First if we take a simplistic view of the ecosystem, if the invasive and native species were competing for resources, then the local extintion of the invasive should release resources for the native to expliot.

Then if your native population has been lowered past their "minnimum viable population", then they are on a train to extintion for they can not recover their nubers, may it be becasue of genetic depression (interbreeding), couples finding it hard to find each other to mate, or random events such as a fire.

That being the case new individuals should be released and if possible with a degree of genetic heterogeneity.

(This is a very simplistic and deterministic view of the ecosystem, many other things can happen, and to be able to predict them you should gather data of that habitat.)

It varies completely on the individual situation of the area in question. Invasive species can impact the quality of the land (e.g. depleting soil nutrients, changing hydrology, etc.) so lingering impacts could still be a problem once eradicated. Some native plant species require time to be fully productive again (e.g. many woody species). So the question becomes what time-scale are you talking about (e.g. years, decades, centuries)? As for returning wildlife, it wholly depends on the current flora and fauna of the surrounding area. If there are no wildlife corridors then the likelihood of returning wildlife is reduced. The distance from existing habitat for repopulation is entirely dependent on the species in question due to their own limitations in motility. Repopulation can occur using both captive and wild species that are re-released. The success of these transfers are highly debatable. Typically most people want to restore habitat for large species with high visibility; however, these organisms need the supporting life (e.g. insects, appropriate soil bacteria) for the ecosystem to thrive. A fully developed plan/approach is necessary before reintroduction of many macro-organisms.

This article is definitely worth a read: http://www.ege.fcen.uba.ar/materias/general/Seminario%20Invasiones%202.pdf

When thinking about eradication it's important to consider the ecological and evolutionary dynamics of the system. There are cases where eradication of invasive species could have serious negative impacts for one or more native species.

A key paper on this topic is Didham et al. 2005: Trends ecol evol

It firstly depends on wether the invasive species was the actual cause of the decline of the native species. In many cases, however, the invaders were just taking advantage of habitat deterioration, whereas the native was suffering from this deterioration. Basically, drivers versus passengers of ecological change. But you can get complex interactions between both situations, where also priority effects pop up.

http://si-pddr.si.edu/jspui/bitstream/10088/6904/1/Didham_Tylianakis_Hutchinson_Ewers_and_Gemmell_2005.pdf

When we remove invasive plants, such as phragmites, the substrate left behind is degraded so the recovery of natives depends on active intervention.

Thank you doctors for answering.

I understand that a lot of abiotic and biotic factors must be taken into account to actually understand how these native organisms would bounce back from their possibly declining population when the alien species were present.

Nevertheless, it would be much appreciated if anyone could tell me, if it is possible for these native species to grow even better in the area previously occupied by the invasive species? And if there are such occurrences, what kind of organisms displayed this?

The Australian Wildlife Conservancy has several sites where it has eradicated foxes/cats/rabbits and stopped their reinvasion using conservation fences and thereafter reintroduced native species. These native species do very well in the absence of the agents of their initial decline, but still struggle wherever they coexist. Their website has some information (www.awc.org) or the 2010 Reintroduction Case Studies book edited by Pritpal Soorae for the IUCN RSG amongst others.

In plant re-introductions the new restored habitat does very well, but it is a combination of factors that include intervention to prevent the recurrence of the invader and also hands-on work to cultivate the restored species. This is particularly true in harsh environments. I am familiar with coastal marsh and succulent thicket restoration. So, yes, restored species can thrive but not without intervention. Once re-established, though, if the correct species has been chosen, they can recover the habitat and multifaceted habitat functions very well. In marsh habitat, invertebrates and fish return quickly. In succulent thicket, there is a long lag while soil ecology recovers, and this is extended if herbivory is not controlled. Not many papers on this but look at Mills and Cowling and Kerley from South Africa. For marshes look at Keryn Blomberg.

In my region we deal with the eradication of House Crows and Parakeets. We find that they have impacted our local avifauna by outcompeting them for breeding cavities or by actively feeding them and their young. However, a domino effect is evident (as yet unpublished data) when th crow numbers are decimated extensively we get a rebound of the Domestic Pigeons (nestlings and eggs taken by crows), which out compete the Collared Doves on the foraging grounds, which in turn allows for an increase in the numbers of Palm Doves. This see-saw is evident also through the complaints registered at the local municipality offices and which reflect the number of attacks by crows or the bother of pigeons on the washing at home!!

Ecological systems are dynamic. Removing invasive species and any other stressors and then allowing the native species to reestablish themselves is referred to as passive restoration. Your objective should be to restore the pre-invasion dynamics of the system. Species abundances may and likely will change over time, so it may not be a precise match to the pre-invasion species frequencies. However if the composition (if not the abundances) and dynamics are reestablished it would be a successful effort.

In some cases invasive species can change cryptic ecological processes such as soil chemistry (increasing salinity), soil biology (changing or eliminating soil mycorrhiza) or soil structure (increasing compaction). Even with the removal of the invasive species these damaged ecological processes can remain and will challenge successful restoration efforts. In such instances, or when native propagules are no longer present, active restoration may be needed.

You did not state whether the invaders were plants or animals. For plant invaders, it is quite likely that a new succession process would start which may not favour the pre-invasion species. Ecological dynamics of the invasion period would have made some dormant or dispersed seeds to thrive after eradication. Most plant invaders are gregarious, often depleting soil nutrients and affecting the hydrology of the place. Thus the need for deliberate intervention to favour the pre-invasion species.

Removal of animal invaders may not guarantee the return or thriving of the former ones (if they did not move away). Reintroduction, creation of corridors or the management of the land to attract them may be necessary.

You ask 'Would these native organisms accordingly return or would there still be a need for an introduction of these organisms that were kept in captivity?': As mentioned in a previous answer, this will depend very much on whether the organism in question is plant or animal.My experience involves reintroducing the West Europen hedgehog back to the wild. The animals are rehabilitated following treatment for injury and/or disease and the hedgehog has been shown (by myself and others) to respond well to hard release i.e. not placed in enclosures for a while prior to their release to the designated area. Long-term monitoring that I have been carrying out proves, beyond all measure of doubt, that hedgehogs do survive from one year to the next, in the areas where they have been released. Where hedgehogs, and other small mammals, are present already in the locality, it should not take long for them to repopulate an area from which they have previously been ousted due to the presence of an invasive species that competed with them in some way. I would look at the local population density of any species that, it is hoped, can be reintroduced to an area where they were previously absent. If time is of the essence then a passive approach, whereby animals find themselves in the intended area mostly by chance, is not the best to adopt; rather an active introduction of the species, having first checked that the area is still compatible for the continuing survival and breeding success of the introduced species.

Several researchers has given their insights into the topic and its great!!!Literature said that the results of eradication are promising in good number of cases but simultaneously its ecological side is highly complex.

Generally, the restorations of any habitat to its original condition just by eradication of an alien species is a function of extent of invasion and also availability of the resources for re-establishment of natives. If an ecosystem has been occupied by an invasive species for a long time, there are every possible chance that it has been altered in several ways, either at a macro/microscale which thereby would not result in restoration to original condition or may be it will occur at much slower pace.

Please see the link for a short review:

http://www.ege.fcen.uba.ar/materias/general/Seminario%20Invasiones%202.pdf

Two large estuaries in coastal Washington (USA) were impacted by Spartina. The Spartina has since been removed and maintenance control is ongoing. Recent investigations have shown that the Spartina likely trapped sediments, as is known to occur, and that the mudflats are now slightly higher in elevation than before. Consequently, although the invasive species has been removed, there are lingering effects, the significance of which are unknown. It is unknown whether or how soon mudflat conditions will return to the former state. However, the mudflats are being used by shorebirds, which was not possible when Spartina was present, so the habitat is again functional. We lack information on prey availability and abundance, and whether the shorebirds find the foraging areas as productive as they might have been prior to Spatina presence.

I agree with what mr. Barrows said. Restoration of ecological processes or the original habitat will not be easy. You must look into the factors/elements that have affected the area or ecological processes.

Before thinking of removing the invasive species, weigh their advantages and disadvantages in that habitat first. Will they be able to help other species than just destroy the native species? Will they be more efficient in that area or not? Should u consider transferring the native species in another habitat instead? Consider the relationships between the species.

As for the question of if the native species will return, it depends upon the species and the situation. The native species will be more likely to return if they'll see their old habitat with other native species or if they sense that the place is safe.

An interesting answer to this question illustrates the complexity of human-modified systems. In southern Brazil, the original (pre-colonization) forests existed since gondwana (in many places) and so the forests tend towards tropical. But, in the time since, weather has changed a lot and frost is now common during winter. Surprisingly, none of the native plants are adapted to frost (or very few) and frost can kill saplings. Prior to deforestation, frost never reached the forest floor but rather only touched the tops of the canopy, probably because the humidity and density of the understory helped keep temperatures higher. Once clear cut, frost reaches the ground and kills saplings. So, if you merely remove the invasive pines and eucalyptus so common there, the native plants do not necessarily return and indeed, OTHER invasive plants (mulberry, privets, among others) can rapidly fill the void. At my place, where I am trying to recover natives, I have had to insulate the native plants so they survive a winter. If climate, such as dryness, cold, etc. can limit germination or survival of seedlings, then merely removing exotics will not favor the return to the native flora.

In general, total eradication of (at least) aquatic species doesn't seem possible as long as persist conditions, which enabled it's settlement. Now, the new species may be an eurythrophic and eurythermal one, and in such case it may withstand changing external conditions and stay put, even if the former regime returns. Then, there's the possibility of genetic adaptation. MB-Y

Carrizo cane (Arundo donax) is an invasive species that dominates the riverbanks of the Rio Grande along the Texas-Mexico border. A program to eradicate the cane is an interagency (USDA, USFWS, BLM) effort that is using multiple methods. In monoculture stands, it does not provide food or habitat for native species. When eliminated, and the areas can be replanted with native plant species, and those areas support a much higher diversity native wildlife community.

For me eradication has a specific connotation, it implies that there are no reproductive individuals left in the target area, and reinvasion is very unlikely - little or no management is needed to keep the invasive species from recolonizing the site. This is rarely achievable except on islands e.g. goats, deer, pigs and rats have been eradicated from a range of islands in the Pacific, or where the populations are at an early stage of invasion (in this latter case they may not have much of an impact on native species). With plants having nearly invisible propagules that may persist in the environment the proposition is much more difficult.

I have never heard of a case where all the invasive species are eradicated as I defined it. So the answer is usually no, the whole suite of native species is unlikely to return to a pre-human contact abundance and distribution. Combine this with the fact that ecosystems are not static anyway - and you are unlikely to ever see a system return to its original state. That being said some eradication programs for invasive mammals on oceanic islands have been immensely successful in allowing population levels of impacted native species (especially animals) to become as dominant as they were prior to invasions.

Towns, David R., Ian AE Atkinson, and Charles H. Daugherty. "Have the harmful effects of introduced rats on islands been exaggerated?." Biological Invasions 8.4 (2006): 863-891.

Simberloff, Daniel. "Eradication of island invasives: practical actions and results achieved." Trends in Ecology & Evolution 16.6 (2001): 273-274.

Its unlikely for an environment to remain unaltered ecologically after invasion by invasive species, so also it is unlikely for the resident species not have undergone some morphological, ecological or physiological adaptations to buffer or survive this invasion.

Thus if and when this invasive species are eradicated, the return to its previous state is unlikely to be an automatic process, most likely and if possible it will take a gradual recuperation phase/process.

The evidences of the invasion are unlikely to be totally eradicated. Most likely some organisms will remain in the realized niche due to the invasion and never really attain their fundamental niche.

There is a question if you are talking about exotic animal or exotic plant species. If it is exotic plants, then 49% of your efforts will be towards eliminating as much of the soil weed seed bank, and then suppressing whatever dormant weed seeds remain, by replanting the proper local native plant species that will do that job using allelochemicals, so you will not have to be out there cleaning up the sprouts for decades into the future. The other 51% of the job, is to restore the ruined native ecosystem, and that can be very expensive, and it usually will not restore itself on its own. You can see a current project at http://www.ecoseeds.com/arastradero.html.

Grazing for any period of time worldwide, has cause spatial extinction of native species. For example in California 99.99% of the land lost its native understory species 100 years ago, so natives are technically extinct on most lands in the State. Then, there are the soil nutrient changes that occurs with grazing, where the animals mine nutrients as they graze and then are taken off the land and slaughtered, and their bones, hooves and hides not returned to the area. There is a soil nutrient threshold that each species of native plant needs, in order to survive as a seedling. When grazing animals drop the soil nutrient levels below that threshold, no amount of native seeds sown on those lands will survive until you add the missing nutrients above the threshold levels.

The native species would return, BUT the community structure would unlikely be the same. One thing that the invasive species do is modifying food webs or even habitats. For example, water buffaloes have changed slope of river bank in Australia due to their behaviour. That change facilitates some native but impede the other. Even if they were removed, the physical structure of the bank would take time to recover or might not at all.

My current project of managing exotic plants on 15 acres in the town of Woodside, California is finding 100-250 year old dormant native seeds coming up, at the rate of one to three seedlings per SQUARE CM!' Plus, we are going from 90% exotic cover to 90% native cover in only 4-5 years. In spring we had 500 billion wildflowers blooming that did not exist a few years earlier, picture attached.

This question, although natural, involves so many variables as to lack a useful answer.

In our experience here, there are numerous responses to culling exotics that depend upon whether the cohort of natives is complete. Then there is the question of whether the natives CAN be managed the way they were for the last ten millennia or so. Here in California for example, we cannot burn as Indians did because we have been so foolish as to accumulate excessive fuel. Yet no modern harvests what their landsapes once produced or hunts as aboriginals once did with the overt intention to maintain system stability for purposes of survival. How then can one reasonable expect the system to remain stable without what they did for so long?

As natives have returned here, we've had some run completely amok for the lack of regular fire at the right time of year. We have seen the lack of insect larvae munching on flowers allow an otherwise inocuous biennial to become a total pain. We have seen natives become invasive, whether by epigenetic transformation or because they were foreign to the immediate area cannot be established. Then there are "natives" with exotic relatives that may in fact be hybrids of both. Finally, there is a serious question re whether we know that a particular "native" is in fact a member of the co-adapted local flora at all, having once been held farther away by the character of aboriginal management, not just a serious consideration here, but worldwide.

I have a lot more to say about this for which I haven't the time. You can read more about these considerations at https://www.wildergarten.org/wildergarten/intro-native.pdf

Mark

Hello, I agree with both Charles Bertelsen

Hey Mark Vande Pol can you describe over time, what you started with and how things changed over time, until you got the re-mark-able amount of native cover today, that probably nobody else on the planet has ever got that close before, within the time frame that you have been doing the project? Last time we estimated, you had produced 99.5% native cover? And all done without sowing a single seed--all from dormant native seeds still in the soil?

Chances of getting native species is high but depends upon site quality and management practices.

...can you describe over time, what you started with and how things changed over time, until you got the re-mark-able amount of native cover today, that probably nobody else on the planet has ever got that close before, within the time frame that you have been doing the project? Last time we estimated, you had produced 99.5%

It's all in Wildergarten, 1700pp of history, anthropology, philosophy, economics, documentation, and methods development. The first introductory chapter is at https://www.wildergarten.org/wildergarten/1trp-intro.pdf.

And all done without sowing a single seed--all from dormant native seeds still in the soil?

This is incorrect. I did buy some B. carinatus and E. glaucus for a grading project, but it is otherwise reasonably accurate. For most species, by aggregate numbers, our native seed bank was D-E-A-D, not all of it mind you, but mostly so with the distinctions by Genera. The lilies made it, as did some fabaeceae, a couple of Angelica, etc. My estimates as to the origins of current populations are annotated in our species list: https://www.wildergarten.org/w-docs/species-list.xls.

At this point, it cannot be determined to what degree the recolonization of the property was from a few remnant individuals in the seed bank or from seed carried in by animals. What I can say is that the process of colonization, dispersal, and adaptation took most of a decade (also documented in the picture book). Remember: Unlike where you are working, our property adjoined the Spanish Road between Missions Santa Clara and Santa Cruz. Hence, the exotic invasions commenced here in 1791 when the road was blown in.