K J Tennessen

122 ENTOMOLOGICALNEWS
THE RATE OF SPECIES DESCRIPTIONS
IN ODONATAi
K.J.Tennessen^
ABSTRACT: The rateofnew species descriptions ofOdonataoverthe last 150years yields an
essentially straight line, indicating that many species are yet tobe discovered within the Order.
Morethan 5,300speciesarenowknown,andtheratesofdescriptionin thesubordersAnisoptera
andZygopterahavebeenrelativelyequal.However,adeclineinthenumberofnewspeciesappear-
inginthethreelargestfamiliesoverthelastsixdecades,despiteanincreasingnumberofauthors,
indicates that theOdonataare now at least halfknown and that fewerthan 10,000 species exist
worldwide.
Examiningthetrendcurveoftherateatwhichspecieshavebeendescribed
indicates whethernearly all species in aparticulargroupare known orwhether
many remain to be discovered. For example, Steyskal (1965) found that the
curve forthe butterflies ofNorthAmericahas leveled off, indicating thatalpha
taxonomy for this group is essentially complete. By contrast, the curves for
fleas, wasps, andmosquitoes showthatmany speciesstill areundescribed.The
curves forAedes mosquitoes (Zavortink 1990) and the stoneflies ofthe world
(Steyskal 1976) show no slowingindescriptiverate. In thispaper, Ipresentthe
statusofthetrendcurvefortheworldOdonata, anOrdergenerallypurportedto
be well-known.
METHODS
Using Bridges' catalogue (1993), I tallied the number of valid species
describedin eachdecade, beginning withLinnaeus in 1758. Subspecies, forms
and known synonyms were omitted from the counts. A decade was defined as
extending from January 1 of the zero year to December 31 of the ninth year
(e.g.,the 1980s beganonJan. 1, 1980andendedonDec. 31, 1989). Exceptions
were made forthe first and lastdecades: the 1750s, from Jan. 1, 1758, to Dec.
31, 1759(2years); andthe 1990s, fromJan. 1, 1990toDec. 31, 1994 (5 years).
I derived the numberofdescriptions appearingafterBridges (1993) by search-
ing literature and abstracts.
RESULTS
Approximately5,300validspecieshavebeendescribedfromLinnaeus'time
through 1994.AsignificantincreaseintherateofOdonatadescriptionsdidnot
begin until 70years afterLinnaeus (Fig. 1). Since then, the rate has been fairly
• ReceivedJune 11, 1996.AcceptedAugust 1, 1996.
2 1949HickoryAve.,Florence,AL35630.
ENT.NEWS 108(2): 122-126,March&April, 1997
Vol. 108,No. 2,March&April, 1997 123
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124 ENTOMOLOGICALNEWS
constant.Thedecadeofgreatestactivitysofarwasthe 1930s, when548species
were described.A relatively low numberofnew species appeared in the 1970s
(208), indicating that the line might begin to level off. However, in the 1980s,
the rate increased again. As Steyskal (1965) pointed out, ". . . if the curve
appears to be leveling off, we have no assurance that we are approaching the
stage at which all species are known."The steeperinclination ofthe line in the
first half of the 1990s, during which more than 235 species were described,
indicatesthat weare farfrom nearingcompletionofodonatespeciesdiscovery.
The numberofspeciesdescribedtodatewithin thetwosuborders,Anisoptera
and Zygoptera, are remarkably similar (about 2,770 and 2,540 species, respec-
tively). In the first halfofthe currentdecade, moreAnisoptera(145) have been
described than Zygoptera (90). a trend opposite that of the 1980s. In the last
three decades, families in which the most species were described were
Gomphidae, Coenagrionidae, Libellulidae, Corduliidae,Aeshnidae, Protoneu-
ridae, Platycnemididae, and Megapodagrionidae, listed in descending order.
Themajority ofnewspeciesdescribedrecentlycame fromtropical areasof
the world (Neotropical. 92: Oriental. 74; Australasian, 40). The fauna of
Europe is almost certainly known, as over 95 percent of the European fauna
was described prior to 1900, and only five new species descriptions have
appeared since then (see Askew 1988). Knowledge of the Nearctic fauna
appears to be nearing completion (Fig. 2), although 8 species have appeared
thus far in the 1990s and a few new species still await description.
The rate of new species discovery in the three largest families (each with
approximately 1,000speciesknown)mightbeacluetothestatusofknowledge
forthe Orderas awhole. Forexample, in theCoenagrionidae and Libellulidae,
thedescriptiverate overthe last4decadeshas slowed(Table 1).Thefuturerate
inCoenagrionidae willprobablybe higherthan intheLibellulidae, however, as
several colleagues have informed me that many undescribed species of these
small, taxonomically-difficultdamselflies exist in tropical areas. In contrast to
thesetwo families,the rate in theGomphidae increased. Reasonsthatnew spe-
cies continue to appear in this family is that gomphid dragonflies are rather
local in distribution, have relatively briefadult flightranges, and are relatively
secretive in their habits, factors which make them difficult to collect. In sum-
mary,thedecliningrate intwoofthe largestfamilies,coupledwiththefactthat
significant numbers of new species are being found in only three of the six
biogeographic regions, indicates that overhalfthe Odonata are now known.
Vol. 108,No. 2,March&April, 1997 125
450
Fig.2.TrendcurveofspeciesdescriptionsforNorthAmericanOdonata.
Furthermore, the increasing number of authors has not increased the rate of
description,althoughodonatologistsarenotevenlydistributedaroundtheworld,
and there are few or none in many ofthe tropical countries where diversity is
greatest. The combinedcircumstances ofthese three factors lead me to specu-
late thatoverhalfthe Odonataare known and thus there are fewerthan 10,000
species in the world.
HigherclassificationwithintheOrderishighlycontroversialandinneedof
revision. Phylogenetic studies ofOdonata have lagged behind studies ofother
aquatic insect Orders. Progress in these two areas will continue to be slow
because funding for such efforts is difficult to obtain. Most currently active
Odonata taxonomists expend personal resources to support research at the
species level and higher, including costs forcollecting, curating, analysis, and
publishing.
126 ENTOMOLOGICALNEWS
Table 1.Changeinnumberofnewspeciesdescriptionsperdecadeinthethreelargestfamiliesof
Odonatafrom 1900to 1994,andtheassociatedpercentincreaseordecreaseinthelastfivedecades
of20thcenturycomparedwithfirsthalfofcentury.