How do introduced species threaten biodiversity

Slower rates of invasion would have given local predators, pathogens, parasites, herbivores and competitors time to adapt to and thus start limiting invaders before the invaders were able to completely displace native species. Today that is not necessarily the case; Alliaria petiolata, Microstegium vimineum and Berberis thunbergii , three exotic plant species that are currently invasive in the US, are indirectly facilitated by a native generalist herbivore, the white-tailed deer, which preferentially grazes co-occurring native plants 33 but see ref.

Not all exotic species introduced by humans experience beneficial enemy release 35 , 36 , 37 , 38 or experience it forever 39 , 40 , but those that do experience it can potentially: live longer 41 , grow larger 42 , reach higher abundances 33 , expand their environmental range 41 , 43 , increase their competitive ability 44 , reproduce more 42 , and reproduce more successfully 45 in their introduced ranges than in their native ranges. These two processes, which stem from human cultivation and the novel evolutionary histories of exotic species, could effectively move some exotic species off a universal tradeoff surface by enabling them to become better colonisers, better competitors or longer-lived than their native counterparts Fig.

Critically, we note that these are not the only processes that could enable species—and exotic species in particular—to move off interspecific tradeoff surfaces 31 Supplementary Table 2 ; we simply use these two prominent, and mechanistically distinct, hypotheses to illustrate our theory.

This combination of decreased mortality, higher seed production and increased competitive ability pose a triple threat of extinction, and—unlike effects of external propagule inputs, which are local—enemy release could occur across an entire area that is invaded. Habitat loss, nitrogen deposition, climate change and changes in disturbance regimes can threaten native species directly, but they can also augment the benefits of high propagule rain 25 , 26 and enemy release 33 , 46 to exacerbate invasions though not always Simulated species extinctions resulting from human-mediated species invasions.

Each line represents a species: the red line indicates the invader after elevating h 10 to 0. In this paper, we use theoretical evidence to determine the conditions for which an exotic species could plausibly lead to local extinctions of its native competitors.

We first present a model that illustrates how competition-colonisation tradeoffs allow species to coexist 11 , We expand the model to incorporate ways in which modern species invasions may disrupt this mechanism of species coexistence, even when the invaders themselves are intrinsically bound by universal tradeoffs. We then analyse ensembles of metacommunity models to identify ways in which human-mediated species invasions, alone and with elevated disturbance, could plausibly drive one or more native species extinct.

Competing species have coexisted for millions of years following past biogeographic migrations In the coexistence model based on a competition-colonisation tradeoff that we use, species are ranked from the best to the poorest competitor s 1 to s N in an N -species metacommunity Species are able to coexist if their colonisation abilities are inversely related to their competitive rank Fig.

The model assumes a perfect competitive hierarchy where superior competitors can displace inferior competitors, and the weakest competitors only colonise vacant sites. Native species may also receive external inputs of propagules if they are planted and cultivated by humans. However, modern exotic species are more likely to experience increased h i because, by definition, they are introduced by humans and associated with humans.

We find that the addition of an external propagule supply h i increases the relative abundance of an invader s 10 in Fig. The probability and extent of species displacement increases over time as the population size of the invader increases, and effects cascade through the populations of inferior competitors Fig. Generally, the further a species is shifted off the tradeoff surface through higher values of h i , and the greater the number of ecologically similar inferior competitors the invader has, the higher the rate and number of extinctions that eventually occur Fig.

Native species that have similar colonisation abilities as the invader but are competitively inferior , and are thus positioned close to the invader on the undisrupted tradeoff surface, are most affected by additions of h i. Native species extinctions resulting from species invasion in species metacommunities where the invader experiences an increase in external colonists h i , and a reduction in mortality m i. For each line in each panel, a new species has been introduced that would normally coexist with the 19 other species in the metacommunity given their natural colonisation and competitive abilities i.

However, input of external colonists h i , shown in panel a and reduced mortality m i , shown in panel b changes in h i and m i are both measured on the X -axis of the invader allows it to overcome this tradeoff and cause extinctions. Where lines plateau, all inferior competitors have been driven to extinction. Species may experience lower rates of enemy attack, including seed predation, when introduced beyond their historic range Rather than discussing the generality of enemy release 48 , 49 and other hypotheses that invoke the novel evolutionary histories of exotic species, which may shift species off the tradeoff surface through changes in biotic interactions 26 , 31 , 50 , here we focus on the potential implications of enemy release—if and when it occurs—on species coexistence.

As with increased h i , decreased m i causes the abundance of the released invader to increase to the detriment of inferior competitors, which may be driven extinct Fig. The greatest number of extinctions, of course, would come from an invader having: more propagules; decreased mortality; and increased competitive ability. Modern biological invasions occur in the context of multiple anthropogenic environmental changes, including climate change, habitat destruction, habitat fragmentation, nutrient deposition and altered disturbance regimes Supplementary Table 2 , each of which could reduce the abundance of one or more native species.

Such reduced abundances might, in combination with human-mediated invasions, cause extinction even if neither factor would do so on its own. To gauge potential interactive effects of invasions and environmental change, we examined diversity consequences of species-specific increases in h i when they are set against a backdrop of elevated disturbance increasing m i of all species in the metacommunity , as would be experienced with increased trampling, mowing, storm damage, fire, and so on.

Increasing mortality rates m i of all species generally heightened the number and rate of extinctions caused by species-specific increases in h i Fig. By itself, elevated disturbance also led to species losses. As with the specific example of habitat destruction 53 , elevated disturbance disproportionately reduces the abundance of poor colonisers which are also better competitors, reflecting tradeoffs , increasing their risk of extinction.

Elevated disturbance correspondingly limits the impact of competitive invaders, which are poor colonisers s 1 in Fig. Species-specific increases in h i and reductions in m i can counteract effects of elevated disturbance, meaning invaders would be less affected by disturbance than native species, all else being equal. Native species extinctions resulting from elevated disturbance and invasion of a species with a supply of external colonists h i. Disturbance increases the mortality rate of all 20 species in the metacommunity from 0.

Other details as in Fig. In all panels, species are lost from the metacommunity due to increased disturbance alone i. Invasion of s 7 causes more extinctions than s 1 under high disturbance shown in b because the natural colonisation rate of s 1 c 1 is too low to cope with elevated mortality, unless increases in h 1 compensate for it.

Each of these exotic species must allocate some of its resources to disperse, establish and reproduce—just like native species.

However, human intervention means that exotics may not experience the full suite of constraints experienced by native species in vicinities where they are introduced and cultivated Compounding the introduction and dispersal advantage of cultivated plants, laws of many countries only allow the importation of exotic plants if they are certifiably free of pests or pathogens, resulting in a joint h i and m i scenario.

This biosecurity requirement is designed to prevent the arrival of new pests and pathogens that could devastate agriculture, horticulture and forestry industries e.

Dutch elm disease, Phytophera. However, exotic species that had been particularly strongly restrained in their native habitat by high enemy loads may therefore be unusually successful in habitats in which they become essentially enemy-free Supplementary Table 1. This dynamic may partially explain why some species with highly restricted native ranges can become globally invasive. For example, Monterey pine Pinus radiata is planted in over 4 million hectares throughout the world for timber production, and is the dominant tree plantation species used in Chile, Australia and New Zealand It has escaped from cultivation and invades native vegetation, and is a threat to biodiversity in much of the Southern hemisphere 56 where its invasiveness is partially attributed to enemy release Species that are rare in their native ranges but highly invasive elsewhere, like Monterey pine, Cootamundra wattle Acacia baileyana , Blue gum Eucalyptus globulus , Small balsam Impatiens parviflora and Yellow start thistle Centaurea solstitialis , may exemplify species that have overcome an interspecific tradeoff because of human introduction.

Demographic data from plant species suggest that invasive species may flout the fast growth versus long survival tradeoff that constrains other species because, unlike the vast majority of non-invasive species, they are able to achieve high reproduction and fast growth rates without compromising survival How these species might be overcoming this tradeoff is largely unexplored, but there is some suggestive evidence that enemy release could be responsible. The ability of Acer platanoides , an invasive tree in the US, to maintain high growth rates under both high and low light 43 has been attributed to it experiencing a three-fold reduction in herbivory in its introduced versus native range 58 , and lower herbivore attack than a native congener Similar observations have been made in tropical 41 and temperate 25 forests for other exotic species Supplementary Table 1.

The notable success of some biocontrol agents in constraining deliberately introduced exotic species, like Prickly pear Opuntia stricta , Salvinia molesta and Gorse Ulex europaeus , demonstrates the importance of confining invaders to their position on interspecific tradeoff surfaces for species coexistence. Our theoretical model predicts that invading species that overcome universal tradeoffs could eventually displace co-occurring native species that occupy similar niches as the invader.

Local extinctions of native species have been observed that may stem from the disruption of interspecific tradeoffs whether via human introduction, enemy release or another mechanism 31 , Supplementary Table 3 , but such extinctions are generally predicted to occur tens to hundreds of generations after the onset of species invasion The ability to overcome interspecific tradeoffs is not necessarily restricted to exotic species though exotics are much more likely to experience enemy release and other benefits of novel evolutionary histories, in particular, than natives , and likely applies to more than just plants.

Patagonian lakes with higher aquaculture intensity have higher abundances of exotic salmonids and lower abundances of native fish 62 , and exotic populations of animals have half as many parasite species and experience lower infection rates per individual than native populations Experiments designed to directly test this theory will no doubt be very informative. By identifying mechanisms through which exotic species introduction and cultivation could prompt the local extinction of native con-trophic species, our theory may help resolve a question that has inspired vigorous debate in ecology and conservation biology 3 , 4 , 5.

These conditions are matched by empirical observations 2 , When considering the two processes that we have focused on—cultivation and enemy release—impacts are likely to be greatest when the invaders are highly competitive yet ecologically similar to native species akin to large fitness but small niche differences 60 , 61 , intensively cultivated or planted, and when they experience high levels of enemy release.

Many species introduced to date may be poor competitors 64 , and only some of these species are likely to have been moved off the tradeoff surface far enough, or for long enough, to meaningfully reduce the abundances of native species. So, while global extinctions are a possible outcome of our proposed mechanism, they have, at least for now, been relatively rare 2.

We appreciate that our theoretical assumptions simplify the real world, and do not suggest that all exotic species will overcome the tradeoff surface or will cause native extinctions. Rather, we illustrate a mechanism by which species invasion, and particularly the introduction of enemy-free species and mass cultivation, could cause the eventual displacement of ecologically similar native species.

Given that species displacement occurs over many generations Fig. We used ensembles of multispecies metacommunity competition models to simulate the dynamics of communities where one species does not conform to a competition-colonisation tradeoff 11 , The metacommunity model was continuous in space, with the abundance dynamics of each population described by the proportion of the total available area they occupy Eq.

Equilibrium abundances for species metacommunities were found by iteratively solving the equations, from the most competitive to the least competitive species. To robustly assess the effect of invaders on extinction rates, we generated a large ensemble of different metacommunities, each with 20 coexisting species. We used this approach, rather than adding an additional species, to ensure we were using a combination of species that would ordinarily coexist, i. We reduced, m a , the mortality of the invader, to simulate enemy release.

When we simulated elevated disturbance, we increased the value of m i for all species in the metacommunity including the invader by the same amount. For each analysis, we designated a particular species to be the invader, and simulated the resulting changes in the metacommunity abundances.

Figure 1c shows the resulting changes in the relative abundance of each species through time, p i t , for a single metacommunity. We defined a species as extinct at time t if its abundance declined below 0. Although the competition-colonisation tradeoff model has been extended and developed 53 , 67 , 68 , we use a simple version to ensure that our points are clear 69 , but note that our modifications could be applied to all variants of the model.

In particular, we used a limited number of scenarios and a simple theoretical model that i does not include niche preemption, ii has uniform mortality across all species, and iii specifies that the release of invaders from natural enemies only results in a reduction in mortality.

If we had used replacement rather than displacement competition i. If we had allowed mortality rates to vary among species such that mortality was part of the tradeoff enabling coexistence i.

This is a necessary simplification of naturally occurring communities, and we acknowledge that the predictive power or applicability to a specific system can be limited with such a simple approach We nevertheless contend that our key points, demonstrated here, apply to more complex systems and more diverse communities because of the role of interspecific tradeoffs in facilitating species coexistence.

Species coexist via multiple tradeoffs, reflecting the myriad resources for which species can compete e. The ideas we have discussed in our paper should be applicable to these tradeoffs.

Results can be reproduced from the models described in the paper, but these data are also available from the corresponding author upon reasonable request.

Tittensor, D. A mid-term analysis of progress toward international biodiversity targets. Science , — Bellard, C. Alien species as a driver of recent extinctions. Simberloff, D. Non-natives: scientists object. Nature , 36 Hulme, P. Challenging the view that invasive non-native plants are not a significant threat to the floristic diversity of Great Britain.

Natl Acad. USA , E—E Thomas, C. The Anthropocene could raise biological diversity. Nature , 7 Gilbert, B. Plant invasions and extinction debts. Global decline in amphibian species : This Limosa Harlequin Frog Atelopus limosus , an endangered species from Panama, died from a fungal disease called chytridiomycosis. The red lesions are symptomatic of the disease.

Lakes and islands are particularly vulnerable to extinction threats from introduced species. In Lake Victoria, as mentioned earlier, the intentional introduction of the Nile perch was largely responsible for the extinction of about species of cichlids.

The accidental introduction of the brown tree snake via aircraft from the Solomon Islands to Guam in has led to the extinction of three species of birds and three to five species of reptiles endemic to the island. Several other species are still threatened. The brown tree snake is adept at exploiting human transportation as a means to migrate; one was even found on an aircraft arriving in Corpus Christi, Texas.

Constant vigilance on the part of airport, military, and commercial aircraft personnel is required to prevent the snake from moving from Guam to other islands in the Pacific, especially Hawaii.

Islands do not make up a large area of land on the globe, but they do contain a disproportionate number of endemic species because of their isolation from mainland ancestors. It now appears that the global decline in amphibian species recognized in the s is, in some part, caused by the fungus Batrachochytrium dendrobatidis , which causes the disease chytridiomycosis.

There is evidence that the fungus, native to Africa, may have been spread throughout the world by transport of a commonly-used laboratory and pet species: the African clawed toad Xenopus laevis. It may well be that biologists themselves are responsible for spreading this disease worldwide. The North American bullfrog, Rana catesbeiana , which has also been widely introduced as a food animal, but which easily escapes captivity, survives most infections of Batrachochytriumdendrobatidis and can act as a reservoir for the disease.

The global warming trend is recognized as a major biodiversity threat, especially when combined with other threats such as habitat loss. Climate change, specifically, the anthropogenic caused by humans warming trend presently underway, is recognized as a major extinction threat, particularly when combined with other threats such as habitat loss.

Scientists disagree about the probable magnitude of the effects, with extinction rate estimates ranging from 15 percent to 40 percent of species by Scientists do agree, however, that climate change will alter regional climates, including rainfall and snowfall patterns, making habitats less hospitable to the species living in them.

Grizzly-polar bear hybrid : Since , grizzly bears Ursus arctos horribilis have been spotted farther north than their historic range, a possible consequence of climate change. As a result, grizzly bear habitat now overlaps polar bear Ursus maritimus habitat. The two kinds of bears, which are capable of mating and producing viable offspring, are considered separate species as historically they lived in different habitats and never met.

However, in a hunter shot a wild grizzly-polar bear hybrid known as a grolar bear, the first wild hybrid ever found. The warming trend will shift colder climates toward the north and south poles, forcing species to move with their adapted climate norms while facing habitat gaps along the way. The shifting ranges will impose new competitive regimes on species as they find themselves in contact with other species not present in their historic range.

One such unexpected species contact is between polar bears and grizzly bears. Previously, these two species had separate ranges. Now, with their ranges are overlapping, there are documented cases of these two species mating and producing viable offspring. Many contemporary mismatches to shifts in resource availability and timing have recently been documented.

Range shifts are already being observed. The same study suggests that the optimal shift based on warming trends was double that distance, suggesting that the populations are not moving quickly enough.

Range shifts have also been observed in plants, butterflies, other insects, freshwater fishes, reptiles, and mammals. Climate gradients will also move up mountains, eventually crowding species higher in altitude and eliminating the habitat for those species adapted to the highest elevations. Some climates will completely disappear. The rate of warming appears to be accelerated in the arctic, which is recognized as a serious threat to polar bear populations that require sea ice to hunt seals during the winter months; seals are the only source of protein available to polar bears.

A trend to decreasing sea ice coverage has occurred since observations began in the mid-twentieth century. The rate of decline observed in recent years is far greater than previously predicted by climate models.

Finally, global warming will raise ocean levels due to glacial melt and the greater volume of warmer water. Shorelines will be inundated, reducing island size, which will have an effect on many species; a number of islands will disappear entirely.

Additionally, the gradual melting and subsequent refreezing of the poles, glaciers, and higher elevation mountains, a cycle that has provided freshwater to environments for centuries, will also be jeopardized.

This could result in an overabundance of salt water and a shortage of fresh water. Privacy Policy. Skip to main content. Conservation Biology and Biodiversity. Search for:. Threats to Biodiversity. Habitat Loss and Sustainability Through increased adoption of sustainable practices, we can reduce habitat loss and its consequences.

Learning Objectives Describe the effects of habitat loss to biodiversity and concept of sustainability. Key Takeaways Key Points Habitat destruction renders entire habitats functionally unable to support the species present; biodiversity is reduced in this process when existing organisms in the habitat are displaced or destroyed.

Clearing areas for agricultural purposes is the main cause of habitat destruction; other principal causes include mining, logging, and urban sprawl. The primary cause of species extinction worldwide is habitat destruction. Sustainability is a term that describes how biological systems remain diverse and productive over time, creating the potential for long-term maintenance of human well-being.

Reducing negative human impact requires three concepts: environmental management, management of human consumption of resources, and awareness of cultural and political concerns to increase sustainability. Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.

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Please support us by making a donation or purchasing a subscription today. Cosmos » Earth » Invasive species threaten biodiversity Share Tweet. Canadian geese are now well established in Europe, posing a serious threat to biodiversity. More on:. Zebra mussels cost North America more than a billion dollars each year. Natalie Parletta Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.

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