How do we know when a threatened species is ‘safe’? A new study featuring one of New Zealand’s iconic kiwi species suggests increasing population size might not be enough – an increase in numbers doesn’t always cut it for conservation…
Saving threatened species tends to be a numbers game. We often use population growth as a proxy for population security. The more individuals of a given species exist, the less endangered we perceive that species to be. Population size and growth/decline are the major criteria for deciding how a species is classified on the IUCN Red List – arguably the most influential international ranking of threatened species.
But what happens when the numbers are misleading? What if population growth hides damaging factors that could threaten the future persistence of a species?
Kiwi hiding inbreeding depression
A new study from my PhD research shows that population growth in little spotted kiwi is masking serious issues caused by inbreeding (mating between relatives). We studied the population of little spotted kiwi on Long Island in the Marlborough Sounds. Amazingly, this population has grown from two founders to around 50 birds in the past 30 years. Unfortunately, our genetic data showed that almost all that population growth is down to continued reproductive success of the founding pair.
Kiwi can live up to 80 years in the wild; the founding pair on Long Island are at least 33 years old and are still producing 1-3 chicks per year. Their offspring, however, are all brothers and sisters with no choice about who to mate with. This makes the Long Island little spotted kiwi family tree a highly inbred mess.
We know that inbreeding tends to be bad news for the fitness of individuals. Accordingly, the inbred offspring of the brother-sister matings on Long Island rarely seem to survive. This population will continue to grow, but only until the founding pair stop producing chicks. The IUCN recently downgraded little spotted kiwi from “Vulnerable” to “Near Threatened”, based on their population growth. This may have been premature given what we know about their genetics.
Hidden dangers for other species
Further afield, the dawn redwood tree presents another example of explosive population growth masking severe genetic issues. Once thought to be extinct, around six-hundred million individual dawn redwoods were bred from 5,000 trees rediscovered in China in the 1940s. However, the regenerated populations of redwoods have very low genetic diversity and the majority of their seeds fail to germinate.
Similar situations may exist in European otters and the Seychelles magpie robin. Unfortunately, for these species, there is currently no genetic data to assess their status. Back in New Zealand, the iconic tuatara often exhibits seriously skewed reproduction, with as few as 30% of males in a population being responsible for all population growth. Without careful management, this too could result in reduced genetic diversity and long term issues. Genetic diversity is also problematic for other New Zealand species that are slowly recovering from drastic population declines. Think Chatham Island black robin, kākāpō, and takahē. Genetic diversity is so low in these, and many other New Zealand bird species, that a major disease event could wipe them out.
Avoiding false conservation targets
Why does any of this matter for conservation? The government’s current targets for kiwi conservation are based entirely on population growth. In return for its recent $11 million investment in kiwi conservation, the government wants to see a 2% growth in kiwi numbers to hit a goal of 100,000 kiwi by 2030. Leaving aside the fact that there are five species of kiwi in New Zealand, each facing different conservation challenges and showing differing population growth or decline rates, this target ignores several issues, including genetics.
As we’ve shown, kiwi can show significant population growth while hiding serious genetic issues that could threaten the long term persistence of each species. If every kiwi population in New Zealand grows by 2%, but that increase is down to a handful of individuals in each case, the genetic consequences could be severe.
Collecting genetic data and monitoring the reproductive success and survival of individuals is challenging and costly, but crucial to effective conservation. If we base conservation targets largely on population growth, we risk ignoring long-term issues that could result in unexpected extinctions.