Tanya Mason
Year: 2006
Degree: PhD
Abstract
Plant invasions of natural communities threaten biodiversity and ecosystem
processes across many biomes and trophic levels. Understanding and
managing invader impacts are therefore significant steps in achieving conservation.
Both causes and management of invasion are dependent on human behaviour
and ecologists must consider this human dimension in developing management
protocols. While control of invaders is routine in many conservation
reserves, assessment of ecological outcomes following control is rarely
enacted. Management is itself a disturbance and the compounding effects
of both invasion and management on biodiversity merit rigorous assessment.
While species-level responses to plant invasion have been widely reported,
functional group and seed bank responses to invasion and management
are
rarely investigated. Generalisations about invader impacts are urgently
required for targeted restoration. In this thesis, I used qualitative
review, field study and meta-analysis to address ecological, social
and land management issues of invasion biology.
Firstly, I reviewed policy instruments and asked: are government, land
managers and the scientific community using available social levers
to achieve optimal
invader management? I found that while important policy principles have
been developed, they require greater enactment. Release of significant
funds will
be required to resource long-term invader control and restoration activities.
Education and voluntary incentives require a strong extension service to
maintain involvement and standards of control. Rigorous monitoring
systems are an under-utilised
but important tool in achieving adaptive invader management.
I then used the invasive shrub bitou bush, Chrysanthemoides monilifera ssp. rotundata, to analyse invasion and management impacts in coastal
dune communities
of eastern Australia. I compared the effects of intensive, manually-based
invader control with those of extensive control (mainly herbicide spraying
from aircraft)
across fore dune communities. I examined one management regime, which
was typically intensive, in hind dune communities. My main aim was to test
the prediction
that intensive methods would achieve better biodiversity outcomes than
extensive approaches in fore dune communities. I found that extensive
management
created
native species complements that diverged from non-invaded sites while
intensive management approached non-invaded site conditions. However, intensively
managed sites were also richer in other exotic species than extensively
managed sites.
Thus, while biodiversity outcomes were better under intensive management,
the risk profile of such sites was increased by the greater array of
exotics
that
could potentially replace the original invader. The positive effects
of management evident on some fore dunes were not found in hind dunes,
where
managed sites
had the highest exotic species richness and similar bitou bush seedling
abundance as both non-invaded and bitou-invaded sites. Hind dune managed
sites were
also compositionally distinct from non-invaded sites. For all sites,
even after
intensive management, follow up control is required to avoid substitution
of the primary invader by other aggressive invaders.
In order to characterise invader impacts beyond species-level comparisons,
I compared bitou bush impacts on vegetation structure, richness of
both native and exotic growth forms and community variability in fore and
hind dunes.
I found that bitou bush impacts were context specific: in fore dune
shrublands,
functionally distinct graminoid, herb and climber rather than shrub
growth forms had significantly reduced species richness following bitou
bush
invasion. However, in forested hind dunes, the functionally similar
native shrub
growth form had significantly reduced species richness following bitou
invasion.
Density
of vegetation structure increased at the shrub level in both fore and
hind dune bitou-invaded, relative to non-invaded communities. Fore
dune ground-level
vegetation density declined at bitou-invaded sites compared with non-invaded
sites reflecting significant reductions in herb and graminoid species
richness. Hind dune canopy-level vegetation density was reduced at
bitou-invaded compared with non-invaded sites. Bitou bush invasion also
affected fore
dune community
variability with significant increases in variability of species abundances
observed in bitou-invaded compared with non-invaded sites. In contrast,
there was similar variability among all hind dune sites. The results
suggest
that
effects of bitou bush invasion are mediated by the vegetation community.
In addition to standing vegetation dynamics, I investigated soil-stored
seed banks affected by bitou bush invasion and management, to further
elucidate levels of community resilience. While management of bitou
bush may have
reduced
the density of bitou bush seeds in the soil, it did not reduce the
richness of other exotic species. Native tree seed richness was significantly
higher in seed banks of sparsely-invaded than either heavily-invaded
or managed
hind dune sites, perhaps indicating a permanent shift in community
structure
following
invasion. However, remaining indices of native seed bank diversity
were similar across all invasion categories, indicating that seed
banks of
many native
species were unaffected by both invasion and management. While examination
of seed
banks is informative in assessing past and potential community dynamics,
low similarity between the standing vegetation and seed bank at all
sites indicated
that many hind dune species had other storage or regeneration modes
and seed banks cannot be relied upon for comprehensive dune restoration.
Finally, I used meta-analysis to contextualise my field studies of
bitou bush invasion using 20 international invasion studies. I
hypothesised that plant
invaders act as biotic filters and more strongly affect representation
of some plant traits over others in the resident community. I analysed
the effects
of graminoid and woody invaders on species richness for native
growth form, longevity, seed mass and flowering phenology traits. I found
that
graminoid
invaders disproportionately affected species with graminoid and
herbaceous
growth forms, perennial strategies and small seed masses. Woody
invaders disproportionately
affected graminoid or shrub species and perennials. Woody invaders
had a consistently greater negative effect on native species than
graminoid invaders,
perhaps
due to pre-emption of light resources. In contrast, it made no
difference to native richness as to whether species flowered at the same
time
as
the invader,
indicating little interaction between invader and native pollination
dynamics.
Plant invaders non-randomly affect resident species in turn changing
community attributes. Current management of invaders may not
re-instate pre-invasion
conditions. Restoration is dependent on recognising the impacts
of an invader and both supplementing native traits adversely
affected by invasion
and
enacting long-term follow-up control of primary and secondary
invaders.
Affiliated Publications
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. and French, K. (2007) Management regimes for a plant invader
differentially impact resident communities. Biological Conservation 136,
246 –259.
Mason, T.J., Lonsdale, W.M. and French, K. (2005) Environmental weed control
policy in Australia: current approaches, policy limitations and future
directions. Pacific Conservation Biology 11(4), 23-45.
