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Recruitment, Dispersal, and Demographic Rescue in Spatially-Structured White-Tailed Ptarmigan Populations ’

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Recruitment, Dispersal, and Demographic Rescue in Spatially-Structured White-Tailed Ptarmigan Populations ’ The Condor 102:503-516 D The Cooper Ornithological Society Zoo0 RECRUITMENT, DISPERSAL, AND DEMOGRAPHIC RESCUE IN SPATIALLY-STRUCTURED WHITE-TAILED PTARMIGAN POPULATIONS ’ KATHY MARTIN Canadian Wildlife Service, 5421 Robertson Rd, RR I Delta, British Columbia, Canada, V4K 3N2, and Department of Forest Sciences, 2424 Main Mall, University of British Columbia, Vancouver, British Columbia, Canada, V6T 124, e-mail: [email protected] PETER B. STACEY Department of Biology, University of New Mexico, Albuquerque, NM 87131, e-mail: [email protected] CLAIT E. BRAUN* Colorado Division of Wildlife, 317 West Prospect Rd., Fort Collins, CO 80526, e-mail: [email protected] Abstract. We studiedrecruitment and dispersalof White-tailed Ptarmigan (Lagopus leu- curus) breeding in naturally fragmented alpine habitatsat four study sites in Colorado from 1987-1998. Almost all recruitment for both sexes, particularly females, was of birds pro- duced outside local populationsand also external to nearby studiedpopulations. Populations were more dependent on female recruitment than on male recruitment to sustainthem, and patterns of recruitment were not correlated with local survival of adults or production of young the previous year, except at one site for females. Over 95% of recruitswere yearlings. Breeding dispersal of adults, an infrequent but regular event, was also important to inter- population connectivity. Our data for multiple populationsallowed us to describemovement patterns among populations to assess consistency with conditions required for a rescue system. After widespread reproductive failure in one year, we expected all populationsthe next year would have low recruitment due to a reduced supply of recruits produced in the region. Recruitment was low, but impact varied among populations.We conductedan over- winter study of radio-marked offspring to determine possible influences of winter site lo- cation and relatives on recruitment patterns.Contrary to expectation,offspring remained on or near breeding sites in winter, but were not located near their mothers or siblings. Re- cruitment location was related to winter site location. White-tailed Ptarmigan exhibit a well developed capacity for external recruitment that allows them to persist in small populations with stochasticconditions for breeding and survival. Extensive external recruitment may be a general pattern for birds. Kev words: demographic rescue, dispersal, Lagopus leucurus, local survival, metapo- pulazon processes, recruitment, White-tailed Ptarmigan. INTRODUCTION and Kodric-Brown 1977). Stacey and Taper Periodic rescue of declining populations through (1992) modeled the importance of immigration recruitment from productive populations (dis- rescue to persistence of small populations, and persal rescue) may be an important feature of showed that external recruitment was vital to en- suring population persistence and stability. Dis- avian population biology. Dispersal rescue de- persal rescue is predicted to work most effec- scribes the processwhereby populations that ex- tively when neighboring populations are in close perience one or several poor years of reproduc- proximity, but are uncorrelated in patterns of an- tion or survival escapeextinction or a population nual production and survival (Stacey et al. bottleneck by immigration of individuals from 1997). If inter-patch dispersal is high, local ex- elsewhere in the metapopulation system (Brown tinction may not occur (Harrison 1991). Thus, dispersal may operate regularly to ensure local I Received 23 April 1999. Accepted 3 1 March 2000. population stability; extinction may be a failed 2 Current address:5572 No. Ventana Vista Rd., Tuc- rescue process. son, AZ 85750. Studies of the patterns and mechanisms of re- [5031 cruitment and dispersal in multiple populations lated across sites and years, and that small pop- allow a direct assessmentof the importance of ulations remained stable despite stochasticity in demographic connectivity among populations. internal population dynamics (Martin et al. Metapopulation processesmay be influenced by 1997, K. Martin, unpubl. data). Here, we present both local and/or regional stochasticity (Hanski data on origins and patterns of recruitment for 1991, 1994, Hanski and Gilpin 1991, Harrison four study populations that showed strong reli- 1991). A knowledge of synchrony in popula- ance on external rescue. For recruits, we present tions within a region is crucial to understanding data on the relative amounts of natal (juveniles) population and multi-population persistence versus breeding dispersal (breeding birds be- times (Harrison and Quinn 1989, Hansson tween years). Settlement patterns in relation to 1991). natal territory are given for offspring recruits in Regardless of whether a metapopulation one population. structure can be demonstrated for a species, in- We also present data on recruitment and pop- vestigating metapopulation processes such as ulation size in a year following widespread re- dispersal and rescue, and how they interact with productive failure, and compare these to recruit- stochastic factors, is vital to understanding the ment patterns from all previous years when re- population dynamics for species, particularly in productive failure varied spatially across popu- fragmented habitats. Alpine habitats, given their lations. Thus, with this natural experiment, we naturally patchy character with regular stochas- examine patterns of recruitment caused by local tic factors, are ideal for examining rescue pro- and regional events. We anticipated that all pop- cesses. Dispersal and other population transfer ulations would have low recruitment in the year mechanisms for vagile vertebratesare poorly un- following regional-level failure due to a reduced derstoodand logistically difficult to study at rel- supply of recruits in the region, but that popu- evant biological and spatial scales (Martin lations with generally higher local survival of 1998). Data are usually lacking on social inter- adults would be impacted less than populations actions or movements in the post-breeding sea- normally requiring a high rate of rescue. We ex- son, factors considered to be important to off- pected that one population that normally recruit- spring making “decisions” regarding dispersal ed a higher proportion of adult females would and recruitment. Most life history studies as- be buffered from this regional-level event. Fi- sume that patterns of offspring settlement show nally, with a telemetry study, we examined pos- a gradual decline away from natal territories, but sible social and behavioral mechanisms for natal there are logistical and analytical problems in recruitment. In one year, we followed radio- estimating numbers and distance of widely dis- marked offspring over winter to the following persing offspring (Koenig et al. 1996, Nichols spring, and examined influences of movements 1996). by mothers and siblings and winter dispersal be- To examine the importance of metapopulation havior on offspring recruitment patterns. processesin naturally fragmented habitats, we studied four populations of White-tailed Ptar- METHODS migan (Lugopus Zeucurus) in Colorado. About STUDY AREAS 95% of both sexes spent their entire breeding life in one population, satisfying the definition We studied four populations of White-tailed of a discrete population suggestedby Harrison Ptarmigan breeding in naturally fragmented high (1991), and patcheswere connected by natal dis- alpine habitats in the Colorado Rocky Moun- persal. Our populations showed dramatic varia- tains (39”34-40’N, 105”35-53’W) from 1986 to tion in offspring production and return (local 1998 (Fig. 1). The life history and breeding bi- survival) of adults acrossyears and sites (Braun ology of White-tailed Ptarmigan are detailed in et al. 1993, Hannon and Martin 1996). Repro- Braun and Rogers (1971) and Braun et al. (1976, ductive failure appeared driven by climatic 1993). Each of the four study areas comprised events and internal ecological processessuch as individual mountain patches separatedfrom the depredation of eggs and young (Braun et al. other areas by distances of l-20 km, and in 1993, K. Martin and K. L. Wiebe, unpubl. data). some directions by areas of unsuitable ptarmigan Preliminary results indicated that variation in re- breeding habitat (forest, wetland, willow Mix productive success and survival were uncorre- spp. shrub). The Mt. Evans (ME) study area was Mt Otter MOUtltill Mt Wilcox 8 -20 km to NW GriZZ%’y Gray Wolf @a MOUtltitl Rosalie Peak I 2km Mountain FIGURE 1. Study sites for White-tailed Ptarmigan in Colorado, including Guanella Pass winter site and ad- jacent mountainsthat were surveyed less intensively in late summer.ME-Mt. Evans, GP-Guanella Pass,ST- Square Tops, and LP-Loveland Pass. The shaded lines on ME study area indicate the four major drainages used to evaluate philopatry of internal recruits. 13.2 km2 in size, and from 1987 to 1996 annual were missing for some years including 1995, the spring population sizes ranged from 26 to 67 year of regional failure. In 1997 and 1998, we birds (Fig. 1, Table 1). Guanella Pass (GP), 6-7 conducted less intensive censusing on ME and km from ME, comprised about 2.8 km*, and ST, and present data only for our over-winter from 1987 to 1996 (excluding 1988), popula- study of radio-marked birds. tions varied from 6 to 20 birds. Square Tops (ST) was l-2 km from GP and 8-11 km from FIELD PROCEDURES ME. Square Tops was 4.75 km2 and during 1992 Field methods and data
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