Background

Re-introductions are considered by some conservation practitioners to be a controversial management option for mitigating threatened plant declines. The use of translocations (including re-introductions) has been criticised for the lack of monitoring and central recording, inappropriateness of the action due to genetic considerations, a lack of knowledge of the demography of the donor populations and inadequate information on the habitat requirements of the species. Despite these arguably justified criticisms, re-introductions are growing in use as practitioners see no other option for meeting management plan targets. Re-introductions have been proposed as options for overcoming habitat loss, habitat fragmentation and reproductive isolation. An extension of this increasingly interventionist approach, often termed assisted colonisation, is being considered as a potential method for preventing extinctions due to climatic shifts too rapid to allow corresponding species‟ distribution changes.

This review evaluates the effectiveness of re-introductions as a conservation tool by using the available evidence to determine in what context plant translocations have improved the status of threatened species.

Objectives

To evaluate the effectiveness of re-introductions as a method for mitigating extinctions of plant species by answering the following question: are re-introductions an effective method of increasing the viability of endangered or vulnerable plant species?

Methods

Ten electronic databases were searched using ten sets of search terms. The library databases of the Joint Nature Conservation Committee, Natural England, Scottish Natural Heritage and the Countryside Council for Wales were searched on behalf of the review team by staff at each of the agencies using search terms provided. The IUCN‟s Re-introductions Specialist Group and the Center for Plant Conservation have both published volumes on re-introductions which were used to identify cases for inclusion and practitioners that might be contacted for details on specific re- introduction attempts. The Botanical Society of the British Isles and Plantlife also provided databases on plant re-introductions in the UK.

Main results

Using systematic review protocols, we identified peer-reviewed and grey literature that provided evidence that re-introductions (using various definitions which some practitioners might describe as conservation introductions) have been attempted, or are planned, for approximately 700 taxa in 32 countries. The USA is the biggest advocate and implementer of re-introductions (228 taxa) and when considered in combination with the high use of the technique in Europe (particularly the UK), this explains why more than 300 taxa are associated with temperate forest biomes. National conservation protection was afforded by 28 countries to 440 taxa although we could not identify levels of protection for about 200 taxa and most (618) had not been evaluated using IUCN Red List protocols making it difficult to discern any links between the perceived extinction threat and use of re-introduction as a management strategy. Threats to target taxa were recorded and agriculture, grazing, competition from invasive or aggressive plants and urban or industrial development were most often cited accounting for 60% of stated causes of decline. When the level of endemism was used to categorise re-introduction targets, approximately one third could be classed as narrow endemics, one third had, at least formerly, wide distributions (cross-continental), and one third fell in between these extremes. This, together with the high level of national protection, indicates that many re- introductions are undertaken because the taxon is declining in part of its range rather than being undertaken as a „last resort‟ to mitigate species-level extinctions.

Metadata analysis of 301 attempted re-introductions of 128 plant taxa generated relative measures of re-introduction success based on propagule survival, population persistence and potential for recruitment of progeny. We found that attempting to summarise re-introduction success based on the results reported in the literature may erroneously imply that re-introductions are mostly successful. This is due to early reporting of outcomes in the literature: the average monitoring time prior to publishing the outcome of a study is about 3 years. Even for annual species this time period is insufficient to judge whether a re-introduction has been successful as populations may succumb to inter-annual variation over longer time scales. Our treatment of the data to discern population persistence (whether extant or extinct at specified time points) is a coarse measure, but illuminating: for those projects that were monitored for more than 10 years, most re-introduction attempts failed i.e. no plants had survived at the last survey. Further, the vast majority of projects initiated more than 5 years prior to this review are unknown in outcome indicating that there potentially exists a vast pool of data that could be used to better evaluate re- introductions if made publicly available.

We used covariates associated with the target organism and intervention (the methods used to re-introduce the target) to discern patterns in the success of re-introduction derived from the proportion of surviving propagules. We found that many factors that might be expected to confer lower risk to a project could not be linked to increased success of threatened plant re-introductions. These factors included removing the cause of original species decline from a site prior to propagule introduction, ensuring the site is within the historic range of the species and sourcing propagules from wild, rather than ex situ, populations.

Conclusions

A narrative synthesis of speculative causes for failure and the absence of empirical evidence that the factors mentioned above can enhance success, are combined to support calls for amended guidelines for future re-introduction projects. Further monitoring and improved dissemination of results of existing re-introduction projects is needed. Plus, more rigorous project design using treatment and site replication in addition to improved monitoring of individuals and populations is required to conclusively elucidate the causes of failure in this increasingly utilised restoration technique.