Review Article |
Corresponding author: F. Dane Panetta ( dane.panetta@gmail.com ) Academic editor: Gerhard Karrer
© 2017 F. Dane Panetta, Ben Gooden.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Panetta FD, Gooden B (2017) Managing for biodiversity: impact and action thresholds for invasive plants in natural ecosystems. NeoBiota 34: 53-66. https://doi.org/10.3897/neobiota.34.11821
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Some studies examining the impacts of invasive plant species in native plant communities have demonstrated non-linear damage functions, whereby community components, such as species richness, are seemingly unaffected by the presence of an invader until it has attained relatively high levels of abundance, whereupon there is a marked decrease with further increases in abundance. Given chronic limitations in the resources available for managing invasive species, it has been argued that the most damaging invaders would be controlled most efficiently by maintaining their abundances below such threshold levels. Because many impact studies do not involve sampling over a wide range of invader abundances it is not possible to estimate the prevalence of threshold relationships. Furthermore, studies that have employed appropriate sampling methods have shown that different life forms exhibit different threshold responses, indicating that maintenance management for biodiversity values should be designed to protect the most sensitive species or groups of species. Since control costs increase with invader abundance, economic and ecological considerations are aligned when invaders are sustainably maintained at relatively low abundances. Adopting such an approach should also minimise negative impacts where damage functions are linear.
Biodiversity asset, diversity, extirpation, maintenance control, species richness, weed impact
It is well-known that invasive alien plants can significantly threaten the structure, function and productivity of natural ecosystems, and are generally associated with declines in diversity and fitness of resident biota (
Given the increasingly high cost and economic burden of controlling invasive species in agricultural and natural ecosystems (e.g. at least $13.6 billion per year in Australia;
For widespread and dominant invasive plants with demonstrably negative effects on native ecosystems, there is growing evidence that ecosystem responses are non-linear, such that they occur only once a particular level of invasive plant abundance has been exceeded; that is, a negative impact threshold relationship (
The concept of invasive species impact thresholds has received attention for at least two decades (see reviews by
The prevalence of impact thresholds throughout invaded ecosystems is poorly known. A recent review of biases and errors in assessments of weed impacts on natural ecosystems by
For example,
Several other studies have also provided evidence for negative impact thresholds for a variety of invasive plants, including the shrub Baccharis halimifolia in Mediterranean saltmarshes (
Conceptual model for negative impact threshold relationships between invasive plant abundance and a natural asset (e.g. number of native plant species) within the recipient ecosystem.
Invasive species impact threshold relationships can be defined as non-linear declines in one or more natural ecosystem properties, such as number of native plant species, with increasing weed abundance. The model curve consists of several components: |
A. Threshold relationships exist when the quality of a particular natural asset does not significantly change (either positively or negatively) at low levels of invader abundance. At point A on the non-linear curve, native plant species are able to coexist with the invasive plant . This initial “zone of maintenance” may vary in extent depending on the type of invaded community, capacity of the native species to withstand invasion and functional activity of the invasive plant. For example, as indicated by the relatively steep light-grey-dotted curve at point A, invasive plants that actively engineer one or more ecosystem properties, such as nitrogen-fixing shrubs, may drive native species decline even at low levels of invasion, due to small changes that accumulate through time. In some instances multiple thresholds have been observed (see |
B. This point lies within the threshold zone: the levels of invasion at which the natural asset in question begins to decrease as weed abundance increases. This represents a transition zone from one natural ecological state (i.e. ecosystem dominated by native species) to an alternative, degraded state (i.e. one dominated by an invasive species, with altered ecosystem properties; |
C. The rate of change (represented by the negative gradient over the stretch of curve at point C) once the threshold zone has been exceeded is unknown in most cases, but can be very high. For example, |
D. The trajectory of the tail-end of a threshold relationship, where weed abundance approaches 100%, has never been examined and therefore is unclear for most invasive species. It is nonetheless an important component of the curve, because it defines the subset of ecological attributes that are tolerant to invasion at high abundances (see |
“Proactive management” is undertaken to prevent weed abundance from reaching threshold zone levels. Delaying control until after the weed has attained high levels of abundance (i.e. “reactive management”) may result in irreversible loss of particularly sensitive species ( |
The ecological processes that underpin impact threshold relationships (Box
Interest in the application of thresholds to the management of weeds in agricultural systems developed as an extension of their use in managing arthropod pests in crops (
Eradication has been defined as the elimination of every single individual (including propagules) of a species from a defined area in which recolonisation is highly unlikely (
The concept of maintenance control for invasive plants in natural ecosystems appears to have originated in relation to the management of aquatic weeds, specifically water hyacinth (Eichhornia crassipes) in Florida during the early 1970s. Until then management of water hyacinth had been essentially reactive (see Box
Maintenance control in a terrestrial context was addressed by
Work rates for control actions in relation to weed cover for different growth habits in KwaZulu Natal, South Africa. Values for each habit are averages for different control methods, calculated from data in
The timing of maintenance control has a significant bearing upon the retention of biodiversity values. Where a maintenance control regime is commenced following control efforts targeting an invader that has achieved a high level of cover, legacy effects (
The aim of maintenance control should be to keep the cover of the targeted species within a range below the impact threshold zone over an indefinite timeframe, without the need for its eradication across the invaded range. The upper limit of this range will be determined largely by two factors, the first being ecological and the second economic. Where the biodiversity value of the asset being managed is very high, there will be a need to protect the life forms (or species) that are most sensitive to the presence of the invader, whether the relevant damage function is linear or non-linear (
Tactics for maintenance control differ qualitatively from those employed when extirpation is the management goal. In essence this means that the exacting standards of extirpation, in particular the control of all aboveground plants and the total elimination of seeds and other propagules, can be relaxed (Table
Tactics for extirpation versus maintenance control. Widespread invaders are generally not good candidates for extirpation because of a continued risk of re-invasion. In the context of asset protection the intensity of control required to keep the invader at maintenance levels will be significantly less than would be the case if the management goal were extirpation.
Activity, process or variable | Extirpation | Maintenance control |
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Search and control | All growth stages targeted | Largest individuals preferentially targeted |
Seed production | Prevented | Reduced |
Vegetative reproduction | Reduced or prevented depending on control effectiveness | Reduced |
Dispersal | Reduced or prevented as consequence of activities targeting seed production per se | Reduced as consequence of activities targeting largest individuals |
Soil seed bank | Eliminated | Controlled as a consequence of reduced seed production |
Return time | Determined by generation time of targeted species | Determined by cover and biomass of targeted species |
Species that reproduce vegetatively warrant special consideration, since clonal growth has been shown to influence the magnitude of the impact of non-native plants on native species richness (
Guidelines for maintenance control. The standard of control here is less exacting than where extirpation is the management goal, but the underlying principles are similar.
1) Maintenance control should be undertaken only if there is a commitment to continued management of a valued asset. As compared to extirpation, where there is a defined management endpoint, maintenance control aims to keep the impact of the invader at an acceptable level. Managers must be prepared to support the latter strategy indefinitely or until an equally effective and more sustainable control method (such as biological control) becomes available. |
2) Control should be implemented in such a way as to minimise the likelihood of rapid increases in weed density. Disturbance resulting from control measures should be minimised. Larger individuals of the targeted species should be prioritised for control. (See (4) below). |
3) Return times should be geared to the life cycle of the targeted species, with more frequent control operations for species with short pre-reproductive periods. While the need to prevent reproductive escape is less stringent for maintenance control than for extirpation, control measures should be timed so as to reduce the level of propagule production. |
4) Larger plants should be prioritised for control. Not only do larger plants contribute more to total cover and thus impact, but they are more fecund than smaller plants, a proportion of which would be pre-reproductive. However, where clonal plants reproduce sexually, care should be taken to detect and control new genets if clones are difficult to manage. |
5) Where travel cost is a significant component of the total cost of management, more time should be spent on site in order to detect and control a larger number of plants. Budget constraints will make it comparatively more difficult to conduct a maintenance control regime when it is relatively expensive to travel to the asset of concern. By increasing search effort (therefore detecting and controlling smaller plants) it may be possible to achieve an acceptable management outcome with greater return times. |
In this piece we have assumed that the biodiversity values of an asset are known and that a management strategy can be formulated on the basis of this knowledge. When considering the management of widespread serious weeds on a larger scale there is a need for an understanding of the biodiversity values of different assets, as well as the urgency of control (see
We thank Guillaume Fried, Marcel Rejmánek, Uwe Starfinger and an anonymous referee for their helpful comments.