摘要:

Mechanisms of ecological succession were investigated by field experiments in a rocky intertidal algal community in southern California. The study site was algal—dominated boulder field in the low intertidal zone. The major form of natural disturbance which clears space in this system is the overturning of boulders by wave action. Algal populations recolonize cleared surfaces either through vegetative regrowth of surviving individuals or by recruitment from spores. Boulders which are experimentally cleared and concrete blocks are colonized within the first month by a mat of the green alga, Ulva. In the fall and winter of the first year after clearing, several species of perennial red algae including Gelidium coulteri, Gigartina leptorhynchos, Rhodoglossum affine, and Gigartina canaliculata colonize the surface. If there is no intervening disturbance, Gigartina canaliculata gradually dominates the community holding 60—90% of the cover after a period of 2 to 3 years. If undisturbed, this monoculture persists through vegetative reproduction, resisting invasion by all other species. During succession diversity increases initially as species colonize a bare surface but declines later as one species monopolizes the space. Several contemporary theories concerning the mechanisms of ecological succession were tested. The early successional alga, Ulva, was found to inhibit the recruitment of perennial red algae. This competition for settling space is an important feature of the successional process. Ulva is the best competitor for this space; it reproduces throughout the year and quickly becomes established on newly cleared substrates. As long as these early colonists remain healthy and undamaged, they preempt colonization by perennial red algae which have highly seasonal recruitment and slower growth. Selective grazing on Ulva by the crab, Pachygrapsus crassipes, breaks this inhibition and accelerates succession to a community of long—lived red algae. Grazing by small molluscs, especially limpets, has no long—term effect on the successional sequence. Their grazing temporarily enhances the recruitment of the barnacle, Chthamalus fissus, by clearing space in the mat of algal sporelings and diatoms which develops on recently denuded rock surfaces. Where locally abundant, middle successional red algae also slow the invasion and growth of the late successional dominant, Gigartina canaliculata. This alga replaces middle successional species because it is less susceptible to damage by desiccation and overgrowth by epiphytes. The results of this study do not support either the classical facilitation model or the tolerance (competitive) model of ecological succession. Once early colonists secure the available space/light, they resist rather than facilitate the invasion of subsequent colonists. Early colonists are not killed by direct interference competition with late successional species which grow up through their canopy; rather, early colonists can successfully inhibit the recruitment and growth of these species. Successional sequences occur because species which dominate early in a succession are more susceptible to the rigors of the physical environment and to attacks by natural enemies than late successional species. Late species colonize and grow to maturity when early species are killed and space is opened. Only late in a successional sequence, when large clearings become a mosaic of small openings, does direct competition with surrounding adult plants of late successional species contribute to the decline in cover of the remaining early species. Studies of succession in a number of terrestrial and marine communities lend support to this inhibition model.

ISSN:0012-9615    

DOI:10.2307/1942484

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Vegetation change during the past 25 000 years in and near the present Mixed Mesophytic Forest Region is inferred from pollen and plant macrofossil analyses of sediment cores from two sites on the eastern Highland Rim of Middle Tennessee, USA. Anderson Pond, in White County, dates from 25 000 radiocarbon years BP to present. During the Farmdalian Interstadial, 25 000 yr BP, northern Diploxylon pines, spruce, and deciduous trees were present at Anderson Pond, indicating a cool, moist climate. From 19 000 to 16 300 yr BP, during the Late Wisconsin glacial maximum, boreal taxa of jack pine, spruce, and fir were dominant. Late—glacial climatic amelioration began in Middle Tennessee at about 16 300 yr BP, with replacement of jack pine—spruce—fir forest by deciduous forest. Ash, ironwood, hickory, birch, butternut, willow, and elm increased in importance first, followed by beech and sugar maple. Mixed mesophytic forest taxa were most abundant during the early Holocene, between 12 500 and 8000 yr BP. A warming and drying trend between 8000 and 5000 yr BP is reflected by high influx values for pollen of oak, ash, hickory, swamp alder, and buttonbush, and by diminishing importance of mixed mesophytic forest taxa. Mingo Pond, Franklin County, Tennessee, contains a pollen record estimated to extend back to 14 000 yr BP. Mixed mesophytic forest taxa were abundantly represented at Mingo Pond during the late glacial and early Holocene, along with pollen representing prairie vegetation. Oak and sweetgum dominated the mid— and late Holocene record from Mingo Pond. Stratigraphic sites from the Missouri Ozarks to the Atlantic Coastal Plain date the full glacial in the southeastern United States between 23 000 and 16 500 yr BP. During the glacial maximum, boreal—like coniferous forests of spruce, jack pine, and fir extended southward to 34°N latitude. Mesic deciduous forest persisted through the full glacial at 35°N at Nonconnah Creek near Memphis. Full—glacial refuges for deciduous forest species may have also existed in south—facing gorges of the Cumberland Plateau and southern Appalachian Mountains in addition to bluffs along major streams in the southeastern United States. During the late glacial, beginning about 16 500 yr BP, boreal—like coniferous forest was replaced in midlatitudes (34° to 37°N) by cool—temperate coniferous—deciduous forest. In the early Holocene, between 12 500 and 8000 yr BP, cool—temperate mixed mesophytic forest prevailed between 34° and 37°N. The Mixed Mesophytic Forest Region assumed its present distribution in the mid—Holocene.

ISSN:0012-9615    

DOI:10.2307/1942485

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

The watershed of Ward Creek, a tributary to oligotrophic Lake Tahoe in the Sierra Nevada, has been investigated since 1971 with the objective of improving our knowledge of processes of nutrient and sediment release and transport to the lake. Quantitative data on selected stream water parameters were collected for 3 yr (1972—1975) at three stations on Ward Creek, two on the main upper tributaries and one near the stream mouth. Comparative data were collected at a stream mouth station on adjacent Blackwood Creek in the 3rd yr. The parameters were selected on the basis of their significance to eutrophication of Lake Tahoe. Precipitation in a normal year is over 90% snow but annual patterns vary widely and rainfall at any time of year can be highly important in sediment and nutrient transport. Water discharge and the flux of suspended sediments, NO3—N, phosphorus, iron and trace metals were dominated by the spring snowmelt runoff from mid—April to mid—June. However, in 1974 heavy fall and summer rains accounted for a large percentage of the annual flux of sediments and nutrients in a total of only 14 d. The spring runoff was characterized by distinct diel water discharge patterns. Similar but not coincident patterns were found to exist for sediments and nutrients, including NO3—N, but not for soluble phosphorus. The Ward watershed has 87% the area of Blackwood but discharge proportionately much lower quantities of sediment and nutrients in comparable water yields per hectare in water year 1975. This contrast in fluxes was probably accounted for in the history of greater disturbance by man in Blackwood Canyon. The principal source of suspended sediments in Ward Creek was streambank erosion in the lower reaches of the channel. The dominant form of inorganic nitrogen in Ward was NO3—N derived from precipitation, symbiotic nitrogen fixation and nitrification of organic nitrogen in forest soil. Phosphorus and iron were almost entirely in particulate form and thus their periods of flux occurred during high flows and sediment transport. Sediment and nutrient loading of Lake Tahoe from the Ward and Blackwood watersheds reflects a history of soil disturbance and vegetation removal. Logging, fire and stream channel diversion have been the dominant perturbations. Conservative extrapolation of annual loading data from this study to the entire basin indicates that algal nutrient levels in the lake probably have increased sufficiently in the century of man's intensive disturbance of the basin watersheds to account for increased phytoplankton and periphyton production that have been measured and observed since 1958.

ISSN:0012-9615    

DOI:10.2307/1942486

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Mature subalpine forest communities were sampled at four locations in central Honshu, Japan–Mounts Fuji and Ontake and Yatsugatake and Chichibu Mountains. Data were subjected to similarity—ordination and association analyses. There are three major groups of communities: Tsuga diversifolia/moss, Abies spp./herb, and conifer/Sasa. Tsuga/moss forest understories are depauperate in vascular plants or dominated by ericads such as Rhododendron and Menziesia and have very stony soils. Abies/herb forests have luxuriant understories in which herbs and ferns are conspicuous; they occur over a wide range of soil conditions. Conifer/Sasa forests have extremely dense layers of dwarf bamboo (Sasa spp.) 1—1.5 m in height and occur on deep, fine—textured volcanic—as soils. Composition and structure of 11 community types and phases belonging to these groups are described. Successional trends appear to favor the more shade—tolerant Abies mariesii and Abies vetichii over Tsuga diversifolia and Picea jezoensis var. hondoensis although this varies with site. Japanese forests have many similarities with those in the eastern United States, but the dwarf bamboo communities are strictly an Asian phenomenon.

ISSN:0012-9615    

DOI:10.2307/1942487

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

The community of epiphytes on the giant kelp Macrocystis pyrifera, is simple, consisting primarily of the bryozoans Membranipora membranacea, Hippothoa hyalina, and Lichenopora buskiana; the serpulid polychaete Spirorbis spirillum; and the hydroids Obelia sp. and Campanulaia sp. Abundances of the organisms vary rapidly, by an order of magnitude or more in 2 wk. In spite of this short—term unpredictability, broad patterns of distribution are predictable. Membranipora, the competitive dominant, is abundant during winter and spring and absent during summer and fall when surface waters are warm and thermally stratified. When present, it occurs primarily in the canopy along the outer edge of the large (1 X 11 km) Point Loma, San Diego, kelp bed. The distribution of the subordinate epiphytes complements that of Membranipora. They occur in the interior portions of bed, and on the lower, older blades of Macrocystis plants. These patterns are maintained by a sophisticated suite of larval behaviors and settlement preferences. The subordinate epiphytes settle preferentially on the older blades of Macrocystis, and avoid blades from plants on the outer edge of the bed where competitive overgrowth by Membranipora is more likely. In addition the extremely short larval life—spans of the subordinate epiphytes (a few hours) mean that offspring will settle near where they were released, in the interior of the bed. Membranipora, in contrast, has a planktonic lifetime of 2 to 4 wk. Nearshore current measurements indicate that as a result its larvae are flushed from the bed, and that the source of larvae ready to metamorphose and settle must be external to the bed, from other kelp beds in the area. This helps restrict Membranipora to the outer edge of the bed, since larvae are removed from the water as it passes through the bed both by settlement on Macrocystis blades and by plankivorous fish that feed on them. The labrid fish Oxyjulis californica is an important predator in this system. It is a curious, actively searching generalist that excludes large, erect epiphytes such as the barnacle Lepas pacifica and the bivalve Leptopecten latiauritus from the nearshore kelp bed. Field and laboratory experiments imply that Oxyjulis forms short—term search images and switches among prey species. One of the species it regulates is the herbivorous isopod Pentidothea resecta, which, when released from regulation, multiples rapidly and destroys the kelp canopy. Oxyjulis, though it buffers the system from extinction due to Pentidothea, is also potentially destructive. Small kelp beds, too small for the filtering action to take effect, become heavily encrusted with Membranipora and can be destroyed by Oxyjulis predation on the encrusted blades. The set of relationships among species in this community can fruitfully be viewed as having coevolved to contribute to persistence in this unstable environment. Indeed, some factors that contribute to instability are the very ones necessary for persistence. Competition, predation, habitat selection, and life history strategies interact to ensure that the number of species in the community and their interrelationships remain constant. There appear to be no mechanisms acting to regulate epiphyte species to equilibrium levels of abundance. This system is thus best described as one that fluctuates unpredictably within well—defined boundaries.

ISSN:0012-9615    

DOI:10.2307/1942488

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY