Playing the Numbers Game: Massive Head-starting of Mojave Desert Tortoises as a Low-Cost Restoration Strategy

James Juvik¹, Kenneth Nagy² and A. Ross Kiester³

¹ Geography & Environmental Studies, University of Hawai`i-Hilo

² Biology, University of California-Los Angeles

³Department of Zoology, Oregon State University

Project Abstract

In this two year project we propose to investigate the potential for increasing the number of desert tortoises (Gopherus agassizii) being reintroduced into the California Mojave by orders of magnitude, ultimately to a level that will more than off-set continuing, anthropogenic and natural losses. We believe this can be accomplished by leveraging the egg producing capabilities from tens of thousands of captive desert tortoise owners in Southern California. In recent years the State of California has formally issued more than 30,000 permits for captive desert tortoise adoption/possession. Due largely to disease transmission concerns, these captive tortoises can never be legally returned to the wild. However, using eggs from these privately held animals will provide a cost effective source of disease-free hatchlings that can then be captive-reared under quarantine (“head-started”) to a “predator-resistant” size and released in large numbers (thousands). Our ultimate aim is to reestablish wild populations in desert areas where tortoises have been recently extirpated or where the species has suffered dramatic population declines. We believe this approach offers the most workable opportunity for large-scale and long-term recovery of this threatened species.

During Year #1 of this proposed project we will develop specific biological (e.g., disease/genetic) legal (State/ Federal) and release site validation protocols through dialog with and input from the scientific, conservation, desert land owner and government regulatory communities currently concerned with the desert tortoise. We will further organize and co-ordinate existing support and participation from the private, tortoise pet-keeping community such as the California Turtle and Tortoise Club (with multiple, Southern California Chapters) with more than 2,000 members. We will evaluate protocols for captive tortoise certification (genetic/ health) and the logistics of marshalling future egg donations to the project, along with other volunteer commitments that could sustain the effort long-term, at relatively low cost. In Year #1 we will also work to obtain the necessary “animal subjects” permits to work with this threatened species in subsequent years.

Subject to successful protocol formulation and “stake-holder buy-in” in Year #1, during the second year of the project (2009-10) we would expect to launch a demonstration scale egg collection, incubation and head-starting effort with an initial goal of 1000 eggs donated during the 2009-nesting season (typically May-July). Based on an average desert tortoise clutch size of 5-6 eggs; we will require a minimum of 200 reproductively active, captive tortoise pairs, certified for project participation during Year #1.

Project Description: Objectives

The desert tortoise (Gopherus agassizii) is listed as a threatened species under the Endangered Species Act. Despite having received various forms of state and federal protection for several decades their populations in the Western and Central Mojave have continued to decrease in recent years as increased human settlement and other anthropogenic factors compete for tortoise habitat (USFWS 1994, Tracy et al. 2004, USFWS 2006). Even under natural conditions eggs and hatchling tortoise typically suffer very high predation rates in the wild. Thus, the intervention strategy known as head-starting which protects eggs and young during their most vulnerable life stage is seen as one way to restore depleted tortoise populations. Pilot studies at Mojave DOD facilities (Edwards Air Force Base, Fort Irwin National Training Center and the Marine Corp Air Ground Combat Center) indicate that this head-starting strategy can be successful in improving juvenile survivorship (Nagy et al. 2007a, b, c). The goal of the present proposal is to demonstrate the feasibility of ramping-up head-starting to a much larger scale that will promote significant recovery of this species over wide areas of its current and former range. This goal hinges on successfully answering three key questions posed to the large and diverse desert tortoise conservation community:

Question 1: Can State and Federal wildlife officials responsible for desert tortoise recovery reach an acceptable consensus on potential release sites for massive head-starting of tortoise that are environmentally suitable and compatible with landowner/management objectives?

Question 2: Can tortoise biologists/geneticists reach a consensus on the level of genetic diversity that is acceptable for these targeted reintroductions sites (i.e. agree on an acceptable range of genetic divergence from the original extirpated meta-population once occupying the reintroduction area)?

Question 3: Can the large (tens of thousands) and conservation-motivated, desert tortoise pet-keeping community in Southern California be organized in a way to annually provide large numbers of desert tortoise eggs and other volunteer support for a large-scale and long term head-starting program at relatively low cost?

This project will attempt to resolve these 3 issues successfully in year #1 and implement a demonstration scale massive head-starting program in year #2.

Desert tortoise restoration is a “numbers game”: massive head-starting will increase needed genetic diversity and eliminate the problems of bootstrapping a population from very low numbers (inbreeding depression). Additionally, large numbers will help to survive demographic and environmental stochasticity, swamp predators, resist disease (through increased genetic diversity), and provide densities necessary for social interactions/reproduction.

During year 1 of this proposed 2 year project we will:

1. develop specific biological (e.g., disease/genetic) and legal (State/ Federal) protocols through dialog with and input from the scientific, conservation, and regulatory communities currently concerned with the desert tortoise.

2. gauge/organize/co-ordinate existing support from the private, tortoise pet-keeping community (e.g. California Turtle and Tortoise Club, with multiple, local, Southern California Chapters, and more than 2,000 members) to evaluate protocols for captive tortoise certification (genetic/ health) and the logistics of marshalling future egg donations to the project, along with other volunteer commitments that could sustain the effort long-term, at relatively low cost.

3. Identify and cost-out needed infrastructure for large-scale incubation and head-starting facilities.

4. Identify potential future Mojave release sites for further detailed environmental/habitat evaluation and validation.

5. seek the necessary “animal subjects” research authorization (State and Federal)

that would be required for initiation of the demonstration phase in year # 2.

Subject to successful protocol formulation and stakeholder ”buy-in” in Year #1, during the second year of the project (2009-10) we would expect to:

1. launch a demonstration scale egg collection, incubation and head-starting effort with an initial goal of 1000 eggs donated during the 2009 nesting season (typically May-July). Based on an average desert tortoise clutch size of 5-6 eggs; we will require a minimum of 200 reproductively active, captive tortoise pairs, certified (health/genetic) for project participation during Year #1.

2. during this demonstration phase a range of experiments will be undertaken evaluating comparative success of egg incubation ex situ (incubators) and in situ (eggs reburied at in-range desert location, protected from predation); and later, various food/water subsidy levels for hatchling growth rate studies at in-range head-starting facilities. The cost/benefit results of past, small scale “low” and ‘high” effort desert tortoise head-starting programs will be carefully evaluated in research design (see: Germano, et al. 2002, Brooks, 2004, and Nagy et al. 2007a, b, c). We will also determine the optimum incubation temperature regime to produce the ideal sex-ratio of hatchlings for the most vigorous population growth.

3. if this initial 2009 demonstration effort proves successful we plan to seek alternative, long-term funding from private corporate (including desert land owners with mitigation issues) and conservation organizations to ramp-up to an annual egg collection of 10,000-20,000 eggs/yr within 4 years (by 2013). Hatchlings would be held for a minimum of 4-5 years in predator-proof desert enclosures before repatriation to the wild. Under this scenario, assuming a liberal 50% loss rate from egg to 5-year old juvenile (due to egg infertility and hatchling survival issues), within 10 years (i.e. 5 years after reaching full production in 2013) we should be able to sustain a release rate of 5,000-10,000 largely predator-resistant juveniles tortoises/year.

Background

The scientific/conservation literature over the past several decades is replete with extended debate over the efficacy of “head-starting” as a useful tool in turtle conservation. Recent reviews (e.g. Bogard, 2006), benefiting from the advantage of greater time-depth in evaluating long-term head-starting projects, now confirm head-starting as a valuable component in several species recovery efforts. A head-starting component in comprehensive tortoise recovery plans dates back several decades and has been adopted for some of the most severely endangered species (Juvik, et al., 1982, Juvik, et. al., 1991). The best-documented success in tortoise head-starting (and most relevant to the current proposal) relates to the recovery of the Espanola Island Galapagos tortoise (Geochelone nigra hoodensis). Depredations by 19^th Century whalers combined with 20^th Century habitat destruction through massive overgrazing by introduced feral goats reduced the total tortoise population on this small island (60 km²) to only 14 animals by 1963 (Cayot and Morillo 1997). This remnant breeding stock was relocated to a captive breeding center in the Galapagos and captive reproduction began in 1971 (artificial egg incubation and head-starting of hatchlings). Repatriation of juvenile tortoises back to Espanola began in 1975 and goats were finally eradicated in 1978. Over the last 30 years more than 2,000 head-started tortoises have been returned to Espanola. More importantly, by 1991, repatriated tortoises from the 1970s had already reached maturity and began successfully reproducing on the Island, the first step to reestablishing a self-sustaining population. One interesting facet to this story (relevant to the desert tortoise issue) is that recent DNA analysis of the Espanola tortoises has revealed that 60% of all the head-started tortoises (2000 plus) have the same father (one of three males in the original captive breeding group of 14 animals). This has raised inbreeding concerns for the currently repatriated population (Minkovitch 2004). With literally tens of thousands of captive desert tortoises to choose from this problem should not arise in the desert tortoise project here proposed.

Other recent turtle head-starting successes have been achieved for the Pacific Pond Turtle (Actinemys mormarata), where the nearly extirpated Washington State population has more than tripled in size through a multi-decades head-starting program sponsored by local zoos (Fish and Wildlife Science 2002). Even sea turtle head-starting programs, long criticized for poorly documented or ambiguous conservation impact, are now showing long-term signs of success (Bell et. al. 2005; Juvik 2007). We also know from multiple studies on turtles, tortoises (and alligators) that these taxa are resilient to egg and hatchling manipulation and can easily make the transition from captivity to free ranging.

Desert tortoise populations have suffered dramatic declines over the past half-century as the Mojave Desert has undergone sustained anthropogenic transformations detrimental to the species, including: opportunistic collecting, habitat alteration through overgrazing, indirect predator subsidy, off-road vehicle impact, disease introduction, urbanization and associated habitat fragmentation by highways and other barriers. Population declines have been particularly severe in the western portion of the Mojave Desert, where human intrusion and influence has been the greatest.

One of the common criticisms of head-starting/repatriation is:”... why put animals back into areas where the situation that lead to their previous decline/extirpation has not been eliminated or substantially reduced?” This forms a basic tenant in most conventional reintroduction guidelines (IUCN 1998). We would argue that massive head-starting itself can potentially swamp some decline factors in the case of long-lived desert tortoises, and may be the only viable strategy, even if initial decline factors cannot be completely eliminated. As an analogy, in Hawai`i the endemic Nene Goose (Branta sandvicensis) is an indigenous wildlife icon and the official “State Bird” As a ground nesting species it was driven nearly to extinction in the mid 20^th Century by introduced alien predators (house cat, mongoose, rats, etc.). Only eggs and young are threatened by these predators, so today a perpetual captive rearing program repatriates adult birds annually and the

species is now commonly encountered in the wild where it helps to restore native ecosystem functions (Kear and Berger, 1980). Although largely non-reproducing in the wild (except in limited areas where intensive predator control is provided), the successful adult repatriation makes the best of an ecologically bad situation. We would argue that the California desert tortoise is in an equally difficult environmental situation.

Scientific issues in Desert Tortoise massive head-starting

Demography. Quantity matters. Releasing large (thousands) numbers of tortoises will eliminate the problems that beset a population with very low numbers. Large numbers will prevent populations from going extinct due to random fluctuations in demography and the environment that can cause extinction when numbers are too close to zero.

Genetics. We believe that the project will insure (through large numbers) the high degree of genetic diversity required for adaptation to the different sites within the Mojave Desert and provide for broad scale adaptation to global change. Earlier genetic studies (Lamb and McLuckie 2002) have suggested a relatively uniform genotype over most of the California Mojave. The genetic composition of populations of tortoises to be potentially released in the Mojave must be set to balance conflicting evolutionary pressures. In particular, there is a trade-off between the adaptedness of a population and the adaptability of the population. On the one hand, natural selection will tend to cause populations to become adapted to their local environment. On the other hand, greater genetic variability provides greater opportunity for adapting to changing environments. Recent work on the desert tortoise (Murphy et al. 2007) had demonstrated genetic differentiation within the Mojave Desert and has hypothesized that these genetic differences may be adaptive. This hypothesis is by no means proven and it may be that the differentiation is the result of genetic drift through isolation or other factors. Their perspective emphasizes the idea of local adaptation and argues the case that reintroduced populations should genetically match the wild population. It assumes that the environment in the Mojave Desert has not changed significantly during the recent evolutionary history of the tortoise. Our perspective follows from Fisher’s Fundamental Theorem of Natural Selection (Fisher 1930; Crow and Kimura 1970; Crow 2000; Turtle Conservation Genetics Working Group 2007) and assumes that the environment of the tortoise has changed in the recent past and will continue to change in the future, due both to local anthropogenic effects and to global climate change. We therefore believe that the best reintroduction strategy is to create populations with high genetic variability to provide the greatest opportunity for evolutionary response to change. We do believe that the population of tortoises to be reintroduced should be descended from parents from the Mojave Desert but that constraining genetic diversity to micro-localities within the Mojave Desert will limit evolution. Therefore we initially propose to genetically screen all captive tortoises (to be certified for future egg contributions) to make sure they are of Mojave stock. Unfortunately we know that captive individuals may be hybrids of Sonoran and Mojave stocks or even hybrids with other Gopherus species, so genetic screening will be an important part of this effort. Taylor Edwards (2008 pers. com.) has recently found significant Mojave/Sonoran mixing and even inter-specific hybrids (with other Gopherus species) in a study of captive desert tortoises in the Tucson area. We must assume the same sometimes will be true in the Southern California pet tortoise population. 

Release site validation: The complex process of choosing sites for future tortoise release will require the development of a thorough protocol involving consultation with state/federal and private landowners/resource managers. Dr. Ross Kiester, a consultant to this project, has been extensively involved in detailed reptile habitat characterization across the Mojave (Heaton, Kiester and Meyers 2006; Mouat, Kiester, and Baker 1998). We obviously would not intend to reintroduce tortoises into areas with healthy existing populations. Choosing areas where tortoises have been extirpated or nearly so will mean deciding that an area nonetheless has reasonable long-term prospects for re-tortoising. Most of these areas will have changed greatly in recent years due to anthropogenic effects and so clearly there will lack existing populations already adapted to these environments. For such areas maximizing tortoise genetic variability is a must so that introduced population have a chance to become adapted to these continually evolving environments. Ultimately, potential reintroduction will lag at least five or more years behind initial hatchling head-starting (as tortoises are captive raised to a “predator-resistant” size), providing ample time to address unresolved issues related to validation of specific release sites.

Disease transmission. By collecting eggs from captive populations we will break the cycle of disease transmission often associated with captive tortoises and provide healthy animals for reintroduction. Recent work on Gopher Tortoises in Florida (Mary Brown and Elliott Jacobson pers. com) and on Desert Tortoises in California (Nagy et al. unpublished results) appears to confirm that maternal tortoise disease transmission does not occur in ova.

Behavior. We know from studies on tortoises, turtles and alligators that these animals are resilient to manipulation and can make the transition from captivity to free ranging (Nussear 2004; Temsiripong et al. 2006; Field et al. 2007; Kahn et al. 2007).

Project components

Here we are requesting funds to begin the Massive Project. We will develop the protocols, begin constituency building, and undertake review by the scientific and conservation communities. We will provide the design, proof-of-concept, and initialization for:

1. Protocol design. We will establish candidate protocols for the entire process. Table 1 shows the components of the complete protocol set.

2. Constituency building. By constituency we mean the population of tortoise owners who will sign up to be part of the effort. These folks will be both participants in and advocates for the effort. This will be truly community-based conservation. Desert tortoises can only be held legally in California with a Department of Fish and Game Permit. More than 30,000 such permits have been issued in recent years, and this number obviously does not include many thousands of additional backyard tortoises held without permits. The primary non-governmental organization that will partner in this initiative is the California Turtle and Tortoise Club. One of us (Nagy) has already received enthusiastic responses to this project concept at personal presentations to various local chapters of the California Turtle & Tortoise Club, the Southwestern Herpetologists Society, and the San Diego Turtle and Tortoise Society.

3. Legal and regulatory setting. We will provide a complete account of how the effort will work within the current regulatory environment. For example it is currently illegal to privately breed desert tortoises in captivity without a State of California “Propagation Permit”. We will need to resolve with the State of California whether a “collective propagation permit” or individual permits will be necessary for this project.

4. Incubation and rearing infrastructure design. We will provide design and cost guidelines for facilities that can successfully handle the large numbers of tortoises that will pass through the system. This will involve a full investigation of “scaling up” issues relative for example to current limited-capacity head-starting facilities such as those now operating at the Marine Corp Air Ground Combat Center (Twenty-nine Palms).

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