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.
Lead Poisoning and the Deceptive Recovery of the Critically Endangered California Condor
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.
California condor
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:
Top image: condor blood lead concentration, 1997 to 2010. The lines in the gray boxes show the median concentration. Bottom image: ratio of lead-206 to lead-207 isotopes in condors (the cloud of dots and triangles) and in various sources of environmental lead (the diamonds and stars at the far right edge). Note that the cloud of condor data points has about the same isotope ratio as the open diamonds which correspond to lead ammunition.
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:
Population growth curves under different management scenarios. Lines with positive slope (>1) denote increasing population, lines and curves with negative slope (<1) indicate decreasing population.
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.
This is a really neat solid paper, I do have a quibble though.
In the first paragraph of their conclusion the authors discuss their statistical model relating a cumulative distribution function of annual condor poisoning to a percentage of contaminated carcasses to support their sense of urgency, but I think their line of reasoning is a bit tortured. Their idea is to show that even ‘low’ levels of carcass contamination can result in high levels of poisoning, but without any way to understand what ‘low’ would mean in the context of the California environment it is really difficult to interpret their model in a meaningful way.
I guess if they could show that the percentage of carcasses that are currently contaminated is high enough to poison to percentage of condors similar to their observed results using their proposed statistical model it would provide significant support for the thesis of the paper and the model. However, absent that, their model could only really attempt to be meaningfully useful after trying to back calculate the percentage of carcasses that aught to be contaminated based on the percentage and age distribution of poisoned birds they observe.
That would actually be kind of cool now that I think about it, but I suspect the authors didn’t do it as it would require some form of proper statistical hypothesis testing or regression, which their data may not be high quality enough to support.
You make some really good points. It definitely struck me as well in reading this paper that the authors were having to make a lot of rather large assumptions about things like lead toxicity and so on.
This sort of thing is a constant problem in conservation research – generally speaking, we frankly lack the kind of background information that we would like to have in order to answer the questions we would like to ask, and we very often lack both the time and the resources to answer those questions in a satisfactory manner – as Michael Soulé put it, conservation biology is a “crisis discipline”, where decisions must often be made quickly, based on scanty information.
California condors are actually relatively *well* understood, at least compared to most other species and systems. They’ve been the subject of a lot of intense research and management for decades now, which is something that can be said about very few species. And yet, as you rightly point out, the authors of this paper made some very large assumptions in drawing their conclusions.
I agree that more research ought to be done. The models in this study could be re-run with fewer assumptions, and we could gain some certainty. In the long run though, the goal is for California condors to be a self-sustaining species – and at very least, this paper casts significant doubt on whether or not that is currently the case, and highlights the importance of looking beyond mere population growth curves in evaluating the success of a conservation program.
The authors claim that scavenging animals, such as the California condor, ingest and are poisoned by pieces of metallic lead bullets present in gut piles of harvested game left in the field by hunters. In turn, self-proclaimed environmental groups rely on these scientific papers that often use the poisoning of the California condor to justify their anti-lead ammunition agenda.
But there are serious scientific questions about the validity of their claims. The failure of the hastily-enacted California lead ammunition ban legislation of 2007 (AB821) suggests that these groups are wrong. AB821 banned the use of lead ammunition in the “condor zone” region of California. It was strong-armed through the legislature, bypassing the usual path involving the more scientifically inclined California Fish & Game Commission, based on the promise that the ban would lower the condors’ elevated blood-lead levels, and solve the lead poisoning problem. But AB821 has not resulted in lower blood-lead levels or otherwise reduced lead poisoning in condors. Despite the California Department of Fish & Wildlife’s acknowledgment that 99% of hunters are complying with the lead ban in the “condor zone” since the law took effect, condors’ blood-lead levels, poisoning and mortality have increased since 2007!
There are obviously other sources of lead in the environment. These alternative sources are likely an industrial lead compound (e.g leaded gasoline, paint or pesticides), which is far more soluble and bioavailable to condors. We have identified some of those potential alternative sources, and we encourage you to join the hunt for the truth with us and learn the real facts! To learn all the facts in the lead ammunition debate, visit http://www.huntfortruth.org.
You’re absolutely right that the current legislation banning the use of lead ammunition in the areas where condors are found has not been effective in lowering the blood lead concentration of condors, despite high compliance. This is something that I mention in my original analysis, actually — the second-to-last paragraph discusses these findings.
However, I would tend to suspect that the problem is not that the lead is entering the condors’ food supply through another source — though this is certainly possible and definitely merits investigation — but rather that the current legislation has not been effective at removing ammunition-based lead from the condors’ environment, perhaps because their range is larger than previously suspected or perhaps because compliance with the legislation is not as high as reports indicate.
The reason that this would be my initial conjecture (though as I have already mentioned I agree that the possibility of lead arising from another source is worth investigating) is that the isotopic analysis of the lead found in condors’ blood matches the isotopic profile of lead ammunition. Unless another environmental source of lead has the same isotopic profile as lead ammunition (a ratio of about 0.84 Pb207/pb206) and that source is present in significant quantities in the condors’ environment, lead ammunition would seem to be by far the most likely source of lead contamination — pointing to a failure of either legislation or enforcement in removing lead ammo from the condors’ environment, rather than poisoning resulting from another source.
The authors did do some of this work in their paper, though admittedly I glossed over it a bit in my analysis. (It’s in the paper on the third page, second column however, if you want to see the original source.) Essentially what they found was that pre-release condors had (low) blood-lead levels consistent with the ambient isotopic profile of lead in California — meaning that they had a small amount of lead in their blood, and that its profile was consistent with just the general small amounts of lead that are inevitably encountered from day to day by anyone living in the area. However, after being released into the wild their blood-lead concentration rose, and its profile changed to match that of lead-based ammunition. This is highly suggestive that the lead in their blood was coming from lead ammunition.
The authors did compare against other sources of lead, mind you — and interestingly, they did find an exception. The exception was a small group of condors (three to five birds) whose blood-lead profiles matched that of lead paint samples taken from a fire tower near where they were found. I will emphasize that this profile was found in only a small minority of the birds studied, but it shows that the authors’ methods were capable of distinguishing between different sources of environmental lead contamination. The large majority of birds had blood-lead profiles matching the isotopic signature of lead ammunition. They
It is also worth mentioning that there is some direct evidence of lead poisoning in condors resulting from lead-based ammunition. I skipped over this in my analysis, but it is present in the paper — there have been six cases in which lead fragments have been found either in the guts of lead-poisoned condors or in carcasses on which condors have been observed feeding, and these fragments have matched the isotopic profiles both of lead ammunition and of the lead found in the blood of the condors from whom the fragments were recovered. Six cases is not a very large number, and this type of study is limited by the great difficulty of systematically observing the feeding habits of condors in the wild.
It is true that they did not check every possible source of lead contamination — the authors only investigated background environmental lead, lead-based ammunition, and lead-based paint. If other sources such as leaded gasoline are present in significant quantities in the environment (though I am skeptical that leaded gasoline is a major contaminant in this day and age) then it would absolutely be worth checking those. It is certainly possible that there is another source of contamination which was missed in this study — I do not know why the authors chose to examine only those three sources, but I would assume that it was because those are the three major sources of lead contamination known to exist in the condors’ environment.
I also am not sure where you are seeing a conflict of interest here. This study was performed by a mixture of academic researchers, nonprofit organizations, and government institutions, none of which have an anti-hunting mission. Academic researchers are ideologically independent — their reputation rests on the strength of their findings, not on their adherence to a particular ideology. The government institution involved was the US Fish and Wildlife service, an organization which has the the interests of hunters as part of its mandate, and the nonprofit was the San Diego Zoo, which has an interest in protecting condors but not in regulating hunting. The goal of this study was not to restrict hunting practices, but rather to simply shed light on the extent and causes of lead poisoning in California condors, so that future policy can be more effective in protecting them from *whatever* source the lead might be coming from.
I will confess, I didn’t realize that lead ammunition bans were so controversial in the hunting community. The Hunt for Truth website certainly seems very vehement in their campaign to preserve the use of lead-based ammunition, and one gets the impression reading it that at least some hunters view restrictions on lead-based ammo as a form of persecution. As a non-hunter, but one who views hunters as potential conservation allies and who tries to be sympathetic to views from all sides of conservation issues, would you mind summarizing for me (and for anyone else who might be reading this) what the reasons are why hunters are resistant to changing ammunition types? Is non-lead ammunition more expensive, or less effective? The high reported rate of compliance to the current legislation seems to suggest that hunters are able to make the switch, so why so much resistance? To me — and admittedly I am ignorant on this issue, so I am asking you to educate me — it seems like a small sacrifice to make. Why so much resistance, then?
Thank you very much for your comment. It is always good to hear from different perspectives. I would tend to disagree with your criticisms of the authors’ conclusions (or at least, I would tend to support their conclusions until I see specific evidence of their error) but I agree that this is an area where research ought to continue. However, at this time the evidence in favor of a major role for lead ammunition in the inadvertent poisoning of this critically endangered species seems strong enough to me that it merits continued policy refinements in that area. I still think that the situation is more likely that the current legislation is simply too weak, rather than that it is targeted at the wrong problem. I would be willing to be persuaded though, given evidence from a well-conducted study in that direction. Can you point to any specific studies which have found lead poisoning in California condors to be coming primarily from a source other than lead-based ammunition? I would be very interested to see such a study, and I promise to read any research that you can find for me on the subject. If you are unable to access a study on such a topic due to it being published in a closed-access journal, I would be willing to help find a way for you to get a copy of that paper.
Thank you again for your thoughts. I have long thought that hunters and conservationists ought to be natural allies (after all, the hunting community was the original source of the conservation movement in the US) and I hope that we can continue this discussion in a thoughtful and considerate way. I appreciate you taking the time to stop by and share your thoughts.