10 - Ecological relationships

Summary

This chapter briefly summarises the findings presented in the previous chapters and attempts a synthesis.

Weather

The weather at the Menindee sites is a sample of that of the Australian sheep rangelands as a whole. Annual rainfall over 100 years has averaged 236 mm with a standard deviation among years of 107 mm, giving a coefficient of variation (CV) of 45%. Serial correlation of rainfall between successive years is very weak at r = 0.13.

Whereas daily temperature has a marked annual cycle it is not paralleled by a similar seasonality of rainfall. The serial correlation between rainfall in the same season across successive years was estimated as r = 0.05. Hence rainfall is unpredictable across a year and unpredictable from one year to the next. In three of ten years the annual rainfall is more than fifty percent above or below the annual average. Floods and droughts are common.

Plants

The high variability of rainfall leads to a much higher proportion of annuals in the pasture than on any other continent. The dominant perennials are low shrubs with deep roots. The pasture layer waxes and wanes according to rainfall. One suite of predominantly annual species germinates with rain in winter. Another suite responds to summer rain.

We saw in Chapter 4 that the pasture biomass could be predicted from rainfall over the previous six months. The relationship was largely independent of time of year and influenced only weakly by soil type.