The effects of flooding and livestock on post‐dispersal seed predation in river red gum habitats

Abstract

SummaryRates of seed predation are influenced by conditions that alter seed supply and the activity of seed predators. In southern Australia the potential seed supply for the dominant floodplain tree species, the river red gumEucalyptus camaldulensis, has been reduced through forest clearing to support grazing by introduced livestock. River regulation and water extraction have reduced the frequency of flooding and thus the conditions that promote seed germination on floodplains. To determine if poor recruitment of river red gums could be caused by low seed supply, as a result of post‐dispersal seed predation, we used field experiments and observations to investigate how post‐dispersal predation on seeds ofE. camaldulensiswas affected by flooding, livestock management and their interaction.Seed predation was measured before and after different flood treatments (0·5 m depth; short flood of 24 h, long flood of 30 days). Flooding of this kind (return frequency of once per year) did not have any significant effect on rates of seed removal by seed predators.Rates of seed predation in floodplain habitats under widespread livestock management regimes changed seasonally. In all seasons seed predation was lowest at sites grazed by sheep. In winter seed predation was highest at ungrazed sites. In spring and summer seed predation was highest at sites grazed by cattle. Ant communities differed between forested and cleared habitats and seed‐eating ant species were most abundant in cleared sites grazed by cattle.Rates of seed predation in forested floodplain sites with different flood histories differed among sites with different livestock management histories. The impact of cattle exclusion on seed predation rates increased as the period since flooding increased.Cattle grazing is widespread on the floodplains of rivers across the southern Murray‐Darling Basin, and tree densities and hence seed supplies are low. In this situation small floods may not result in significant recruitment to river red gum populations because seed predation may reduce seed supply before and following flooding. Decreases in the frequency of flooding owing to river regulation and water extraction are likely to have exacerbated the influence of livestock on seed supply and thus reduced potential recruitment even further.Efforts to rehabilitate large floodplain rivers based solely on the return of more natural flow regimes may fail if the effects of factors such as livestock grazing are not managed concurrently.