Continuous and episodic components of demographic change in arid zone shrubs: models of two Eremophila species from Western Australia compared with published data on other species

1 Time-varying matrix models were constructed for the population dynamics of two species of arid zone shrub (Eremophila maitlandii and E. forrestii) using parameters estimated from a long-term sheep grazing study (1983-93). 2 The models were used to assess the relative contributions of episodic versus continuous mortality and recruitment processes to total population change over 48-year periods. In 'baseline' models that used best estimates of return times for droughts and wet episodes, 15-40% of total deaths and 50-70% of total recruitment occurred during episodes. In none of the models, using a range of event return times, did event-driven mortality exceed 60% or event-driven recruitment exceed 85%. 3 Because these conclusions could not easily be accommodated within the current event-driven world view, we compiled other published quantifications of arid zone shrub dynamics. For long-lived species, drought episodes have not generally been found to cause population decline of more than about 20%. There are no documented examples of recurring recruitment episodes. Single recruitment episodes reported have been highly variable in magnitude and often patchy in space. 4 Continuous processes of recruitment and mortality have been recorded as background in most arid zone shrub populations, but the contribution of this low level change to overall dynamics is often perceived as small in comparison to episodic events. We conclude that population dynamics of arid zone shrubs should be perceived as driven both by continuous processes and by episodes of more substantial mortality or recruitment.