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Extinction, colonization, and distribution patterns of common eider populations nesting in a naturally fragmented landscape K.G. Chaulk, G.J. Robertson, and W.A. Montevecchi
Abstract: Spatial distribution, patchy environments, and population turnover have many fundamental implications for conservation ecology. Common eider (Somateria mollissima L., 1758) population processes were investigated in Labrador, Canada, between 1998 and 2003. We predicted that local colonies would exhibit population turnover, that extinction would be negatively related to colony and patch size, that colonization would be negatively related to island isolation, and that intraspecific incidence–abundance relationships would be positive. We found that small colonies were prone to extinction, but patch size was not a significant predictor of extinction, nor was colonization related to isolation. The overall observed annual extinction and colonization rates were 0.11 ± 0.02 and 0.41 ± 0.06, respectively, and showed variation across archipelagos. At two spatial scales we found that mean colony size was a positive predictor of island occupancy (incidence), and these relationships were maintained across years. Our findings show that common eider colonies in Labrador are dynamic and have greater turnover rates than previously expected in a species that is considered highly philopatric. Our findings support the notion that highly mobile organisms such as migratory birds can exhibit characteristics associated with metapopulation processes. Re´sume´ : La re´partition spatiale, les environnements contagieux et le remplacement des populations ont des implications fondamentales en e´cologie de la conservation. Nous avons e´tudie´ les processus de´mographiques de l’eider a` duvet (Somateria mollissima L., 1758) au Labrador, Canada de 1998 a` 2003. Nous avons pre´dit que les populations locales subiraient un remplacement, que l’extinction serait relie´e ne´gativement a` la taille de la colonie et de la tache, que la colonisation serait relie´e ne´gativement a` l’isolement de l’ıˆle et que les relations intraspe´cifiques d’incidence–abondance seraient positives. En re´alite´, les petites colonies sont vulne´rables a` l’extinction, mais la taille des taches ne permet pas de pre´dire l’extinction de fac¸on significative et la colonisation n’est pas relie´e a` l’isolement. Les taux annuels globaux d’extinction et de colonisa` deux e´chelles tion observe´s sont respectivement de 0,11 ± 0,02 et de 0,40 ± 0,06 et varient d’un archipel a` un autre. A spatiales, la taille moyenne des colonies est un facteur de pre´diction positif de l’occupation des ˆıles (occurrence) et ces relations se maintiennent au cours des anne´es. Nos re´sultats montrent que les colonies d’eiders a` duvet au Labrador sont dynamiques et qu’elles posse`dent un taux de remplacement plus grand qu’attendu ante´rieurement chez une espe`ce conside´re´e comme fortement philopatrique. Nos donne´es appuient la notion que les organismes tre`s mobiles comme les oiseaux migrateurs peuvent afficher des caracte´ristiques associe´es aux processus des me´tapopulations. [Traduit par la Re´daction]
Introduction Andrewartha and Birch (1954) were among the first to recognize that many populations are discontinuous, often split by patchy environments into multiple local populations. Subsequently, MacArthur and Wilson (1967) investigated the relationship between population turnover and community dynamics, and Levins (1969) then developed a mathematical model to explain patterns of local extinction. Since then, ecologists have come to understand that spatial distribution, Received 20 March 2006. Accepted 10 August 2006. Published on the NRC Research Press Web site at http://cjz.nrc.ca on 10 November 2006. K.G. Chaulk.1 Box 382, Station C, Goose Bay, NL A0P 1C0, Canada. G.J. Robertson. Canadian Wildlife Service, 6 Bruce Street, Mount Pearl, NL A1N 4T3, Canada. W.A. Montevecchi. Department of Psychology, Memorial University of Newfoundland, St. John’s, NL A1B 3X9, Canada. 1Corresponding
author (e-mail:
[email protected]).
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patchy environments, and population turnover have many fundamental implications for conservation ecology. Populations that occupy patchy habitats, exhibit both local extinction and colonization, and have limited connectivity with adjacent populations are often labeled metapopulations (Hanski 1991). Metapopulation concepts have been widely applied (Gulve 1994; Hanski and Thomas 1994; Appelt and Poethke 1997; Barbraud et al. 2003), mainly to organisms with limited dispersal ability (Szacki 1999). According to Esler (2000), if migratory birds exhibit both spatial structuring and philopatry, a metapopulation approach can be useful in describing their population dynamics. The common eider (Somateria mollissima L., 1758) is a migratory species that has never been described using a metapopulation approach. Typically eiders are researched at the colony level or managed at the subspecies level, and limited attention is paid to population processes and colony dynamics at spatial scales between these extremes. Johnsgard (1979) differentiated six subspecies of common eider. Of these, four regularly breed in Canada, and only the American (S. m. dresseri) and northern (S. m. borealis) sub-
doi:10.1139/Z06-138
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species breed in Labrador (Mendall 1980; Knapton 1997; Goudie et al. 2000). While the existence of subspecies remains controversial (Zink 2004), there is substantial morphological variation in eiders nesting across North America (Mendall 1980; Knapton 1997; Goudie et al. 2000). The existence of distinct populations that can be grouped based on morphology implies limited mixing across the species’ range (Wright 1940), which in turn is consistent with metapopulation assumptions (Hanski 1991), albeit at large spatial scales. Based on the presence of two eider subspecies in Labrador and the subsequent inference that these are not completely panmictic, patchy colony distribution (Chaulk et al. 2004), female philopatry (Coulson 1984; Goudie et al. 2000) reaching 98% (Swennen 1990), and anecdotal evidence suggesting colony turnover (Chaulk 2006), we hypothesized that common eiders breeding in Labrador could be described using a metapopulation approach. We predicted that local nesting populations would exhibit colony turnover, that extinction would be negatively related to both colony size and island size, that colonization would be negatively related to island isolation, and that intraspecific incidence–abundance relationships would be positive (i.e., as average colony size within a given area increases, the occupancy rate of adjacent islands will also tend to increase).
Materials and methods Study area Archipelagos near the communities of Nain and Hopedale were surveyed six times from 1998 to 2003. The archipelago adjacent to the community of Makkovik was surveyed once in 1999, and the one near Rigolet was surveyed four times from 2000 to 2003 (Fig. 1). The Nain study area was approximately 3383 km2, containing 1000 islands ranging in size from 0.01 to 44 800 ha. The Hopedale study area was approximately 566 km2, containing 650 islands ranging in size from 0.01 to 3875 ha. The Makkovik study area was approximately 763 km2, with 300 islands ranging in size from 0.01 to 3396 ha, and the Rigolet study area was approximately 2834 km2, containing 335 islands ranging in size from 0.02 to 5204 ha. All archipelagos shared similar environmental characteristics including a northern maritime climate and vegetation composed primarily of mosses, lichens, forbs, grasses, and sedges. The archipelagos of Nain, Hopedale, Makkovik, and Rigolet comprised barren islands with sparse vegetation and very limited nesting cover. All four archipelagos are classified as coastal barrens (Lopoukhine et al. 1978) and are considered to have a high-boreal ecoclimate (Meades 1990) and a Low Arctic oceanographic regime (Nettleship and Evans 1985). Census methods Archipelagos were surveyed for evidence of breeding eiders. Geodetic information from these surveys was plotted on 1 : 50 000 digital base maps and linked to tables containing information on species occurrence, nest abundance, and density. In all areas, islands were selected for study based on random or haphazard sampling (Chaulk et al. 2004). For
1403 Fig. 1. General location of archipelagos surveyed for common eider (Somateria mollisima) nests between 1998 and 2003 on the Labrador coast.
logistical reasons, we limited our searches to islands that were estimated to be smaller than 30 ha. As large islands require significantly more effort to search, we focused on smaller islands that could be easily censused by small field crews during restricted time periods. Goudie et al. (2000) reported that common eiders preferred nesting on islands smaller than 100 ha. Other researchers have used size thresholds to help identify islands for investigation during eider research (Nakashima and Murray 1988; Robertson and Gilchrist 1998; Merkel 2004) or have focused on small islands during breeding surveys (Korschgen 1977; Go¨tmark ˚ hlund 1984). and A Ground censuses were conducted using standard search methods employed by the Canadian Wildlife Service (Nettleship 1976; Chaulk et al. 2004); these consisted of two to four people systematically walking over the islands searching for signs of eider nesting. Colony sizes were based on all detected nests (active, depredated, etc.). Islands in the four archipelagos had extremely limited cover and were primarily barren, so both hens and unattended nests were easily detected. In all cases an island was considered occupied if it contained at least one eider nest with eggs. Extinction and colonization Extinction events were the change of colony state from occupied to unoccupied from one survey year to the next. Though surveys did not always occur in successive years, in most cases they did. Similarly, colonization events were a change of state from unoccupied to occupied. To model the extinction – colony size relationship we used binary logistic regression (Minitab Inc. 2003); our predictor was log10 of mean colony size. Similarly, to model extinction – island size relationships, we used binary logistic regression with the predictor log10 of island size. For the colonization – isolation relationship, we used binary logistic regression with number of islands within a 5 km buffer as the predictor. MINITAB’s logit function was used to calculate event probabilities (Minitab Inc. 2003); for significance tests, event probabilities were plotted as fitted lines along with turnover rates (extinction and colonization). #
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1404 Fig. 2. An example of the grid systems used in the incidence and abundance analysis, based on surveys of the Nain archipelago in the year 2000. The figure shows a subset of grid system 1 (support = 104 km2) and shows cells that contain surveyed islands.
Turnover rates were calculated by dividing the number of changes in occupancy by the total number of opportunities to make the change (e.g., an island could become extinct only if it had been occupied the previous year). Only islands that were surveyed in 2 or more years were used in the analysis. Island size was derived from 1 : 50 000 digital base maps. Island isolation was quantified by creating a 5 km buffer around each island and using the structured query language in MapInfo Professional 7.0 (MapInfo Corp., Troy, New York) to determine the total number of islands inside this buffer. Colonization rate to extinction rate ratios were calculated using overall mean rates, while the standard error of this ratio was calculated using the delta method (Williams et al. 2002). Colonization to extinction ratios were compared using simple Z-tests (Pollock et al. 1990) using an adjusted critical alpha of 0.017 (0.05/3) to compensate for multiple tests. Incidence and abundance Two rectangular grid systems encompassing all four archipelagos were created in MapInfo Professional 7.0 using a spherical projection system (Fig. 2). The support or cell size (Perry et al. 2002) of grid system 1 was 104 km2, and that for grid system 2 was 455 km2. Structured query language was used to limit each grid network to cells that contained three or more surveyed islands. The placement of each grid system was the same across all years. For the incidence and abundance analysis, incidence was the number of islands where eiders were present divided by the total number of surveyed islands in the cell. For example, a grid cell with 6 surveyed islands, 4 of which were occupied by eiders, with a total of 40 eider nests would have an incidence of 4/6 = 0.67 and a mean abundance of 40/6 = 6.67 eider nests/island.
Can. J. Zool. Vol. 84, 2006 Fig. 3. Observed colony extinction rates versus average colony size (log10) of common eiders nesting in Labrador, 1998–2003.
We used a fully nested hierarchical binary logistic regression (Sokal and Rohlf 1995; Hosmer and Lemeshow 2000; Minitab Inc. 2003). Model terms included year, support nested within year, and log10 of mean abundance nested within support. When appropriate we present P values for the Hosmer–Lemeshow goodness of fit test (Minitab Inc. 2003), which assesses the fit of the logistic model against actual outcomes and is a form of a Pearson w2 statistic; P > 0.05 suggests that the model fits the data well (Peng et al. 2002). All statistical tests were two-tailed and critical alpha was set at 0.05.
Results As colony size increased, extinction probability decreased, resulting in a negative relationship between eider colony size and extinction probability (G = 45.899, df = 1, P = 0.000, = –3.12 ± 0.60, odds ratio = 0.040, Hosmer–Lemeshow = 0.309, Fig. 3). We found no significant relationship between island size and extinction probability (G = 0.353, df = 1, P = 0.553, = –0.23 ± 0.39, odds ratio = 0.79, Hosmer– Lemeshow = 0.181). The number of islands within a 5 km buffer was not a significant predictor of colonization rate (G = 0.507, df = 1, P = 0.476, = –0.006 ± 0.008, odds ratio = 0.990), and the model did not fit the data well (Hosmer–Lemeshow = 0.002), suggesting no relationship between colonization probability and island isolation. Extinction rates varied by archipelago (G = 14.7, df = 2, P = 0.001), but colonization rates did not (G = 1.3, df = 2, P = 0.529; Table 1). The overall colonization to extinction (C/E) ratio was significantly higher than 1 (Table 1), indicating that more islands were colonized than went extinct. In addition, the C/E ratio varied significantly by archipelago (all three paired tests, Z > 46.3, P < 0.001), indicating regional variation with respect to both colonization and extinction rates. With respect to the incidence and abundance relationship, mean abundance (log10) was a significant positive predictor of incidence for eiders at both spatial scales in all years (G = 147.380, df = 23, P = 0.000, Fig. 4), and the model fit the data well (Hosmer–Lemeshow = 0.404). This means that as eider abundance increased in a given area, more islands in that area were occupied. #
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1405 Table 1. Observed local colony extinction and colonization rates (mean ± 1 SE) for common eiders (Somateria mollissima) on the Labrador coast, 1998–2003.
No. of islands No. of extinction trials No. of colonization trials No. of possible turnovers No. of extinctions No. of colonizations Extinction Colonization C/E ratio
Nain 39 97 15 112 4 7 0.04±0.02 0.47±0.13 11.36±6.44
Hopedale 62 123 54 177 23 20 0.19±0.04 0.37±0.07 1.98±0.52
Rigolet 24 30 5 35 1 3 0.03±0.03 0.60±0.24 18.00±19.44
Overall 125 250 74 324 28 30 0.11±0.02 0.41±0.06 3.62±0.82
Note: The C/E ratio is the colonization rate divided by the extinction rate. Extinction and colonization trials are the number of opportunities for the event to occur.
Discussion Extinction and colonization Understanding local extinction and colonization dynamics is fundamental to conservation ecology and is essential to the development and implementation of long-term conservation plans. Taken out of context, local extinction events can appear more negative and local colonization events can appear more positive than they actually are for a given population or species. We found that colony size was a significant negative predictor of local extinction. This is consistent with theoretical and empirical findings of other researchers across taxa (Schoener and Spiller 1987; Thomas 1994; Schoereder et al. 2004). That smaller colonies have higher extinction probability is consistent with the Allee effect and inverse density dependence, which state that small populations have increased extinction risks (Allee 1931; Courchamp et al. 1999). Generally, eiders were in a higher state of flux than we anticipated. High turnover was not expected because site tenacity and philopatry are traditionally thought to be high in common eiders (Cooch 1965; Reed 1975; Wakeley and Mendall 1976; Swennen 1990). Extensive periods of nonbreeding have been documented (Coulson 1984) but to our knowledge have not been reported to lead to colony turnover. However, it is possible that extensive non-breeding could increase extinction probability in small colonies. In terms of extrinsic factors, the effect of annual environmental variability on local population turnover is currently unknown. Eider nest initiation date is thought to be negatively related to the amount of sea ice (Chaulk 2006). Chaulk et al. (2004) found that nest initiation was delayed in 2002, probably in response to heavy ice conditions. While not the focus of this paper, it is possible that local ice conditions and mammalian predators could also influence colony turnover rates (Chaulk 2006). While turnovers were more frequent than expected, colonization rates were higher than extinction rates. For the same time period and study area, we documented significant population increases for eiders in Labrador (Chaulk et al. 2005). Our colony turnover findings are consistent with these general population trends, as one might expect to find a C/E ratio > 1 in a growing population. While some researchers have investigated colony-site dynamics in colonial bird species (Erwin et al. 1998; Nichols et al. 1998; Barbraud et al. 2003), there is limited information on colony
turnover rates relative to population growth with which we can compare our results. However, Martı´nez-Abraı´n et al. (2003), in their investigation of Audouin’s Gull (Larus audouinii Payraudeau, 1826), found similar patterns whereby population increases were accompanied by increased colonization and vice versa. It is possible that some of the patterns we have observed in terms of eider colonization and extinction could be driven by synchronous non-breeding (Coulson 1984). However, we feel that this is unlikely. For example, one would expect to observe either sudden decreases in abundance and incidence during non-breeding years or vice versa. However, this was not the case; instead, over the 6-year study period we observed steady increases in abundance and incidence with an overall C/E ratio greater than unity. We feel that the rates of colonization and extinction that we documented are typical of common eiders that nest on small colonies in complex archipelagos. On the Labrador coast there are thousands of islands on which eiders could nest (Chaulk et al. 2004); however, elsewhere in the breeding range there may be fewer choices for colony selection, and consequently eiders in these regions may nest in a few large and stable colonies. Greater colony stability in other areas might explain why colony turnover in nesting common eiders is rarely reported. We found that patch (i.e., island) size was not a significant predictor of extinction risk, although this has been suggested by other researchers for other taxa (Kindvall and Ahlen 1992; Hanski 1999; Schoereder et al. 2004). This was not unexpected, since eiders are thought to avoid large islands (>100 ha) to decrease the risk of mammalian predation (Goudie et al. 2000). Aerial surveys of the Labrador coast in 2006 suggested that this is indeed the case: eiders rarely associated with islands larger than 100 ha (K.G. Chaulk, unpublished data). We also found that colonization rates were not related to island isolation, although this may have been due to the isolation measure we used in our analyses. For example, we had incomplete knowledge of all colony locations; instead, we counted all islands within a given radius of each surveyed island, regardless of island size. Furthermore, the dispersal distance of migratory birds is so vast that larger spatial scales (>100’s km2) may be required to detect relationships between colonization and isolation. However, it could be that isolation from other colonies is unimportant in the population dynamics of nesting eiders. #
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Fig. 4. Intraspecific incidence versus abundance curves by survey year for breeding common eiders on the Labrador coast, 1998–2003. Two spatial scales were investigated: one grid cell size (support) was 104 km2, and the other was 455 km2. Mean abundance (log10) is total number of nests per grid cell divided by number of islands surveyed.
In some regions, eiders have extremely high rates of philopatry (Swennen 1990) and strong tenacity to brood-rearing areas (Bustnes and Erikstad 1993). Other researchers have shown that eiders occasionally move between adjacent colonies across breeding years (Milne 1974; Schamel 1977; Mehlum 1991; Merkel 2004). Overall, eiders probably have strong philopatric tendencies to islands and (or) island clusters, which supports the idea of local population structure and provides some evidence of mixing at local scales but limited population mixing across larger spatial scales. Differential mixing at both local and regional scales might help explain the evolution of the various common eider subspecies. Incidence and abundance Many species have been shown to have positive incidence (distribution) and abundance relationships (Hanski 1982, 1999; Nee et al. 1991; Gaston and Curnutt 1998) and there is a growing body of research that explores this phenomenon (Bock and Ricklefs 1983; Brown 1984; Wright 1991; Gaston and Blackburn 1996; Venier and Fahrig 1996; Gaston et al. 1997; Johnson 1998). Gaston and Curnutt (1998) found that the distribution of the common grackle (Quiscalus quiscula (L., 1758)) increased as the population declined, but more typically a declining population will probably translate into a reduced distribution. One explanation for the positive intraspecific incidence and abundance curve is the rescue effect from adjacent local populations (Brown and Kodric-Brown 1977). The rescue effect is based on the notion that individuals from densely occupied patches are available to colonize nearby patches, increasing the local occupancy rate. In eiders, and likely most migratory birds, individuals are not restricted by dispersal ability, but large colonies may act as source populations from which individuals disperse and settle on nearby
islands, consistent with either the ideal free distribution (Fretwell and Lucas 1970) or source–sink population dynamics (Pulliam 1988). Other mechanisms such as competition for local resources are also used to explain avian colony distribution patterns (Furness and Birkhead 1984; Cairns 1989), but it is not clear whether these processes would result in significant positive incidence and abundance relationships. Understanding trends in incidence and abundance relationships and their covariance is very important for understanding overall population processes. For example, a population with locally increasing abundance and decreasing distribution may require a different management strategy than one with increasing abundance and increasing distribution. In terms of eiders nesting in Labrador, we found that positive incidence and abundance relationships were repeated at two spatial scales across all years. This is important given the large volume of ecological literature that stresses pattern assessment at multiple spatial and temporal scales (Weins 1989; Levin 1992; Schneider 1994; Turner et al. 2001; Scott et al. 2002). It should be noted that niche breadth, sampling errors, and locally abundant resources have also been suggested as potential causes of positive incidence and abundance relationships (Brown 1984; Wright 1991; Venier and Fahrig 1996). In conjunction with our findings on local colonization rates, we interpret the positive intraspecific incidence–abundance relationship as evidence of local population connectivity and a rescue effect, recognizing that local resources (food and shelter) and environmental conditions (predators, ice, disease) are important to overall population dynamics and distribution. A variety of theories, models, and hypotheses have been developed to describe spatial population structure (Levins 1969; Fretwell and Lucas 1970; Pulliam 1988; Stamps 1988). Within the context of the evidence presented here, the spatial structure of nesting islands, local colony turnover, #
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and positive intraspecific incidence–abundance relationships lead us to conclude that concepts derived from the metapopulation framework can be applied to nesting eider populations. Though particular metapopulation characteristics or even the metapopulation label can be argued, it is clear that a comprehensive understanding of population dynamics can be achieved only by simultaneous consideration of space and time (see Heath et al. 2006). We suggest that conservation planners dealing with eiders and other colonial bird species consider metapopulation or other spatially explicit models to assist in developing ecologically relevant conservation strategies, such as protecting networks of islands that will sustain local population processes.
Acknowledgements We thank Judy Rowell and Bruce Turner for their longterm support of seabird research in Labrador; Jolene Jackman and Shawn Broomfield for assisting with data preparation; and Tariqul Hasan from the Department of Statistics, Memorial University of Newfoundland, for his review of the fully nested binary logistic model. I also thank Bryn Wood for his help. We thank Dan Esler, Grant Gilchrist, Alejandro Martinez Abrain, and several anonymous reviewers for their helpful comments on earlier versions of this manuscript. We thank the following organizations for financial support: the Labrador Inuit Association, Canadian Wildlife Service, Memorial University of Newfoundland, Nasivvik Centre for Inuit Environment and Health, the University of Laval, Northern Ecosystem Initiative, and the Northern Scientific Training Program.
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