REINTRODUCTION STRATEGY –

HOW MANY EGGS, HOW MANY BASKETS?

Corresponding Readings in Primack, Richard B. Essentials of Conservation Biology.
Chapter 13: pages 337-360

Introduction:

The goal of captive breeding should be to successfully re-establish one or more wild populations, and go out of business. Why do some introductions succeed, and others fail? What factors might explain differences in success? Drawing upon the same logic of population viability analysis, are there "thresholds" of population size below which success is unlikely? Is it wiser to release as many individuals as one can, in the best possible habitat; or to spread the risk, releasing smaller numbers in various locations? In general, how can we better manage the re-introduction phase of a species rescue plan?

Successes and Failures – Case Studies:

Several authors present case studies and statistics on the fate of introductions.

We shall see that explanations for failure fall into two main camps --- the area where the species was released was not suitable (circumstances -- a deterministic explanation), and the released propagule was too small to withstand the risks of rarity (chance -- a stochastic explanation).

Despite what we might expect from the global spread of non-indigenous species, failures are common. Hawaii has experienced at least 50 (mostly intentional) bird introductions. Because of Hawaii’s tropical environment, one might expect tropical birds to have a better chance. However, 7 of 24 tropical species failed to establish, compared to 10 of 23 temperate species. Not much of a trend, but lends little support to the expectation that species will do best under circumstances most similar to their original home.

Inference -- Failure is due to chance: Fish and game agencies have made many introductions of game birds and game fishes. According to one study, the USFWS released 130,000 game birds into 44 states and 3 territories between 1960 and 1970, and releases continued until 1978 in 23 states. This study selected seven species for further study, on the basis that all succeeded somewhere. Of 424 releases, some 360 (85%) failed. When success rate is plotted against propagule size, an S-shaped curve results. A release of at least 200 birds results in a success rate of about 15%, and that is as good as it gets.

This study concludes that:

• since most releases fail, we need many releases toincrease the chances of hitting the right time and place.
• The release shouldn’t be less than 75 individuals, because chance of failure is very high below this release threshold
• maximum success for a single release is about 15%, and 200 birds is enough

A review of insect introductions in Canada, for biological control, provides supporting evidence. Only 10% of introductions of < 5,000 individuals succeed, up to 40% success occurs with introductions of 5,000-30,000 individuals, and up to 78% success with even higher releases. Why are the numbers so much higher for insects? Recall that organisms with small body size tend to exhibit more extreme fluctuations in abundance.

Inference -- Failure is due to circumstances: Another study of intentional releases of native birds and mammals in Australia, Canada, Hawaii, New Zealand and USA found at least 93 species were involved in nearly 700 translocations annually. Wildlife managers were surveyed for information about these programs.

The "typical" translocation consisted of six releases over three years. Many (46%) released 30 or fewer animals and most (72%) released 75 or fewer animals. Note that a release isn’t defined as a single event, but rather as a series of events, presumably constituting a program to establish that species in that location.

The success rate was 86% with native game species, 46% with threatened/endangered /sensitive species.

Success was affected by a number of circumstances.

higher success rates lower success rates

  native game species threatened species

good habitat quality sub-optimal habitat

core of range periphery of range

herbivores carnivores, omnivores

absence of competitors presence of competitors

early breeder/large clutch opposite

Using such information, fully 81% of the introductions could be classified as successful or not, using information about the circumstances of the release.

A Comparison of the Two Studies: The first study asks what is the success of an independent, single release, which is something we wish to know. The second study asks what is the success of a concerted effort to establish a species, which also is something we wish to know. The second study includes information about circumstances that should be relevant, and finds that they are; the first study fails to include such information. However, a potential weakness of the second study is that at least some of the assessment of circumstances likely was very subjective and after the fact. Managers asked to evaluate those circumstances presumably knew if the translocation was successful or not, and this may have influenced their assessment of the variables thought to be causal.

Clearly, there are abundant data to help guide managers in deciding how many organisms to release, in how many separate releases. Species recovery plans almost always call for the establishment of multiple sub-populations, to guard against the risk of extinction of an individual sub-population. It would be useful to conduct further studies that attempt to quantify the role of circumstances, such as good habitat; and chance, representing the sum of all of the demographic and genetic risks facing small populations.

Links:

1. CBSG News: Newsletter from this organization that provides more information on captive breeding programs for the conservation of species.

2. South China Tiger Studbook Analysis and Masterplan: an excerpt from the the full text of this plan for the preservation of tigers.