Infection by a native parasitic plant had strong negative effects on the physiology and above-ground biomass allocation of an introduced species and was correlated with increased plant mortality. The greater impact of the parasite on the introduced host may be due to either the greater resources that this host provides or increased resistance to infection by the native host. This disparity of effects between introduced host and native host indicates the potential for Cassytha to be exploited as a control tool.

Summary 1.Plants that grow beneath trees in arid systems may frequently experience both water and light limitation, although protection from high radiation loads during drought may compensate for a loss of productivity due to reduced light availability when water is plentiful. 2. We examined the effects of shading, during an imposed water deficit, on the carbon gain, stomatal conductance ( g s ) and shoot water potential ( Ψ s ) of seedlings of three shrubs: Atriplex vesicaria (Heward ex Benth.), a C 4 species, and Enchylaena tomentosa (R. Br.) and Rhagodia spinescens (R. Br.), which are restricted to shaded sites beneath trees. 3. Under conditions of limiting water, photosynthetic rates measured at saturating light ( A max ) were negative in high-light grown Enchylaena plants but remained positive in shade-grown plants. When water was not limiting, A max was reduced in shade-grown Atriplex but shade did not affect carbon gain in the other two species. 4. Atriplex Ψ s was higher in shaded than in unshaded plants, but in unshaded plants positive carbon gain was maintained at Ψ s below − 10 MPa. Stomatal conductance and A max decreased more slowly with increasing water deficit in shaded conditions in all species. 5. Atriplex was tolerant of a broader range of light and soil moisture conditions than Enchylaena , with Rhagodia intermediate between these two species. The interactive effect between drought and shade and the ecophysiological tolerances of these three species have consequences for their field distributions.

Infection with Cassytha pubescens R.Br, an Australian native hemiparasitic plant, can lead to death of the invasive shrub, Cytisus scoparius L. Link (Scotch broom). We examined the influence of C. pubescens on photosynthetic physiology of C. scoparius to determine whether this might contribute to death of infected plants. Infected C. scoparius had significantly lower photosynthetic rates, stomatal conductance and transpiration, and higher Ci (internal [CO2]), than uninfected plants. Rapid light response curves, determined using chlorophyll fluorescence, indicated significantly lower light-saturated electron transport rates and lower quantum yields for infected plants relative to uninfected plants. However, Rubisco content did not differ between infected and uninfected plants, suggesting the lower photosynthetic rates were most likely due to stomatal closure, rather than lower photosynthetic capacity. As a consequence of lower assimilation rates, PSII efficiency was lower in infected plants than uninfected plants across the diurnal cycle. Infected plants also had significantly lower pre-dawn Fv/Fm values and slower recovery from exposure to high light than uninfected plants. Our results suggest that infected C. scoparius are more susceptible to photodamage than uninfected plants. Combined with lower carbon fixation rates, this could contribute to the poor performance and even death of infected plants.

Invasive non-native plants are a major driver of native biodiversity loss, yet native biodiversity can sometimes benefit from non-native species. Depending on habitat context, even the same non-native species can have positive and negative effects on biodiversity. Blackberry (Rubus fruticosus aggregate) is a useful model organism to better understand a non-native plant with conflicting impacts on biodiversity. We used a Communicated by Pe ´ter To ¨ro ¨k.

Interactions between species pairs are almost always mediated by interactions with other species. The outcomes of these multispecies interactions are often difficult to predict and are rarely studied. In addition to their theoretical importance, multispecies interactions are also important for management situations. Where multiple agents are used to control invasive species, interactions between agents may either enhance or reduce the impacts on the target species, or may simply have additive effects. In this study, conducted in a Mediterranean-type woodland in Australia, we examined how the interaction between an invasive legume, Cytisus scoparius (Leguminosae), its pollinator, Apis mellifera and a seed predator, Bruchidius villosus (Bruchidae), are modified by a native hemiparasitic vine, Cassytha pubescens (Loranthaceae). The parasite had a direct negative effect on C. scoparius, reducing flowering by 50% and consequently fruit and seed production. Despite having fewer flowers, infected plants had the same proportion of 'tripped' flowers, an indirect measure of pollinator visitation, as uninfected plants; although fruit formed on infected plants it was more likely to abort prematurely. Seed predation by B. villosus was lower on parasite-infected C. scoparius plants than in uninfected plants. Although Ca. pubescens had an antagonistic effect on B. villosus, in consort, the two agents reduced overall seed production by an average of 62%. The acquired parasite Ca. pubescens was more effective in reducing reproductive output than the introduced seed predator, B. villosus, and shows potential as a biocontrol agent for C. scoparius. We documented a subadditive effect of two biological enemies on the invasive species, where the acquired parasite had a stronger effect than the introduced seed predator.