29.10:
Conservation of Declining Populations
The conservation of declining populations focuses on the ecological causes of the population decline. This approach preserves populations that are still large, but declining, by studying the species history and determining the reasons for decline and species needs for survival.
The rebound of the Eastern Bluebird in the United States is one successful example of this approach. Bluebirds are secondary cavity nesters, meaning they rely on natural cavities, abandoned nesting sites of other species, or artificial cavities, such as old fence posts.
Eastern Bluebird populations began to dwindle in the 1950s, reaching minimum levels in the 1960s. This decline is attributed to several factors, including pesticides and invasive, non-native species.
House sparrows, introduced in the 1950s, frequently outcompete bluebirds for nesting cavities, often with deadly consequences. These threats compounded natural pressures, such as harsh winters, predators, and competition with native species.
In the 1960s, designated trails were outfitted with nesting boxes to provide cavities for bluebirds. Scientists and volunteers monitored the boxes, removing invasive competitors and evaluating the breeding success of different nesting box configurations.
This work identified optimal features for bluebird nesting boxes that reduce the competitive advantages of non-native, invasive species. For example, an entry hole with a maximum width of 3.8 centimeters prevents disturbance by European Starlings.
These conservation efforts have helped Eastern Bluebird populations recover and actually increase in many areas. This success story demonstrates that, while human activities often contribute to species decline and endangerment, conservation efforts can help these populations recuperate.
29.10:
Conservation of Declining Populations
Conservation of declining population focuses on ways of detecting, diagnosing, and halting a population decline. The approach uses methods to prevent populations from going extinct.
Conservation efforts often utilize scientific approaches to identify the reasons, or the agents, causing the population to decline. This approach then devises steps to remove, oppose, or neutralize the agents.
Conservation efforts may also introduce a test group to determine the probable cause of the decline. The translocation of the remaining population to unaffected areas follows if the cause is confirmed. In cases where the remaining population is too low, or at risk of further reduction, a protected stock is bred rapidly and released to bolster the population. Subsequent monitoring confirms the success of population re-establishment.
The conservation approach has successfully restored the population of an endangered Australian bird—the Lord Howe Woodhen. These flightless birds are endemic to Lord Howe Island off the Australian coast. The population of these birds began dwindling since the beginning of human inhabitation within the island. The population had reached a state of extinction when an ornithologist, Dr. Ben Miller, identified the situation.
Methodical testing and rejection of several hypotheses identified the feral pigs as responsible for the contraction of the population size of these birds. These pigs, introduced into the island with human settlement, killed and ate the incubating birds and also destroyed their nests and eggs.
Miller’s finding resulted in the elimination of the pigs and instituting a breeding program to rear these endangered birds within an enclosed facility. The progeny from the breeding program was released into the wild in batches and monitored closely. Subsequently, the released birds started breeding, and stabilizing their population and saturating all the suitable habitat on the island.
Suggested Reading
Collen, Ben, Louise McRae, Stefanie Deinet, Adriana De Palma, Tharsila Carranza, Natalie Cooper, Jonathan Loh, and Jonathan E. M. Baillie. "Predicting How Populations Decline to Extinction." Philosophical Transactions of the Royal Society B 366, no. 1577 (2011): 2577-586. [Source]
Peery, M. Zachariah, Steven R. Beissinger, Scott H. Newman, Esther B. Burkett, and Tony D. Williams. "Applying the Declining Population Paradigm: Diagnosing Causes of Poor Reproduction in the Marbled Murrelet." Conservation Biology 18, no. 4 (2004): 1088-098. [Source]