low latitudes (the tropics) species richness is greater than at higher latitudes (temperate and polar regions). ... Tropic of Cancer (231/2 âN) and the Tropic of.
Hydrobiologia 202: 175-183, 1990. 0 1990 Kluwer Academic Publishers.
Printed
175
in Belgium.
Species richness in tropical fresh waters of Australia R. J. Shiel’ & W. D. Williams2 ‘Murray-Darling Freshwater Research Centre, Albury, N. S. W. 2640, Australia; ‘Department Zoology, University of Adelaide, G. P. 0. Box 498, Adelaide, S. A. 5001, Australia
of
Received 12 January 1989; in revised form 11 July 1989; accepted 10 August 1989
Key words: species richness, tropical fresh-waters,
Australia,
latitudinal
gradients
Abstract General ecological expectations about the relationship between latitude and species richness are that at low latitudes (the tropics) species richness is greater than at higher latitudes (temperate and polar regions). Recent work suggests that this may not be the case for several habitat types and biological groups in Australia. Results are conflicting: on present evidence (admittedly sparse) it appears that in Australian tropical fresh waters species richness is generally depressed in zooplankton and littoral microfaunal communities, but not in macroinvertebrate communities in typical streams and in fish communities (and perhaps also in amphibian and reptile communities). The situation is indeterminate for tropical phytoplankton and macrophyte communities.
Definitions In the context of this review, we understand tropical to be that area of the globe between 30 ON and 30 “S latitude, i.e. bounding and between the Tropic of Cancer (231/2 “N) and the Tropic of Capricorn (23*/2 OS) (subtropical may be used to include 20-35 ’ latitude). Temperate zones are those extending beyond the Tropics towards the Polar Circles (i.e. 30-60 “N and S). Tropical Australia is that area north of 30 ’ S, approx. 40 % of the continental area, with generally low relief (< 500 m). For full details of climatic influences in the Australian region, see Linacre & Hobbs (1977). Introduction General ecological theory would have limnologists believe that, paralleling trends in tropical
rainforests and coral reefs, species diversity is high in tropical bodies of inland water. As Payne (1986) puts it: ‘it has largely become axiomatic that tropical communities contain a high diversity of organisms’. Indeed, the inverse gradient between latitude and species diversity assumes ‘paradigm’ status in most general ecological texts (e.g. Krebs, 1972). Begon et al. (1986) put it plainly: ‘perhaps the most widely recognized pattern in species diversity is the increase that occurs from the poles to the tropics’. Among the most frequent explanations advanced are concepts involving climatic stability, geological age, habitat heterogeneity, high productivity, predator-prey and comparative interactions relationships, (Pianka, 1966, 1974). Recently, limnologists have begun to examine this matter further, and to consider the composition of tropical aquatic communities on a basis other than checklists and distribution records.
176 Two recent reviews are of particular interest: those by Payne (1986) and Lewis (1987). In both, the conclusion is that, apart from special cases such as ancient tropical lakes and their fish faunas, tropical aquatic communities are either equally or less diverse than those in non-tropical zones. These conclusions are in line with earlier
contributions on this subject by Patrick (1966) and MacArthur (1972). Both Payne and Lewis stressed the rather thin data base upon which their conclusions were based. Nevertheless, they did not consider Australian work; this scarcely rated passing mention, despite the numerous references to Australian work on tropical waters provided by Dussart et al. (1984). Conversely, many Australian limnologists working on tropical waters have largely ignored the global context of their work: although brief reference is sometimes made to papers concerning non-Australian waters, most of their analyses have been confined to a more or less intracontinental framework. There is clearly room for improvement by both sets of limnologists. Authors concerned with global patterns of species diversity (and other ecological attributes of communities) should be better aware of the extensive and increasing amount of work that has and is now being done on Australian tropical waters. And Australian limnologists working on tropical waters must attempt to integrate better their findings with those from other tropical regions. These matters are raised for several reasons. Not the least is to underscore the important role that studies of Australian inland waters have in the development of a truly ‘global’ limnology (Williams, 1988). In this connection, we note that about half of the total annual runoff from Australia is in tropical rivers. Moreover, there are special reasons why Australia is a particularly apposite place to compare temperate and tropical waters: Australia, unlike other tropical countries, lacks serious human diseases vectored by aquatic animals; it is safer to study tropical limnology in Australia than anywhere else. Second, Australia lacks the political frontiers which frequently impede limnological studies in, for example,
Africa and South America. Third, the geography of Australia, spanning more than 30 ’ of latitude, allows an almost uninterrupted study of wetlands arranged in a north-south pattern in the east. Finally, the climate is moderated by oceanic influences, which, in the absence of high mountain ranges acting as climatic barriers, reduce the climatic range experienced compared to other continents at similar latitudes. This has significance in permitting range extensions of aquatic organisms. The aims of this paper are threefold: generally to inform limnologists studying tropical lakes and streams about Australian studies; to underscore the need for Australian work on tropical lakes and streams to relate to global questions concerning tropical limnology; and, by systematic examination of Australian work thus far, to consider whether the biotic diversity in Australian tropical waters reflects the trends observed by Payne (1986) and Lewis (1987).
Systematic
survey
Phytoplankton
Extremely complex algal assemblages were reported from Magela Ck (N.T.) billabongs by Thomas (1983) (diatoms) and Ling and Tyler (1986) and Thomasson (1986) (other algae). All noted affinities of the flora with that of the IndoMalaysian region and a diversity ‘typical’ of tropical regions. Thomasson listed 850 taxa from 8 small samples, and considered the species diversity ‘almost comparable with that of the Amazon basin’. Similar studies are lacking for temperate Australian waters, so on present evidence the ‘remarkable spectrum of morphological diversity’ (Ling & Tyler, 1986) seen in the tropical algae must remain unchallenged. Macrophytes.
According to Aston (1973) and Specht (1981), many more genera of aquatic macrophytes occur in southern Australian waters (including southeastern Queensland) than in northern waters. Williams (1983; Table 10.1) provided a summary
Table 1. Comparative
species diversity, tropical vs. temperate Australia studies
(representative
Group
Tropical
Phytoplankton
Thomas (1983) Ling & Tyler (1983) Thomasson (1986)
None comparable?
Inconclusive
Zooplankton
Tait et al. (1984) Julli (1986) Hawkins (1988) Timms & Morton (1988)
Shiel (1981)
Temp. > Trop.
Litt. microfauna
Koste (1981) Smirnov & Timms (1983) Timms (1988)
Koste & Shiel (1986, 1987)
Temp. > Trop.
Macroinvertebrates
Marchant (1982) Outridge (1987)
Marchant (1984)
Temp. > Trop.
Fish
Payne (1986)
of their data: of the dicotyledonous macrophytes, 13 genera occur in northern waters and 23 in southern waters ; of monocotyledonous macrophytes, 26 occur in northern waters, 38 in southern ones. Even so, freshwater swamps as a whole are said to have more genera in northern Australia than in southern regions (Specht, 198 1). According to Beadle (198 l), about the same number of species occur in tropical and temperate waters of Australia. At the individual site level, species richness may be low in northern waters, at least in certain habitats, as shown by the study of a series of plant communities in the Alligator Rivers Region of the Northern Territory (Taylor & Dunlop, 1985). The lowest species richness (N3) was found by them to occur in Eleocharis and Oryza swamps. Southern wetlands of apparently similar type often have many more species (cf. Boon et al., 1990). Many tropical fresh waters of different type also have many more species (e.g. lakes of the Atherton Tableland, Queensland; Kershaw, 1978) especially those in the Alligator Rivers Region not subject to trampling by introduced buffaloes. Whether low macrophyte species richness does characterise tropical fresh waters in Australia vi+ci-vis temperate waters on the same continent must remain indeterminate at present, for as Briggs (1981) amongst many has noted, the
Temperate
studies
177
groups only)
et al.
Diversity
Temp. > Trop.
macrophytes of Australian wetlands are still relatively poorly studied. In our view, it seems likely that, as for all aquatic macrophytes, local physical conditions are very important and as such much more likely to determine macrophyte species richness in tropical fresh waters of Australia than are factors related to latitude. Zooplankton.
Published work on tropical zooplankton was reviewed by Dussart et al. (1984), who noted that the zooplankton of tropical Australia was the least known of all tropical regions. Since that review some gaps have been filled by Tait et al. (1984) and Julli (1986) from Magela Creek billabongs, and Hawk& (1988) from Solomon Dam. These studies are not readily comparable: the Magela billabong ‘plankton’ contained a high proportion of littoral incursions with resultant high species diversity, a consequence of habitat heterogeneity, and many species are better considered as littoral microfauna. The limnoplankton from the 1984 Magela study are included in Table 2. Few littoral incursions were recorded in Solomon Dam, which is compared in Table 2 with specific lakes elsewhere in the tropics and in temperate Australia. Both the Parakrama Samudra and Sunda Lakes studies included littoral incursions: a less complex true plankton is indicated. Temperate waters appear to support a
178 Table 2. Zooplankton
assemblages of Asian/Australian
Site
Latitude
waters (after Hawkins
1988)
Species of: Copepoda
Reference Cladocera
Rotifera
L. Lana0
8”N
2
4
I
Lewis, 1979
Parakrama Samudra
8”N
6
17’
32’
Rajapaksa & Fernando, 1983
Sunda Lakes
o-8”N
6
13
23
Ruttner,
Magela Ck
13”s
5
10
24
Tait et al., 1984
Solomon Dam
19”s
1
12
35’
Hawkins,
Mt Bold Res
35”s
8
10
32
Shiel et al., 1987
L. Mulwala
36”s
8
12
30
Shiel, 1981
(*L. Garcia
42”s
4
6
34
Koste et al., 1988)
1952
1988
[*2 samples only; + includes littoral incursions]
higher microcrustacean species diversity, but a similar rotifer assemblage. A more comprehensive study of the crustacean zooplankton of fresh waters in tropical Australia was that of Timms & Morton (1988). These authors found that throughout tropical Australia the eulimnetic zooplankton comprised 7 species of calanoid copepods, 5 of cyclopoid copepods and 12 of the Cladocera. They concluded that ‘these assemblages are not only less complex than in temperate Australia, but also are not as rich as in most other tropical regions’.
Littoral microfauna. Table 2 compares littoral microfaunal assemblages from some tropical and temperate fresh waters in Australia. Protozoa are not included; the group has been neglected since the work of Playfair in the early 1900’s. With regard to littoral rotifers, some 250 species have been recorded from northern Australia thus far (Koste, 1981; Koste & Shiel, 1983; Tait et al., 1984; Hawkins, 1988), most of Indo-Malaysian affinity. Some 400 rotifer taxa are known from temperate Australia, but from considerably more samples. Except for a single
1980 collection from Winmurra billabongs (80 taxa, including littoral incursion species), most single net tows from Magela Creek billabongs contained 15-30 rotifer species. Recent intensive study of billabongs on the Murray, and in Tasmanian waters comparable to mainland billabongs, has suggested equally (if not more) diverse rotifer faunas. Some representative comparisons are given in Table 2. Ninety-six cladoceran species are known from tropical Australia, with up to 44 taxa recorded in areas of the Northern Territory and Queensland by Timms & Morton (1988), presumably including limnetic taxa. Twenty-eight of 35 cladoceran taxa recorded by Tait et al. (1984) were littoral in habit; 37 of 43 recorded by Julli (1986) were littoral species. Fewer taxa were reported from a Goulburn billabong (Shiel, 1976), viz. 3 1 taxa; however, recent collections from Murray billabongs below Lake Hume (Frey & Shiel, unpublished) have yielded at least 60 cladoceran taxa (including limnetic species). Momentary species composition also appears to be higher in these samples than reported from the tropics by Timms & Morton (1988) (cf. Table 2), although approximately comparable to the cladoceran component
179 Table 3. Littoral Site
microfaunal
assemblages, tropical vs. temperate Australia Latitude
(single collections)
Species of:
Reference
Copepoda
Cladocera
Rotifera
Nankeen 15.vi.79
13”s
4
9
29
Koste, 1981
Leichhardt 13.vi.79
13”s
3
6
20
Koste, 1981
Winmurra 15.iv.80
13”s
4
9
62
Shiel & Koste, 1983
Ryans2 MDFRC 09.xi.87
36”s
4
8
32
Unpublished
Ryans 1 MDFRC 15.i.88
36"s
4
7
31
Unpublished
Ryans 1 MDFRC 04.iii.88
36"s
3
3
58
Unpublished
Dune pool
43"s
3
5
34
Koste et al., 1988
42"s
4
8
27
Koste et al., 1988
02.x.87
L. Garcia margin 02.x.87
of tropical collections made by one of us (RJS). Without distinguishing between littoral and limnetic cladocerans, Timms (1988), in a review of the biogeography of Cladocera in tropical Australia, concludes that although species richness varies widely throughout the region considered, ‘in general there is a decrease from north to south’. At present, it seems likely that temperate waters are equally or more diverse in littoral cladocerans than are tropical waters of Australia, but further work is required to confirm this. Long term quantitative studies (cf. Kratz et al., 1987) are lacking. Information on littoral copepods is more limited than it is for limnetic copepods, but it appears that the diversity of tropical littoral Copepoda is low. Three or four of the 12 species reported by Tait et al. (1984) appear to be littoral incursions; four littoral copepod taxa were reported by Julli (1986); 10 spp. were regarded as littoral strays by Timms & Morton (1988). In temperate Australia, 14 littoral copepod taxa were recorded from a Goulburn billabong (Shiel,
1976), and 20 species (including some ‘heleoplankters’) occured in temporary waters in Victoria (Morton & Bayly, 1977). Table 2 compares some available momentary species data; diversity in these samples is similar. Although the data base is meagre, there is no evidence that tropical waters in Australia support a more diverse littoral copepod fauna than occurs in temperate waters on the continent. Macroinvertebrates. High species diversity of Magela Creek macroinvertebrates was reported by Marchant (1982) and Outridge (1987). The latter recorded 101 species, ‘the greatest diversity.. .yet recorded in an Australian or tropical lentic system’. Classilication by Outridge of the Magela billabongs as ‘lentic’, however, is curious, for although it therefore ostensibly permitted comparison with a series of temperate Australian lentic environments, the latter are totally unlike the lotic/lentic Magela environment. Had comparisons been made with Australian lotic systems, the con-
180 elusions would have been reversed: cf. 240 taxa identified by Arthington etal. (1982), and 337 taxa by Marchant et al. (1984). More intensive collection efforts are represented in the latter study, but the inference is that temperate macroinvertebrate species diversity is greater than that of the reported tropical studies. Unpublished work on some River Murray billabongs, where 240 macroinvertebrate taxa have been identified to date (T. J. Hillman, personal communication) indicates species richness considerable greater than recorded from tropical billabongs. On the other hand, species richness in truly lotic tropical fresh waters may well be as high if not higher than it is in temperate streams. Thus, Pearson & Smith (1986) in a careful study of the diversity and abundance of the macroinvertebrates of Yuccabine Creek, a tropical rainforest stream in north-eastern Queensland, found that the total number of macroinvertebrate taxa from one site was high compared to temperate streams in Australia (and even when compared to tropical streams outside Australia). The mean number of species found by them per sample was 105 (with a range 85-134), and the total number of species recorded from a single 50 m stretch of riffle was 267. These findings are generally in accord with the earlier conclusions of Stout & Vandermeer (1975) that tropical streams have greater faunal diversity than temperate counterparts. Nevertheless, not all tropical streams, it might be added, have high number of species: one of us (W. D. W.) remains impressed with the poverty of species in a small stream in Kimberleys region of northern Western Australia (Williams, 1979). Fish and other aquatic vertebrates.
The freshwater fish fauna of tropical Australia is greatly impoverished vis-a-vis the fish faunas of other continents (cf. Lowe-McConnell, 1987), but in general more species are known from northern Australia than elsewhere on the continent (Fig. 1). This relationship, also holds when species richness in individual tropical localities is compared with that in temperate ones, as can now be done following investigations of several northern rivers (Taylor, 1964; Jeffree & Williams,
1975; Allen & Hoese, 1980; Beumer, 1980; Bishop et al. ; 198 1). Bishop & Forbes (in press) concluded from such a comparison that fish diversities - relative to areas of drainage basin indeed seem to be higher in northern Australian coastal rivers than in southern temperate analogues. They also concluded that fish diversities in northern Australian coastal rivers appear not to be lower than in similar floodplain rivers elsewhere. These conclusions run counter to expectations and require explanation. In our view, explanations probably involve three facts. First, agreeing with Bishop & Forbes (in press), the widely accepted notion that the freshwater fish fauna of Australia is depauperate vis-a-vis other continents is strictly not applicable to northern Australian freshwater fish faunas. Second, the low numbers of fish species in southern Australian rivers reflect the severe effects of recurring periods of climatic aridity (cf. McDowall, 1981). Third, there are many long, low-gradient rivers in northern Australia which could provide suitable avenues for estuarine fish to penetrate and adapt to inland conditions. For fish at least, it seems reasonable to conclude that species richness is more strongly determined by factors other than those directly related to latitude. Much less can be said about other aquatic vertebrates, particularly with regard to comparisons involving communities at particular sites. On a broad biogeographic basis, however, it can be said that, despite geographical discrepancies in research effort, amphibians and reptiles are known to be more diverse in tropical than temperate Australia. The extent to which this is true of waterbirds is less certain. Within the Northern Territory at least, there is certainly a very close relationship between the number of frog species and latitude (and thus rainfall) (M. J. Tyler, personal communication).
Conclusions The data base from which to draw conclusions concerning the relative species richness of
181
y/h 7.7
0 Fig. 1. Distribution
of freshwater
40-49
species:
20-29
species;
lo-19
species.
fish in Australia according to major drainage areas and number of species in each a Redrawn after Williams (1983).
Australian tropical waters is thin. Even so, the available information does suggest that in Australia too, tropical species richness is generally depressed in zooplankton and littoral microfaunal communities. However, this does not seem to be so for macroinvertebrate communities in typical streams and for tropical freshwater fish communities (and perhaps also for amphibian and reptile communities); for these communities, species richness is either higher in tropical fresh waters of Australia or not noticeably lower than in the temperate part of the continent. The situa-
tion with regard to phytoplankton and macrophyte communities remains indeterminate. Clearly, many questions of both a descriptive and determinative sort remain concerning the relative richness of the tropical freshwater biota of Australia. Resolution will undoubtedly be aided by consideration of limnological studies in other tropical areas; equally, attempts to resolve them have the potential to add substantially to our understanding of wider limnological and ecological questions. Of one thing, however, we can be quite certain: simple answers are quite unlikely
182 (pace Begon et al., 1986; see also Dixon et al.,
1987), as the paper already suggests.As Nix (1982)notes: ‘the distribution of organisms and populations in space and time is a complex function of the coupling of ecophysiological responsesand environment’.It would be surprising indeedif suchcorrelationsof the aquaticbiota and latitude were as uncomplicated as perhaps suggestedby Payne (1986) and Lewis (1987). Ridpath (1985), who considered latitude and speciesrichnessof the (mostly) terrestial biota of Australia, cameto much the sameconclusion.In his view, and ours, many determinantsof species richnessother than distancefrom the equatorare likely to be important in Australia, particularly rainfall patterns and the seasonality of plant growth. Acknowledgements
We thank Dr Terry Hillman, The Murray-Darling Freshwater Research Centre, for unpublished information. Dr Peter Kilham, University of Michigan, is thankedfor drawing our attention to someearlyliterature. Oneof us (WDW) was supported by the Australian Water ResearchAdvisory Council (AWRAC) during the period this paper was prepared.Field work in the Alligator Rivers Region,N.T., (RJS) was courtesyof Pancontinental Mining. References Allen, G. R. & D. F. Hoese, 1980. A collection of fishes from the Jardine River, Cape York Peninsula, Australia. J. R. Sot. West. Aust. 63: 53-61. Arthington, A., D. L. Conrick, D. W. Connell & P. M. Outridge, 1982. The ecology of a polluted urban creek. Aust. Water Res. Council Tech. Pap. 68. Aston, H. I., 1973. Aquatic Plants in Australia. Melbourne Univ. Press, Melbourne. Beadle, N. C. W., 1981. The Vegetation of Australia. Cambridge University Press, Cambridge. Begon, M., J. L. Harper & C. R. Townsend, 1986. Ecology: Individuals, Populations and Communities. Blackwell Scientific Publications, Oxford. Beumer, J. P., 1980. Hydrology and fish diversity in a north Queensland tropical stream. Aust. J. Ecol. 5: 159-186.
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