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Vertebrate Biodiversity on Australian Rice Farms
An Inventory of Species, Variation Among Farms, and Proximate Factors Explaining that Variation
by J. Sean Doody, William Osborne, David Bourne, Bronwyn Rennie, and Rachel A. Sims February 2006
RIRDC Publication No W05/198 RIRDC Project No UCA-6A
Two major factors underpinning existing biodiversity on rice farms were: productive rice bays and remnant vegetation patches. Rice bays were inhabited by large numbers of frogs, snakes, turtles, and waterbirds. We estimated that hundreds of millions of frogs of one common species, the spotted grass frog, are produced on rice farms annually, based on pitfall captures of dispersing frogs. This species probably represents the highest biomass of any vertebrate on rice farms. The importance of frogs on rice farms is further highlighted by our anecdotal finding that brownsnakes consumed frogs on rice farms, despite their documented habit of consuming mostly lizards and small mammals. We found a rapid increase in the average richness and abundance of birds on farms after the flooding of rice bays (11+ species, 140 individuals) compared to pre-flooding (7 species, 40 individuals), and a similar decrease in bird richness and abundance was associated with draining of the rice bays. Based on capture rates and bird counts in ‘wet’ and ‘dry’ habitats within farms, rice farms are likely to have a higher abundance of frogs, snakes, turtles, and birds, and higher species richness of birds, than dryland farms. Farms with remnant vegetation patches were occupied by more lizard species (on average, 6 vs. 2) and woodland specialist birds (on average, 7 vs. 2) than were farms without remnants. Lizards were also more abundant on farms with vegetation remnants. Farms with remnant vegetation patches are likely to have higher species richness of birds, frogs, lizards, snakes, and mammals (mostly ferals), and higher abundance of birds, than farms without remnant vegetation patches.
The apparent importance of vegetation remnants on birds in the general survey prompted us to study the relationships between characteristics of remnants (e.g., size, shape, condition) and bird diversity. An intensive study of 23 Black Box remnants in the Coleambally Region during 2002 indicated that (1) patch area and shape of remnants was positively related to DECREASER bird and WOODLAND bird richness, but inversely related to INCREASER bird abundance (see text for explanations of terms); (2) habitat complexity, or the condition of remnants was positively related to WOODLAND bird richness and abundance; (3) some woodland bird species known to be in decline in the region were found only in large remnants with a high area-to-perimeter ratio in relatively good condition (i.e., some understory); (4) smaller remnants generally contained bird species known to be common in the region; and (5) noisy minor abundance did not appear to influence species richness or abundance of any other bird groups.
An intensive study of the habitat utilisation, movements, and diet of longneck turtles revealed that (1) turtles exploited the seasonally-available flooded rice bays, using them as a surrogate for natural wetlands; (2) however, invertebrate prey productivity was apparently not higher in rice bays than in irrigation channels; (3) turtles used all major aquatic habitats on rice farms; (4) some turtles moved long distances between farms; (5) turtles became more selective of prey types as prey richness increased; (6) timing of movements between habitats differed between males and females; and (7) mortality and injury occurred when turtles attempted to move through dethridge wheels.
Our study provided evidence in support of management recommendations provided by others, and these recommendations can be amalgamated into two major on-farm recommendations: (1) improving existing vegetation; and (2) revegetation. Vegetation remnants and riparian vegetation are among the rarer habitats in and around rice farms in the Riverina. Accordingly, fauna associated with these habitats are less common. Other recommendations include (3) conserving existing vegetation, in the context of any expansion of the industry; (4) maintaining biodiversity in rice bays. Finally, other studies support some additional recommendations that we did not address in the present studies, but need to be integrated. These include (5) avoiding over-grazing of areas with remnant vegetation; (6) avoid removing fallen timber, bark, rocks, and leaves from areas with remnant vegetation; and (7) improving and revegetating riparian areas.
We suggest that the best conservation strategy to protect, maintain, or enhance biodiversity on rice farms should include the following three approaches: First, a monitoring program should be developed and initiated, whereby populations of selected species are sampled every few years.
Second, an experimental approach improving rare habitats (e.g., planting or enhancing an understory in vegetation remnants) would provide a low-risk conservation measure for key species.
Third, ecological research into selected species (e.g., species occupying rarer habitats) would provide critical information on habitat utilisation on farms, thereby facilitating their monitoring and management. The latter should include threatened species to uphold the goal of maintaining current levels of biodiversity.
Specific recommendations stemming from the ‘birds and remnants’ study include (1) maintaining, setting aside, or revegetating remnant vegetation patches > 10 ha (however, patches > 50 ha are best, and any patch is better than none); and (2) improving habitat complexity (e.g., understory structure) in remnant vegetation patches. Specific recommendations from the ‘turtles in rice’ study include (1) general revegetation or plantings as refuges for turtles traveling overland; (2) avoiding the use of fences that form barriers to overland movement of turtles; and (3) research to quantify the impact of dethridge wheels on mortality and injury of turtles.
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