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Review

Mechanisms and Impact of Acacia mearnsii Invasion

by
Hisashi Kato-Noguchi
* and
Midori Kato
Department of Applied Biological Science, Faculty of Agriculture, Kagawa University, Miki 761-0795, Kagawa, Japan
*
Author to whom correspondence should be addressed.
Diversity 2025, 17(8), 553; https://doi.org/10.3390/d17080553
Submission received: 15 July 2025 / Revised: 1 August 2025 / Accepted: 1 August 2025 / Published: 4 August 2025
(This article belongs to the Special Issue Plant Adaptation and Survival Under Global Environmental Change)
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Abstract

Acacia mearnsii De Wild. has been introduced to over 150 countries for its economic value. However, it easily escapes from plantations and establishes monospecific stands across plains, hills, valleys, and riparian habitats, including protected areas such as national parks and forest reserves. Due to its negative ecological impact, A. mearnsii has been listed among the world’s 100 worst invasive alien species. This species exhibits rapid stem growth in its sapling stage and reaches reproductive maturity early. It produces a large quantity of long-lived seeds, establishing a substantial seed bank. A. mearnsii can grow in different environmental conditions and tolerates various adverse conditions, such as low temperatures and drought. Its invasive populations are unlikely to be seriously damaged by herbivores and pathogens. Additionally, A. mearnsii exhibits allelopathic activity, though its ecological significance remains unclear. These characteristics of A. mearnsii may contribute to its expansion in introduced ranges. The presence of A. mearnsii affects abiotic processes in ecosystems by reducing water availability, increasing the risk of soil erosion and flooding, altering soil chemical composition, and obstructing solar light irradiation. The invasion negatively affects biotic processes as well, reducing the diversity and abundance of native plants and arthropods, including protective species. Eradicating invasive populations of A. mearnsii requires an integrated, long-term management approach based on an understanding of its invasive mechanisms. Early detection of invasive populations and the promotion of public awareness about their impact are also important. More attention must be given to its invasive traits because it easily escapes from cultivation.

Graphical Abstract

1. Introduction

Acacia mearnsii De Wild., commonly known as black wattle, is an evergreen tree belonging to the Fabaceae family. It was formerly classified as Acacia mollissima Wild. or Acacia decurrens Willd. var. mollis Lindl. The plants grow to a height of 6–15 m and a diameter of 10–45 cm and have well-branched structures. Its leaves are 8–12 cm long, bipinnate, and consist of 8–21 pairs of pinnae per leaf and 15–70 pairs of leaflets per pinna. Small spherical glands, up to 0.8 mm in diameter, are arranged along the upper surface of the rachis and on the petiole of the leaves. The globular inflorescence contains 20–40 small, pale-yellow flowers [1,2,3,4,5]. Flowering occurs between September and November in the Southern Hemisphere and between March and May in the Northam Hemisphere [2,5,6,7,8]. The fruits are leguminous pods that are dark brown, 4–15 cm long, and 0.4–1 cm wide. They contain an average of seven seeds, ranging from one to fourteen seeds per pod. A. mearnsii has a life span of 10–20 years [1,2,3,4,5] (Figure 1).
A. mearnsii is native to southeastern Australia, including Tasmania. It has been introduced to over 150 countries in Africa, southern Europe, South Asia, East Asia, and South America for its economic value. The trees are used for timber, paper pulp, fuelwood, charcoal, and to rehabilitate degraded lands. They are also used as shade trees for crop production and as ornamental trees. The wood is hard but easy to process [9,10,11,12,13]. The bark of A. mearnsii contains significant amounts of condensed tannin, known as wattle tannin, which consists of flavonoid units (flavan-3-ols). This tannin has been used in the production of tanned leather for over 100 years. It has also been used in water-resistant adhesives and as a dietary supplement for human health [13,14].
The cultivation of A. mearnsii has been reported to span over 170,000 to 200,000 hectares in Brazil, 110,000 to 160,00 hectares in South Africa, 30,000 hectares in East Africa, 20,000 hectares in India, 15,000 hectares in Indonesia, and 10,000 hectares in China [11,15,16,17]. In rural villages of the Drakensberg region in South Africa, the coverage of A. mearnsii increased from 7% in 1952 to 21% in 1975 and 48% in 2000 [9]. A. mearnsii seeds are still exported from Australia to other countries around 300 times a year because cultivating them brings considerable economic benefits [10].
However, A. mearnsii easily escapes from cultivation through seed dispersal, and establishes monospecific stands across plains, hills, valleys, and riparian habitats, including species hotspots, national parks, and forest reserves [2,18,19,20,21,22]. As previously mentioned, A. mearnsii was planted on 110,000 to 160,00 hectares in South Africa. However, its population has spread to over 2,500,000 hectares [23]. In just six months, the A. mearnsii population expanded 1800 m along the airport boundary [24]. The extensive planting of A. mearnsii has created opportunities for its invasive population to increase worldwide [2,18,19,20,21,22]. The invasion of A. mearnsii has also been reported to have significant negative impacts on the environment and species diversity in the introduced ranges [2,3,25]. Therefore, A. mearnsii has been listed among the world’s 100 worst invasive alien species [26]. Additionally, global climate change may exacerbate the invasion risk of A. mearnsii [27].
The life history traits of A. mearnsii, including its growth, reproduction, and adaptation to different environmental conditions and biotic stressors, have been reported. However, no review has focused on the invasive mechanisms of this species. This is the first review to provide an overview of the invasive mechanisms and their impact on abiotic and biotic ecosystem processes. A combination of online search engines was used to search the literature: Scopus, ScienceDirect, and Google Scholar. The following terms were searched in relation to A. mearnsii: botany, invasion, growth, reproduction, habitat, adaptation, drought, pathogen, allelopathy, and impact.

2. Invasive Mechanism

2.1. Growth

A. mearnsii is a species that demands light and has a rapid early stem growth rate of up to 3 m per year during the first three to five years [16]. The photosynthetic CO2 assimilation rate of three-week-old A. mearnsii was 7.15 g per m2 per day. After 12 weeks of cultivation under greenhouse conditions, the seedlings increased to 7.68 g of dry mass per plant [28,29]. The above-ground biomass of two-year-old A. mearnsii trees in a plantation at Rio Grande in Bazile was 182 kg per hectare. Leaves and branches accounted for 43% of the biomass, whereas bark and wood accounted for the remaining [30]. The biomass of the four-year-old A. mearnsii trees in the same plantation was 73 tons per hectare. It consisted of 64% wood, 11% roots, 10% bark, 7% branches, 5% dead branches, and 3% leaves [31]. After 11 years of cultivation in the Cane River, Victoria, Australia, A. mearnsii trees reached a height of 10.5 m and stem diameters of 11.2 cm at breast height. Their annual net primary production ranged from 2.1 to 4.3 tons per hectare, and their total above-ground biomass was 75 tons per hectare [32,33]. Biomass accumulation increased with stand age and reached 189 tons per hectare in mature A. mearnsii plantations in the Pearl River Delta in southern China [34]. Topography and location influenced tree height and biomass differently [35]. Among Acacia species, A. mearnsii showed a relatively high growth rate [28,29]. Additionally, A. mearnsii lives with Rhizobium and fixes nitrogen. It fixes nitrogen at a significantly greater rate than the indigenous, coexisting nitrogen-fixing species Acacia caffra (syn. Senegalia afra) in riparian habitats in South Africa [36]. The rapid stem growth pattern of A. mearnsii during its sapling stage may give it a competitive advantage in acquiring resources, including light perception, and may contribute to its establishment in given habitats.

2.2. Reproduction

A. mearnsii was observed to bear flowers on 20-month-old plants and seeds on three-year-old plants [37]. However, it only produces significant quantities of seeds after five to six years [4]. A. mearnsii primarily produces seeds through cross-fertilization and, to a lesser extent, through self-fertilization. The cross-fertilization rate was estimated to be between 48 and 100%. The plant is pollinated by wind and insects, including bees, beetles, and flies [4]. Seeds mature 12–14 months after flowering. A. mearnsii produces a large number of seeds. Its seed rain forms large seed banks around the mother plants. These banks have densities ranging from 7500 to 20,000 seeds per m2 [38,39,40]. Seeds are carried short distances by the wind and long distances by mammals and birds, as well as by water flow, including floodwater [2,10,38]. Heavy rainfall carried A. mearnsii seeds along the airport boundary, expanding the population to a length of 1800 m. The density of the seedlings was between one and ten plants per m2 [24]. Additionally, humans have carried a large quantity of seeds from Australia to many other countries [2,10,38].
The seeds of A. mearnsii were hard-coated and long-lived [41]. The seed viability of A. mearnsii in the seed bank was approximately 90%, as determined by a tetrazolium test [39,42,43]. However, the germination rate was only 10–30% due to the physical dormancy caused by the impermeable epidermal layer of the seeds. The germination rate increased to over 95% when the seeds were manually scarified with a scalpel or subjected to hot water treatment [42,44]. The germination rates of fresh and mature seeds of A. mearnsii were 1% and 7%, respectively. The fresh seeds were collected during the seed fall period, and the mature seeds were stored for one year. The germination rate increased to 70% for both fresh and mature seeds with a five minute thermal treatment at 110 °C and to 80% with a ten minute thermal treatment at 110 °C [43]. The emergence of A. mearnsii seedlings increased greatly in areas where there was a fire [45,46]. Wildfires may stimulate the germination of A. mearnsii seeds in the seed bank, which contributes to its reestablishment and domination in affected areas. Additionally, sprouts arise from the remaining parts and/or underground parts of A. mearnsii after wildfires burn the vegetation [47]. This may also contribute to its reestablishment and domination in burned fields.
A. mearnsii can also be propagated through vegetative reproduction. Its shoot segments, which contain at least one adventitious bud, will sprout and root when placed in an appropriate medium [4,48,49]. However, it is unclear whether vegetative reproduction occurs naturally.
A. mearnsii creates a large seed bank of long-lived seeds. These seeds are dispersed by wind, mammals, birds, water flow, and human activity. Some seeds are carried for long distances, contributing to its large geographic and even international expansion. Wildfires promote the germination and contribute to the reestablishment and dominance of A. mearnsii in burned fields.

2.3. Adaptative Ability

The chromosome number of A. mearnsii is 2n = 26 [2,16]. Both diploid and tetraploid forms exist [50,51,52]. However, the tetraploid form is likely artificially produced [50]. Genetic variation among A. mearnsii populations from different geographic regions is low to moderate when examined using various marker genes [48,53,54]. Nevertheless, A. mearnsii exhibits significant variation in growth rate, adaptation to drought and low temperatures, as well as wood and bark characteristics [54,55,56,57]. These findings suggest that A. mearnsii has greater epigenetic variation than genetic variation.
A. mearnsii prefers a warm humid climate. However, it can tolerate various climate zones, ranging from temperate and subtropical lowlands to tropical highlands. Its habitat is not strictly limited, as it grows in plains, hills, mountains, and valleys. It also grows in coastal lowlands and riparian habitats [57]. A. mearnsii grows best in relatively deep, moist, and well-drained soils with moderate to low fertility. However, it can also grow in moderately heavy and shallow soils. It grows in loam, sandy loam, and podzols. The pH of these soils ranges from 5.0 to 6.5 [2].
It grows where the mean temperature ranges from 21 to 27 °C in the hottest month, and from −1 to −7 °C in the coldest month [58,59]. The annual mean temperature ranges from 14 to 28 °C [59,60,61]. A. mearnsii populations from warmer regions cannot survive temperatures below −4 °C. However, populations from high altitudes can tolerate temperatures between −6 and −8 °C. Different populations within Australia, and between Australia and South Africa, also exhibit varying degrees of cold tolerance [57,62]. However, the interaction between genotype and the environment was low. The cold tolerance of the species increases with age [63,64]. These tolerant populations induce several cold-stress-related proteins, including those that protect cell membranes and the photosynthesis apparatus during cold acclimation. These populations also produce proteins involved in transporting other proteins [65].
A. mearnsii grows in areas with annual precipitation ranging from 400 to 2400 mm [59,60,61]. Under drought conditions, the stem xylem of A. mearnsii exhibited a lower water potential than that of native tree species, such as Metrosideros angustifolia (Myrtaceae) in South African riparian ecotones [66]. This lower water potential is due to osmotic adjustment in cells, which is caused by an increased solute concentration [67,68]. Osmotic adjustment maintains cell turgor and plant vigor under drought conditions [69]. Therefore, A. mearnsii is more tolerant of drought conditions than native species.
Riparian habitats experience seasonal variations in water table height and soil oxygen concentration. These habitats also experience high-water events, such as waterlogging conditions, which stresses the plants. Another Acacia species, Acacia melanoxylon, grows in Australian riparian habitats and shows high tolerance to waterlogging and low oxygen conditions [70,71]. These species tolerate stressful conditions through morphological and physiological adaptations, such as aerenchyma formation and anaerobic respiration [72,73,74,75,76,77]. A. mearnsii also grows in riparian habitats, likely experiencing waterlogging and low oxygen conditions. However, its tolerance of waterlogging conditions remains unclear and should be investigated in the future, as it is a riparian species.
These findings demonstrate that A. mearnsii thrives in diverse habitats. This species is also tolerant of low temperatures and drought stress. Its ability to adapt to different conditions may be due to epigenetic rather than genetic variation within the species.

2.4. The Significance of Herbivores and Pathogens

Biotic stressors, such as herbivores and pathogens, significantly impact the germination, growth, and reproduction of plants. These stressors affect plant abundance, distribution, and survival [78,79,80,81,82,83,84]. The specific pathogens and monophagous herbivores of certain invasive plant species are limited in their introduced ranges due to a lack of coevolutionary history with these invasive plants [85].
In its native range of Australia, many insects have been reported to damage A. mearnsii. Among them, the leaf-eating fire blight beetle, Pyrgoides orphana (Coleoptera), is a serious pest in Australian plantations [86]. Twenty-five insect species were considered economically important in South African plantations. Most of these insects are leaf feeders. The wattle bagworm, Kotochalia junodi (Psychidae), is the most widespread and destructive leaf feeder. This insect is indigenous to South Africa and feeds on other indigenous relative species of A. mearnsii [87]. The brown wattle mirid, Lygidolon laevigatum (Miridae), feeds on Fabaceae plant species, and the cottony cushion scale, Icerya purchase (Monophlebidae), feeds on over 80 families of woody plants. These insects cause serious problems in plantations in Zimbabwe and Tanzania, respectively [88]. The twig girdler beetles, Oncideres impluviata and Oncideres dejeanii (Cerambycidae), are native to South America and feed on Fabaceae species. They have caused considerable damage in plantations in Brazil [89]. Therefore, several oligophagous and polyphagous insects cause damage to A. mearnsii in plantations within its introduced ranges.
The most common disease of A. mearnsii is caused by the gall rust fungus Uromycladium tepperianum (Pileolariaceae), which is found in Australia [18,90]. Gummosis is the exudation and accumulation of gummy substances on the plant surface. In A. mearnsii, gummosis is caused by pathogens, herbivores, adverse weather conditions, and mechanical damage, especially in plantations within the introduced ranges. Gummosis is a significant problem in plantations because it reduces bark quality [87]. Several diseases, including rusts and rots, have been observed in the plantations within the introduced ranges [87,88,89]. Rhizoctonia spp. (Ceratobasidiaceae) and Amauroderma rude (Ganodermaceae) are common wood-rotting pathogens [87]. Uromycladium acacia (Pucciniales) causes rust diseases in South African plantations [91,92,93].
The bark extracts of A. mearnsii have traditionally been used as treatments for microbial infections in humans and its condensed tannin has been reported to exhibit antimicrobial activity [94]. Aqueous acetone extracts of A. mearnsii bark suppressed the growth of wood-rotting fungi, such as Coriolus versicolor (Polyporaceae) and Tyromyces palustris (Incrustoporiaceae) [95]. Additionally, aqueous extracts of A. mearnsii bark suppressed the growth of Microcystis aeruginosa (Microcystaceae) [96]. M. aeruginosa is a freshwater cyanobacterium that causes toxic blooms [97,98]. The bark extracts increased the permeability of the cell membranes of M. aeruginosa, leading to membrane leakage and even lysis. The extracts also significantly reduced the expression of photosynthesis-related genes in M. aeruginosa [99]. Additionally, a low molecular weight proanthocyanidin isolated from A. mearnsii bark exhibited anti-termite activity [95]. Therefore, A. mearnsii may contain compounds that affect certain bacteria, fungi, and insects. However, the active principles remain unclear. The role of these toxic substances in its protection against herbivore and pathogen attacks is also unclear.
As described in this section, several herbivores and pathogens can significantly impact A. mearnsii populations in the plantations within their introduced ranges. However, these observations were made in cultivated populations. Therefore, the impact of these herbivores and pathogens on the abundance, distribution, and survival of invasive A. mearnsii populations remains unclear, as these populations continue to increase. It is possible that none of these herbivores or pathogens significantly reduce the abundance or distribution of invasive populations in the introduced ranges. Several invasive plant species have been reported to exhibit morphological and physiological defense responses against herbivores and pathogens [100,101,102]. These species also produce several toxic substances. These functions likely contribute to their invasive characteristics [103,104,105]. A. mearnsii may also be able to tolerate herbivores and pathogens. Further research is needed to investigate these defense responses and toxic substances.

2.5. Allelopathy

Neighboring plant species are another type of biotic stressor. These species compete for resources, such as water, nutrients, and light. Those with stronger competitive ability gain more resources. Invasive plant species have often been reported to exhibit allelopathic activity [106,107,108,109]. Allelopathy enables invasive plants to increase their competitiveness by suppressing the germination and growth of neighboring plant species. This process involves releasing allelochemicals into the surrounding environment [106,107,110,111,112].
Aqueous extracts of the leaves, stems, and pods inhibited the germination of two small native tree species, Lagerstroemia indica (Lythraceae) and Callicarpa formosana (Lamiaceae), and two native herbaceous plant species, Patrinia villosa (Valerianaceae) and Corchoropsis tomentosa (Malvaceae), in a extract concentration-dependent manner [113]. The aqueous extracts of the leaves, stems, and bark of A. mearnsii also suppressed germination of the cosmopolitan grass species African lovegrass, Eragrostis curvula (Poaceae), and the crop species cabbage (Brassica oleracea; Brassicaceae) and maize (Zea mays; Poaceae) [114]. These results suggest that the extracts have allelopathic activity and contain certain allelochemicals. However, the allelochemicals responsible for this activity have not yet been identified.
Flavonoids, such as quercetin, catechin, gallocatechin, and myricitrin, were identified in the leaves, stems, and bark of A. mearnsii [115,116,117,118,119]. These flavonoids have also been found in various invasive plant species, including Bidens pilosa (Asterales), Lantana camara (Verbenaceae), Leucaena leucocephala (Fabaceae), Mimosa pigra (Fabaceae), and Fallopia japonica (syn. Reynoutria japonica; Polygonaceae), and have been reported to exhibit allelopathic activity against different plant species as allelochemicals of these invasive plants [120,121,122,123,124]. Therefore, these flavonoids may also act as allelochemicals in A. mearnsii (Figure 2). Due to their similar structures, these flavonoids may have similar binding sites and modes of action. However, the ecological significance of allelopathy on the invasive traits remains unclear. For instance, it is un unclear how and to what extent these flavonoids are released into the environment from A. mearnsii.

3. Impact on Abiotic Environment

The annual total evaporation from the native grasslands and fynbos shrublands of South Africa was estimated to be between 600 and 850 mm [125]. Many of these plant species are seasonally dormant. In these areas, A. mearnsii infestation replaces the native grasslands and shrublands. This species forms dense stands and maintains high rates of evaporation throughout the year due to its evergreen nature and high green leaf index [126,127]. The annual total evaporation from A. mearnsii-dominated vegetation was estimated to be 1500 mm. Among the invasive plant species in these areas, A. mearnsii uses the most water, consuming 25% of the total amount. It was estimated to reduce annual surface water flow by 7% [125,128]. Reduced streamflow in A. mearnsii dominates catchments, especially during the dry seasons, and threatens valuable water resources in areas that are already facing shortages [129].
On the other hand, during extreme rainfall events, streamflow was significantly higher in A. mearnsii-dominated catchments than in the native grassland catchments in the Nilgiri region of southern India. This was attributed to the spreading roots of A. mearnsii, which reduce water retention and enhance shallow subsurface flows, resulting in increased streamflow [130,131]. A. mearnsii-dominated habitats have a thick layer of its leaf litter on the soil surface. This layer also prevents water from entering the soil [132]. Although A. mearnsii root systems stabilize soil, the roots are shallow. Soil can easily be washed away during extreme rainfall events, such as flooding. Additionally, A. mearnsii collapses into streams, blocks bridge arches, and floods control structures [127]. A. mearnsii also increases riverbank erosion because it is less adapted to flash floods than native plants [133,134].
The Cape Floristic Region in South Africa is a global biodiversity hotspot, characterized by nutrient-poor soils and streams. The nitrogen cycle is also very slow [135,136,137,138]. A. mearnsii reabsorbed 76% of the phosphate contained in its leaf litter. However, the species reabsorbed only 30% of the nitrogen because it relies on a symbiotic relationship with Rhizobium for nitrogen fixation. The remaining 70% of the nitrogen in its leaf litter enriched the nitrogen concentration in the soil and streams [139]. The infestation of A. mearnsii increases nitrogen in the soils and streams, altering nitrogen cycles in the entire ecosystem [140,141].
In addition to nitrogen, A. mearnsii infestation alters the chemical and physical properties of soil. There is an increase in total carbon, phosphate, and potassium concentrations, electrical conductivity, and cation exchange capacity, and a decrease in soil pH and zinc concentration [142]. Soils under A. mearnsii are compact, with a high litter mass [143]. Conversely, lower carbon stocks in the litter and soil were observed in 50-year-old A. mearnsii stands in the Eastern Cape of South Africa [144].
The infestation of A. mearnsii in grasslands reduces the amount of photosynthetically active radiation that reaches ground levels due to its canopy closure [145]. This reduction negatively affects the growth of native herbaceous plant species [146].
As described in this section, A. mearnsii affects hydrology, the chemical and physical properties of the soil, and solar irradiation in infested areas. These effects include reduced water availability, increased erosion and flooding risk in riparian areas, and changes to the nitrogen cycle. These alterations may impact ecosystem functions in the invaded ranges.

4. Impact on Biotic Environment

Infestations of A. mearnsii were found to reduce the abundance and diversity of plant species in infested areas in species-rich fynbos in South Africa [136,147]. In the Mandhu Shola forest of the Western Ghats in India, a biodiversity hotspot, the A. mearnsii infestation reduced native plant diversity, including ground vegetation due to competition for resources. Large areas of the Shola grasslands on the Nilgiri Plateau and Palani Hills in the Western Ghats have already been lost due to A. mearnsii infestation [148]. In addition to altering the chemical and physical properties of soil, A. mearnsii alters the bacterial populations in the soil, including ectomycorrhizal populations [149,150]. These alterations reduce grass production [24,151]. Altering the ectomycorrhizal population suppresses the growth of the native plant Quercus suber (Fagaceae) [149].
The infestation of A. mearnsii and the replacement of native vegetation may alter ecosystem structures and functions. A. mearnsii impact fauna by affecting food availability. It has been reported that A. mearnsii affects arthropod populations. The abundance and β-diversity of arthropods in habitats dominated by the native plant Virgilia divaricate (Fabaceae) were significantly higher than in habitats dominated by A. mearnsii due to the availability of food in forests and fynbos vegetation in South Africa [152]. The α-diversity and β-diversity of arthropods were also higher in native riparian vegetation dominated by Brabejum stellatifolium (Proteaceae) and Metrosideros angustifolia (Myrtaceae) than in vegetation dominated by A. mearnsii in South Africa [153]. A. mearnsii also threatens several species of protected Odonata dragonflies in South Africa [154].
The infestation of A. mearnsii reduces the abundance and diversity of plant and arthropod species, as well as soil bacterial populations. Even after these populations are removed, it takes a long time for the original flora and fauna to return [147,151,153,155,156]. Since the infestation of A. mearnsii replaces native vegetation, it may impact populations of other invertebrates and vertebrates, including reptiles, birds, and mammals, by affecting their food availability and habitats. However, this information remains unclear.

5. Control

Cutting is a common control technique for Acacia species [157]. Fire burning is an effective method of controlling the seedlings and saplings of A. mearnsii. However, a high population of mature trees survive after fires [158]. New shoots sprout from the remaining parts, stumps, and/or underground parts after burning or cutting treatments [45,158]. Additionally, fire burning stimulates the germination of A. mearnsii seeds in its seed bank [46,47]. Foliar spraying with herbicides, such as glyphosate, dicamba, and picloram, is also an effective method of controlling A. mearnsii seedlings and saplings. However, applying these herbicides to cut stumps and/or drilled holes in adult trees is necessary [2,159].
Several insects and pathogenic fungi have been evaluated as biological control agents [160,161]. The fungus Cylindrobasidium laeve (Physalacriaceae) attacks A. mearnsii stumps and prevents resprouting [160]. The seed weevil Melanterius maculatus (Curculionidae) feeds on A. mearnsii seeds [162]. The midge Dasineura rubiformis (Cecidomyiidae) forms galls in the flowers and prevents pod development [163,164]. M. maculatus and D. rubiformis were introduced into South Africa as biocontrol agents in 1994 and 2006, respectively. However, their effectiveness against the invasive population of A. mearnsii remains unclear [161,165]. A. mearnsii is also cultivated in plantations. This makes it difficult to release biocontrol agents.
To eradicate its invasive populations, an ongoing, long-term integrated management approach based on an understanding of the mechanisms by which A. mearnsii becomes invasive is required. This approach incorporates various herbicides, smart decision-making tools, and innovative equipment. Early detection of its invasive populations and increased public awareness of their impact are also important [25,166,167,168]. A. mearnsii easily escapes from cultivation through seed dispersal [2,19,20,21,25]. Large-scale invasions are likely to result from the extensive planting of A. mearnsii unless they are managed efficiently. Over time, its escaped populations have increased, and management costs will exceed the benefits [169]. Therefore, its invasive traits must be considered more carefully. Additionally, global warming increases the invasive risk of A. mearnsii [27].

6. Invasive Potential and Risk

A. mearnsii has spread to over 150 countries in Africa, southern Europe, South Asia, East Asia, and South America for its economic value [1,2,3,4,5,9,10,11,12,13,14,94]. However, it easily escapes from cultivation and establishes monospecific stands across plains, hills, valleys, and riparian habitats, including protected areas such as national parks and forest reserves [2,18,19,20,21,22]. A. mearnsii has been listed among the world’s 100 worst invasive alien species because of its negative ecological impact [26].
A. mearnsii exhibits rapid early stem growth and reaches the early reproductive stage [4,16,28,29,30,31]. It produces a large number of seeds. Its seed rain forms large seed banks containing long-lived seeds [4,38,39,40]. Seeds are carried short distances by the wind and long distances by mammals, birds, water flow, and human activity. Wildfires stimulate its germination [45,46,47]. The pattern of rapid early stem growth may give A. mearnsii a competitive advantage in acquiring resources, contributing to its establishment in certain habitats.
A. mearnsii can grow in different environmental conditions and can tolerate various adverse conditions, such as low temperatures and droughts [2,57,60,61,62,63,64]. Its low-temperature-tolerant populations induce several cold-stress-related proteins [65]. The stem xylem of A. mearnsii exhibits a lower water potential and maintains cell turgor and plant vigor under drought conditions [66].
Several herbivores and pathogens can seriously damage A. mearnsii populations in plantations within introduced ranges [87,88,89,91,92,93]. However, these observations were made in cultivated populations. The impact of herbivores and pathogens on the abundance, distribution, and survival of the invasive populations of A. mearnsii remains unclear, as these populations continue to increase. Therefore, A. mearnsii may be able to tolerate herbivores and pathogens. A. mearnsii exhibits allelopathic activity against certain plant species [113,114]. It contains several flavonoids that may act as allelochemicals [115,116,117,118,119]. However, the ecological significance of allelopathy on the invasive traits remains unclear. Defense responses and toxic substances against herbivores and pathogens also remain unclear.
The infestation of A. mearnsii affects abiotic processes in ecosystems. It reduces water availability and increases the risk of erosion and flooding in riparian areas [125,126,127,128,129,130,131,132,133,134]. It also alters the chemical composition of soil, including the nitrogen cycle, and obstructs solar light irradiation [135,136,137,138,139,140,141,142,143,144,145,146]. These alterations may affect ecosystem functions in the infested areas. The infestation of A. mearnsii reduces the abundance and diversity of plant and arthropod species, as well as soil bacterial populations [24,136,147,148,149,150,151,152,153,154] (Table 1). However, its impact on other invertebrates and vertebrates remains unclear.
The characteristics of A. mearnsii, such as its growth and reproductive abilities, adaptability to different environmental conditions, and biotic stressors, may contribute to its infestation and population expansion within introduced ranges. Removal of the invasive populations of A. mearnsii has not successfully restored the original flora and fauna [147,151,153,155,156]. Eradicating these populations requires an ongoing long-term integrated management approach based on its invasive mechanisms. Early detection of its invasive populations and increased public awareness of their impact are also important [25,166,167,168]. A. mearnsii easily escapes from cultivation through seed dispersal [2,19,20,21,25]. Therefore, more attention should be given to its invasive traits during cultivation. To better understand its invasive mechanisms, the defense responses and functions of A. mearnsii against herbivores, pathogens, and competitive plant species should be investigated. Additionally, the abundance and species richness of vertebrates and other invertebrates in areas infested by A. mearnsii should be evaluated to determine its impact on the ecosystem as a whole.

Funding

This research received no external funding.

Data Availability Statement

No new data were created or analyzed in this study.

Conflicts of Interest

The authors declare no conflict of interest.

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Diversity 17 00553 g001
Figure 1. Acacia mearnsii inflorescence and leaves.
Figure 1. Acacia mearnsii inflorescence and leaves.
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Figure 2. Compounds involved in the allelopathy of Acacia mearnsii. 1: Quercetin, 2: Catechin, 3: Gallocatechin, and 4: Myricitrin.
Figure 2. Compounds involved in the allelopathy of Acacia mearnsii. 1: Quercetin, 2: Catechin, 3: Gallocatechin, and 4: Myricitrin.
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Table 1. Invasive mechanisms and the impact of Acacia mearnsii.
Table 1. Invasive mechanisms and the impact of Acacia mearnsii.
Invasive MechanismReferences
➢
Rapid stem growth in juveniles and early reproductive maturity
[4,16,31,32,33,34,35,36,37,38]
➢
A large seed bank containing long-lived seeds
[4,38,39,40]
➢
Germination stimulation through wildfire
[45,46,47]
➢
Adaptability to different environmental conditions
[2,57,60,61,62,63,64,65,66]
➢
Defense responses against herbivores and pathogens
Unclear
➢
Allelopathy
[113,114]
Impact on the abiotic environment
➢
Reduction of water availability
[125,126,127,128,129]
➢
Increased risk of erosion and flooding
[127,130,131,132,133,134]
➢
Alteration of soil chemical composition, including the nitrogen cycle
[135,136,137,138,139,140,141,142,143]
➢
Obstruction of solar light irradiation
[145,146]
Impact on the abiotic environment
➢
Reduction of plant abundance and diversity
[136,137,138,139,140,141,142,143,144,145,146,147,148,149,151]
➢
Reduction of arthropod abundance and diversity
[152,153,154]
➢
Alteration of bacterial populations
[149,150]
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Kato-Noguchi, H.; Kato, M. Mechanisms and Impact of Acacia mearnsii Invasion. Diversity 2025, 17, 553. https://doi.org/10.3390/d17080553

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Kato-Noguchi H, Kato M. Mechanisms and Impact of Acacia mearnsii Invasion. Diversity. 2025; 17(8):553. https://doi.org/10.3390/d17080553

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Kato-Noguchi, Hisashi, and Midori Kato. 2025. "Mechanisms and Impact of Acacia mearnsii Invasion" Diversity 17, no. 8: 553. https://doi.org/10.3390/d17080553

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Kato-Noguchi, H., & Kato, M. (2025). Mechanisms and Impact of Acacia mearnsii Invasion. Diversity, 17(8), 553. https://doi.org/10.3390/d17080553

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