摘要:

For 3 yr we studied the reproductive responses of desert rodents in the Owens Valley of eastern California (average annual precipitation 14 cm): four nocturnal heteromyids–the kangaroo rats Dipodomys microps and D. merriami and the pocket mice Perognathus formosus and P. longimembris–and one diurnal sciurid, the antelope ground squirrel, Ammospermophilus leucurus. Reproductive status was assessed by autopsies of adults trapped at approximately monthly intervals. Reproduction differed conspicuously among the five species. Our analysis illustrates effects of body size, phylogenetic association, and adaptation to the desert environment upon reproductive performances and associated life—history parameters. Most breeding occurs in late winter and early spring. Winter rains cause a series of pulses in vegetation growth and an attendant increase in availability of water in food plants, which contribute to rodent reproduction. Among the four heteromyids, onset of breeding is sequential according to body size, with the largest first. Pocket mice hibernate (P. formosus typically 3½ mo, P. longimembris 6½ mo), which restricts their breeding season compared to that of Dipodomys, but breeding normally begins following hibernation. The males of all species precede females in reproductive readiness; sperm production begins 1@2 to 2 mo before mating begins. Some male D. merriami remain spermatogenic throughout the year, and the mating season of this species is the longest (typically 2½ mo) and most variable of any of the species. D. merriami typically produces only two young, which are weaned in just less than 3 wk. It can breed repeatedly under favorable conditions and is the only species in which we observed reproductive maturity of both male and female juveniles in the season of birth. D. merriami has the highest annual reproductive potential of any of the five species studied. D. microps, although larger than D. merriami and sharing similar traits of small litter size and rapid growth, has a more restricted mating season (typically 1½ mo), but its breeding success generally exceeds that of D. merriami. The diet of saltbush leaves consumed by D. microps is atypical within this generally granivorous rodent family. Saltbush is a perennial shrub with highly predictable spring growth of leaves that are used by lactating mothers and developing young. Consequently the breeding season of D. microps is less variable and shorter than that of D. merriami. D. microps typically produces one litter per year and the juveniles typically do not mature sexually in the season of their birth. Due to their small size, seasonal dormancy, and restricted reproductive season, pocket mice are more prone to reproductive failure than are Dipodomys. We observed a complete reproductive failure in both species of Perognathus in a year when winter—spring temperature was below average and precipitation only 47% normal. Perognathus have larger litter size (°5 young) than Dipodomys. Consequently, the total annual reproductive potential of Perognathus is close to that of Dipodomys. The relative energy investment and attendant risks for production of a given litter are considerably greater in Perognathus than in Dipodomys, particularly in P. longimembris, which is at the lower limit of body size in rodents. Nonetheless both species of Perognathus have the potential for breeding twice inan unusually favorable year. The pattern of reproduction of the marmotine sciurid A. leucurus contrasts sharply with that of heteromyids. It breeds only once a year, at a fixed time and with a mating season that lasts only 2 wk. Litter size is larger (average 8 or 9) and more variable (range 5—14) than that of any of the heteromyids. Growth and development are slow: 8 wk to weaningin contrast to<3 wk in the heteromyids. The breeding strategy of A. leucurus differs quantitatively from that of other marmotine rodents; it remains homeothermic throughout the year, never hibernates, and has one of the largest litter sizes and slowest growth rates of any sciurid. We suggest that Dipodomys and Ammospermophilus can reproduce when annual precipitation is as low as one standard deviation below the mean, whereas Perognathus generally require more precipitation for successful reproduction due to the short available time between the end of hibernation and the onset of summer drought. The five species differ in year—to—year flexibility in response to variable food supply. Dipodomys show little variation in litter size, but adjust frequency of reproduction and time for maturation of young. Perognathus have a greater range of potential litter size and less potential for adjusting frequency of litters. For A. leucurus frequency is fixed but litter size is highly adjustable; at early stages (ovulation, fertilization, implantation) it can be high, and later adjusted downward during gestation by resorption of of embryos and during lactation by nestling attrition. Dipodomys and Ammospermophilus represent alternative strategies of reproductive timing and flexibility, and each responds to environmental seasonality and unpredictability on a different time scale. D. merriami is a "pulse matcher"; it responds directly and quickly to the occurrence of pulses of food production. A leucurus is a "pulse averager"; its slow and prolonged reproductive effort is programmed to match the historical probability distribution of precipitation and food production. Despite the diversity of reproductive tactics in the five species, they are all long—lived and maintain relativley stable population levels, which is critical not only to reproduction itself but to survival between successive breeding opportunities in a desert environment.

ISSN:0012-9615    

DOI:10.2307/2937128

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY

摘要:

The population cycles of meadow voles (Microtus pennsylvanicus) were studied in southeastern Manitoba, Canada, from 1968 to 1978 on a 32—ha old field and in surrounding boreal forest. Vole density was monitored on mark—recapture live—trapping at 2— to 4—wk intervals throughout the year. This was supplemented by a 3—d spring and a 30—d summer snap—trapping survey. Multiannual and annual fluctuations in density of Microtus were independent of a diversity of patterns seen in other small mammal species. Principal components analysis of demographic statistics was used to characterize changes in seasonal reproduction, survival, and dispersal that accounted for the observed numerical fluctuations. Conditions during a catastrophic winter population decline in 1970—1971 were compared with those in 1974—1975, when numbers recovered quickly after a modest spring decline. Voles were in good condition before and during both declines, as evidenced by normal body composition indices. Large body masses, characteristic of peak populations, preceded both population declines. The main conditions associated with the 1970—1971 decline were unusual patterns in social behavior and dispersal. Changes in social behavior in the field were reflected in unprecedented numbers of multiple captures in Longworth traps before and during this decline. Field observations were supported by unusually high frequencies of amicable behavior in neutral arena bouts staged between males removed from a control population. Little dispersal was detected with conventional live—trapping during the winter period of chronic mortality in 1971. In contrast, a period of acute mortality, resulting in the near extinction of the population in March 1971, was associated with considerable dispersal. We propose two alternative hypotheses consistent with the unpredictable and episodic nature of vole population declines: (1) an immunological dysfunction exists in voles that makes them susceptible to naturally occurring microparasites or (2) opportunistic, pathogenic microparasites exploit the changes in dispersal and social contacts that occur as a normal part of the vole population cycle.

ISSN:0012-9615    

DOI:10.2307/2937129

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY

摘要:

The effects of two sources of biological disturbance–predation and sea urchin grazing–on the structure of benthic communities inside and outside beds of the horse mussel, Modiolus modiolus, were examined in the rocky subtidal zone off the Isles of Shoals, New Hampshire, USA. Multivariate analysis revealed three major communities between 8 and 30 m in depth: (1) a Modiolus community; (2) a 30 m community; and (3) an 8—18 m community. At all depths, mussel beds contained significantly higher densities of infauna than did other subtidal habitats. The hypothesis that Modiolus beds provide a refuge from predation for the associated community was tested in five manipulative field experiments. Three members of the mussel bed community, the bivalve Hiatella arctica, the sea urchin Strongylocentrotus droebachiensis, and the ophiuroid Ophiopholis aculeata, were used as experimental prey. Consumption of these prey by a guild of generalist predators was observed outside but not inside mussel beds, which indicates that subtidal Modiolus beds provide a spatial refuge from predation. Deaths from predation were significantly higher at night than during the day for Hiatella but not for Strongylocentrotus. At night, crab (Cancer borealis, Cancer irroratus) and lobster (Homarus americanus) predation accounted for all attacks that were directly witnessed, while fish (Tautoglabrus adspersus, Pseudopleuronectes americanus) predation accounted for 71% of the total prey consumed during the day. Such diel differences in predation corresponded with predator abundance patterns. The sea urchin Strongylocentrotus droebachiensis was the most significant agent of biological disturbance during the 1979—1984 study period. Strongylocentrotus intensively grazed the benthos at 8 m in 1982, causing a 79% reduction (from predisturbance levels) in the mean population density of invertebrates outside the mussel beds. This event served as a natural experiment in which to test the hypothesis that Modiolus beds function as a refuge from severe grazing disturbance. Re—sampling of communities inside and outside mussel beds immediately after the grazing disturbance indicated that the Modiolus community exhibited less change in species composition, dominance, and diversity than communities outside the mussel beds. Such comparisons indicate that Modiolus beds afford protection from severe grazing disturbance for infaunal invertebrates. Long—term photographic monitoring of marked mussel beds at 8, 18, and 30 m depth showed that Modiolus beds at all depths persisted for more than 5 yr. Mortality rates of adult Modiolus were low; however, mortality was highest at the shallow site (8 m). Mussel beds successfully resisted the grazing disturbance which eliminated all other biogenic habitats except those created by crustose coralline algae. Modiolus beds are effective refuges because they persist for many years and resist biotic disturbance. In the subtidal communities examined here, lower levels of disturbance inside mussel beds can account for the abundance and spatial distribution of Modiolus—community species. These results demonstrate the functional significance of mussel beds in cold—temperate subtidal regions where predation and sea urchin grazing are major determinants of community organization.

ISSN:0012-9615    

DOI:10.2307/2937130

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY

摘要:

The main objective of this study was to evaluate how physical stress and herbivores influence the distribution, abundance, size frequency, and mortality of the giant kelp, Macrocystis pyrifera, in the southeast Pacific. These factors were studied for the kelp and the echinoid herbivore Loxechinus albus over a wide latitudinal range (56°—42° S) of South America on RV Hero cruises in 1972 and 1973. There were large kelp forests with many large plants in the southern Isla de los Estados and Tierra del Fuego sites. Sources of kelp mortality in these sites include entanglement with drift plants and heavy encrustation of bivalves. This resulted in a considerable amount of drifting kelp, and the sea urchins appeared to have abundant food and did not attack living kelp plants. Nevertheless, the urchin densities usually were relatively low and the Loxechinus size frequencies skewed to larger size classes, suggesting that their populations may be limited by larval availability in the Westwind Drift. There were also some large kelp forests in the northern Golfo Corcovado and Isla Chiloe areas, but here the kelp forests seemed relatively ephemeral and Loxechinus appeared to be harvested by humans. The intermediate latitudes (54°—46° S) in areas semiexposed to oceanic waves were characterized by situations in which Loxechinus overexploit the kelps and maintain urchin—coralline algae "barren grounds." In these areas kelps occur only where urchin access is restricted by wave exposure, ephemeral clumps of Desmarestia ligulata, and rarely, by a predator, the asteroid Meyenaster gelatinosus. In many areas the shallow distribution of Macrocystis was restricted by competition with the large fucoid Durvillaea antarctica. These relationships were evaluated by removal and addition experiments. Much more important than latitudinal relationships to both kelp and urchin distribution and abundance patterns were exposure gradients, from shores subject to strong oceanic swells to protected fjords. Transect data suggest that Loxechinus disappear from protected waters before Macrocystis. Thus in some semiprotected habitats there are very dense patches of kelps without grazers; kelp removal experiments in these habitats suggest strong density—dependent kelp interactions. In still more protected sites, the Macrocystis growth form changes, the plants appear to have low fertility, and there is very low recruitment. Summarizing, the Loxechinus population seems restricted by larval recruitment in the face of the Westwind Drift in the far south, perhaps by sedimentation or fresh water in protected fjords, and by human fishing in the north. The very low Loxechinus densities in the far south are correlated with large kelp forests in which the main kelp mortality results from drifting Macrocystis plants. The kelp populations appear limited by physical factors in the fjords and by Loxechinus grazing in most other areas.

ISSN:0012-9615    

DOI:10.2307/2937131

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY

摘要:

I examined the recruitment of reef fishes to isolated experimental standard—size coral colonies supporting residents of each of four planktivorous fishes: (a) Dascyllus aruanus, (b) D. reticulatus (both at several densities), (c) Chromis caerulea, or (d) Pomacentrus popei. I removed all recruits at approximately weekly intervals, and summed larval recruitment over each of two settlement seasons. I found two types of interactions between resident fishes and recruits: (1) There was increased settlement of three species in sites where there were resident conspecifics. This appears to be due to habitat selection by settling larvae. More larval D. aruanus, D. reticulatus, and C. caerulea recruited to corals with resident conspecifics than to corals with other congeners or vacant control corals. In addition, corals with the highest densities of resident D. aruanus and D. reticulatus received the most conspecific recruits. (2) The presence of high densities of resident D. aruanus and D. reticulatus reduced recruitment of heterospecific larvae in all cases for which data were sufficient to allow a powerful test. The presence of C. caerulea reduced recruitment of some species but not others. This interspecific effect could be due to habitat selection by settling larvae, and/or emigration and mortality caused by aggressive interactions with residents. Some species were unaffected by the presence of resident P. popei, but the data for most species were inconclusive. Interactions of the first type may be the cause of the clumped distributions of other reef fishes, though this does not seem to hold in some known instances. Interactions of the second type do not seem to be directly related to resource availability. There are biological and statistical reasons why the likelihood of finding such interactions will be positively correlated with the supply of recruits. The general significance of these interactions for the structure of reef fish assemblages will depend on (1) the level of larval supply, and (2) the number and abundance of species that show this kind of behavior.

ISSN:0012-9615    

DOI:10.2307/2937132

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY