In terrestrial ecosystems, many plant species (ca. 1% of all plant species) have
evolved a parasitic strategy by which they acquire many (and sometimes all) of the
resources they need for growth and reproduction by tapping into the vascular
systems of neighbouring plants (i.e. hosts) using specialised organs called
haustoria. The impacts and wider dynamics of hemiparasites (parasitic plants with
photosynthetic capabilities) in natural communities are still poorly understood, but
this knowledge is imperative for conservation biology and successful natural
resource management, as well as for fundamental understandings of plant-plant
interactions and species coexistence. The aims of the present dissertation were to
uncover what is currently known about host impacts of hemiparasitic plants and to
investigate some of the major gaps in our knowledge, with focus on relevant
spatial and molecular aspects. Spatial investigations provide a means for studying
impacts of neighbouring plants in communities and molecular investigations
enable the examination of impacts and variation on an intraspecific level. In the
present dissertation, two Australian Acacia species (Acacia ligulata and A.
acuminata) were investigated in combination with three species of multi-host
hemiparasites that are capable of parasitising multiple hosts simultaneously. Two
of these hemiparasites (Santalum spicatum and Exocarpos aphyllus) attach to
their hosts belowground and the third (Cassytha melantha) attaches to its hosts
aboveground.
Locations recorded for all A. ligulata shrubs (living and dead, with or
without C. melantha) and root hemiparasitic shrubs within a 1.1-ha study site
provided the basis for different methods of spatial analysis. Evidence is found for
the root hemiparasitic shrubs to grow in areas where the density and aboveground
biomass of A. ligulata are at a medium level. This evidence, along with
similar evidence from other studies of root hemiparasitic herbs, provides the
foundation for a suggested ‘Goldilocks hypothesis’ of root hemiparasites,
according to which the parasites seem to follow a “just-right” strategy. The
hemiparasites may not only passively rely on their hosts for optimal conditions
within the vegetation, but may also actively be modulating the vegetation to their
benefit, for example by suppressing the hosts, potentially lowering the level of
above-ground competition. Such a behaviour is generally considered as a form of
ecosystem engineering. Despite not finding the root hemiparasites to negatively
affect the above-ground volume of the neighbouring A. ligulata shrubs, mortality of
the A. ligulata shrubs surrounding the root hemiparasites appeared to be higher
than what would be expected under randomness. The hemiparasitic vine (C.
melantha) appears to have a greater impact on the A. ligulata shrubs than do the
root hemiparasites. The mortality amongst (potential) hosts is higher and the
above-ground volumes of the Cassytha-infected A. ligulata are smaller than for the
A. ligulata shrubs neighbouring the root hemiparasites.
The hemiparasites may further impair reproduction of the host and impact
other trophic levels associated with the host. Traits of Acacia ligulata were
therefore investigated further in terms of seeds per seedpods, seed mass and
phyllode C/N ratios. Indications are found of a higher C/N ratio in the phyllodes of
A. ligulata parasitised by C. melantha, which in turn could result in lower
palatability for insect herbivores. Phyllode palatability was investigated for both
natural insect herbivores and an insect herbivore in a laboratory setting. The
present study is the first to report that larvae of the native moth Dasypodia
selenophora (southern old lady moth) feed on A. ligulata phyllodes. Nocturnal
observations during the primary annual feeding season, where numbers of D.
selenophora larvae and presence-absence were recorded, did not show
correlations with hemiparasite locations, A. ligulata vegetation density or A. ligulata
above-ground volume. Young A. ligulata phyllodes were preferred by D.
selenophora over mature phyllodes. Laboratory feeding experiments with larvae of
Epiphyas postvittana (light brown apple moth) using phyllodes from A. ligulata did
not result in differences of larval mass.
Finally, to further investigate the belowground-aboveground link, phyllodes
from A. acuminata were used for an RNA-sequencing (RNA-Seq) experiment to
investigate gene expression in individuals, non-parasitised or parasitised by
Santalum spicatum. Two transcriptomes were obtained, one from each of two
single shrubs growing under either of these two treatments. Indicative findings
suggested higher transcript levels of genes involved in the biosynthetic pathway of
flavonoids for the non-parasitised shrub. However, these findings require further
validation. The annotated transcriptomes together with identified markers enable
further relational and gene expression studies with either shoot or root
hemiparasites in either natural communities or plantations.