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Explora: Environment
and Resource Data should determine biocontrol success

seed production before the agents were released. This Besides few data being available for many aspects of the
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is based on these plants having persistent seed banks general biology and ecology of invasive Australian Acacia
with seeds being able to remain dormant in the soil for spp., the determination of further control of these plants is
over 50 years. There is, however, a misconception in the exacerbated by different methods having been used over
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interpretation, as it is often implied that the seed banks of time to investigate the same factors relating to these plant
invasive Australian Acacia spp. in their entirety (i.e., all invasions in South Africa. For example, distributional data
seeds in the soils) are persistent in the long term. However, on these plant invasions have been kept since the 1990s but
seed bank data suggest otherwise, with the seed banks of have not consistently been reported in the same format over
three evaluated Australian Acacia spp. being short-term time. 30,34,35 Furthermore, the distributional data for the same
(<5 years) and long-term (>5 years) persistent. Most of plants have not been indicated in subsequent publications,
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the seeds in the seed banks of invasive Australian Acacia making assessments of the increase or decline of these
spp. are short-term persistent, and only a small proportion plant distributions in South Africa problematic. Another
are long-term persistent. 16,54,56,62,86,90,91 Milton and Hall example is Impson et al. not expressing average seed bank
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and Holmes et al. also demonstrated that seed banks of size as previously reported by authors (i.e., Table 3) but as
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invasive Australian Acacia spp. decline with 80% lost within box-and-whisker plots.
4 – 6 years of populations having been cleared through
felling and with more than 94% lost within 7 – 8 years after Besides considering the distribution and seed bank
felling followed by burning. The recorded seed banks after dynamics of invasive Australian Acacia spp., data on
the release of their biocontrol agents are, therefore, not a population changes can also be considered over time. For
consequence of seed production before their release but example, the area covered by a population at a stand scale
rather in their presence. This was further demonstrated over time can be assessed to ascertain whether biocontrol
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by the pod production and seed rain of Australian Acacia agents have caused a decline in the extent of invasion.
spp. (A. longifolia, A. pycnantha, and A. saligna) being 131 This assessment of biocontrol agent effectiveness was
– 437 pods m and 307 – 1942 seeds m , respectively, even used to determine the impact of Uromycladium morrisii
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when 45 – 444 pods m and 5.6 – 41.9% of seeds were lost (Pucciniales) (Figure 2) (formerly Uromycladium
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respectively to their seed-reducing biocontrol agents. 14,15 tepperianum) on A. saligna populations in South Africa. 29,69
Consequently, there is no quantitative data to support Morris, and Wood and Morris have argued that U.
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the notion that there is a paucity in the decline in the morrisii has caused a decline in the area occupied by A.
seed banks after the release of their associated biocontrol saligna on a population level. This view is based on the
agents due to a seed dormancy mechanism. Seed bank data observation that the gall-rust fungus has caused a decline
recorded after the release of the biocontrol agents indicates in the number of trees within fixed plots. However, the
that invasive Australian Acacia spp. populations are able to area covered by these trees was never quantified, and
maintain their area of occupancy with potentially the same consequently, these results could be due to self-thinning
number of trees having to be managed over time. 13 (Figure 3A). Strydom et al. 13,75 conducted experiments at
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This conclusion is further supported by seed bank and the same localities as Morris and Wood and Morris,
seedling density data recorded after fire events (Table 3). and suggested that the study populations should be
Invasive Australian Acacia spp. populations are exposed to monospecific and should have a near-close to closed
frequent natural and increasingly frequent human-induced canopy cover. Consequently, even if U. morrisii causes
fires. During these events, seed banks are often reduced the death of A. saligna, this will not cause a decline in the
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to a fraction of the pre-fire seed bank size with post-fire area of occupancy but will cause populations to be more
Acacia seed banks of 6 – 100 seeds m being able to recruit variable in age, tree size, and tree density (Figure 3B).
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2 – 88 seedlings m (Table 3). These seedling densities This has, therefore, potentially introduced another level of
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are enough to form stands with a complete canopy cover. management complexity as these biocontrol agents may
For example, Strydom et al. worked in mature stands maintain seed banks of A. saligna at optimal levels for these
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with closed canopies where within tree densities of stands plants to recover after disturbance events. 13,14 For example,
were often found to be 0.14 trees m in older parts of the where trees within populations would be of similar age and,
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population. The high prevalence of fire over the distribution with time, produce less seed as they mature and go into
range of invasive Australian Acacia spp. and its influence senescence, U. morrisii has resulted in a stand of mixed-age
on seed banks further support the observation that seed trees and has caused seed banks to reaccumulate within
banks observed after the release of the biocontrol agents is populations as seedlings are recruited into populations
the consequence of current seed input and that seed banks where trees have died, so maintaining reproductive vigor of
are able to recover after disturbance events. populations. 13,14 Consequently, U. morrisii could potentially

Volume 2 Issue 1 (2025) 9 doi: 10.36922/eer.5876