Restoration of Ecological Function in Terrestrial Systems Impacted by Invasive Species

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Invasive species are responsible for the decline or extirpation of many species around the world. When those lost species provide essential ecological functions, the system may further degrade over time. Restoration ecology aims to restore these systems and associated ecological functions. It is important to first understand the invaders and their direct and indirect impacts to the native ecosystems. This requires a thorough understanding of the system and functions pre-invasion. Once these links and mechanisms are understood, managers must decide on a course of action to control or halt the spread of the invasive species and prevent further ecological degradation. Managers must determine what types of control are most appropriate for their systems as well as to what levels an invader must be controlled before restoration actions lead to improved ecological function. Deciding on specific restoration actions will vary considerably from system to system, but must involve considerations such as topography, landcover, feasibility, scale, social impacts, and timing. Specific details about habitats and species natural history are important to incorporate into planning models. Finally, monitoring and adaptive management throughout the course of the restoration and beyond are crucial to long-term success.

Related Article(s):

Contributors: Dr. Hugo Thierry, McKayla M. Spencer, Ann Marie Gawel, and Dr. Haldre Rogers

Key Resources:

Introduction- Invasion Biology

Because of the increased ease and frequency of transportation of people and goods across the globe, almost all ecosystems have species introduced by humans that do not share an evolutionary history with the native members of the ecosystem. Only some of these species survive to reproduce, and even fewer cause harm[4]. Invasive species are recognized as having been transported to a novel geographic area, establishing in that area, and then causing ecological or economic harm to the systems in that geographic region[5][6][7]. Several attempts have been made by researchers in the field to distinguish “invasive” from “non-native,” “alien” and “exotic”[8][9]. Invasive species were defined in The President's Executive Order 13112 (1999) as, “an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health”. The Global Invasive Species Program of the International Union for the Conservation of Nature accepts a similar definition of “invasive alien species” as “This subset of alien species that become established in a new environment, then proliferate and spread in ways that are destructive to native ecosystems, human health, and ultimately human welfare…”[10]. Invasive species are one of the greatest threats to ecological and economic well-being of the planet. Developing common definitions was essential given the prevalence and urgency of the impacts.

Efforts focused on early detection and rapid response are preferable to trying to control a species once it has established[11]. However, in many cases, it can be difficult to identify potential invasive species until they have started causing obvious detrimental effects.

Once a species has been identified as invasive, there are some key questions that need to be asked and answered to attempt restoration of ecological function within an ecosystem. The return of ecosystems to their original state may not be financially feasible or even technically possible due to extinctions, invasive species, or climate change, but these ecosystems still have tremendous value[1], and managing them to maximize that value requires an understanding of how these systems function. In places where the cause of species loss and species endangerment are still present and the invasive species removal appears intractable, managers may need to utilize the strategy of “intervention ecology” (Figure 1)[1], restoring function within these novel systems without attempting to restore the original ecosystem[12].

Figure 1. When an invasive species cannot be eradicated, and disrupts important ecological processes, then, an intervention ecology approach is required to restore function and stability to the system.

A well-known example of an invasive species that caused detrimental effects to an entire ecosystem, where the intervention ecology approach is now being applied, is the brown treesnake (Boiga irregularis) on the island of Guam. The snake was introduced to the island at the end of WWII, likely a stowaway aboard U.S. military cargo ships. Within approximately 40 years the snake had spread throughout the entire island and eliminated 9 of the 11 species of native forest birds[13][14]. While the brown treesnake may be the most infamous, other introduced species also have detrimental effects on Guam’s ecosystems. Rats (Rattus sp.), feral pigs (Sus scrofa), and Philippine deer (Rusa mariannae) are well-established and numerous arthropod pests, including the little fire ant and coconut rhinoceros beetle are taking a noticeable toll on local species.

Identifying the Impacts of Non-native Species on the Ecosystem

When a species has been identified in an ecosystem, it is essential to determine how it has impacted the stability, composition, and diversity of the ecosystem. This may be done by comparing changes over time, if data exist from prior to the invasion, or comparing across space if comparable areas exist nearby. Experiments that compare areas where the invasive is excluded to areas where it is present (aka ‘exclosure experiments’) may also shed light on how the system would operate in the absence of the invader.  

Invasive species may cause the decline or extirpation of native species that provide essential ecological functions in the ecosystem. For example, the Hemlock Woolly Adelgid (Adelges tsugae), an invasive insect from Asia, has led to the destruction of up to 80% of the hemlock trees in the Eastern United States, which then impacted overall forest composition. In Florida, the burmese python (Python molurus) became a destructive invasive species in less than 20 years by causing severe declines in mammal populations through predation[15].

Invasive species also cause problems for human health and economies. They can be disease vectors, such as the Asian tiger mosquito (Aedes albopictus), which can transmit Dengue Fever and Chikungunya. Invasive species can impact local and even large economies[16][17]. Brown treesnakes on Guam climb onto powerlines or into transistor stations, and are linked to nearly 200 power outages per year costing approximately $4.5 million[18]. Species may also impact economically important animals and plants[17].

The nearby islands of Saipan, Rota, and Tinian, together with Guam, comprise the inhabited southern islands of the Mariana Island archipelago. Saipan, Tinian, and Rota have flora and fauna similar to Guam but do not have the invasive snake. Comparing Guam, Saipan, Rota, and Tinian offers a unique accidental experiment to test the effects of an invasive predator and its cascading effects on a forest system, particularly through the loss of native forest birds and their accompanying ecological roles. We use this as an example for designing restoration approaches to restore function to a system with an intractable invasive species problem.

It took several decades after the introduction of the brown treesnake to Guam for it to be identified as the culprit behind bird declines, and even longer to identify the cascading ecological effects of bird loss. Because the islands to the north of Guam have similar forests but still retain their bird populations, it was possible to set up comparative studies to determine impacts. Since 5 of the bird species were frugivores, the loss of seed dispersal stands out as a major impact on the forests of Guam[19]. With reduced seed dispersal, seeds fall underneath their parent tree unhandled by frugivores, which results in a lower chance of germination[2]. In addition, treefall gaps are filled almost exclusively by the trees adjacent to the gap, which has led to reduced species richness in treefall gap seedling communities[20]. In addition, spiders are more numerous in Guam compared to the other islands, presumably due to a lack of vertebrate predators[21]. The full impact of the snake is still being uncovered.

Preventing Further Degradation by Managing the Invasive Species

Once a species has been identified as being invasive in an ecosystem, it is essential to act as quickly as possible in order to prevent further impacts. In some systems, eradication may be impossible, in which case control methods should aim at containing the population. This step is challenging and calls for the cooperation of many actors such as scientists, policy holders and funding agencies to identify methods to control the invasive species across the landscape.

Many tools exist and are being developed to manage invasive species[22], but the specific tools to use depend on the species and habitat. Mechanical control methods, such as hunting and invasive plant removal, are manually intensive and can be costly if an invasive species has reached high densities. Chemical control methods such as application of herbicides and pesticides have proven effective but are often costly and may be socially unpalatable[17].

Sterilization and gene drive techniques are being developed for use in invasive species control. Sterilization involves sterilizing either males or females of a species and releasing those sterilized individuals into a population where they are mating but not physically able to produce offspring. Sterilization has been used successfully to suppress fruit fly populations[23] and has shown some success in pilot studies targeting mosquitoes[24]. Gene drive approaches manipulate a genome to increase the likelihood specific alleles will be inherited[25]. Gene drive control methods are being pursued for rats and some other invasive species[26][27], but remain controversial because of uncertainty of its safety and fears that effects may reach beyond target populations[28].  

In the case of invasive brown treesnakes on Guam, the snakes already occupied the majority of the island by the time management began, so management efforts were directed at control rather than eradication. The snake had spread throughout the entire island by the time it was linked to the loss of birds in the mid-1980’s. Dr. Julie Savidge ruled out other suspected agents such as disease and pathogens, and temporally and geographically aligned the spread of the snake with the disappearance of birds across the island[13], making a convincing case that the snake was the culprit. However, she still faced difficulties convincing others that a single species of introduced snake was responsible for the alarming disappearance of native birds, so control of the snake didn’t start occurring until 1993. The goal of control has largely been to keep the snakes from establishing on other islands, such as Hawaii. Initial control methods implemented included snake trapping, visual searching, and detector dogs to inspect cargo and planes leaving the island, and concrete barriers to quarantine cargo originating from Guam on snake-free islands[29]. Recently a chemical control method has been developed. This method consists of dropping dead mice with an attached acetaminophen tablet (a toxin for brown treesnakes) from helicopters to the forest canopy where snakes will encounter and consume them. These “mouse-drops” need to occur at regular intervals over a prolonged period to ensure all snakes in an area will be affected[30]. Areas may be enclosed with snake barriers in order to prevent incursion and reach eradication. The barriers erected for this purpose in Guam also serve as effective exclosures for invasive deer and pigs. Researchers are also trying to predict indirect effects of invasive control that may require further management, such as increases in invasive small mammals with the eradication of snakes or increases in weedy plant species with the eradication of ungulates.

Restoring Degraded Ecological Functions in the Ecosystem  

Figure 2. Conceptual framework for restoring ecological functions in an ecosystem. First, it is essential to identify the ecological services that need to be restored and the species that may be able to provide those services. The next step is to conduct a landscape-level evaluation of the potential spatial distribution of both the service and its provider. Then, both should be combined to identify optimal rewilding areas. Finally, societal constraints should be taken into account to develop effective management strategies and policies.

In an ideal situation, the invasive species would be eradicated, and then native species restored across the landscape, in turn restoring ecological function. However, complete eradication is often impossible. Instead, invasive population reduction or local eradication combined with restoration of native species or surrogate species within protected areas may be possible. In this section, we describe a proposed conceptual framework (Figure 2) to use when planning invasive control and rewilding scenarios for the purposes of restoring ecological functions.

Identifying the Key Ecosystem Function Providers

The degradation of an ecosystem is often caused by the extirpation of ecological functions provided by keystone or foundational species. If these functionally important species are not entirely extirpated from the system, the first step is to identify strategies for increasing the population, via conserving or enhancing essential habitats for these species and controlling the invasive species. In an ideal situation, one would be able to conduct an ecological function analysis to assess the abundance of the species required to provide sufficient ecological function[31]. On the other hand, if these species are already extinct from the ecosystem, then rewilding will be necessary to restore the extirpated ecological functions. Rewilding can be defined as introducing locally extinct or ecologically similar species to perform ecosystem functions analogous to those of extinct species[32][33][34] . In some cases, the species that has been extirpated can be found in ther systems. This is the most favorable case, with the possibility of translocating populations back into the ecosystem from these other sources. In other cases, if the species is globally extinct, rewilding can be done by selecting a species that can perform the same ecological function. This requires a great deal of caution, however, since predicting the extent of possible impacts on an ecosystem is very difficult. Many case studies have underlined the risks of such an approach[34].

Figure 3. Seed dispersal kernels of five avian seed dispersers and 15 tree species on Saipan. Each panel represents a modelled species-specific interaction. For each interaction, we show standard dispersal curves, median and 99th percentile dispersal distances (numbers in top right of panel), and probability of seed dispersal greater than 500 m (filled dots represent probabilities greater than 0 and hollow dots represent probability = 0). The final column represents potential seed dispersal in two dimensions for bird species dispersing that given plant species (Rehm et al.,2019).

The first step towards restoring a functional ecosystem in Guam was to identify frugivorous species that were effective seed dispersers[35] . Nearby islands to Guam were unaffected by the brown treesnake and host avian communities similar to those historically present in Guam. This allowed us to prioritize reintroducing native species over introducing non-native species to Guam, and to assess which native frugivores were the most effective dispersers. We conducted studies to evaluate the effect of gut passage on germination of native forest tree species, and to study the movement of the candidate avian frugivore species (Figure 3)[36]. The Micronesian Starling was identified as an ideal candidate for rewilding because of its extensive diet, probability of dispersing seeds a long distance, and propensity for crossing from native to degraded forest, and thus assisting with forest regeneration[35]. Other strong candidates for rewilding are the Mariana Fruit Dove (Ptilinopus roseicapilla) and the Mariana Fruit Bat (Pteropus mariannus), because they also disperse many tree species. Furthermore, Micronesian Starlings and Mariana Fruit Bats are still present in small populations on the island of Guam, even in the presence of the brown treesnake. Restoration of seed dispersal can be accomplished through a combination of increasing the population and extending the range of the two extant species and rewilding the extirpated species[2][20][37].


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