Invasion Debt Research Articles

The effects of landscape history and time-lags on plant invasion in Mediterranean coastal habitats

Human-driven landscape changes may promote plant invasions by increasing propagule pressure and providing favourable conditions for non-native species to establish and spread. The increase in invasion levels might not be immediate but rather exhibit a time-lag (i.e. invasion debt). Moreover, the relationship between invasion and landscape history (i.e. changes in landscape composition) might be extremely complex as it might also include extinction debts of invasive species currently in regression. In order to understand the effect of landscape history on plant invasion, we studied the invasion level in coastal plant communities affected by intense human-driven landscape alteration in the last 50 years. We identified all non-native plant species within 809 grid cells (250 × 250 m) along ~500 km of the Spanish SW coast. We tested the association of (1) non-native richness conditional on the overall presence of non-natives (at least one species), and (2) the occurrence of the most frequent non-native species, with the percentage of human land-cover categories in 1956, 1991 and 2007 using a multimodel inference approach. We used these models to project future invasion patterns in the region. We found non-native richness to be more associated with land-cover variables in 1956 than in 2007, suggesting the existence of an invasion debt, but not for the overall presence of non-natives. For most frequent species, the effects of past landscape alteration depended on the introduction pathway and the region of origin. Species used in agriculture were more related to past cropland area, while most of the species used in gardening or forestry showed higher affinity for recently altered areas. These results highlight the relevance of the species’ introduction history and landscape history in assessing future long-term invasion trends.

Red Imported Fire Ant in Australia: What if we lose the war?

SummaryIn Australia, a national eradication programme for the Red Imported Fire Ant (Solenopsis invicta Buren), one of the world's most invasive species, has been in operation since 2001 when the pest was first detected in Brisbane, Queensland. Since that time, four separate incursions of this ant have been successfully eradicated from this country, but the main Brisbane population remains. Cost‐benefit analyses already conducted put the likely impact of Red Imported Fire Ant in Australia, if not eradicated, at between A$8.5 and A$45 billion. Despite this, ongoing funding for the eradication programme is not assured. A recent external review has concluded that it remains technically feasible, cost beneficial and in the national interest to eradicate. In support of previous analyses, our study highlights some of the potential impacts of Red Imported Fire Ant in Australia in more detail and provides case examples. Results show that adverse impacts are likely in most sectors of the economy, and will be felt not only by agricultural industries, but also the building and construction, tourism, electrical and communications industries. In addition to industry effects, there will also be negative impacts on public health and lifestyle, the environment and infrastructure such as main roads, airports and schools. Our estimates of potential cost impacts in the case examples where extrapolation was possible exceed A$1.65 billion/year and support previous predictions. We conclude that increased spending is justified to avert ‘invasion debt’ – the future cost of battling pests that escape today. This is a war that Australia cannot afford to lose.

Out of the weeds? Reduced plant invasion risk with climate change in the continental United States

Identifying invasion risk is critical for regional prioritization of management and monitoring, however, we currently lack a comprehensive assessment of the invasion risk posed by plants for the United States. We aim to quantify geographic invasion risk for currently established terrestrial invasive plants in the continental U.S. under current and future climate. We assembled a comprehensive occurrence database for 896 terrestrial invasive plant species from 33 regional collections of field and museum data and projected species ranges using MaxEnt species distribution models based on current (1950–2000 average) and future (2040–2060 average) climate. We quantified geographic invasion risk as differences in species richness, invasion debt, range infilling, and identification of hotspots. Potential invasive plant richness was higher than observed richness, particularly in eastern temperate forests, where as many as 83% of species with suitable climate have not yet established. A small percentage (median=0.22%) of species' potential ranges are currently occupied by them. With climate change, potential invasive plant richness declined by a median of 7.3% by 2050. About 80% of invasive plant hotspots were geographically stable with climate change, with the remaining 20% shifting northward. Invasion hotspots and current invasion debt reveal extensive, ongoing risk from existing invasive plants across the U.S., particularly in the Southeast. Climate change alters the spatial distributions of focal species for monitoring and is likely to reduce overall invasion risk in many areas. Early detection and rapid response programs could be most effective in stemming the spread of invasive plant species in areas with increased risk under climate change, while areas with persistent high risk are candidates for containment and control. The areas with reduced risk are prime locations for invasion of new imports from tropical and subtropical climates, highlighting the simultaneous need for prevention strategies.

Towards using thermal stress thresholds to predict salmonid invasion potential

The salmonids rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta are amongst the world’s worst invaders with devastating impacts where introduced. Information on their invasive potential is key to predicting range expansion in recipient environments. Salmonids were introduced into the Keiskamma River system, South Africa in the late 1890s, established and spread throughout its headwaters. This provided an opportunity to document their distribution in relation to stream thermal regimes. Salmonids conformed to documented thermal tolerances predominantly occupying cool-water sites (13.7 ± 3.5 °C). Salmonid probability of occurrence was assessed using seven-day moving average of daily mean water temperatures (MWAT) and exceedances of 100 and 59 days reduce the probability of occurrence for S. trutta and O. mykiss to 0.5 respectively, with probability of occurrence falling to zero at 150 days for both species. Greater tolerance of MWAT exceedance was observed in impoundment (233 days) rather than riverine (146 days) habitats. Epilimnetic releases from impoundments increased temporal heat loads and average temperatures were 3 °C higher below than immediately above impoundments. Salmonids did not occupy all sites within their thermal preferendum, indicating invasion debt and further potential to spread. These models highlight subtle ecological niche differences between both salmonids with O. mykiss more likely to survive warmer water temperatures than S. trutta, but at the expense of a lower altitudinal range and with a greater susceptibility to extinction with an increase in water temperatures. Aquaculture and fisheries enhancements using salmonids are increasing, and these results may contribute to risk assessments for salmonid invasions globally.

Invasion debt – quantifying future biological invasions

AbstractAimWe develop a framework for quantifying invasions based on lagged trends in invasions (‘invasion debt’) with the aim of identifying appropriate metrics to quantify delayed responses at different invasion stages – from introduction to when environmental impacts occur.LocationWorld‐wide; detailed case study in South Africa.MethodsWe define four components of invasion debt: the number of species not yet introduced but likely to be introduced in the future given current levels of introduction/propagule pressure; the establishment of introduced species; the potential increase in area invaded by established species (including invasive species); and the potential increase in impacts. We demonstrate the approach in terms of number of species for 21 known invasive Australian Acacia species globally and estimate three components of invasion debt for 58 Acacia species already introduced to South Africa by quantifying key invasion factors (environmental suitability, species invasion status, residence time, propagule pressure, spread rate and impacts).ResultsCurrent global patterns of invasive species richness reflect historical trends of introduction – most acacia species that will become invasive in southern Africa have already invaded, but there is a substantial establishment debt in South and North America. In South Africa, the likely consequence of invasion debt over the next 20 years was estimated at: four additional species becoming invasive with an average increase of 1075 km2 invaded area per invasive species. We estimate that this would require over US$ 500 million to clear.Main conclusionsOur results indicate that invasion debt is a valuable metric for reporting on the threats attributable to biological invasions, that invasion debt must be factored into strategic plans for managing global change, and, as with other studies, they highlight the value of proactive management. Given the uncertainty associated with biological invasions, further work is required to quantify the different components of invasion debt.

Melaleuca parvistaminea Byrnes (Myrtaceae) in South Africa: Invasion risk and feasibility of eradication

We document and assess management options for the first reported invasion of Melaleuca parvistaminea Byrnes (initially identified as M. ericifolia) in the world, in the context of a South African wetland ecosystem. Delimitation surveys indicate that the entire invasion is restricted to three sites between Tulbagh and Wolseley and that populations are only associated with areas currently or previously covered by pine plantations (primarily Pinus radiata). To estimate abundance we surveyed 42% of the three identified areas and found ~26,000 plants over 1800ha (condensed canopy area of 1.15ha). At least 63% of recorded plants were seedlings or juveniles, mostly <4yrs old, and most occurred in seasonally inundated (but not waterlogged) habitats. M. parvistaminea creates monospecific stands that overtop the native shrubland vegetation (Breede Shale Renosterveld) and is thus considered a potential transformer species. Species distribution modelling also revealed large areas of climatically suitable habitat in the Western Cape, pointing to substantial invasion debt for the species in South Africa. Felling triggers seed release from serotinous capsules, resulting in prolific seedling recruitment after winter rains (up to ~18,000seedlings/m2). No evidence of a soil-stored seed bank was found, and when plants are cut at ground level or treated with herbicide after cutting, plants do not resprout. The invasive populations of this water-dispersed species are close to major rivers (the Berg and Breede), but the intervening countryside is largely transformed and is unfavourable for establishment. Much of the area downstream from the invaded area is open vegetation that is unsuitable for major recruitment but easy to survey and detect small plants. Consequently, although the extent of invasion is large (potentially 9185ha), the invasion can be delimited with some confidence, and eradication is considered achievable since seeds only survive for about a year, seedlings achieve maturity after 4years, and because the species is an obligate reseeder. Given the threats posed, eradication is desirable and M. parvistaminea should be listed as a category-1a invader (requiring compulsory control) under the proposed invasive species regulations under South Africa's National Environmental Management: Biodiversity Act (10/2004). We estimate that search and destroy operations could eradicate the species by 2021 at a cost of ZAR 3 475 000 (US$ 355 400).

Casuarina: biogeography and ecology of an important tree genus in a changing world

Important insights on the invasion ecology of woody plants are emerging from the study of model groups, but it is important to test how widely such results can be generalised. We examined whether drivers of introduction and invasion in the genus Casuarina are similar to those identified for other groups. We reviewed the literature, mapped current global distributions, and modelled the potential global distribution of the genus. We assessed the rationale behind introductions, impacts of invasions, and the evolution of management approaches. Casuarinas have been introduced to about 150 countries. Ten of the 14 species in the genus have been introduced outside their native ranges, but only three species are recorded as naturalized or invasive (C. equisetifolia, C. cunninghamiana and C. glauca). Species with large native ranges are more likely to be introduced, and the three invasive species have the largest native ranges and the most records of introduction. There were no clear relationships between life-history traits (e.g. seed mass, plant height, or resprouting ability) and the extent of invasion. About 8 % of the Earth’s land surface is potentially suitable for casuarinas and large-scale plantings in some climatically suitable areas have not yet resulted in large-scale invasions; there is a substantial global Casuarina invasion debt. Experiences in Florida and the Mascarene Islands highlight that casuarinas have the potential to transform ecosystems with significant control costs. The challenge is to develop management approaches that minimise the impacts of invasions while preserving economic, environmental and cultural values of species in their introduced ranges.

Biological invasions: What's worth fighting and what can be won?

Recent critics of the effort to control biological invasions argue that the extent of the damage they cause is overblown or that, whatever the damage, the effort to control them is largely futile. The best-known invasion impacts – predation, herbivory, competition, parasitism, and hybridization – are at the population level and are joined by myriad other, more idiosyncratic impacts. Many of these impose staggering ecological, economic, and public health costs and are surely worth combating. Although major effects are not yet known for the majority of established introduced species, three facts suggest that many more impacts will ultimately be detected. First, only a small minority of introduced populations have been studied in detail. Second, some substantial impacts are subtle and have become evident only after intensive research, often at the ecosystem level. Third, an invasion debt exists, such that some introduced populations remain innocuous for an extended period before rapidly growing and spreading to become highly invasive. Many invasive populations have been eradicated or successfully managed at low levels, and technological advances have increased the range of invasions that can be attacked successfully. An effective early-warning, rapid-response system would greatly facilitate eradication of invaders and could be aided by organizing citizen volunteers and publicizing standard reporting procedures. For all these reasons, permitting new introductions should be done with great caution, and introductions detected quickly, before the species has spread widely, should be targeted for eradication if a feasible method can be identified. In the absence of eradication, maintenance of many invasive populations at low levels has proven possible by biological control, chemical control, and physical or mechanical control. New approaches from allied fields promise an even wider range of management possibilities. However, managing long-established invaders should be considered on a case-by-case basis, and in some cases ecological or social concerns may argue against management.

Abundance, rarity and invasion debt among exotic species in a patchy ecosystem

Community assembly through species invasions is a long-term process, for which vital information regarding future trends can be contained in current patterns. Using comparative analyses of native and exotic plant assemblages from meadow patches on islands in British Columbia, Canada, we examined multiple lines of evidence for ‘invasion debt’, a latent expansion of exotic species populations. We show that: (1) short-dispersing species are underrepresented compared to their long-dispersing counterparts in exotic species only; (2) among species that are invasive elsewhere in North America, a greater proportion of long dispersers are common in the study area and a greater proportion of short dispersers are rare; and (3) time since arrival in the study region is positively related to number of occurrences in exotic species. In addition, we show that a suite of exotic species possesses the facility of rapid long-distance dispersal and ability to establish viable populations on even the most isolated and least disturbed patches. While some highly-dispersive exotic species can rapidly colonize new areas, short dispersers appear to exhibit invasion debt, with their potential distributions only being realized in the long term. Removing or even reducing populations of many rapid colonizers could be extremely difficult; however, for species exhibiting patterns most consistent with invasion debt, an opportunity exists for monitoring and removal to help reduce potential competition with native species.

Ancient and recent alien species in temperate forests: steady state and time lags

Temperate forests are relatively little affected by invasions globally. However, it remains unclear if these low invasion levels can be attributed to high invasion resistance, or if particularly long time lags in forests have lead to wide-spread invasion debt at the landscape and habitat scales, thus masking the invasibility of forests. Using linear and generalized linear mixed-effects models, we tested which factors determine the level of invasion of old (archaeophytes, pre-1500 alien species) and new (neophytes, post-1500 alien species) invaders across 30 study sites (5 ha size) and 16 transects in lowland forests in eastern Austria, Central Europe. We analysed the importance of different forest types, adjacent habitats, introduction pathways, species’ local residence time and proximity to human settlements as proxy for propagule pressure. We recorded 109 neophyte and 50 archaeophyte species, with significant differences between both groups with respect to region of origin, pathways, and life form. The number of neophyte species per study site varied by more than an order of magnitude and was dependent on both proximity to settlements and residence time in a non-linear manner, with proximity to settlements showing a particularly strong effect for recently introduced neophytes. In contrast, archaeophyte species numbers were only marginally affected by distance to the nearest settlement. Within habitats, transect plots close to the forest edge and adjacent to settlements showed significantly higher numbers and cover of neophyte species, whereas for archaeophytes and native species no such effect could be detected. Our results suggest that time lag phenomena stemming from dispersal limitation (e.g. aggravated by isolation of forest patches), heterogeneity in species’ local residence time as well as increases in local levels of propagule pressure (e.g. created by expanding settlements), may result in wide-spread invasion debt at the landscape and habitat scales, hence masking invasibility of forests. Spread of new invaders may ultimately, although possibly only over long time periods, result in increased frequency and impact of neophyte species in temperate forests.

Past approaches and future challenges to the management of fire and invasive alien plants in the new Garden Route National Park

The recently established Garden Route National Park (GRNP) along the Cape south coast of South Africa occurs in a landscape where indigenous forests, fire-prone fynbos shrublands and fire-sensitive plantations of alien invasive trees are interspersed. We used the area as a case study in the challenges facing conservation managers in the achievement of biodiversity goals in a fire-prone environment. We explored the context within which fire management was practised during the past century by interviewing former catchment managers and reviewing forestry and catchment management policies. Mountain fynbos adjacent to plantations was subjected to burning regimes aimed at the protection of commercial timber resources rather than the preservation of fynbos biodiversity. Prescribed burning of fynbos adjacent to the plantations was typically done in multiple belt systems at rotations of about 4-8 years during spring, summer and autumn, to avoid the winter berg wind season. Such short-rotation and low-intensity fires favour resprouting graminoids over slow-maturing reseeders, and likely account for the compositional impoverishment observed in fynbos near plantations. Current and future challenges faced by the GRNP include (1) balancing conflicting fire management requirements for plantation safety against fynbos conservation; (2) the continual invasion of fynbos by fire-propagated alien pines sourced from plantations; (3) inadequate resources to redress the 'invasion debt' caused by the socio-economic legacy and past management neglect; and (4) fragmentation of land use between conservation and forestry threatening the sustainability of the region at large. We provide recommendations for management actions and research priorities to address these challenges.

Risk assessment, eradication, and biological control: global efforts to limit Australian acacia invasions

AbstractAim Many Australian Acacia species have been planted around the world, some are highly valued, some are invasive, and some are both highly valued and invasive. We review global efforts to minimize the risk and limit the impact of invasions in this widely used plant group.Location Global.Methods Using information from literature sources, knowledge and experience of the authors, and the responses from a questionnaire sent to experts around the world, we reviewed: (1) a generalized life cycle of Australian acacias and how to control each life stage, (2) different management approaches and (3) what is required to help limit or prevent invasions.Results Relatively few Australian acacias have been introduced in large numbers, but all species with a long and extensive history of planting have become invasive somewhere. Australian acacias, as a group, have a high risk of becoming invasive and causing significant impacts as determined by existing assessment schemes. Moreover, in most situations, long‐lived seed banks mean it is very difficult to control established infestations. Control has focused almost exclusively on widespread invaders, and eradication has rarely been attempted. Classical biological control is being used in South Africa with increasing success.Main conclusions A greater emphasis on pro‐active rather than reactive management is required given the difficulties managing established invasions of Australian acacias. Adverse effects of proposed new introductions can be minimized by conducting detailed risk assessments in advance, planning for on‐going monitoring and management, and ensuring resources are in place for long‐term mitigation. Benign alternatives (e.g. sterile hybrids) could be developed to replace existing utilized taxa. Eradication should be set as a management goal more often to reduce the invasion debt. Introducing classical biological control agents that have a successful track‐record in South Africa to other regions and identifying new agents (notably vegetative feeders) can help mitigate existing widespread invasions. Trans‐boundary sharing of information will assist efforts to limit future invasions, in particular, management strategies need to be better evaluated, monitored, published and publicised so that global best‐practice procedures can be developed.

Long-term correlations in European socioeconomic conditions create a bias to conclusion that an invasion debt occurs

Essl et al. (1) tested whether historical socioeconomic conditions have created an invasion debt in Europe (i.e., a suite of nonnative species that have not yet been discovered but whose establishment has been determined by previous socioeconomic conditions). Essl et al. (1) tested this theory by asking whether the cumulative numbers of nonnative species presently established in 28 European countries were more closely correlated to socioeconomic conditions in the year 1900 or 2000. In this letter, we describe a flaw in their logic that biases the analysis to concluding that an invasion debt does occur.

Reply to Keller and Springborn: No doubt about invasion debt

We recently showed (1) that, for a wide range of taxa, the current numbers of established alien species in 28 European countries were generally more closely related to socioeconomic indicators from the year 1900 than 2000. Thus, the establishment of alien species seemed to lag considerably behind one of the main drivers of alien species introductions (2). We concluded that current high socioeconomic activity could result in considerable additional accumulation of alien species in the future, a phenomenon that we have called invasion debt.

Socioeconomic legacy yields an invasion debt

Globalization and economic growth are widely recognized as important drivers of biological invasions. Consequently, there is an increasing need for governments to address the role of international trade in their strategies to prevent species introductions. However, many of the most problematic alien species are not recent arrivals but were introduced several decades ago. Hence, current patterns of alien-species richness may better reflect historical rather than contemporary human activities, a phenomenon which might be called "invasion debt." Here, we show that across 10 taxonomic groups (vascular plants, bryophytes, fungi, birds, mammals, reptiles, amphibians, fish, terrestrial insects, and aquatic invertebrates) in 28 European countries, current numbers of alien species established in the wild are indeed more closely related to indicators of socioeconomic activity from the year 1900 than to those from 2000, although the majority of species introductions occurred during the second half of the 20th century. The strength of the historical signal varies among taxonomic groups, with those possessing good capabilities for dispersal (birds, insects) more strongly associated with recent socioeconomic drivers. Nevertheless, our results suggest a considerable historical legacy for the majority of the taxa analyzed. The consequences of the current high levels of socioeconomic activity on the extent of biological invasions will thus probably not be completely realized until several decades into the future.

HUMAN IMPACTS, PLANT INVASION, AND IMPERILED PLANT SPECIES IN CALIFORNIA

Invasive species are one of the fastest growing conservation problems. These species homogenize the world's flora and fauna, threaten rare and endemic species, and impose large economic costs. Here, we examine the distribution of 834 of the more than 1000 exotic plant taxa that have become established in California, USA. Total species richness increases with net primary productivity; however, the exotic flora is richest in low-lying coastal sites that harbor large numbers of imperiled species, while native diversity is highest in areas with high mean elevation. Weedy and invasive exotics are more tightly linked to the distribution of imperiled species than the overall pool of exotic species. Structural equation modeling suggests that while human activities, such as urbanization and agriculture, facilitate the initial invasion by exotic plants, exotics spread ahead of the front of human development into areas with high numbers of threatened native plants. The range sizes of exotic taxa are an order of magnitude smaller than for comparable native taxa. The current small range size of exotic species implies that California has a significant "invasion debt" that will be paid as exotic plants expand their range and spread throughout the state.