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Keywords:

  • population bottleneck;
  • immunocompetence;
  • phytohaemagglutinin skin test;
  • Petroica australis;
  • New Zealand

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Inbreeding resulting from severe population bottlenecks may impair an individual's immune system and render it more susceptible to disease. Although a reduced immune response could threaten the survival of highly endangered species, few studies have assessed the effect of population bottlenecks on immunocompetence. We compared the counts of leucocytes and external, blood and gastrointestinal parasite loads in two populations of the endemic New Zealand robin Petroica australis to assess the immunocompetence of birds in a severely bottlenecked population relative to its more genetically diverse source population. Despite similar parasite loads in both populations, robins in the severely bottlenecked population showed lower counts of both total leucocyte and total lymphocyte numbers. When the immune system was experimentally challenged using the phytohaemagglutinin skin test, robins in the severely bottlenecked population exhibited a significantly lower immune response than the source population, suggesting that birds passing through a severe bottleneck have a compromised immunocompetence. Our results confirm that severe bottlenecks reduce the immune response of birds and highlight the need to avoid severe bottlenecks in the recovery programmes of endangered species.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

A bottleneck occurs when a population drops to a low number (Frankham, Ballou & Briscoe, 2002). Even if such a population recovers, severe bottlenecks may increase inbreeding as survivors are forced to mate with relatives (Keller, 1998). Inbreeding caused by a bottleneck may render a population more vulnerable to environmental stress, disease and extinction, as reduced genetic variation in inbred individuals could lead to their immune systems being less adaptable or defective due to the loss of specific resistance alleles (O'Brien et al., 1985; Thorne & Williams, 1988; Spielman et al., 2004; Swinnerton et al., 2005). Reduced genetic diversity may also limit the ability of a population to adapt to new pests, climatic changes, habitat changes, and introduced or co-evolving parasites (Altizer, Foufopoulos & Gager, 2001). It has been suggested that bottlenecked populations may be particularly at risk of disease outbreaks and are more prone to collapse (O'Brien & Evermann, 1988; Frankham, 1995; Zekarias et al., 2002). For example, birds in a song sparrow Melospiza melodia population that survived a severe population bottleneck on Mandarte Island were a non-random subset of the pre-crash population (Keller et al., 1994). Those individuals that survived were comparatively outbred compared with the individuals that died (Keller et al., 1994). Although bottlenecks and inbreeding may not be the only factors in determining how a population responds to disease, they may dictate which individuals survive a disease outbreak. However, few studies have experimentally tested the relationship between severe population bottleneck size and reduced immunocompetence (Reid, Arcese & Keller, 2003; Hawley et al., 2005).

Many species around the world are currently experiencing severe population declines due to habitat loss, excessive hunting and the introduction of exotic predators. Conservation management in many endangered species has thus focused on translocating individuals to safer or more suitable areas. For example, in New Zealand translocation to predator-free islands has been used to save at least a dozen species of endangered birds, although in most cases the number of individuals translocated is often quite small (average about 30 birds) and sometimes as few as four individuals (Craig et al., 2000). Likewise, translocations in other parts of the world for conservation purposes average about 75 individuals, but can be as low as 30 individuals (Griffith et al., 1989). Such small founder populations are often quite successful in establishing new populations (e.g. Swinnerton et al., 2005; Taylor, Jamieson & Armstrong, 2005); however, little consideration has been given to the genetic implications of this management strategy. The rapid speed at which new pathogens can be spread around the world (Dobson & Foufopoulos, 2001; Friend, McLean & Dein, 2001) could consequently threaten many endangered species if passing through a severe bottleneck reduced their ability to mount an immune response.

In this study we test the effect of population bottlenecks on immunocompetence using two closed island populations of the New Zealand robin Petroica australis: an ancestral source population on one island and a new population founded 33 years ago by translocating five individuals to a second island. Thus, unlike many critically endangered species, we were able to evaluate the immune response of birds both before and after passing through a severe population bottleneck. We used surveys of parasite loads and leucocyte profiles to measure immune response to current pathogens, and then experimentally challenged birds with the phytohaemagglutinin (PHA) skin test to estimate the strength of their cell-mediated immune system. Our hypothesis is that individuals in a population that has gone through a severe bottleneck will have reduced immunocompetence and could be more vulnerable to future potential outbreaks of diseases and parasites.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Study populations

The New Zealand robin is a medium-sized passerine (35 g) that feeds on invertebrates on the forest floor (Heather & Robertson, 2000). Robins were once widespread across New Zealand and have declined dramatically in range and density since human settlement, but survive on several predator-free offshore islands. Our experiment was carried out on two such islands: Motuara Island (41°5′S, 174°16′E) and Nukuwaiata Island (40°53′S, 174°4′E), in the Marlborough Sounds of the South Island. In 1973 five robins were transferred from Nukuwaiata Island to nearby Motuara Island where robins were once present but disappeared when the island was cleared for farming. This transfer proved successful (the vegetation had also recovered in the interim) and the population now exceeds 600 individuals (Byrne, 1999). However, recent studies show high hatching failure, lower clutch size and fewer clutches per year, suggesting that the Motuara Island robin population is suffering from inbreeding depression (Mackintosh & Briskie, 2005). Genetic analyses confirm that the Motuara population has a significantly lower genetic variation than its source population on Nukuwaiata Island (Ardern et al., 1997; Miller & Lambert, 2004).

Robins can be readily sexed by plumage and size differences (Heather & Robertson, 2000) and we analysed our data for each sex separately. Hatch-year birds (<1 year) can also be distinguished from adult birds by plumage differences. As parasite loads and the development of the immune system may differ with age (e.g. young birds have not been exposed to as many potential pathogens as adults), only adult robins were used in our study.

Parasite loads

During January–February 2005 (post-breeding autumn period) and July–August 2005 (pre-breeding spring period), we caught adult robins using a Potter trap baited with mealworm larva (Tenebrio sp.). All birds were colour banded for identification, and an estimate of feather mite density was obtained for each individual by examining the primary feathers of the left wing. Feather mite density was given a category score from 0 to 5: 0=no feather mites, 1=0–10, 2=10–100, 3=100–1000, 4=1000–10 000 and 5=10 000+ feather mites. The number of hippoboscid flies (Ornithomya spp. and Ornithoica spp.) seen on or flying off the bird was also counted. Hippoboscid flies are obligate blood-feeding ectoparasites and are known to be vectors of blood-borne diseases (Hutson, 1984). Faecal samples were collected for analysis for gastrointestinal nematodes and the parasitic protozoan coccidia. During the autumn, faecal samples were collected from the holding bags shortly after capture; however, because we found diurnal variation in the shedding of coccidian oocysts, faecal samples in the spring were collected over the full 6-h holding period of the PHA experiment using trays in the holding cages (see below). Faecal egg counts to estimate coccidia and nematode burdens were estimated using standard faecal flotation methods carried out by a commercial laboratory (New Zealand Veterinary Pathology Ltd, Hamilton, New Zealand). Sporulation of the coccidia oocysts revealed that they were from the genus Isospora (K. Townsend, pers. comm.).

Leucocyte counts

A drop of blood was obtained from the right wing of each adult robin via brachial venipuncture to measure leucocyte parameters. Blood was smeared onto a glass slide, fixed in methanol and stained using a modified May–Grünwald Giemsa staining method (Lucas & Jamroz, 1961). Blood smears were then viewed under a light microscope and the following measurements were taken: (1) estimated total leucocyte number (henceforth referred to as leucocyte count) and (2) leucocyte differential. The leucocyte count was calculated by counting all white blood cells in 10 consecutive 400 × fields of view for each bird. Counts were averaged to give an estimate for each individual (Fudge, 2000; Walberg, 2001). A leucocyte count gives an indication of the health status of the individual at the time of sampling. A high count, or leucocytosis, is characteristic of inflammatory diseases and parasitic infection (Woerpel & Rosskopf, 1984; Fudge, 2000). Although avian blood is comprised of five types of leucocyte, we focused on the two most common types: lymphocytes and heterophils. Lymphocytes are divided into two cell types: T lymphocytes, which play a role in the cell-mediated immune response, and B lymphocytes, which are involved in the humoral immune response and antibody production. Both cell types work to generate a pathogen-specific immune response; however, due to the difficulty in distinguishing the two cell types in peripheral blood smears, for the purpose of this study we have treated them as one cell type. A high lymphocyte count is correlated with marked immune stimulation (Fudge, 2000); however, a low lymphocyte count can indicate immunosuppression, viral infection, severe stressors or a lack of parasitic infection (Ots & Horak, 1998; Horak et al., 1999; Fudge, 2000). Heterophils are phagocytic cells and high numbers can indicate either inflammation or stress (Fudge, 2000). They play a key role in initiating the innate immune response and the recognition of pathogens by detecting molecules unique to invading organisms (Swaggerty et al., 2005). To obtain a differential leucocyte count and thus estimate both lymphocyte and heterophil numbers, each blood smear was examined under oil immersion (1000 ×) and the relative frequency of the five different types of leucocytes was determined for a total of 100 leucocytes. We then calculated the heterophil/lymphocyte ratio (H:L), which has been used as an index of stress in both poultry (Gross & Siegel, 1983; Maxwell, 1993) and wild birds (Tompkins, Mitchell & Bryant, 2006). Finally, the blood smear was scanned for 3 min to detect any blood-borne parasites. A cross-sectional (up-across–down-across–up, etc.) method of scanning the slide was used to prevent scanning the same area twice.

Cell-mediated immune assay

The PHA skin test is a standard procedure for quantifying one aspect of avian acquired immunity, the T-cell-mediated immune response, to a novel challenge. The cell-mediated immune system is largely responsible for removing virus-infected host cells and is involved in the defence against fungi, protozoans, cancers and intracellular bacteria (Ritchie, Harrison & Harrison, 1994). The PHA test works by activating white blood cells in the peripheral blood and causes temporary inflammation at the point of injection. The resultant swelling can then be measured. A large swelling is considered a strong immune response and may provide a measure of the health and condition of the birds (Norris & Evans, 2000).

We followed the protocol of Smits, Bortolottie & Tella (1999) by injecting 50 μL of a 5 mg mL−1 PHA suspension [Sigma (St. Louis, Missouri) PHA-P; L8754] in phosphate-buffered saline (Sigma P4244) into the left patagium of adult robins. All robins were held in cages for the 6 h between injection of PHA and the final measurements. Cages measured 300 mm by 600 mm and birds were provided with ad libitum water, mealworms and a perch. Cages were placed in a quiet and shady location for the duration of the experiment. We measured a total of 45 adult robins on Nukuwaiata and 101 adult robins on Motuara. Robins were tested in both spring and autumn to determine any seasonal effect on immunocompetence. Birds were banded to ensure that no individual was tested twice.

We measured patagium thickness at the site of injection to the nearest 0.01 mm using a digital micrometer (Mitutoyo 0–1 inch, Tokyo, Japan). Three measurements were taken immediately before the injection and again 6 h after the injection. To reduce measurement error, all measurements were conducted by the senior author. The three measurements were averaged because wing web thickness is shown to have high repeatability (Moreno, Sanz & Arriero, 1999). The cell-mediated immune response was calculated as the difference between the pre- and post-injection measurements of patagium thickness. For some individuals the difference between pre- and post-injection patagium thickness was negative. The reason for this is unknown, but is likely due to measurement error or because these individuals became dehydrated during the post-injection holding period (J. E. Smits, pers. comm.). Although all individuals were supplied with water, we were not able to control the amount they consumed. Statistical analyses were run both with and without these individuals included, but it had no effect on the outcome of the analysis and we only present the results with all individuals included.

Statistical analyses

We performed all statistical analyses in Statistica 6 (StatsSoft Inc.). We used general linear models to determine which variables explained a significant proportion of variation in the cell-mediated immune response, the leucocyte counts, the counts of lymphocytes and heterophils, and the H:L ratio. We used the non-parametric Kruskal–Wallis test to determine seasonal variation in parasite abundance across both islands and also within island by season comparisons. Pearson correlations were used to determine whether the H:L ratio correlated with leucocyte count or PHA response for island by season combinations.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Parasite loads and bottleneck size

Robins in both populations were found to harbour feather mites and hippoboscid flies, but there was no significant difference in the prevalence of ectoparasites between the source population on Nukuwaiata and the bottlenecked population on Motuara in either autumn or spring (Fig. 1). Hippoboscid numbers were significantly higher in autumn than in spring (H1,146=73.43, P<0.001), and this pattern held for both islands when analysed separately (Motuara: H1,101=59.33, P<0.001; Nukuwaiata: H1,45=14.80, P=0.001). In contrast, feather mite loads were higher in spring than in autumn (H1,143=90.82, P<0.001), and this was true for both islands (Motuara: H1,98=67.30, P<0.001; Nukuwaiata: H1,45=26.17, P<0.001). Ectoparasite loads were also not correlated with either leucocyte counts or PHA responses (Pearson correlation coefficients, all P>0.05). Coccidia was the only endoparasite isolated from faecal samples, but it was only present in 3/101 (3.0%) individuals on Motuara Island and 1/45 (2.2%) individuals on Nukuwaiata Island. Oocyst counts of coccidia were low to moderate (200–2600 oocysts g−1) in all infected individuals. No nematodes were found and no blood parasites were seen from the blood smears in any robin from either population. Thus, there was no indication from surveys of parasites that robins in the severely bottlenecked population presently suffer significantly greater loads of parasites than do birds in the source population.

image

Figure 1.  Difference in parasite loads between autumn and spring for New Zealand robins in a source population on Nukuwaiata Island compared with birds in a bottlenecked population on Motuara Island.

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Blood cell counts and bottleneck size

Blood parameters differed in a number of ways between the bottlenecked population on Motuara Island and their source population on Nukuwaiata Island (Fig. 2). Leucocyte counts were significantly different between seasons (F1,130=76.81, P<0.001) and sexes (F1,130=8.31, P<0.05), but robins on Nukuwaiata Island had a significantly higher leucocyte count than robins on Motuara Island (F1,130=13.28, P<0.001; Fig. 2). Similarly, there was a significant difference in lymphocyte count between seasons (F1,130=95.65, P<0.001) and sexes (F1,130=7.77, P<0.01), and robins on Nukuwaiata Island again showed significantly higher counts than birds on Motuara Island (F1,130=12.82, P<0.001; Fig. 2). In contrast, heterophil counts did not differ significantly between the two populations or sexes, although they were higher in autumn than in spring for both islands (Fig. 2). There was also no difference in H:L ratio for either island or season (Fig. 2). Within each island and season, H:L ratios were not correlated with leucocyte counts (Pearson correlation coefficients, all P>0.05), indicating that the leucocyte response was not due to one particular type of leucocyte. PHA response was also not correlated with H:L ratio, leucocyte count, or heterophil or lymphocyte counts within each island or season (Pearson correlation coefficients, all P>0.05). The higher leucocyte and lymphocyte counts in the source population suggest that robins in the bottlenecked population have a reduced immune response, although this reduction was not evident in other measures such as heterophil counts or H:L ratios.

image

Figure 2.  Blood profiles showing relationships between leucocyte counts for females (a) and males (b) in autumn (closed bars) and spring (open bars) for New Zealand robins in a source population on Nukuwaiata Island and their bottlenecked population on Motuara Island; lymphocyte counts for females (c) and males (d) during both seasons and on both islands; (e) heterophil count and (f) heterophil/lymphocyte (H:L) ratio in females and males during both seasons and on both islands. Sexes combined in (e) and (f).

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Cell-mediated immune response and bottleneck size

Although there was no overall difference between the cell-mediated immune response in the PHA experiment between Motuara and Nukuwaiata islands (F1,138=1.20, P=0.28), there was a significant interaction between population and season (F1,138=11.90, P<0.001). When each season was examined separately, robins in the source population on Nukuwaiata Island had a significantly higher cell-mediated immune response in autumn than robins in the bottlenecked population on Motuara Island (F1,81=10.47, P<0.05; Fig. 3). However, the response of birds did not differ significantly between the two populations in spring (F1,57=2.80, P>0.05; Fig. 3). These results confirm that the cell-mediated immune system of robins in the bottlenecked population was weaker than in the source population when experimentally challenged, but this difference varied with season.

image

Figure 3.  Phytohaemagglutinin (PHA) response, calculated as the difference between pre- and post-PHA injection, for New Zealand robins in a source population on Nukuwaiata Island compared with robins in a bottlenecked population on Motuara Island during autumn (black bars) versus spring (open bars). The same letters above the bars indicate no significant difference between bars.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The genetic consequences of severe population bottlenecks are well documented (e.g. O'Brien et al., 1985; Saccheri et al., 1998; Westemeier et al., 1998; Briskie & Mackintosh, 2004). Even if such populations subsequently recover through conservation initiatives, individuals in the post-bottlenecked population are expected to have reduced genetic diversity through inbreeding and genetic drift (Frankham et al., 2002; Reed & Frankham, 2003). Whether such a loss is manifested in decreased fitness has been less clear, but increased inbreeding as a result of a bottleneck should lead to increased susceptibility to pathogens and a compromised immune system (Keller et al., 1994; Spielman et al., 2004). Several disease outbreaks in endangered wild populations support the link between reduced genetic variation and susceptibility to pathogens (Altizer et al., 2001; Spielman et al., 2004). For example, feline infectious peritonitis virus in cheetah Acinonyx jubatus (O'Brien et al., 1985), canine distemper in the black-footed ferret Mustela nigripes (Thorne & Williams, 1988) and a number of infectious diseases found in the Florida panther Puma concolor coryi (Roelke, Martenson & O'Brien, 1993) have all been linked to the negative effects of bottlenecks experienced by these species.

When we examined a variety of internal and external parasites in a highly bottlenecked population of the New Zealand robin, we found no differences in parasite loads compared with that found in their source population in either pre-breeding or post-breeding seasons. However, at a cellular level, the immune systems of robins appeared to be less responsive in the severely bottlenecked population. Robins in the bottlenecked population on Motuara Island had significantly fewer leucocytes (including fewer lymphocytes) than their counterparts in the source population on Nukuwaiata Island, and when the immune systems of robins were experimentally challenged with PHA, we again found that birds in the severely bottlenecked population responded less strongly than birds in the source population, at least in one season. Our experimental results thus add to the growing body of evidence (e.g. Reid et al., 2003; Hawley et al., 2005; Whitman et al., 2006) that populations passing through a recent severe bottleneck have a lower cell-mediated immunocompetence and may therefore be more susceptible to parasites and diseases.

The lack of a difference in current parasite loads on birds in both populations would at first glance run counter to our expectations based on reduced genetic diversity in a bottlenecked population: if severe bottlenecks reduce immunocompetence, one would expect birds in the post-bottlenecked population to be particularly prone to infestation and therefore carry higher burdens. However, using current pathogen loads to infer the strength of a bird's immune system is subject to errors that can mask any underlying difference in immunocompetence. For example, severe infestations might quickly lead to mortality, and such birds would be under-represented in any survey of either population, through either death or a reduced likelihood of capture. Likewise, some pathogens are only likely to become costly when an individual is stressed, and as we only sampled birds in the non-breeding season (when birds are probably under less stress than in the breeding season), it is possible that pathogen loads differ at other times of the year. Finally, for practical reasons we could not assess every possible pathogen or parasite (e.g. tapeworms, flukes) and it is possible that other types of parasites and pathogens might differ between the two populations.

The fact that we found similar parasite loads and H:L ratios in both the source and bottlenecked robin populations suggests that neither population is currently subject to greater environmental stresses than the other. Such a difference might be expected when comparing two populations because defence against infections, including acquired immunity, can be modulated by environmental factors such as nutrition, stress and age (Gustafsson et al., 1994) and these are likely to be different in different study sites. Furthermore, extreme environmental conditions are known to increase susceptibility to disease and parasites (Hoffmann & Parsons, 1991). Thus, differences in the levels of environmental stress could be a confounding factor in many comparative studies of immunocompetence. However, the proximity of our two populations means that they share similar environmental conditions, temperatures and habitat structure. Both are also located on small offshore islands with similar size and geography. This suggests that the differences we observed in cell-mediated immunocompetence are likely the result of a severe population bottleneck rather than different levels of stress that might be associated with differing environmental conditions between the two study sites.

As elevated leucocyte counts are indicative of an individual mounting an immune response (Fudge, 2000), the higher leucocyte counts of robins in the source population would suggest that either they are responding more effectively to immune challenges than birds in the post-bottlenecked population or birds in the latter population are simply unable to respond as effectively. Alternatively, the difference in leucocyte counts could arise from one population dealing with a disease epidemic not found in the other population. However, we found no physical evidence, as would be expected with such an epidemic, that either population was currently suffering from a disease outbreak of any sort. The results of the PHA test, in which we experimentally challenged the cell-mediated immune system of birds in both populations, suggest that the lower leucocyte counts in the post-bottlenecked population are more likely due to the inability of robins to mount as strong an immune response as their counterparts in the source population, even when faced with apparently similar parasite loads. Further information on how leucocyte counts change in response to specific pathogens (e.g. coccidia) is now needed to determine whether reductions in immunocompetence occur uniformly across different types of immune system challenge (Matson et al., 2006).

One striking result of our study was the seasonal changes in the reaction of robins to the PHA immune challenge. Robins in the post-bottlenecked population displayed a reduced immune response only in the autumn and not in the early spring before breeding began. Seasonal differences in parasite loads were also noted, with hippoboscid flies more common in the autumn and feather mites more common in the spring. Such seasonal differences in immunocompetence and parasite loads have been noted previously, and it has been suggested that this pattern results from a trade-off between increased reproductive effort and reduced immunocompetence (Deerenberg et al., 1997; Merino, Møller & deLope, 2000; Dubiec & Cichon, 2001; Lozano & Lank, 2003; Møller, Erritzoe & Saino, 2003). However, Møller et al. (2003) suggest that seasonal differences in immune response reflect the impact of parasites on their hosts and that it would be beneficial to mount an immune response when parasites are most abundant. The numbers of some parasites tend to peak during and just after the reproductive season of their host (Møller et al., 2003). The high numbers of hippoboscid flies that we found on robins fit with this pattern and may explain the higher response to the PHA at this time of the year. However, further information on the immune response of birds across the entire year and in relation to their condition is needed before the significance of seasonal fluctuations is better understood. Our results nevertheless provide a caution to other researchers drawing conclusions on the immunocompetence of birds based on data from only one season.

The immune system of birds is complex and there is no single assay available that can assess all kinds of immune response simultaneously. Because we only challenged one aspect of the acquired immune system, we cannot rule out the alternative that resources were directed towards different immune components in the Motuara robin population. For example, more resources may have been invested in the humoral immune response than the T-cell-mediated immune response during the autumn period (Westneat & Birkhead, 1998; Norris & Evans, 2000; Adamo, 2004) and this could explain the difference between our two study populations. However, the low reaction to PHA and the low leucocyte counts suggest that robins from the post-bottlenecked population on Motuara may be less capable of mounting an immune response in any form. At present, the population on Motuara appears stable and self-sustaining, but our results suggest that if a new pathogen appeared on the island it might have more severe consequences than in the source population.

Theoretical evidence suggests that the individuals most likely to survive adverse conditions and disease outbreaks are those with the greatest genetic diversity (Frankham, 1995, 1998). Genetic diversity of the immune system is known to cause differences in resistance to infectious pathogens in chickens (Zekarias et al., 2002), and two studies on wild animals found that reduced genetic diversity strongly influenced response to PHA (Reid et al., 2003; Hawley et al., 2005). However, Hawley et al. (2005) did not find a relationship between heterozygosity and humoral immune response, suggesting that there may be differences in the way in which each component of the adaptive immune response is influenced by genetic diversity. In vertebrates, mediator proteins such as the major histocompatibility complex (MHC) are probably responsible for linking the different events of the immune response and they play a major role in the recognition of viruses (Benjamini, Coico & Sunshine, 2000). The MHC is the most highly polymorphic gene in the vertebrate body (O'Brien et al., 1985), and maintaining a large number of MHC genes is thought to increase pathogen resistance. A recent study of MHC variation in the two robin populations used in the present study confirms a loss of MHC diversity in the Motuara Island robins compared with their source population (Miller & Lambert, 2004). As disease is known to have been the main contributing factor of population decline and/or extinctions for several endangered species (van Riper et al., 1986; Wikelski et al., 2004), the loss of MHC diversity may be the genetic mechanism by which bottlenecked populations become more susceptible to pathogens.

The population of robins that we studied on Motuara Island went through an extremely severe bottleneck of only five individuals and thus it is not too surprising that their immune systems might be compromised as a result. Such severe bottlenecks have occurred in a variety of endangered species around the world (e.g. black robin Petroica traversi; Butler & Merton, 1992; Mauritius kestrel Falco punctatus; Groombridge et al., 2001) and thus it is important to understand the fitness consequences on individuals in the post-bottlenecked population for effective management. Nevertheless, conservation management of endangered birds usually involves founding new populations with a greater number of individuals (usually 30–75 founders; Griffith et al., 1989) than that experienced by New Zealand robins on Motuara Island. In a recent study, Briskie & Mackintosh (2004) found that bottlenecks below ∼150 individuals resulted in increased hatching failure, but whether bottlenecks in this higher range also reduce the immunocompetence (and hence survival) of birds in the post-bottlenecked populations is not known at present. Such information is urgently needed if conservationists are not to unwittingly increase the susceptibility of endangered species to parasites and pathogens as a consequence of their actions of founding new populations without considering the genetic consequences on immunocompetence.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We thank Kathryn Atkinson, Peter Hale and Jackie Hale for assistance with fieldwork and Dr Raphael Didham for statistical advice. The Department of Conservation granted permission to work on Motuara and Nukuwaiata islands and we thank Peter Gaze, Bill Cash, Mike Aviss and the Maud Island staff for their support. New Zealand Veterinary Pathology Ltd (Hamilton) assisted with faecal analyses and Brett Gartrell provided advice on haematology. Medlab Hamilton Ltd and Southern Community Laboratories (Christchurch) also provided laboratory space and use of equipment. Funding was provided by the Royal Society of New Zealand and the University of Canterbury. R. Didham, D. Tompkins and two anonymous reviewers provided valuable feedback on an earlier draft of this paper. This study was approved by the Animal Ethics Committee of the University of Canterbury.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • Adamo, S.A. (2004). How should behavioural ecologists interpret measurements of immunity? Anim. Behav. 68, 14431449.
  • Altizer, S., Foufopoulos, J. & Gager, A. (2001). Conservation and diseases. In Encyclopedia of biodiversity: 109126. Levin, S.A. (Ed.). San Diego: Academic Press.
  • Ardern, S.L., Lambert, D.M., Rodrigo, A.G. & McLean, I.G. (1997). The effects of population bottlenecks on multilocus DNA variation in robins. J. Hered. 88, 179186.
  • Benjamini, E., Coico, R. & Sunshine, G. (2000). Immunology: a short course. New York: Wiley-Liss.
  • Briskie, J.V. & Mackintosh, M. (2004). Hatching failure increases with severity of population bottlenecks in birds. Proc. Natl. Acad. Sci. USA 101, 558561.
  • Butler, D. & Merton, D. (1992). The black robin: saving the world's most endangered bird. Oxford: Oxford University Press.
  • Byrne, A.J. (1999). Effects of population bottlenecks on the South Island robin, Petroica australis australis. MSc thesis, University of Canterbury, Christchurch, New Zealand.
  • Craig, J., Anderson, S., Clout, M., Creese, B., Mitchell, N., Ogden, J., Roberts, M. & Ussher, G. (2000). Conservation issues in New Zealand. Annu. Rev. Ecol. Syst. 31, 6178.
  • Deerenberg, C., Arpanius, V., Daan, S. & Bos, N. (1997). Reproductive effort decreases antibody responsiveness. Proc. Roy. Soc. Lond. Ser. B 264, 10211029.
  • Dobson, A. & Foufopoulos, J. (2001). Emerging infectious pathogens of wildlife. Philos. Trans. Roy. Soc. Lond. B 356, 10011012.
  • Dubiec, A. & Cichon, M. (2001). Seasonal decline in health status of great tit (Parus major) nestlings. Can. J. Zool. 79, 18291833.
  • Frankham, R. (1995). Conservation genetics. Annu. Rev. Genet. 29, 305327.
  • Frankham, R. (1998). Inbreeding and extinction: island populations. Conserv. Biol. 12, 665675.
  • Frankham, R., Ballou, J.D. & Briscoe, D.A. (2002). Introduction to conservation genetics. Cambridge: Cambridge University Press.
  • Friend, M., McLean, R.G. & Dein, F.J. (2001). Disease emergence in birds: challenges for the twenty-first century. Auk 118, 290303.
  • Fudge, A.M. (2000). Laboratory medicine: avian and exotic pets. Philadelphia, PA: W.B. Saunders Company.
  • Griffith, B., Michael, S., Caprenter, J.W. & Reed, C. (1989). Translocation as a species conservation tool: status and strategy. Science 245, 477480.
  • Groombridge, J.J., Bruford, M.W., Jones, C.G. & Nichols, R.A. (2001). Evaluating the severity of the population bottleneck in the Mauritius kestrel Falco punctatus from ringing records using MCMC estimation. J. Anim. Ecol. 70, 401409.
  • Gross, W.B. & Siegel, H.S. (1983). Evaluation of the heterophil/lymphocyte ratio as a measure of stress in chickens. Avian Dis. 27, 972979.
  • Gustafsson, L., Nordling, D., Andersson, M.S., Sheldon, B.C. & Qvarnstrom, A. (1994). Infectious-diseases, reproductive effort and the cost of reproduction in birds. Philos. Trans. Roy. Soc. Lond. B 346, 323331.
  • Hawley, D.M., Sydenstricker, K.V., Kollias, G.V. & Dhondt, A.A. (2005). Genetic diversity predicts pathogen resistance and cell-mediated immunocompetence in house finches. Biol. Lett. 1, 326329.
  • Heather, B.D. & Robertson, H.A. (2000). The field guide to the birds of New Zealand. Auckland, New Zealand: Viking.
  • Hoffmann, A.A. & Parsons, P.A. (1991). Evolutionary genetics and environmental stress. Oxford: Oxford University Press.
  • Horak, P., Tegelmann, L., Ots, I. & Møller, A.P. (1999). Immune function and survival of great tit nestlings in relation to growth conditions. Oecologia 121, 316322.
  • Hutson, A.M. (1984). Keds, flat-flies and bat-flies. Diptera: Hippoboscidae and Nyceteribiidae. Handbook for the identification of British insects 10(7). London: Royal Entomological Society of London.
  • Keller, L.F. (1998). Inbreeding and its fitness effects in an insular population of song sparrows (Melospiza melodia). Evolution 52, 240250.
  • Keller, L.F., Arcese, P., Smith, J.N.M., Hochachka, W.M. & Stearns, S.C. (1994). Selection against inbred song sparrows during a natural population bottleneck. Nature 372, 356357.
  • Lozano, G.A. & Lank, D.B. (2003). Seasonal trade-offs in cell-mediated immunosenescence in ruffs (Philomachus pugnax). Proc. Roy. Soc. Lond. Ser. B 270, 12031208.
  • Lucas, A.M. & Jamroz, C. (1961). Atlas of avian hematology. Agriculture monograph, Vol. 25. Washington: United States Department of Agriculture.
  • Mackintosh, M.A. & Briskie, J.V. (2005). High levels of hatching failure in an insular population of the South Island robin: a consequence of food limitation? Biol. Conserv. 122, 409416.
  • Matson, K.D., Cohen, A.A., Klasing, K.C., Ricklefs, R.E. & Scheuerlein, A. (2006). No simple answers for ecological immunology: relationships among immune indices at the individual level break down at the species level in waterfowl. Proc. Roy. Soc. Lond. Ser. B 273, 815822.
  • Maxwell, M.H. (1993). Avian blood leucocyte responses to stress. World's Poul. Sci. J. 49, 3443.
  • Merino, S., Møller, A.P. & DeLope, F. (2000). Seasonal changes in cell-mediated immunocompetence and mass gain in nestling barn swallows: a parasite-mediated effect? Oikos 90, 327332.
  • Miller, H.C. & Lambert, D.M. (2004). Genetic drift outweighs balancing selection in shaping post-bottleneck major histocompatibility complex variation in New Zealand robins (Petroicidae). Mol. Ecol. 13, 37093721.
  • Møller, A.P., Erritzoe, J. & Saino, N. (2003). Seasonal changes in immune response and parasite impact on hosts. Am. Nat. 161, 657671.
  • Moreno, J., Sanz, J.J. & Arriero, E. (1999). Reproductive effort and T-lymphocyte cell-mediated immunocompetence in female pied flycatchers Ficedula hypoleuca. Proc. Roy. Soc. Lond. Ser. B 266, 11051109.
  • Norris, K. & Evans, M.R. (2000). Ecological immunology: life history trade-offs and immune defense in birds. Behav. Ecol. 11, 1926.
  • O'Brien, S.J. & Evermann, J.F. (1988). Interactive influence of infectious-disease and genetic diversity in natural-populations. Trends Ecol. Evol. 3, 254259.
  • O'Brien, S.J., Roelke, M.E., Marker, L., Newman, A., Winkler, C.A., Meltzer, D., Colly, L., Evermann, J.F., Bush, M. & Wildt, D.E. (1985). Genetic-basis for species vulnerability in the cheetah. Science 227, 14281434.
  • Ots, I. & Horak, P. (1998). Health impact of blood parasites in breeding of great tits. Oecologia 116, 441448.
  • Reed, D.H. & Frankham, R. (2003). Correlation between fitness and genetic diversity. Conserv. Biol. 17, 230237.
  • Reid, J.M., Arcese, P. & Keller, L.F. (2003). Inbreeding depresses immune response in song sparrows (Melospiza melodia): direct and inter-generational effects. Proc. Roy. Soc. Lond. Ser. B 270, 21512157.
  • Van Riper, C., Van Riper, S.G., Goff, M.L. & Laird, M. (1986). The epizootiology and ecological significance of malaria in Hawaiian land birds. Ecol. Monogr. 56, 327344.
  • Ritchie, B.W., Harrison, G.J. & Harrison, L.R. (1994). Avian medicine: principles and applications. Lake Worth, FL: Wingers Publishing Inc.
  • Roelke, M.E., Martenson, J.S. & O'Brien, S.J. (1993). The consequences of demographic reduction and genetic depletion in the Florida panther. Curr. Biol. 3, 340350.
  • Saccheri, I., Kuussaari, M., Kankare, M., Vikman, P., Fortelius, W. & Hanski, I. (1998). Inbreeding and extinction in a butterfly metapopulation. Nature 392, 491494.
  • Smits, J.E., Bortolottie, G.R. & Tella, J.L. (1999). Simplifying the phytohaemagglutinin skin-testing technique in studies of avian immunocompetence. Funct. Ecol. 13, 567572.
  • Spielman, D., Brook, B.W., Briscoe, D.A. & Frankham, R. (2004). Does inbreeding and loss of genetic diversity decrease disease resistance? Conserv. Gen. 5, 439448.
  • Swaggerty, C.L., Ferro, P.J., Pevzner, I.Y. & Kogut, M.H. (2005). Heterophils are associated with resistance to systemic Salmonella enteritidis infections in genetically distinct chicken lines. FEMS Immunol. Med. Microbiol. 43, 149154.
  • Swinnerton, K.J., Greenwood, A.G., Chapman, R.E. & Jones, C.G. (2005). The incidence of the parasitic disease trichomoniasis and its treatment in reintroduced and wild pink pigeons Columba mayeri. Ibis 147, 772782.
  • Taylor, S.S., Jamieson, I.G. & Armstrong, D.P. (2005). Successful island reintroductions of New Zealand robins and saddlebacks with small numbers of founders. Anim. Conserv. 8, 415420.
  • Thorne, T.E. & Williams, E.S. (1988). Disease and endangered species: the black-footed ferret as a recent example. Conserv. Biol. 2, 6674.
  • Tompkins, D.M., Mitchell, R.A. & Bryant, D.M. (2006). Hybridization increases measures of innate and cell-mediated immunity in an endangered bird species. J. Anim. Ecol. 75, 559564.
  • Walberg, J. (2001). White blood cell counting techniques in birds. Sem. Avian Exotic Pet Med. 10, 7276.
  • Westemeier, R.L., Brawn, J.D., Simpson, S.A., Esker, T.L., Jansen, R.W., Walk, J.W., Kershner, E.L., Bouzat, J.L. & Paige, K.N. (1998). Tracking the long-term decline and recovery of an isolated population. Science 282, 16951698.
  • Westneat, D.F. & Birkhead, T.R. (1998). Alternative hypotheses linking the immune system and mate choice for good genes. Proc. Roy. Soc. Lond. Ser. B 265, 10651073.
  • Whitman, N.K., Matson, K.D., Bollmer, J.L. & Parker, P.G. (2006). Disease ecology in the Galapagos hawk (Buteo galapagoensis): host genetic diversity, parasite load and natural antibodies. Proc. Roy. Soc. Lond. Ser. B 273, 797804.
  • Wikelski, M., Foufopoulos, J., Vargas, H. & Snell, H. (2004). Galapagos birds and diseases: invasive pathogens as threats for island species. Ecol. Soc. 9 [online] URL: http://www.ecologyandsociety.org/vol9/iss1/art5.
  • Woerpel, R.W. & Rosskopf, W.J. (1984). Clinical experience with avian laboratory diagnostics. Vet. Clin. N. Am.: Small Anim. Prac. 14, 249286.
  • Zekarias, B., Ter Huurne, A.A.H.M., Landman, W.J.M., Rebel, J.M.J., Pol, J.M.A. & Gruys, E. (2002). Immunological basis of differences in disease resistance in the chicken. Vet. Res. 33, 109125.