Tanya
Mason investigated the impacts of bitou invasion on dune
vegetation as part of her PhD research. Specifically, she examined
effects on vegetation structure, the richness of both native and
exotic plants and variability in fore and hind dune plant communities.
By measuring the presence and relative abundance of species within
a site, Tanya found that bitou bush differs in its impact on fore
and hind dune zones. In fore dune shrublands, there was significantly
reduced species richness of climber, graminoid (grass-like) and
herb growth forms following bitou bush invasion. However, there
was no difference in species richness of shrubs. In forested hind
dunes, the shrubs did have significantly reduced species richness
following bitou invasion. Density of the vegetation increased at
the shrub level in both fore and hind dune bitou-invaded, relative
to non-invaded communities. In the fore dunes, the density at the
ground declined at bitou-invaded sites compared with non-invaded
sites reflecting the reductions in herb and graminoid species.
In hind dunes, canopys were more open in invaded habitats.
It is clear that the effects of bitou invasion differ between
the fore and hind dune zones of coastal vegetation. This is likely
to be influenced by the differences that already exist naturally
between these two zones. For example, the diversity of fore dune
shrub vegetation is naturally lower than that of the hind dune.
Therefore, the effects of invasion (as described above) do not
affect diversity because there are fewer species to be replaced
by the invader.
Emile Ens has investigated the impact of invasion on physiological
stress levels and reproductive output in four native plants; Monotoca
elliptica, Corea alba and Lomandra longifoli as part of her PhD.
She measured physiological stress in terms of photosynthetic yield
and counted reproductive and vegetative buds on plants throughout
a year. While summer is the period of least stress in northern
hemisphere plants, our results show that summer is the most stressful
season for native plants on coastal dunes with plants showing maximum
yield in winter. However, we found no differences in stress levels
between plants in invaded versus uninvaded plants. For most plants
there were no differences in the numbers of reproductive buds or
vegetative buds, however there was evidence of higher variability
in reproductive output in some species.
One important aspect of Emilie’s work is the identification
of a difference in the population structure of the species
she studied. For all 3 species, there were few new recruits in
the
invaded sites, indicating that weed invasion is interrupting
population growth. Furthermore, this highlights the fact that
competition
is likely to be most important as the seedling stages.
The impact of invasion on soil seedbanks has been investigated
for hind dunes and is currently being investigated in foredunes.
The results are included on the Impacts
on Seedbanks page.
Implications for management
If the impacts of invasion differ between the fore and hind
dune, it is possible that the resilience and recovery of these
communities
will also differ. For these reasons it may be necessary to
alter the management of bitou bush according to the habitat it
has
invaded. We found little evidence for a serious impact on
adult growth and reproduction. Instead our results have highlighted
that the changes in communities are likely to be occurring
at
the seedling establishment phase.
The impacts of current
bitou control methods are discussed further in the management
section of the webpage
Affiliated publications
Ens, E and French, K (2008) Exotic woody invader limits the
recruitment of three indigenous plant species Biological Conservation141,
590-595
Mason T. J., French K. and Russell K.G. (2007). Moderate impacts
of plant invasion and management regimes in coastal hind dune
seed banks. Biological Conservation 134: 428-439.
Mason T. J., French K. (2007) Management regimes for a plant
invader differentially impact resident communities. Biological
Conservation 136, 246-259.
