Effects of disturbance on population dynamics of the threatened orchid Prasophyllum correctum D.L. Jones and implications for grassland management in south-eastern Australia

Natural and anthropogenic disturbances can have dramatic consequences for population growth, particularly for small populations of threatened plants. We analysed census data for the largest population (124 individuals) of the critically endangered orchid Prasophyllum correctum between 1992 and 2003, to identify environmental factors associated with annual changes in emergence and flowering, and to develop management prescriptions for its conservation. Fire frequency effects were analysed by comparing life stage transition matrices between plants subject to <3 year and >3 year fire intervals; climate effects were investigated using cross-correlation plots to relate total emergence, and numbers of sterile and flowering plants to rainfall, and grazing impacts were investigated by experimentally protecting plants in 1996-1998. Plants rarely emerged for more than two consecutive years or flowered for more than a single year. The total number of plants that emerged was significantly negatively correlated with autumn/winter rainfall in the previous year, perhaps due to on-going competitive effects of increased grass growth under wetter conditions. The proportion of reproductive adults was greater when fire intervals were <3 years, and a greater proportion of the population remained dormant and non-reproductive when fire intervals exceeded 3 years. Grazing had a significant negative effect on the orchid population in the first 2 years after fire. A management regime that includes frequent burning is likely to benefit conservation of the population by reducing competition from grasses, shortening dormancy periods, reducing mortality, enhancing flowering and, by implication, possibly increasing recruitment.