jorn kohler, diversity in amphibians frank glaw and ... · diversity, species diversity and...

Jorn Kohler,Frank Glaw andWolfgang Bohme

Diversity in AmphibiansThe term "biodiversity" is often confused with "species diversity." Biodiversity is far moreinclusive and describes diversity in all aspects of biology—that is, the morphological,physiological, ethological, ecological and genetic diversity in populations, species or highertaxonomic categories.

The Significance of Researchon Diversity

Research on systematics and taxonomy, largelyignored in an era of genetics and biochemistry,is experiencing a comeback under the moderndesignation "research in biodiversity." Thisresurgence answers an urgent need, for a fewyears ago it became apparent that only a smallpercentage of the earth's species diversity wasscientifically known. Since 1758, the officialbeginning of zoological nomenclature, some 1.7million species have been described. Today, theactual species pool has been estimated toinclude anywhere from 10 to 100 millionspecies. At the same time, it is becomingincreasingly evident that the near future willbring species extinction on a scale that hasoccurred only a handful of times in the earth'shistory. In other words, innumerable species willbecome extinct before we will have had theopportunity to study and know them.

The species that remain hidden from scientificand general knowledge are by no means incon-spicuous insects or worms. Major groups of ver-tebrates are still unknown. Today, more than5,000 amphibian species have been described,and the number increases on a yearly basis byapproximately 70 to 100 newly discoveredspecies. Never before has the rate of discoveringnew amphibian species been as great. Since1994 the number of known recent amphibianspecies is larger than that of mammals (approx-imately 4,700 species). With some 4,370 species(as at the end of 1997), frogs and toads (Anura)are by far the most diverse group; only one-tenth (436 species) are salamanders (Urodela),and a mere 163 species are counted among thecaecilians (Gymnophiona).

With the exception of oceans, amphibiansinhabit a variety of zones, from deserts to thesubpolar region, from sea level to snow line,every imaginable type of fresh water, from theground right up to the highest treetop.Amphibians seem to have once again reached alevel of diversity comparable to their first"golden age," the Carboniferous and thePermian. Equipped with lungs and limbs, theywere the first vertebrates to leave water in theDevonian, some 350 to 360 million years ago,and to occupy the land masses previouslyuncolonized by vertebrates. In the process they

developed an enormous diversity in form andspecies, including representatives that were sev-eral yards/meters in length and had very littleresemblance to present-day amphibians. Thefossil evidence of Paleozoic amphibians endswith the Triassic, but amphibians did notbecome entirely extinct. The first frog-like crea-ture appears in the Triassic in Madagascar, stillsporting a short tail, but already displaying thefirst signs of a saltatory mode of life. In theJurassic, many recent frog families alreadyexisted, such as tongueless frogs (Pipidae) anddisk-tongued frogs (Discoglossidae).Salamanders and caecilians followed in theCretaceous. From that time forward, amphibiansexperienced a second golden age that has lastedinto the present.

Differences in Diversity

Amphibians need water. It comes as no surprisethen that the moist environment of the tropicsis home to their greatest diversity. High temper-atures and constant access to water (precipita-tion) create ideal conditions, and the number ofamphibian species increases the nearer one getsto the equator. But in reality the situation is farmore complex and should be examined notonly against the background of current climaticconditions. Instead, past climatic changes mustalso be considered, since they often createdenvironments that were hostile to amphibiansurvival and caused the extinction of manyspecies.

Climatic change has the least impact on moun-tain slopes, where flora and fauna simply wan-der uphill when temperatures increase, ordownhill when they decrease. In other words,mountain slopes act as climatic buffer zonesthat offer a refuge and help curb species extinc-tion in times of climatic change. This is why wecan still observe the greatest diversity ofamphibians on humid mountain slopes, such asthe Andes in South America or in easternMadagascar. The gigantic rain forests in SouthAmerica's Amazon basin or Africa's Congobasin, on the other hand, are inhabited by com-paratively fewer species.

Of all zoogeographic regions, the Neotropics—that is, the tropical and subtropical region inCentral and South America—display by far thewidest amphibian diversity. Over two-thirds of

A 1.11.2 „„

all newly described amphibian species in thelast 10 years originate from that region. Ofcourse, current species descriptions do not nec-essarily reflect the true status of existing speciesdiversity, because different levels in researchintensity should also be considered. Researchmay be more intensely focused in Latin Americathan in Africa and Asia. Nevertheless, currentstudies show that there still is no end in sight tothe discovery of new species in tropical SouthAmerica. It is therefore probable that amphibian

diversity is most concentrated in the Neotropics.Many areas in South America present a highdegree of diversity: 84 species near SantaCecilia (Ecuador), 67 species near Panguana(Peru), 72 species near Cocha Cashu (Peru). Bycontrast the diversity falls drastically as wemove toward more temperate latitudes. AcrossEurope, from Portugal to the Ural Mountains,there are only 35 Anura species, and Germanyis home to a mere 14 anuran and 6 urodelespecies.

Comparison of amphibianspecies diversity in theUnited States and the for-mer Soviet Union. Largeareas of the states of theSoviet Republic have a verylow diversity in species,which is understood as aresult of the predominantlycold climate in thoseregions where 47 percent ofland surface is subject topermafrost. Altogether only41 amphibian species occuracross an area that coversmore than 8.4 millionsquare miles (22 millionsquare kilometers). In thesoutheastern United Statesthe diversity in species ismuch higher. The increasein species to the southeastis especially noticeable:these are regions with awarm climate and high lev-els of precipitation. On thesouthern tip of Florida thenumber of species declinesagain, a result of the so-called peninsula effect. Thishypothesis postulates thatthe diversity in speciesdeclines on the tips of largepeninsulas (after Kuzmin,1996, and Duellman andTrueb, 1986).

Diversity in Amphibians 33

Diversity, Species Diversity andDistribution of Recent Amphians

1

2

3

4

5

6

CAECILIA (GYMNOPHIONA)FAMILY

Caeciliidae RAFINESQUE-SCHMALTZ, 1814

Ichthyophiidae TAYLOR, 1968

Rhinatrematidae NUSSBAUM, 1977

Scolecomorphidae TAYLOR, 1969

Typhlonectidae TAYLOR, 1968

Uraeotyphlidae NUSSBAUM, 1979TOTAL (status: end of 1997)33

NUMBER

GENERA

21

2

2

2

5

1

163

OF

SPECIES

88

37

9

5

20

4

Hypogeophis rostratus (Caeciliidae). Distribution of Caecilians

1

2

345

67

8

9

URODELAFAMILY

Ambystomatidae HALLOWELL, 1856Amphiumidae GRAY, 1825Cryptobranchidae FITZINGER, 1826Dicamptodontidae TIHEN, 1958

Hynobiidae COPE, 1859Plethodontidae GRAY, 1850

Proteidae GRAY, 1825Salamandridae GOLDFUSS, 1820

Sirenidae GRAY, 1825TOTAL (status: end of 1997)

NUMBER

GENERA212

28

292

162

64

OF

SPECIES32

337

41280

6

613

436

Plethodon jordani (Plethodontidae). Distribution of Urodela

In contrast to the Anura, which are distributedworldwide with the exception of the polar region,the Urodela are primarily limited to the NorthernHemisphere, with the exception of the lunglesssalamanders (Plethodontidae) of South America.The distribution of caecilians is limited to thetropical regions of the earth. The distribution mapfor individual amphibian families is included inthe chapter on systematics (pp. 36 to 63).

ANURA

FAMILY

1 Allophrynidae GOIN, GOIN ft ZUG , 19782 Arthroleptidae MR/ART, 18693 Brachycephalidae GUNTHER, 18584 Bufonidae GRAY, 18255 Centrolenidae TAYLOR, 19516 Dendrobatidae COPE, 18657 Discoglossidae GUNTHER, 18598 Heleophrynidae NOBLE, 19319 Hemisotidae COPE, 186710 Hylidae RAFINESQUE, 181511 Hyperoliidae LAURENT, 194312 Leiopelmatidae MIVART, 186913 Leptodactylidae WERNER, 189614 Microhylidae NOBLE, 193115 Myobatrachidae SCHLEGEL, 185016 Pelobatidae BONAPARTE, 185017 Pelodytidae BONAPARTE, 185018 Pipidae GRAY, 182519 Pseudidae FITZINGER, 184320 Ranidae RAFINESQUE-SCHMALTZ, 181421 Rhacophoridae HOFFMAN, 193222 Rhinodermatidae BONAPARTE, 185023 Rhinophrynidae GUNTHER, 185824 Sooglossidae NOBLE, 1931

TOTAL (status: end of 1997)

NUMBER

GENERA172

34310411

40192

506623101

52

441011

2339

OFSPECIES

1743

4101251861858

773233

49723211191092293

746236

213

4371

The Northern- andSouthernmost Margin ofAmphibian Distribution

Among amphibians, species of theRanidae (true frogs) have the north-ernmost distribution. In the Palearctic,the distrbution of the Grass frog (Ranatemporia) extends beyond the ArcticCircle and to the "northernmost pointof Europe," the North Cape(71°10'21"N). (The neighboring CapeKnivskjellodden, at a northern latitudeof 71°11"N, is even farther north.) TheMoor frog {Rana arvalis) on the KolaPeninsula near Murmansk extends toalmost the same northern limit (69°northern latitude).

The distribution of the Siberian(Manchurian) salamander(Salamandrella keyserligii,Hynobiidae), one of the most cold-resistant amphibian species, alsoextends beyond the Arctic Circle. InNorth America, the Wood frog (Ranasylvatica] is distributed farthest to thenorth. This particularly cold-resistantfrog, which exhibits many similaritiesto the European grass frog (Rana tem-poraria), populates major parts ofNorth America and extends to theArctic Circle in Alaska.Among recent amphibians, severalspecies of Bufo (Bufonidae, truetoads)-for example, Bufo variegatus-are distributed farthest to the south.They reach almost to the southern-most tip of South America (approx.

Distribution of Anura Hyla versicolor (Hylidae).

Diversity in Amphibians 35