Which is the most likely explanation for why invasive species take over communities into which they have been introduced?

Global Perspective

Invasive species aggressively invade new continents so that these species become dominant in their new geographical areas. Benign components of their original habitats, invasive species include plants, mammals, birds, fish, amphibians, reptiles, arthropods, mollusks, and plant and animal diseases. Some invaders familiar in North America include the black rat, house sparrow, and kudzu (Rattus rattus, Passer domesticus, Pueraria montana var. lobata). Once established, invasive species cause many problems for humans in that they degrade natural communities, and damage agricultural species with pests and diseases. Invasive aquatic species can cut off local commerce by cutting off boating along rivers, and cause local electricity emergencies by clogging the operation of hydroelectric dams, particularly in countries such as New Zealand that rely on hydroelectric power.

Invasive species are those species that arrived on continents after the sixteenth century after human global travel, commerce, and migration increased. While species have moved between continents for millennia, global travel by humans has greatly accelerated the rate of intercontinental movement of species. New invasions of species have paralleled the movements of humans worldwide, and the associated explosion of invasive species has caused the decline of native species on their continents of origin. This relatively free movement of biota between continents has created a ‘New Pangea’. Continents will have more species as new invasive species arrive, but the displacement and extinction of native species caused by invasive species ultimately will cause the overall worldwide number of species to decrease. Exotic species introductions are generally unintentional, although there are many documented cases of species being transported to other continents for horticultural or agricultural purposes.

From a philosophical perspective, invasive species are not just another species for the species-richness list because invasive species cause environmental degradation. While some authors criticize the furor over invasive species as being akin to xenophobia, the perspective that invasive species are problematic in natural areas is fundamentally dissimilar to the idea that foreigners can cause harm to a society. The concern of ecologists over invasive species is due to the damage invasive species cause to natural plant communities.

In North America, there are 50 000 nonindigenous species, 3000 of which are invasive. Hawaii has more than its share of invasives with 860 invasive species. Each year, US$137 billion per year are spent to eradicate invasive species in the United States (Figures 1 and 2), and US$26 billion are spent to combat crop weeds.

General Invasion Patterns

Invasive species are commonly observed to exhibit sustained, rapid rates of population growth (Elton, 1958; Parker et al., 1999). This in turn tends to promote high rates of range expansion and may result in extreme levels of dominance and the displacement of native species (Kolar and Lodge, 2001; Shea and Chesson, 2002). Although observations by early explorers, both predating and including the travels of Darwin (1859), noted the aggressive spread and competitive dominance of certain introduced species in novel environments, the potential for subsequent negative consequences of nonnative species were generally unforeseen by those who introduced them into a new environment. Indeed, the successful colonization of the planet and development of human enterprises has been inextricably dependent on the transportation and establishment of new sources of food, fiber, forage, and other goods and services obtained mostly from species not native to the region they are produced. Moreover, the vast majority of introduced species fail to become established and when they do frequently require habitat modifications and repeated introductions to eventually form a persistent population (Sax and Brown, 2000; Blackburn et al., 2011; Zenni and Nuñez, 2013).

Terrestrial plant introductions that escape cultivation and spread to natural settings are the largest and most thoroughly investigated group of invasive organisms (Blackburn et al., 2011; Stohlgren et al., 2011). Few introduced plant species have the potential to escape cultivation and become naturalized in surrounding areas. Fewer still are likely to become invasive and swiftly spread into new habitats while having disproportionately negative effects on native species and environmental conditions (Williamson, 1996). When this does occur, the spread and increased abundances of nonnative species frequently occur along a continuum that transitions from introduction to naturalization and then invasiveness (Vitousek, 1990; Blackburn et al., 2011; Richardson and Pyšek, 2012). The naturalization process is often accompanied by a time-lag, oftentimes many decades, where the introduced species remains at relatively low densities and within a restricted range prior to experiencing a population “explosion” characterized by rapid density and distribution increases (Williamson, 1996; Sakai et al., 2001; Hierro et al., 2005; Simberloff, 2013). The occurrence of this time-lag prior to the onset of invasiveness has typically been thought of as being demographic in nature, requiring a sufficient number of adult generations spreading new offspring into the surrounding area that subsequently become established (Williamson, 1996; Lockwood et al., 2005). However, recent evidence suggests that evolutionary alterations in the nonnative plant itself also may promote this accelerated expansion toward invasiveness after a time-lag of multiple generations as the species genetically adapts to new selective pressures in the introduced environment (Siemann and Rogers, 2001, 2003; Müller-Schärer et al., 2004; Bossdorf et al., 2005).

Case Study 2: Nile Perch in Lake Victoria – Good Intentions Gone Wrong

Perhaps the best-known example of biodiversity impacts from an aquatic invasive species is the introduction of Nile perch (Perca niloticus) to Lake Victoria (Africa) in 1954. This introduction was intended to augment fisheries in the lake, and thus to provide an extra source of protein, jobs, and income for locals. For a number of years the species thrived, and to this day it supports a large, mainly export, fishery. Although the original intentions for introduction were initially met, they have come at a large cost to biodiversity and indigenous fisheries.

Lake Victoria is famous for its enormous diversity of endemic cichlid fish, which although never fully counted probably once numbered around 450 species. It is estimated that approximately half of these species have been driven to extinction – or at least to undetectably low abundances – by Nile perch predation. Additionally, because many of these cichlid species were algae eaters, it is likely that the Nile perch introduction is at least partly responsible for an increased incidence of nuisance algal blooms in the lake. These algal blooms reduce visibility, which further interferes with mate recognition for the endemic cichlids.

Nile perch support a valuable fishery in Lake Victoria, and even if the will existed it is unlikely that they could be eradicated. Indeed, most efforts to remove populations of invasive fish rely on general purpose fish poisons, so the consequences of such attempts would be unacceptable. Recent declines, possibly as a result of overfishing, in the Nile perch population have, however, allowed for a recovery of some cichlid species. Thus, the future of many of the remaining cichlid species may hinge on the ways that society manages the Nile perch fishery.

Abstract

Invasive species may be one of the worts environmental problems facing the conservation of natural areas, because of their role in changing ecosystem function. At the same time, invasive species cause much human suffering and economic loss. The approach to eliminating invasive species can be improved by a better understanding of the various types of invasive species, and the scientific hypotheses surrounding their ability to invade novel environments. Despite the billions of dollars spent each year, invasive species are difficult if not impossible to eliminate after they have established. Various methods of eliminating plant species from natural communities are described in this review. An increased understanding of the nature of invasive species including their genetic relationship to their progenitors, hypotheses regarding their invasive qualities, and effective approaches for their removal from ecosystems are all sorely needed. Volunteers can help in the invasive species effort by working on local plant/animal removal projects, reporting invasive species sightings to appropriate officials, or working with scientists to collect basic data for ecological research.

Detection of invasive species

Invasive species pose a threat to endemic biodiversity, as their establishment can lead to dislocation or extirpation of local, often rare, species. Early detection is crucial to effectively manage invasive species eradication and to prevent establishment (Mehta et al., 2007; Sepulveda et al., 2020). If invasive species become established, they are often very difficult to eradicate and can have negative effects on local biodiversity, threaten endemic species and incur huge economic cost (Sepulveda et al., 2020). The high sensitivity of eDNA-based assays can offer successful early detection of invasive species, even preceding early stages of invasions (Sepulveda et al., 2020). The standards and methods of invasive species eDNA development and implementation are very similar and are closely linked to those used in conservation biology. Some examples of invasive species that are routinely surveyed for using eDNA include the bluegill sunfish (Lepomis macrochirus) (Takahara et al., 2013), the New Zealand mudsnail (Potamopyrgus antipodarum) (Goldberg et al., 2013), and the red swamp crayfish (Procambarus clarkii) (Tréguier et al., 2014).

Invasive species committees (ISC)

Invasive species are a prominent stressor for Hawaiʻi’s native ecosystems, including mesic grasslands and shrublands. The State has developed a group of invasive species committees (ISC’s) to focus on priority threats identified by each island. The ISC’s are voluntary island-based partnerships on Kauaʻi (KISC), Oʻahu (OISC), Maui (MISC), Molokaʻi (MOMISC), and the Big Island (BIISC) that work with government agencies, non-profit organizations, and private businesses and landowners to protect each island from the most threatening invasive species with a proactive approach. The ISC’s target species that have high potential to severely impact the economy, environment, agriculture, human health, and quality of life (HDLNR, 2015).

The ISC’s are invaluable to the protection and restoration of mesic grasslands and shrublands because of the significant impacts invasive plants and animals have on these communities. The ISCs work contributes directly to the conservation of mesic grasslands and shrublands through removal and reduction of invasive plants and animals in mesic grassland and shrubland areas and in restoration of these sites across the Hawaiian islands.

Invasive Species

Invasive species are also experiencing changes in phenology and distributions, which may exacerbate the threats of climate change to native species. Climatic changes are likely to result in increases in invasive species’ survival, abundance, and range expansions. Experiments and field observations provide evidence of the tendency for invasive species to outcompete native species in a changing climate (Verlinden and Nijs, 2010; Willis et al., 2010). First, invasive species have a higher propensity than native species to adjust their phenology in accordance with climatic changes. The more adaptable phenologies of invasive species facilitate community invasions and also lead to increases in the abundance of invasive species (Willis et al., 2010). Moreover, characteristics common to invasive species such as high dispersal abilities, high growth rates, short generation times, and broad climatic tolerances facilitate rapid range expansion in accordance with the rapid changes in climatic conditions (Schweiger et al., 2010; Hellmann et al., 2008a). Bioclimatic models can forecast the extent of a species’ invasive potential in a changing climate by projecting the distribution of suitable climate for the invader. Forecasts of potential invasions from climate-change-induced expansions comprise most of the recent research on the interaction of invasive species and climate change. Generally, these models predict expansions of the invaders’ ranges (Bradley et al., 2010; Jarnevich and Stohlgren, 2009). However, in regions where they anticipate contractions, forecasts can provide useful guidance for restoration of sites that are no longer suitable for an existing invader (Bradley and Wilcove, 2009). Incorporating invasive ranges into bioclimatic models may also be useful for improving the forecasts of species distributions by more accurately approximating fundamental climatic niches (Beaumont et al., 2009).

Climate-change-induced range shifts of native species may challenge the traditional definitions of nonnative and invasive species as ranges expand beyond species’ historical distributions. Previously noninvasive species have the potential to become invasive in a new region without the biological controls provided by interspecies interactions present in the previous community (Hellmann et al., 2008a). Bioclimatic models do not predict these changes in fitness or species interactions and so may underestimate the potential for ecosystem invasions from previously noninvasive species.

Invasive species committees

Invasive species committees (ISCs) are voluntary partnerships of government, private and nonprofit organizations, and concerned individuals working to address invasive species issues in Hawai‘i. These committees have been formed on individual islands (island of Hawaiʻi/Big Island [BIISC], Maui [MISC], Moloka‘i [MoMISC], O‘ahu [OISC], Kaua‘i [KISC]) and focus on the specific issues of their island. The overall goal of the ISCs is to “prevent, eradicate, or control priority incipient plant and animal species that threaten Hawai‘i’s most intact federal, state, and private conservation lands” (HDLNR, 2015). For example, fountain grass has been identified as a target species for OISC and cooperative project species (work with others to control) for BISC (BISC, 2018; OISC, 2018). See Table 5 for a list of conservation actions undertaken by ISCs.

3.1.4 Invasive species management

Invasive species that are alien and nonnative plants usually have a competitive advantage over native plant populations (Kumar et al., 2019b). This is primarily due to the absence of natural factors that can regulate the population of introduced plants. As a result, the nonnative plants often flourish much and outcompete the native plants. Invasive species are characterized by rapid growth rates, extensive dispersal capabilities, large and rapid reproductive output, and broad environmental tolerance. Invasive species are damaging a number of ecosystems including agriculture and forest lands. They often pose threat to human health. Few invasive species that need immediate action to control their species include Parthenium and Lantana. The selected strategies identified for the prevention and control of invasion in the landscape are as following:

There is a need to identify the causes of spread and to initiate action to prevent those causes triggering invasion process.

Application of seed for agriculture that are weed free.

Avoiding manure and mulch that are contaminated with spores and seeds of invasive species.

Application and transfer of top soil to a new area must be done after ensuring removal of any contamination to stop the spread of invasion.

Any viable population of contaminated nursery must be isolated and not to be used.

Introduction

Invasive species destroy key natural resources used to produce market and nonmarket goods and services causing damages that outweigh any benefits they may generate (Elton, 1958; Kareiva, 1996; Lodge et al., 2009). Their expanding global numbers play a primary role in global environmental change (ESA, 2004; Mack et al., 2000; Sala et al., 2000). In response, invasive species policies have expanded in scope and magnitude. For instance, in the USA, Executive Order 13112 points out the need to “minimize the economic, ecological, and human health impacts that invasive species cause” (also see Miller and Fabian, 2004; National Invasive Species Counsel (2001); CENR, 1999; FICMNEW, 1998). Internationally, countries can propose trade restrictions to prevent introductions of invasive species through the World Trade Organization's (WTO) Agreement on Sanitary and Phytosanitary (SPS) measures (see Peterson and Orden, 2008; Calvin and Krissoff, 1998; Josling et al., 2004; Smith, 2003).

For these public policies to be cost-effective, they should be informed by both economic and ecological principles (Finnoff et al., 2009a, b, 2010a, b). Invasive species risks are jointly determined by economic and ecological interactions (Crocker and Tschirhart, 1993). Economic activity introduces many invasive species who find the new ecosystem suitable; they then spread through natural migratory patterns or hitch-hiking on commerce and recreation. Both economics and ecology and the feedback loops between the two systems play a key role (feedback loops here are defined as ecological responses from human actions having environmental consequences, followed by human behavioral responses to ecological changes having economic consequences). Effective public policy also requires detailed knowledge about the portfolio of risk reduction technologies – prevention, control, and adaptation (see Shogren, 2000). (These guiding principles have generated an explosion of bioeconomic research, as shown in special issues of Ecological Economics, Volume 52, 2005; Agricultural and Resource Economics Review, Volume 35, 2006; and Resource and Energy Economics, Volume 32, 2010, a book edited by Keller et al. (2009) and a book edited by Perrings et al. (2010).) This article defines “prevention” as investments to reduce the probability of an invasive species being introduced and subsequently becoming established in a new habitat; “control” is defined as actions to curtail invasive species population or spread to reduce the damage an invasive species causes after becoming established in a new habitat; “adaptation” is defined as actions to reduce the consequences of invasive species damage.

This article combines the idea of endogenous risk into an optimal control setting to provide a framework to address the challenge on how to best manage the prevention and control of invasive species (Kamien and Schwartz, 1991). Endogenous risk captures the idea that private citizens and public servants have some control over the set of probabilities and outcomes that define the relevant states of the world (Ehrlich and Becker, 1972; Shogren and Crocker, 1991). The dynamic endogenous risk approach used here captures the following: (1) the biological and economic circumstances of invasions, (2) the interdependency of economic and ecological systems, and (3) accounts for inherent uncertainty associated with the bioeconomic parameters. This framework provides a means to identify the feedback loops between systems and to evaluate the trade-offs inherent in designing and implementing cost-effective public policies directed at the threats of invasive species. The discussion begins by presenting the problem of invasive species in the dynamic endogenous risk optimal control framework.