Close
Compiled from notes by S. Brooks, G. Boxshall, G. Kattel and R. Flower
Aquatic invertebrates are an essential component of any aquatic ecosystem. The inland waters of the Falkland Islands do support an invertebrate fauna but it is rather impoverished in terms of both diversity and absolute abundance. However, the 2001 survey focussed on sampling a geographical region of lake and pond types and it may well be that some sites, relatively rich in aquatic invertebrates, were missed. Although quantitative surveys were not undertaken, sampling indicates that the Halfway House stream in Lafonia possessed the most invertebrates with an abundance of freshwater gasteropods, caddis larvae and gammarids. Small fish, the native 鈥榯rout鈥� (Aplochiton zebra) were also common at this site.
Compared with good trout streams in the UK, the Falkland Island aquatic invertebrate fauna is remarkably poor with Ephemoptera, Odonata and Heteroptera all being probably absent. Some absences could be accounted for by the lack of limestone area (e.g. Ostracoda), however, absences, such as the large-winged dragonflies (Odonata), may be related to the persistent strong winds. The heteropterans, and most notably the Corixidae (water bugs), seem entirely absent in the Falkland Islands yet this group is a common representative of acid upland UK waters (Macan 1963). Corixid adults do fly but it is summarized that high winds are unlikely to completely suppress this group and absence could be caused by poor dispersal mechanisms and the geographical isolation of the Falkland Islands.
Regarding food resources for birds and fish, only the gammarids, chironomids and cladocerans (and perhaps trichopterans and copepods) are likely to be important in this respect. Aquatic macroinvertebrates such as the gammarids are generally scarce in most of the sites surveyed but are likely to constitute a useful food resource in some locations. Similarly, the copepods, were sparse in the turbid lowlands lakes and in the upland tarns. The presence of large predatory copepods at several sites indicates a lack of fish predation. Cladocerans and, at a few sites, ostracods could well constitute a food resource for fish. Fish usually predate larger cladocerans and it is well known that a switch from听Daphnia听迟辞听Bosmina听can indicate increasing fish predation (Brooks & Dodson 1965). We know nothing about past cladoceran abundances at these sites but听Daphnia听was most common at Goose Pond and Half Way House stream, and small fish were abundant at the latter site. On the other hand,听Bosmina听spp. were particularly common at Lake Arthur and Sand Pass Pond and it is very unlikely that the latter small acid site has a fish population. We think that local physicochemical conditions (water- level fluctuations, often turbid wind-stressed conditions, and flushing of the smaller sites) probably exert more influences on cladoceran species distributions than do biological factors.
Overall, the aquatic invertebrate fauna at some lowland Falkland Island freshwater sites probably does help sustain fish populations (McDowell听et al.听2001) and encourage waterfowl (Weller 1972). There is no evidence that the low abundance and diversity of aquatic invertebrates results from anything other than isolation, low nutrients and generally harsh environmental conditions. This fauna is perhaps more interesting from a biogeographical aspect. Many taxa show regionally limited distributions and the copepods appear to be represented entirely by species limited to south American and Antarctic habitats. On the other hand, the cladocerans seem to be mainly cosmopolitan types. This indicates that taxonomic discrimination of the cladoceran taxa was not carried out sufficiently to recognize regional endemics, or that this group is generally better at dispersal and colonization of new environments than copepods.
The only endemic aquatic invertebrate genus so far found in the Falkland Islands inland waters is within the Amphipoda. The gammarid genus听Prefalklandiella听is probably the most notable feature of the Islands鈥� benthic invertebrate fauna. The two representative species appear to have penetrated freshwaters on the Falkland Islands following recent speciation from marine crangonyctid relatives (cf. Stock & Platvoet 1991). Doubtless, these unusual taxa are adjusted to oligotrophic waters and would be threatened by any significant water pollution problems.
References
- Brooks JL & Dodson SI (1965) Predation, body size and composition of plankton.. Science 150: 28-35.
- Macan TT (1963) Freshwater Ecology. Longmans, London. 338pp.
- McDowell RM, Allibone RM and Chadderton WL (2001) Issues for the conservation and management of Falkland Islands freshwater fishes. Aquatic Conservation: Marine and Freshwater Ecosystems 11: 473-486.
- Stock JH & Platvoet D (1991) Freshwater amphipods of the Falkland Islands. J. Nat. Hist. 25: 1469-1491.
- Weller MW (1972) Ecological studies of Falkland Islands鈥� waterfowl. Wildfowl 23: 25-44.
听