Using an age-structured population model to define management requirements for conservation of egrets in the Murray-Darling Basin, Australia.

Major modifications to river systems and their associated floodplains throughout the world have detrimental consequences for biodiversity. A reduction in the frequency of large-scale flooding events is one change that could have significant negative effects. In the Murray-Darling Basin of Australia many species of waterbirds breed in large colonies on inland floodplain systems when appropriate flooding occurs. Across the Murray-Darling Basin populations of many of these species are now at very low levels and breeding events occur much more infrequently than they did historically. I used an age-structured population model of a generic egret to assess how frequently breeding must occur to maintain populations. Using simulation I explored how sensitive results were to demographic parameter values. The results indicated that: (1) most egrets must breed every one to two years for populations to persist; and (2) the results are sensitive to adult survival, suggesting that management may also need to consider habitat availability between breeding events to ensure high adult survival. The results highlight a need to determine over what spatial scale egret populations function so that a sufficient frequency of breeding events is provided at the appropriate scale. Results were less sensitive to juvenile survival, but juvenile survival rates that average between 25 and 35% per year are probably required for persistence. Hence it is important that future research also determines whether the recent practice of managing breeding events only until fledging is achieved does not have a detrimental effect on juvenile survival. Maintaining a high frequency of flooding events that lead to successful breeding will be necessary for any species with similar life-histories. [ABSTRACT FROM AUTHOR]