摘要:

Strongylocentrotus drobachiensis and S. franciscanus exhibited strong algal preferences in laboratory studies. A generalized ranking of algae from most to least preferred for both sea urchins includes: Nereocystis luetkeana, Costaria costata, Laminaria saccharina, L. groenlandica, Monostroma fuscum, Opuntiella californica, Agarum fimbriatum and A. cribrosum. Similar but weaker preferences were exhibited by S. purpuratus. Preference experiments showed that substance(s) readily sloughed from the fronds of Nereocystis attract urchins whereas substances from A. fimbriatum repel or are not detected by urchins. In the field urchins in contact with or on the fronds of Agarum spp. revealed a low proportion of feeding individuals except during winter. Food preferences were inversely correlated with the caloric content of algae. Although preferences generally were weakly correlated with caloric intake, this relationship is an artifact of the high feeding rates on preferred algae. Absorption efficiencies for the three urchins were strongly correlated with food preference rankings. Mean values for the three species ranged from 84% to 91% on Nereocystis, 77%—83% on Costaria, 64%—78% on L. saccharina, 62%—71% on Callophyllis flabellulata, 28%—56% on J. fuscum, 40%—56% on A. cribrosum and 36%—52% on A. fimbriatum. Growth in diameter and weight of S. drobachiensis fed specific algae for 15 mo correlated positively with algal preference rankings: urchins raised on Nereocystis or L. saccharina grew faster and produced significantly larger tests than animals raised on either species of Agarum. At the end of 15 mo, 50% of the animals were placed on a reciprocal diet from that on which they were originally grown, i.e., from preferred to nonpreferred algae or vice versa. The other half were retained on the original diet as controls. At the end of 5 mo urchins maintained on or transferred to preferred algae produced the largest tests (diameter and weight). Growth rates, however, were highest with urchins transferred from A. cribrosum to Nereocystis. Conversely, poorest growth occurred with the reciprocal transfer. Similarly, reproductive values, gonad indexes, and gonad weights were highest for urchins maintained on a continuous (20—mo) diet of Nereocystis. The second highest reproductivv values occurred with animals transferred from A. cribrosum to Nereocystis. Lowest values occurred with urchins maintained continuously on a diet of Agarum. Field observations on feeding at seven sites showed that S. drobachiensis and S. franciscanus fed primarily on Nereocystis (27%—40%) and ulvoids (Monostroma—Ulva spp.) (24%—28%). Other species accounted for less than 10%; brown and green algae comprised 64% and 26%, respectively, of the diet of these urchins. Qualitative and quantitative (density, frequency percentage, relative biomass) measures of algal availability showed that ulvoids and Alaria dominated shallow (0—4 m) subtidal depths. With few exceptions A. cribrosum dominated deeper depths (4—12 m) at most sites and the areas in which most feeding observations were made. Nereocystis dominated only one site (in terms of relative biomass). Unlike A. cribrosum, ulvoids, Laminaria spp., and other understory species, Nereocystis was not readily available to urchins on the bottom because of its large surface canopy, narrow stipe, and small holdfast. It became more available, however, when dislodged by grazers and storms. Feeding by sea urchins in nature is a compromise between urchin preferences and algal availability. The disproportionately high feeding percentages on Nereocystis relative to its physiognomy and low abundance is due to selective feeding. Similarly, feeding on L. saccharina and possibly filamentous diatoms is interpreted as selective. The high feeding percentage on ulvoids is a reflection of availability. With algae of intermediate preference in laboratory experiments, feeding in nature is interpreted as the result primarily of availability, e.g., Alaria, L. groenlandica and L. complanata. Costaria, although highly preferred in experiments, was taken more in proportion to availability in the field. With the dominant and most available prey (Agarum spp.) and with Opuntiella there was an active avoidance by urchins suggesting that these algae have evolved chemical or less likely physical defensive systems that reduce browsing by sea urchins. Field measurements of sizes (diameter and weight) and reproductive capacities (gonad weights) of S. drobachiensis within food—limited, relatively food—limited, and food—unlimited environments revealed significant differences between populations in all but test diameter. These observations are in good accord with laboratory experiments. Urchins in the food—unlimited and relatively food—limited sites had available ample amounts of preferred and nonpreferred prey, respectively, e.g., Nereocystis, Costaria, and Laminaria spp. vs. Agarum. The growth and reproductive capacities of these populations parallel laboratory experiments. This algal—herbivore interaction has led to important adaptations and strategies by both the sea urchins and algae. Important or dominant subtidal algae in the San Juan Islands have evolved an opportunistic strategy (r—selection), especially those species highly preferred by urchins, or a perennial strategy (K—selection), especially those species not preferred by urchins. Sea urchins, on the other hand, have evolved a keen sense of prey detection, one that increases their intake and absorption of prey, their growth, and their reproductive potential. These measures can be considered 'components of fitness' and the ensuing selective feeding behavior as an optimization strategy. The question of what is being optimized by urchins was examined using Emlen's model. The food value of algae (based on urchin growth and reproduction) was important to the net gain of urchins but differences in absorption efficiencies and feeding rates had an even greater effect on net gain in Emlen's model. Urchins in this study selected algae which were ingested rapidly, absorbed efficiently, and provided the greatest nutritional value. These urchins are energy maximizers, and by feeding selectively are maximizing fitness. Thus, urchins with well—developed preferences will on the whole, grow faster, reproduce earlier, and contribute more offspring to future generations than individuals with indiscriminate feeding behavior.

ISSN:0012-9615    

DOI:10.2307/1942173

出版商:Ecological Society of America    

年代:1977

数据来源: WILEY

摘要:

Living biomass, organic matter distribution, and organic matter production budgets were determined for plant communities of a small watershed dominated by 450—yr—old Pseudotsuga menziesii (Mirb.) Franco forests. Dominant trees in the communities were large, up to 175 cm diam and 80 m tall. Aboveground tree biomass of the various communities ranged from 491.8—975.8 tonnes/hectare, total aboveground living biomass ranged from 500.4—982.5 t/ha, total leaf biomass ranged from 10.4—16.3 t/ha and total organic matter accumulations ranged from 1,008.3—1,513.7 t/ha. Total tree biomass in the various communities was more related to past mortality than habitat differences. Biomass of standing dead trees and fallen logs was generally inversely related to aboveground tree biomass. Amounts of woody detritus were large, ranging from 59.0—650.6 t/ha or 4.3%—43.0% of total community organic accumulation. Aboveground tree biomass increment was negative in all communities, ranging from —2.9 to —6.2 t/ha. Positive increment by shrubs and trees<15 cm dbh, produced overall aboveground biomass increment —2.5 to —5.0 t/ha. Mortality averaged 1% of standing biomass. Aboveground net primary production in the various communities ranged from 6.3 to 10.1 t°ha—1°—1and was roughly proportional to standing biomass. Net primary production consisted entirely of detritus. Total community autotrophic respiration ranged from 102.9—203.7 t°ha—1°yr—1of which °70% was by foliage. Gross primary production ranged from 111.2—216.8 t°ha—1°yr—1of which only 6.0%—7.9% was net primary production. Net ecosystem production ranged from 0.12—5.6 t°ha—1°yr—1, entirely as an accumulation of woody detritus on the soil surface. Available evidence indicates larger peak biomass in seral P. menziesii than in climax Tsuga heterophylla forests. These communities may be in the process of declining from seral peak to steady—state climax biomass.

ISSN:0012-9615    

DOI:10.2307/1942174

出版商:Ecological Society of America    

年代:1977

数据来源: WILEY

摘要:

The Campbell—Dolomite uplands comprise a small area (140 km2) of outcropping, faulted dolomite, limestone, and shale east of the Mackenzie River Delta, ~ 40 km south of the northern limit of trees. The major landforms are bedrock ridges and plateaux, steep colluvium, stable slopes, shorelines, and depressions. A principal component analysis of vegetation—cover data from 150 stands suggest that much of the variation within the heterogeneous vegetation is correlated with these broad habitat categories. Stable surfaces bear an open spruce woodland with alder, tree and dwarf birch, and a varied lichen—heath—Dryas ground vegetation. A glacially modified karstic (solution) depression contains a small (8 ha), relatively deep (22 m), apparently meromictic lake, which yielded a 12,000—yr core of sediment. A conventional percentage diagram, an influx diagram, and numerical analysis (principal components) suggest a sequence of pollen assemblage zones as follows: (1) Salix—Gramineae—Artemisia: 13,000 to 11,300 radiocarbon yr ago, (2) Betula (shrub)—Salix—Gramineae—Artemisia: 11,300 to 10,300, (3) Betula—Populus: 10,300 to 9,700, (4) Betula—Populus—Juniperus: 9,700 to 8,900, (5) Picea—Betula (tree and shrub)—Juniperus: 8,900 to 6,500, and (6) Picea—Betula—Alnus: 6,500 to present. Both percentage data and numerical analyses show that none of the pollen assemblage zones 1 to 5 has a modern analogue. With 1 exception, these patterns of change in pollen spectra can be interpreted parsimoniously without reference to regional environmental change. They suggest an initial phase of migration of willow and herbs from adjacent unglaciated Megaberingia (North Yukon and Alaska), followed rapidly by dwarf birch and later poplar. Megaberingian floristic elements (e.g., Plantago canescens, Selaginella sibirica) reached the area during this early phase of migration. Subsequently arriving from the south along the Mackenzie valley were juniper, ericads, spruce, and finally alder, which intensified competition and restricted the early Megaberingian herb types to open, unstable habitats where they persist today. Slow soil development (humus accumulation, rising permafrost table) probably favoured the spread of the palynologically 'silent' elements–lichens, ericads, and Dryas (the dominants of the modern ground vegetation). Changes in the influx values of Picea suggest a climatically induced increase in tree density and/or pollen production during the period 9,000—7,000 BP (Zone 5).

ISSN:0012-9615    

DOI:10.2307/1942175

出版商:Ecological Society of America    

年代:1977

数据来源: WILEY

摘要:

Community development was followed for 2 1/2 to 3 1/2 years on unglazed ceramic tile plates (232 cm2), suspended horizontally beneath the Duke University Marine Laboratory dock, in Beaufort, North Carolina. Series of 3 or 4 plates were submerged at approximately the 1st of each month from May—November 1971 and from April—November 1972. Percentage cover for each species that settled and grew on the lower surface was estimated at 6— to 8—week intervals, using 75 points randomly positioned over the plate area. Samples were nondestructive; plates were resubmerged after each census. Larval recruitment was estimated at 1— to 3—week intervals on newly submerged plates. Temperature and salinity were also measured. Initial community development was relatively unpredictable. Larval recruitment patterns varied markedly from year to year and as a result, different patterns of initial community development were observed both within and between years. Instead of preparing the way for subsequent arrivals, most resident adults strongly inhibited the recruitment and growth of other species. Species varied in their ability to resist subsequent invasion as adults and in their ability to invade occupied substrate as larvae. After an unpredictable initial developmental phase, subsequent changes in species composition depended in part on the degree to which larvae were able to invade existing adult assemblages. This in turn depended on the identity of the resident adults and the identity of the invading larvae. As a result, the direction and rate of community development, dependent on the order of initial invasion and subsequent recruitment, were difficult to predict although an equilibrium number of 8—10 species/plate was often observed. Adult residence time was generally<1 year and the mortality and/or disappearance of these adults produced 20—60% free space on an approximately annual basis. This free space was usually occupied by recruits of a different species than the original occupant. The combined addition of species through larval recruitment and subtraction of species as a result of adult mortality produced dramatic changes in community structure each year. There is no reason to believe these changes will ever cease. We conclude that succession in the classical sense (Odum 1969) does not occur in this system because initial development was variable, residents impeded subsequent development instead of enhancing it, and there was no stable climax. There is good reason to believe similar processes occur in other temperate and subtropical fouling communities. We believe these communities are fundamentally different from terrestrial plant communities, where succession may occur, for 3 reasons: (1) the organisms do not alter the substrate they occupy, i.e., “prepare” it for later arrivals, (2) there is no possibility of “storing” dormant “seeds” of successional species. Colonization of free space is generally by animals which have a short—lived, even nonfeeding larvae, (3) most adults are extremely short—lived. An emerging paradigm of marine benthic community organization postulates the existence of competitive hierarchies in which 1 or a few species win in the absence of disturbances. The fouling community appears to lack such dominants.

ISSN:0012-9615    

DOI:10.2307/1942176

出版商:Ecological Society of America    

年代:1977

数据来源: WILEY