It has become customary to regard the impact of humans upon the environment as entirely deleterious. In Australia, documentation of the frequency of extinctions, particularly among birds and marsupials (Ovington 1978; Kennedy 1990), has led to a sense of shame and apologia for events that have taken place progressively over the last 200 years.

Concern about the global decline of amphibian populations, summarised by Phillips (1994), has not addressed causal factors other than in a superficial way. An exception is the Action Plan for Australian Frogs in which numerous contributors were provided with an opportunity for input (Tyler 1997). The media has given considerable exposure to the potential deleterious effects of chemical compounds applied as sprays to control insects and plants, and to any other possible causal factors of anthropogenic origin.

We wish to highlight evidence of current human activity which is enhancing the abundance of some frog populations. We emphasise that we are in no way underestimating or ignoring the serious plight of many species of frogs.

Within Australia, three frog species can be considered commensals: the arboreal frogs Litoria caerulea, L. rothii and L. rubella. All exist within the northern half of the continent, and live within or upon dwellings and associated buildings. They benefit from obtaining shelter, shade, water and food.

Litoria caerulea is the best known commensal, found commonly within houses (Tyler 1979). At high latitudes it is often found beneath the rim of toilet pedestals. In June 1994, Michael Tyler and David Williams visited Nutwood Downs Station, approximately 100 kilometres east of Daly Waters, Northern Territory. Frogs commonly can be found there (at the homestead) in and around toilets. In one toilet cistern we found 33 adult L. caerulea and one adult L. rothii. A further two L. caerulea were floating in the toilet bowl.

On the same date we observed Litoria rothii in the ablution block at the same locality. Within this small building we counted more than 100 adults plus two L. caerulea. The frogs were all on the walls and more than one and a half metres above the ground. The majority were higher than two metres. Litoria rothii was the most abundant species on the homestead site. We estimated that at least 300 individuals were within or directly outside the buildings.

There are numerous examples in Australia of the creation of new breeding sites for frogs. For example, within the Northern Territory, the majority are the shallow quarries (borrow pits, gravel scrapes) adjacent to major highways. They are approximately 0.5 metre deep in the wet season but dry during the rest of the year (Tyler 1979). In the absence of predatory fish, the frogs flourish.

A further example exists in the Kakadu National Park at Retention Pond No. 2 (RP2) of the Energy Resources of Australia Ranger uranium mine site at Jabiru. Prior to its excavation there were no frogs there or in the surrounding area, and no breeding sites (Tyler, pers. obs. 1974). Following the excavation and filling of the retention ponds, frogs migrated to the area, and the aquatic species Litoria dahlii became established there. There it has become extremely abundant at the periphery, and at times the population numbers involve many thousands of frogs.

In temperate south-eastern Australia there has been extensive habitat modification since European settlement, primarily as a result of agricultural development. There is no doubtthese changes have resulted in loss or substantial change to many aquatic habitats, particularly ephemeral wetlands. Historical reductions in the areas of freshwater meadows and shallow freshwater marshes in Victoria have been 33% (102.69 km²) and 55% (436.99 km²) respectively (Anon. 1988). These, presumably, were significant habitats for some species of anurans. There has been a substantial increase in the numbers of farm dams. These artificial small freshwater habitats provide breeding sites for three widespread and abundant species: Limnodynastes tasmaniensis, Litoria ewingii and Ranidella signifera (Tyler, pers. obs). Victorian 1:25,000 Map Series sheets were selected so that one map was obtained from each of the eleven statistical subdivisions of the State. Statistics for areal extent of farmed land are available for these subdivisions (Anon. 1994). They approximate the biophysical regions found across the area. By choosing ten grid squares (area = 1 km²) at random from each map and counting the number of farm dams in each square, an estimate of the number of farm dams on agricultural land was obtained. These estimates are shown in the table.

Table. Estimated incidence of artificial dams in Victoria

Statistical Division	Farmland (km2)	Average number of dams/km2
Melbourne	2030	2.8
Barwon	4290	7.1
Western District	17,160	2.3
Central Highlands	7650	3.6
Wimmera	20,960	2.8
Mallee	25,950	1
Loddon/Campaspe	12,850	1.4
Goulburn	12,290	3.1
Ovens/Murray	6350	5.7
East Gippsland	8400	4.2
Gippsland	5810	3.9

A large number of small artificial habitats have been produced as a result of human modification of the environment. Whereas these sites are only suitable for some species of anurans, these habitats have undoubtedly had a positive and profound effect on the population size and, most likely, the distribution of those species able to use the sites.

Frogs are not the only animals to benefit from dam construction. Kingsford (1992) has reviewed the evidence that maned ducks (Chenonetta jubata) have benefited from human-modified habitats. The number of ducks on any property is directly linked to the number of dams. Larger artificial water bodies constructed as evaporation basins for the disposal of ground water have similarly become significant habitats for waterbirds and waders (Roberts 1995).

There is no doubt that throughout the world many frog species are experiencing serious declines attributable to direct or indirect human impact. It is therefore important to acknowledge beneficial contributions through habitat enhancement (albeit unintended) when that occurs.

Acknowledgments
We thank the Australian Research Committee and Mount Isa Mines (Holdings) Pty Ltd for funding that provided the opportunity for these observations, incidental to our principal objectives, to be made.

References
Anon. 1988, State of the Environment Report 1988 Victoria's Inland Waters, Office of the Commissioner for the Environment, Melbourne.

Anon. 1994, Victorian Year Book, Australian Bureau of Statistics, Melbourne.

Kennedy, M. (ed.) 1990, Australia's endangered species, The extinction dilemma, Simon & Schuster, Brookvale, Australia.

Kingsford, R.T. 1992, 'Maned ducks and farm dams: a success story', Emu, 92,163-169.

Ovington, D. 1978, Australian endangered species, Mammals, birds and reptiles, Cassell, Stanmore, Australia.

Phillips, K. 1994, Tracking the vanishing frogs, St Martin's Press, New York.

Roberts, J. 1995, 'Evaporation basins are wetlands', Aust. J. Environ. Mgt, 2,7-18.

Tyler, M.J. 1979. 'The impact of European man upon Australasian amphibians', in The status of endangered Australasian wildlife, ed. M.J. Tyler, Royal Zoological Society of South Australia, Adelaide.

Tyler, M.J. 1997, 'The Action Plan for Australian Frogs', Wildlife Australia.