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Demographic characteristics of Peromyscus leucopus inhabiting a natural dispersal sink
DAVID T. KROHNE Department of Biology, Wabash College, Crawfordsville, IN 47933, U. S. A.
Received July 22, 1988
KROHNE, D. T. 1989. Demographic characteristics of Peromyscus leucopus inhabiting a natural dispersal sink. Can. J. Zool. 67: 232 1 - 2325.
The population biology of white-footed mice (Peromyscus leucopus) inhabiting a 1.4-ha naturally occurring dispersal sink was studied for 5 years in west-central Indiana and compared with that in surrounding old-growth habitat. Densities in the dispersal sink were consistently lower than in prime habitat. Autumn and winter survival were practically nil. The sink was recolonized by adults in the spring following extreme low winter densities or extinction. Summer reproductive rates and the pattern of territoriality were similar to those in prime habitat. Mice colonized elsewhere when empty prime habitat was made experimentally available. The data suggest that few dispersing mice can be accommodated by the dispersal sink.
KROHNE, D. T. 1989. Demographic characteristics of Peromyscus leucopus inhabiting a natural dispersal sink. Can. J. Zool. 67 : 2321 - 2325.
La biologie d'une population de Souris B pattes blanches (Peromyscus leucopus) dans un bassin de dispersion nature1 de 1'4 ha a fait l'objet d'une etude d'une duree de 5 ans dans le centre-ouest de ]'Indiana et a 6t6 comparee B celle de la population de l'habitat environnant, une for& Agee. La densite de la population du bassin de dispersion etait toujours plus faible que celle de la population de 17habitat principal. La survie au cours de 17automne et de 17hiver etait pratiquement nulle. Le bassin etait recolonis6 par des adultes au printemps lorsque la densite au cours de 17hiver precedent etait extremement faible ou lorsqu7elle tombait B zero. Les taux de reproduction et la territorialit6 6taient semblables B ceux enregistres dans 17habitat principal. Les souris s76tablissaient ailleurs lorsque des territoires devenaient disponibles (expkrimentalement) dans l'habitat d70rigine. Les donnkes indiquent que peu de souris migratrices peuvent s'ktablir dans le bassin de dispersion.
[Traduit par la revue]
Introduction demography may result (Krebs et al. 1969). Much of our
Most of our information about population biology is derived from populations inhabiting prime or even pristine habitats. One reason for this is that areas with dense populations are preferred by researchers because they generate larger sample sizes. Furthermore, for climax species, the processes observed in pristine habitat are less likely to be confounded by the effects of human disturbance. One can thus be more certain that the demographic characteristics in fact evolved in that ecological context. Nevertheless, there is ample evidence that many populations inhabit regions that are a mosaic of micro- habitat qualities (Evans 1942; Krebs 1966; Cockburn and Lidicker 1983; Garrett et al. 1982). Coincident with this microhabitat variation is spatial variation in density within what I would call a population. Low density populations may be instructive in at least two respects. First, inclusion of information from these populations is necessary to expand our understanding of population regulation. If regulation is a process that includes interactions among subpopulations and habitats of different quality, low density populations are of inherent interest. Second, spatial variation in density is a com- ponent of the environment that can be exploited by dispersers and thus may be used to test hypotheses concerning the evolu-
information about dispersal sinks is derived from removal studies (Dobson 1981; Krohne and Miner 1985; references in Gaines and McClenaghan 1980). The demography of sinks created by removal trapping may be artificial because the sinks are generally created in high-quality habitat or at least in areas that have relatively high natural densities. Little information is available from sinks not created by removal trapping.
This paper reports the results of a study of a naturally occur- ring low density area that serves as a dispersal sink in a popu- lation of Peromyscus leucopus. It accumulates dispersing animals from higher density populations in prime habitat. The basic population biology of the dispersal sink was compared with that in prime habitat. The following questions were addressed: Is the demography of the dispersal sink funda- mentally different from that in prime habitat or does it simply represent a scaled-down version of that in prime habitat? What role does the dispersal sink play for dispersing animals from prime habitat? How many can be accommodated? I test the hypothesis that the dispersal sink represents a refuge for large numbers of dispersers from surrounding habitat.
Materials and methods tion of dispersal.
The study was completed at the Allee Memorial Forest, located Lidicker ( 1975) referred to low density areas available to 1 3 km of Rockville, parke County , Indiana. The field ~ t a - dispersers as dispersal sinks. They may be empty tion consists of 80 ha of old-growth deciduous forest. Except for 1.4
habitat Or suboptimal habitat where at least short-term survival ha, the forest is either virgin timber or has had minor disturbance in is possible. These sinks accumulate dispersing animals and the last 200 years. The low density dispersal sink consists of a succes- relieve population pressure in high density areas. If no dis- sional area 1.4 ha in size on the site of an agricultural field abandoned persal sinks are available, dispersal is frustrated and atypical in 1938. The boundary between this areaind the undisturbed forest
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CAN. J. ZOOL. VOL. 67, 1989
- - - . g o r g e b o u n d a r y
H
FIG. 1. Arrangement of live-trapping grids at Allee Memorial Woods.
is very sharp. Succession has proceeded undisturbed for the past 50 years. Presently, the forest there is dominated by tulip (Liriodendron tulipifera) and red maple (Acer rubrum). Some individuals are more than 15 cm diameter at breast height (dbh). Invasion of hardwoods including black oak (Quercus velutina), beech (Fagus grandifolia), and sugar maple (Acer saccharum) has begun. While the original dis- turbance was a result of human activities, the sink is considered natural because a similar succession would result from natural dis- turbance and, more importantly, densities of mice were not manipu- lated to create the sink.
A live-trapping grid (grid 4) consisting of 55 stations at 15.2-m intervals was established in the successional area (Fig. 1). A single large Sherman trap baited with wild bird seed was placed at each station. Four other grids were established in old-growth forest (Fig. 1). Although there were some demographic differences among these four grids, they were generally similar (Krohne et al. 1984). Grid 1 was used for comparison with grid 4 because it was live- trapped in all years of the study, with no manipulations. Both grids were livetrapped for two to four nights at least monthly and occasion- ally bimonthly during the breeding season from 1980 to 1985.
Density estimates were made with the direct enumeration method (Krebs 1966).
To test the tendency of mice to colonize grid 4 if prime habitat is available, all mice were removed biweekly from September 1982 to November 1983 from grid 3, immediately adjacent to the dispersal sink. Animals were marked on all other grids. Thus, the source of many colonists of grids 3 and 4 was known. I compared the frequency of colonization of grid 4 by marked animals when empty prime habitat was available with that observed when normal populations occupied grid 3.
Results Densities on grid 4 were consistently lower than those on
grid 1 (Fig. 2). Both grids showed annual peaks in abundance and a moderate but statistically significant correlation of densi- ties (Fig. 3). Henttonen's index of density variation, s (Hen- tonnen et al. 1985), was 0.293 for grid 4 and 0.200 for grid 1. Peak densities on grid 1 showed greater variation than those on grid 4. However, the very low densities on grid 4 each
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FIG. 2. Density changes on grid 1 (A) and grid 4 (a). Shaded regions represent the winter months, December -February.
G R I D 1 D E N S I T Y (no./ha)
FIG. 3. The association between densities on grids 1 and 4. Each point represents a monthly density estimate.
winter followed by summer recovery resulted in higher overall variation there.
The population on grid 4 went extinct in two winters and nearly so in three others (Fig. 2). Recolonization occurred in the spring and early summer via immigration from surround- ing areas. A high proportion of first captures on grid 4 com- prised animals first marked elsewhere (0.20; n = 184). This figure was considerably higher than that reported in previous work (Krohne and Baccus 1985) for adjacent grids 2 and 3 (0.04 and 0.03, respectively; comparison of grid 4 with grids 2 and 3: X2 = 20.7; df = 1; p < 0.001). On grid 4 the pro- portions of juveniles, subadults, and adults at first capture were 0.14, 0.19, and 0.68, respectively. These differed sig- nificantly from grid 1 where the corresponding values were 0.26, 0.21, and0.54 (x2 = 12.95; df = 2 ; p < 0.002; grid 4, n = 184; grid 1, n = 393).
The overall proportions of males and females of all ages first caught on grids 1 and 4 were very similar (grid 4: 55 % male, n = 185; grid 1: 54% male, n = 393; x2 = 0.147, df = 1, p < 0.70). A three-way G-test of independence indicated that there were significant differences in sex ratio among years on both grids (G = 89.6, df = 4, p < 0.001) but not between grids (G = 2.0, df = 1, p < 0.80) in any year.
Residence times for mice on grid 4 were significantly less than those on grid 1 (Igrid4 = 26.5; Igrid I = 45.0; t = 2.99;
df = 435; p < 0.001). This difference was partly a result of the extremely low overwinter survival on grid 4. Three per- cent of mice present in the fall (September - November) were captured the following spring on grid 4 compared with 21 % on grid 1 (x2 = 5.01; df = 1; p < 0.05). Furthermore, many colonists were first captured on grid 4 as adults and thus had less of their life expectancy remaining. Few grid 4 animals were subsequently caught elsewhere.
There was no difference between grids 1 and 4 in the pro- portion of males in reproductive condition at some time in their lives (Table 1). The proportion of all adult females preg- nant while on grid 4 was lower than that for grid 1 but the difference was not statistically significant. The capture of very young juveniles in summer suggests that reproduction in situ contributes to summer density. However, the relative impor- tance of reproduction and immigration is not known.
Secondary succession is not proceeding at the same rate on all parts of grid 4. A large proportion of the trees larger than 15 cm dbh is found in the northeast corner of the grid where hardwood mast-bearing trees are invading. I analyzed micro- habitat use on grids 1 and 4 by dividing the grids into 578-m2 quadrats composed of six trap stations and tallying the number of individuals caught in each quadrat. The number using each quadrat was compared with the number expected if quadrats were used at random. On grid 1, mice used the quadrats at ran- dom (x2 = 21.6, df = 14, p < 0.086). However, on grid 4 some quadrats were used significantly more frequently than others (x2 = 32.5, df = 11, p < 0.001). The areas of highest use were those where succession has proceeded furthest.
These data indicate that dispersing mice exercise some microhabitat selection. To further examine the choices made by dispersers, empty prime habitat was created on grid 3, adja- cent to grid 4. Under such conditions, no marked animals colonized grid 4 compared with 2 1.3 % marked mice in the other years (x2 = 3.84; df = 1 ; p < 0.05). Removal trap- ping of grid 3 eliminated one source of marked colonists for grid 4. However, this cannot by itself account for the decrease in marked animals on grid 4 in 1983. Previous studies showed that grids 1 and 2 normally supply approximately 65% of the marked mice captured on grid 4, whereas grid 3 provides only 29 % (Krohne and Baccus 1985).
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2324 CAN. 1. ZOOL. VOL. 67, 1989
TABLE 1. Proportions of all mice and of mice that reached adulthood that became reproductively active at least once
-- -
Proportion reproductive Proportion of adults repro- during lifetime ductive during lifetime
Male Female Male Female - -
Grid 4 0.57 (99) 0.31 (86) 0.59 (78) 0.37 (74) Grid 1 0.57 (178) 0.43 (145) 0.61 (146) 0.49 (127) x2 0.004 2.94 0.08 2.88 d f 1 1 1 1 P < 0.95 <0.08 < 0.77 <0.09
NOTE: Females were scored as reproductive if palpably pregnant or lactating. Males were scored reproductive if testes were descended. Values in parentheses are sample sizes.
Krohne (1986) showed that home range size determined by livetrapping is sensitive to sample size in Peromyscus leuco- pus. Because residence times and densities were low on grid 4, it was difficult to obtain the 10 captures required for home range measurement (Krohne 1986). However, in the summer of 1981 sufficient data were obtained for 10 coexisting indi- viduals (5 male; 5 female). Home ranges of coexisting males were nonoverlapping. Males usually overlapped at least one female (percent overlap: x~~~~~~~~~~ = 24.0). Similarly, females maintained intrasexual territories (percent over- lap: xfemale = 12.8). In other years the same pattern was apparent with smaller samples.
Discussion During the summer the dispersal sink was similar to a
scaled-down version of the population in prime habitat. Densi- ties were consistently lower than, but correlated with, those on grid 1. Sex ratios were similar. The pattern of territoriality was like that observed in prime habitat and reported for Pero- myscus leucopus by Wolff (1986). The main difference between grid 4 and grid 1 in summer was the greater adult bias on grid 4.. This was perhaps a reflection of the early spring colonization by adults and a slightly decreased reproductive rate of resident females there. The winter extinction in the sink had profound effects on the population. The low fall and winter survival contributed to the consistently low density and affected the residence times of animals there.
The low winter survival on grid 4 could result from seasonal emigration or low survival in situ. Few mice from grid 4 were subsequently caught elsewhere as expected if emigration is important. At least two factors could lead to increased mortal- ity. First, because of the successional status of the sink, few large mast-producing trees are present to provide food in the fall and winter. Second, the dispersal sink is less structurally complex. Since succession is far from climax, there is no senility and windthrow of canopy trees; hence, there is less down wood, branches, and logs to provide cover. Consequently, predation may be more intense there than in old-growth areas.
The dispersal sink can accommodate a small number of dis- persers. In summer, densities reached a peak far below that on grid 1. A typical territorial system with little overlap was maintained. Only parts of the sink were used; few individuals and no permanent territories were found in portions of the grid at younger stages of succession. Much dispersal occurs in prime habitat during summer (Krohne et al. 1984; Wolff et al. 1988) when the sink reaches its highest density. The dispersal sink is surrounded by at least 80 ha of prime habitat in which
densities are higher and relatively homogeneous. Thus, the dispersal sink can probably accommodate only a fraction of the dispersing animals in this 80-ha forest. Most dispersers must integrate into a social system in prime habitat where densities are higher or they must wander for large distances. We know something about the demographic context in which dispersal occurs (Gaines et al. 1979; Tamarin 1977; Krohne et al. 1984) and the success of dispersers in the new area (Krohne and Burgin 1987), but we know very little about how the new area is chosen or how a disperser establishes itself.
The dispersal sink is clearly suboptimal habitat. Colonists and their philopatric offspring probably have low fitness owing to extremely low autumn and winter survival there. The microhabitat selection observed within grid 4 and between grids 3 and 4 indicates that mice perceive the area as subopti- mal habitat. The evolutionary origin and persistence of pre- saturation dispersal require that it provides positive fitness benefits to the dispersing individual (Dobson 1981 ; Green- wood 1983). Immigration to grid 4 is not possible for many dispersers. Thus, the fitness advantages of presaturation dis- persal must accrue to those who can successfully enter prime habitat despite its higher densities. Similarly, among the few mice who can enter grid 4, the only successful ones are those whose offspring manage to leave and enter a population in prime habitat.
I suspect that the biology of low density populations such as this one is highly idiosyncratic. Many factors, including size, successional status, or habitat quality, dispersal barriers, and the dynamics of surrounding populations, will determine the unique demographic patterns of low density populations. Single examples probably cannot be generalized. However, analysis and documentation of the biology of low density habitats may be important to expand our understanding of population dynamics and regulation. Regional populations are clearly a mosaic based on microhabitat variation and demo- graphic heterogeneity. Dispersal connects these subunits of a population. Our knowledge of the role of dispersal in homo- genizing density is scant. The data presented here indicate that removal of animals from grid 3 in 1983 affected the coloniza- tion of grid 4. Note in Fig. 2 that 1983 was also one of the lowest density years on grid 1. Is this attributable to the grid 3 removals? From how far are animals drawn to low density areas? On what scale does dispersal homogenize densities? Studies of these and related questions would be a fruitful line of inquiry.
In summary, the demographic patterns of this dispersal sink during the summer months were qualitatively similar to those in nearby high density populations. Nevertheless, the low winter survival had profound effects on density which were manifest in density differences that persisted all year. Appar- ently this particular dispersal sink can accommodate only a fraction of dispersing animals and it represents a suboptimal choice for those forced to colonize it. I suggest that studies of the frequency of immigration to high density prime habitats and the characteristics of the dispersers able to colonize such areas will be important information to complement that from dispersal sinks.
Acknowledgements
I thank Mark Miner, Chris Krupp, Brad Dubbs, Jerry Noll, and Alex Burgin for their assistance in the field. This work was supported by grants from the National Science Foundation (No. SP1-80256687) and the Indiana Academy of Sciences.
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NOTES 2325
The financial support of the Treves Fund is also gratefully acknowledged. The McLain - McTurnan - Arnold Fund sup- ported a leave of absence to pursue this work in 1982.
COCKBURN, A., and LIDICKER, W. Z., JR. 1983. Microhabitat hetero- geneity and population ecology of an herbivorous rodent, Microtus californicus. Oecologia, 59: 167 - 177.
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EVANS, F. C. 1942. Studies of small mammal populations in Bagley Wood, Berkshire. J. Anim. Ecol. 11: 182-197.
GAINES, M. S., and MCCLENAGHAN, L. R. 1980. Dispersal in small mammals. Annu. Rev. Ecol. Syst. 11: 163-196.
GAINES, M. S., VIVAS, A. M., and BAKER, C. L. 1979. An experi- mental analysis of dispersal in fluctuating vole populations: demo- graphic parameters. Ecology, 60: 8 14 - 828.
GARRETT, M. G., HOOGLAND, J. L., and FRANKLIN, W. M. 1982. Demographic differences between an old and new colony of black- tailed prairie dogs. Am. Midl. Nat. 108: 5 1 - 59.
GREENWOOD, P. J. 1983. Mating systems and the evolutionary conse- quences of dispersal. In The ecology of animal movement. Edited by I. R. Swingland and P. J. Greenwood. Oxford University Press, Oxford.
HENTTONEN, H., MCGUIRE, A. D., and HANNSON, L. 1985. Com- parisons of amplitudes and spectral analyses of density variations in long-term data sets of Clethrionomys species. Ann. Zool. Fenn. 22: 221-227.
KREBS, C. J. 1966. Demographic changes in fluctuating populations of Microtus californicus. Ecol. Monogr. 36: 239 - 273.
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WOLFF, J. 0. 1986. Life history strategies of white-footed mice (Pero- myscus). Virginia J. Sci. 37: 208 -220.
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Exploratory behavior and dispersal: a graphical model
MICHAEL L. JOHNSON Kansas Biological Survey, 2291 Irving Hill Drive, Lawrence, KS 66045, U.S.A. and Department of Systematics and
Ecology, University of Kansas, Lawrence, KS 66045, U. S. A.
Received September 1, 1988
JOHNSON, M. L. 1989. Exploratory behavior and dispersal: a graphical model. Can. J. Zool. 67: 2325 -2328. A graphical model is developed that describes the costs and benefits of exploratory behavior, and qualitatively predicts the
optimal exploration time. An increase in density results in a decrease in exploration because of the decreased benefits and increased costs. This decrease in exploration time is hypothesized to result in the decreased dispersal rate that is observed in populations of small rodents. By incorporating predation, the graphical model explains why the dispersal rate in the decline phase of a microtine density cycle is lower than the dispersal rate at comparable densities of the increase phase.
JOHNSON, M. L. 1989. Exploratory behavior and dispersal: a graphical model. Can. J. Zool. 67 : 2325 -2328. On propose ici un modkle graphique capable de dkcrire les coQts et gains reliks au comportement d'exploration, et de prkdire
qualitativement la dude optimale de l'exploration. Une augmentation de la densitk entraine une diminution de l'exploration rksultant de l'augmentation des coQts et de la diminution des gains. I1 est possible que cette diminution de la durke d'exploration soit responsable du taux rduit de dispersion observk chez les populations de petits rongeurs. En tenant compte de la prdation, le modkle graphique explique pourquoi le taux de dispersion chez une population de microtinks en phase de dkclin est plus faible que le taux de dispersion chez une population de densitk comparable en phase de rkcupkration.
[Traduit par la revue]
During studies in which small mammals are livetrapped, it is designate "movers. " These were animals who, within a trap- common to capture an animal many times within a small area. ping session, travelled a distance greater than the mean + 1 However, the same animal occasionally may be captured a standard deviation. Several differences in demographic param- long distance from its home range. Radio-tagged animals are eters between movers and nonmovers were found, indicating often located some distance away from the area where most that long-distance movements were not randomly distributed activity is concentrated (see review by Madison 1985). among individuals in the population. Often, long-distance Recently, Baird and Birney (1982) used a statistical criterion to movements have been interpreted as exploratory behavior Printed in Canada 1 Imprime au Canada
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