The California condor (Gymnogyps californianus) is one of the most widely-known success stories in conservation biology. From a population of just 22 individuals in 1982, the species’s numbers have grown to almost 400 birds over the last thirty years, thanks to an intense captive breeding and reintroduction program which is ongoing to this day. However, the species is hardly out of the woods yet – and there are many signs that the wild population may not yet have reached a self-sustaining state. In today’s paper from 2012, a collaboration between researchers from several organizations including UC Santa Cruz, the San Diego Zoo, and the US Fish & Wildlife Service try to assess that sustainability and to quantify the effect of one of the condor’s greatest threats: lead poisoning.
Finkelstein ME, Doak DF, George D, Burnett J, Brandt J, Church M, Grantham J, and Smith DR. Proceedings of the National Academy of Sciences (PNAS) 2012. doi:10.1073/pnas.1203141109
Stories about the condor’s supposed successful recovery are easy to find, and the recent history of condor conservation is well-known in conservation circles. California condor populations crashed in the latter half of the 20th century due to a variety of factors including illegal hunting, thinning of eggshells from DDT, and the contamination of their food sources with lead from hunters’ ammunition. (condors are vulture-like scavengers, and feed on carrion including carcasses left in the field by human hunters.) In 1980 a major conservation effort was initiated, involving tracking wild condors via radio transmitters and the collection of wild condor eggs for hatching and raising in captivity. By the mid-’80s it was clear that this was insufficient, and all wild condors were captured and brought into captivity for breeding and protection. Over the next few decades, condors have been regularly re-released into the wild and their population (still very carefully monitored) has been steadily growing ever since.
credit: USFWS Pacific Southwest Region
The researchers in this paper wanted to improve our understanding of that growth. Specifically, they wanted to address the question of whether the increase in the California condor’s population has been due to their success in the wild, or whether the recovery is dependent on ongoing active conservation efforts. This paper focuses specifically on the question of lead poisoning in condors; it tries to determine the extent of the problem today, its primary causes, and whether or not its effects are likely to be severe enough to significantly hinder the natural recovery of this species. In the abstract they lay out both their questions and their main conclusions:
Endangered species recovery programs seek to restore populations to self-sustaining levels. Nonetheless, many recovering species require continuing management to compensate for persistent threats in their environment. Judging true recovery in the face of this management is often difficult, impeding thorough analysis of the success of conservation programs. We illustrate these challenges with a multidisciplinary study of one of the world’s rarest birds—the California condor (Gymnogyps californianus). California condors were brought to the brink of extinction, in part, because of lead poisoning, and lead poisoning remains a significant threat today. We evaluated individual lead-related health effects, the efficacy of current efforts to prevent lead-caused deaths, and the consequences of any reduction in currently intensive management actions. Our results show that condors in California remain chronically exposed to harmful levels of lead; 30% of the annual blood samples collected from condors indicate lead exposure (blood lead = 200 ng/mL) that causes significant subclinical health effects, measured as >60% inhibition of the heme biosynthetic enzyme d-aminolevulinic acid dehydratase. Furthermore, each year, ~20% of free-flying birds have blood lead levels (=450 ng/mL) that indicate the need for clinical intervention to avert morbidity and mortality. Lead isotopic analysis shows that lead-based ammunition is the principle source of lead poisoning in condors. Finally, population models based on condor demographic data show that the condor’s apparent recovery is solely because of intensive ongoing management, with the only hope of achieving true recovery dependent on the elimination or substantial reduction of lead poisoning rates.
To summarize the summary, they find that 30% of wild condors have blood lead levels high enough to cause sub-clinical effects, and that 20% have blood lead levels high enough to cause illness or death. They determine through isotopic analysis (analyzing the type of lead atoms present) that the primary source of lead poisoning in condors is lead-based ammunition. They also find, though population modeling, that the recovery of the California condor’s population is not self-sustaining and that removing lead from their environment is a critical part of the path to a stable population. Let’s look at how they got to those conclusions.
The researchers began by analyzing lead concentrations in blood and feather samples taken from birds during standard monitoring procedures between 1997 and 2010. They also evaluated blood enzyme activity rates to get a sense of the clinical effects that might be caused by that lead. They also compared the isotopic makeup of the lead in their samples to that of lead from various other sources, including hunting ammunition, to determine the source of the lead found in their samples. So far pretty simple, as the below figures show:
credit: Finkelstein et al, 2012
They found, in summary, that about 70% of wild California condors had blood lead concentrations exceeding the recommended safe maximum of 100 ng/mL, and that about 20% of condors had blood lead concentrations exceeding 450 ng/mL, which is the threshold beyond which the CDC recommends chelation therapy be employed for lead-exposed children – and they were able to match the profile of the lead to that of lead ammunition. This is obviously an unwelcome finding, but the logical next question is the question of what this actually means for the health of the condor population as a whole. To get at this, the authors perform some population modeling simulations, under four scenarios:
Although continued mortality because of anthropogenic causes occurs in most recovering populations, this added mortality must be low enough that populations can be, at a minimum, stable. … Therefore, we used demographic analyses to explore the self-sustainability of the free-flying condor population within California (i.e., without considering future releases of captive-reared birds) under four scenarios: (i) status quo, where current management interventions to mitigate lead poisonings are continued, (ii) cessation of management interventions to mitigate lead poisonings, with the result that birds die when blood lead levels are ≥3,000 ng/mL, (iii) cessation of management interventions to mitigate lead poisonings, with the result that birds die when blood lead levels are ≥1,000 ng/mL, and (iv) no lead-related mortalities….
To do this, they incorporate their lead poisoning rates from the earlier part of their study, along with existing demographic information about California condors, into something like a PHVA. (Population Habitat Viability Analysis is a common modeling framework often used for simulating populations in conservation management, both for captive and wild populations.) The results of these simulations are encapsulated in this chart:
credit: Finkelstein et al, 2012
As you can see, both of the scenarios in which active conservation management ceases and in which mortality due to lead poisoning is present lead to decreasing populations of free-flying condors, indicating that the California condor population is likely dependent upon ongoing management intervention and that left to its own devices it would collapse. The silver lining here though is that if we can reduce lead poisoning in condor populations to the point where there are no significant mortalities from that cause, the population grows even in the absence of active management!
The key piece of the puzzle, as you have probably already realized, is the contamination of condors’ food sources with lead from hunting ammunition. Therefore, if we seek to create a self-sustaining and stable condor population, the obvious recommendation would be to ban or severely restrict the use of lead-based hunting ammo. Lead-free ammo certainly exists, so like so many things in conservation the problem becomes one of public acceptance and political will. Fortunately, many hunters already use lead-free ammunition and in fact, California already has a partial ban on lead hunting ammunition in areas where condors are found, and reports indicate that there is good compliance with those laws. Unfortunately, that legislation had been in place for two years at the time that data collection ended for this study, and the researchers found no decline in the prevalence of lead poisoning in that time.
Clearly, more is needed. Additional research is (as always) required in this area, to determine why the existing legislation has apparently been ineffective and to determine what could be done to improve matters. There is obviously a long way to go, and a surprising amount that we do not know given how celebrated and intensively-studied the California condor is, both as a species and as a “success story” in conservation. If nothing else, this study highlights the importance of looking beyond raw population numbers when evaluating the success of conservation efforts, and stresses the fact that no matter how much effort we put forth in saving a species, all that effort is in vain if we cannot also address the underlying causes that imperiled that species in the first place.