摘要:

The influences of predation, competition, biological disturbance, exposure to wave action, and inclination and heterogeneity of the substratum on the structure of the rocky intertidal community of New England were studied with a combination of experiments and observations at six areas in Maine and Massachusetts from 1972—1975. Several aspects of community structure (seasonal utilization of primary and canopy space, relative abundances of predators, trophic structure, species richness) were determined at five areas spanning a wave exposure gradient. Primary space at exposed areas was dominated by the barnacle Balanus balanoides (high intertidal) and the mussel Mytilus edulis (mid—intertidal). Fucoid algae and mobile carnivores and herbivores were scarce and species numbers were low at these areas. The availability of free primary space is usually<10% at exposed areas, except in late winter and early spring when large areas of substratum are cleared of animals by storms, and up to 90% of the primary space may be free of sessile species. The only obvious similarity in community structure at exposed and protected areas is the usual dominance of the high intertidal by B. balanoides. The mid—intertidal of protected areas is usually characterized by a relatively large proportion of free primary space (40%—90%), a luxuriant fucoid canopy (Fucus spp. or Ascophyllum nodosum), relatively dense populations of carnivores and herbivores, and relatively high species richness. The primary prey of the only common intertidal predator at these areas, the snail Thais lapillus, are M. edulis and B. balanoides. The relative simplicity of this system permitted a relatively simple experimental design. Stainless steel mesh cages (3 ° 10 ° 10 cm) were used to exclude Thais and test for competition for space between B. balanoides and M. edulis; sideless cages tested for shading effects of cages, and undisturbed areas served as controls. Replicated experiments were established at points along a vertical gradient (high— to mid—intertidal) at each area. To detect variations in the effects of competition and predation in different microhabitats, cage sets were established on surfaces differing in substratum inclination, substratum heterogeneity, and algal canopy. Finally, some experiments were designed to determine possible effects of biological disturbance from herbivore activity. The major factors influencing community "structure" within the high intertidal zone are apparently largely those factors affecting the population ecology of B. balanoides, i.e., intraspecific competition, and various physical stresses. Neither predation nor interspecific competition had a significant influence on patterns of space utilization within this zone. However, the lower distributional limit of B. balanoides is evidently determined at the more exposed areas by competition for space with M. edulis and at less exposed areas by predation by Thais. The angle of inclination in the high intertidal has little effect on observed patterns of space occupancy; however, the effect of substratum heterogeneity is to extend the range of Mytilus and Thais upward. As an apparent result of such heterogeneity, the interface between the high— and mid—intertidal is a patchy mosaic of Balanus, Mytilus, and bare space. The mid—intertidal of exposed areas is structured primarily by interspecific competition. Space cleared experimentally or naturally in late winter (March) is rapidly occupied by B. balanoides (April—June). However, mussels settle in summer (at least June—September) and usually outcompete barnacles by August—October on horizontal and inclined substrata. If mussels are excluded, barnacles persist, which supports the hypothesis that M. edulis outcompete B. balanoides for space. The rate of interspecific competition is slowed on vertical substrata, and B. balanoides may monopolize space on these substrata for up to 2 yr before M. edulis outcompete them. Predators have no effect on space utilization at exposed areas. However, primary space utilization on all substrata in the mid—intertidal at protected areas is determined largely by predators. My experiments indicate that only when Thais are excluded does interspecific competition occur between barnacles and mussels. Otherwise, predation usually prevents either species from monopolizing primary space. Disturbance from fucoid whiplash inhibits B. balanoides settlement but does not alter the eventual outcome of the experiments. Herbivore disturbance may also inhibit barnacle settlement but this effect was detected only at artificially high herbivore densities. If B. balanoides are removed before M. edulis settle, the latter fail to monopolize space on relatively flat, bare rock, indicating space dominance by mussels is at least partly dependent on either the presence of a competitor or substratum irregularity or both. The results of experiments where three—way competition between barnacles, mussels, and Fucus spp. occurred indicated the survival of the latter is inhibited by mussels and enhanced by barnacles. The scarcity of fucoids at exposed areas may be a result of competition with mussels. Since many of the species present at relatively protected areas are directly or indirectly dependent on free primary space, observed variations in local species richness are evidently partly a function of the activity of Thais. Although this system is structurally very simple and occurs in a relatively harsh physical environment, its organization is characterized by strong interactions which have a powerful influence on observed structural patterns. A key problem in the development of a general theory of community organization is understanding the mechanism(s) behind variations in the effectiveness of predators along gradients of environmental rigor.

ISSN:0012-9615    

DOI:10.2307/1942563

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

The pollen and plant—macrofossil stratigraphy of Wolf Creek, an extensive marsh within one of the oldest drumlin fields in central Minnesota, is described for the period 20,500 to 9150 yr ago. The pollen stratigraphy is presented both as percentages and as "absolute" influx. Both the pollen and macrofossil data have been analyzed by numerical zonation procedures to reveal the major stratigraphic patterns with the two data sets. The vegetational reconstructions presented are based on the pollen percentage and influx data, on macrofossil assemblages, on comparisons of modern and fossil pollen spectra, and on the present ecology of the individual taxa concerned. From 20,500 to 14,700 yr ago, a mosaic of tundra—like vegetation prevailed, with wide—spread "tundra—barrens" of Dryas integrifolia, Arenaria rubella, Silene acaulis, and Vaccinium uliginosum var. alpinum. Between 14,700 and 13,600 yr ago, shrub vegetation with dwarf birch, Empetrum, willows, alder, and Shepherdia canadensis developed as a transitional phase prior to the establishment of spruce—dominated woodland 13,600 yr ago. Between this time and 10,000 yr ago, little change occurred in the upland vegetation, except for the expansion of some deciduous elements ° 12,250 yr ago. Local changes in wetland assemblages occurred, primarily in response to changes in lake level. At °10,000 yr ago, Pinus banksiana expanded at the expense of spruce to form a mixed coniferous—deciduous forest. Climatic interpretation of this vegetational development suggests a progressive increase in temperature since 14,700 yr ago. This unidirectional trend in climatic development contrasts with the glacial history of the state for the time period of interest, as the Lake Superior ice lobe advanced three times between 16,000 and 12,000 yr ago. The lack of correspondence between the pollen stratigraphy and the glacial history is discussed, and hypotheses are presented to explain this apparent anomaly.

ISSN:0012-9615    

DOI:10.2307/1942564

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

Various physical and biological factors affecting coral community structure were investigated by direct observation and periodic censusing (supplemented with laboratory observations and experiments) on three coral reefs off the Pacific coast of Panama from 1970 to 1975. The physical environment has a strong control over coral growth at shallow depth; physical factors are also important subtidally (light, sediment transport). However, paralleling the pattern on temperate shores, biological processes (competition, predation, bioturbation, mutualism) assume an increasing influence on community structure in deeper and more diverse reef assemblages. Coral zonation is marked on these biologically simple and small reefs; the following assemblages are recognized: drying reef flat–live coral cover moderate, species diversity low; reef crest and upper reef slope–highest cover, lowest diversity; lower reef slope and reef base–cover moderate to low, diversity highest. Coral populations in the different zones, though spatially close, are affected by unique sets of conditions. Recurrent extreme tidal exposures devastate reef flat corals (Pocillopora mortality = 40%—60%). The mortality rate of pocilloporid corals is higher than for other corals; this has a diversifying effect on the reef flat assemblage. Acanthaster normally feeds in deep reef zones and numerically its major prey are the predominant pocilloporid corals (most often small colonies and broken branches). Electivity indices and prey choice experiments indicate that less abundant, nonbranching corals are preferred over Pocillopora. Large, branching pocilloporid colonies harbor crustacean symbionts (Trapezia and Alpheus) which can repulse Acanthaster and therefore protect this group of corals. Experimental removal of the symbionts results in a shift of prey preference from nonbranching corals towards the branching pocilloporids. Crustacean symbionts were present in all large Pocillopora colonies sampled, but the density of Trapezia in colonies on the reef flat was about twice that in colonies from deep zones where Acanthaster forages. Further, small pocilloporid colonies and fragments contained relatively few (and a high proportion of juvenile) symbionts. The variety of preferred coral prey present along the seaward reef flanks and the relatively low abundance of Pocillopora in this habitat are considered important factors affecting the distribution of Acanthaster. In addition, a continuous live cover of pocilloporid corals, which Acanthaster avoids, can protect reef zones (e.g., the reef flat) or preferred prey species from attack. The selective destruction of nonpocilloporid corals by Acanthaster tends to lower both live coral cover and species diversity (H'). This trend is evident on the Uva Island study reef where a significant decline in coral cover (47%—18%) and H' (1.06—0.58) occurred over a 4—mo period.

ISSN:0012-9615    

DOI:10.2307/1942565

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

The general pattern of changes in avian communities along latitudinal, successional, or moisture gradients have been clarified in recent years. However, few studies have compared avifaunas of similar vegetation types and climates in two tropical regions. In this study I asked two questions. How similar ecologically are the avian communities in two tropical areas (Panama and Liberia) with similar climate? To what extent might differences be due to historical factors? In general, there is convergence in the structural and functional characteristics of species and communities in the two tropical regions. However, because historical factors have been different in detail, avifaunas of varying faunal complexity have developed. The forest fauna of Panama is richer than that of Africa but the situation is reversed in grassland and savanna. This seems to be due to the geographic extent of these habitats and their degree of fragmentation during the Pleistocene. Food and habitat specializations are more striking in the neotropics than in Africa. In addition, several feeding guilds, especially bark—gleaners, hover—gleaners, and salliers, are less well represented in Africa. A lower number of bird species feeding on fruit and flower resources in Africa parallels a similar trend in bat communities. The reasons for these differences are complex and outweigh the trends toward convergent evolution in areas of similar climate. Differences of historical geography in the two regions are correlated with the differences in the two avifaunas. These include long— and short—term effects of weather and climate, differences in distribution and abundance of dispersal barriers, trophic role replacements, variation in coevolutionary interactions among groups, and varying histories of human influence. In brief, it appears that the avifaunas of Central America and Africa would be more similar if (1) there were fewer isolating barriers (especially mountains and rivers) in South America, (2) Central America were more isolated from the barriers and consequent high speciation rates of South America, (3) man's influence had been of longer duration in Central America, and (4) the Central American dry season was longer and/or less predictable. These factors and their interactions have produced faunas that are clearly tropical but which are different in a number of respects.

ISSN:0012-9615    

DOI:10.2307/1942566

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY