摘要:

The foraging ecology and trophic dynamics of the striped swamp snake, Regina alleni, were studied from 1974—1977 in a south Florida water hyacinth community. The mean standing crop of R. alleni was 30.79 kg (1289 individuals)/ha of water hyacinths. This biomass estimate is greater than that known for any snake species and is related to (1) the high primary productivity of water hyacinths; (2) the trophic positions, diets, and concomitant abundances of the snake's prey; (3) the availability of alternate habitats for exploitation; and (4) release from certain competitive and predatory pressures. Snake density was lowest in summer, increased in fall and winter, then decreased in spring. Changes in density apparently represent seasonal movements of snakes between the hyacinth habitat and the surrounding marshes, in response to fluctuations in water level, prey availability, and predator pressure. R. alleni showed dramatic seasonal and ontogenetic shifts in food habits. Young R. alleni (120—200 mm snout—vent length [SVL]) fed primarily on libellulid odonate naiads but switched to seasonally abundant palaemonid shrimp and astacid crayfish. Between 200 and 300 mm SVL a major dietary shift occurred and crayfish gradually replaced odonates and shrimp. This change occurs throughout the range of the species and also involves a reorganization of snake feeding behavior to accommodate prey of different shapes. The ecological and energetic consequences of this dietary shift are interpreted in terms of the divergent life histories and growth patterns of the predator and its prey. Within the specialized trophic niche of R. alleni, food availability and predator size largely determined the types and sizes of prey consumed, but interacted with the thermal regime, energetic requirements, and foraging efficiencies of individual snakes to determine the rate of food intake. In most seasons juvenile R. alleni consumed more but smaller prey than adults, and the mean percent of snake body mass eaten per day was higher for adults. However, because juveniles fed on odanate naiads that were higher in protein but lower in ash content than the decapods taken by adults, the energetic intake per gram snake body mass was generally higher in juveniles. The predatory impact of R. alleni may be a significant factor limiting the growth of both the snake and its prey populations. During fall, when measured rates of food intake were highest, adult R. alleni consumed °9.6% of the adult crayfish population. In contrast, juveniles accounted for removal of °90.7% of the odonate population in the same time period. When given an unlimited food supply in the laboratory, the daily energetic intake of juveniles was more than two times that of maximum field estimates.

ISSN:0012-9615    

DOI:10.2307/1942651

出版商:Ecological Society of America    

年代:1980

数据来源: WILEY

摘要:

Mole crabs of the genus Emerita are successful colonizers of exposed sandy beaches, in spite of the severity of the physical environment. An adult population of Emerita talpoida (Say) was studied for 2 yr to determine possible changes of its life history pattern as a consequence of its physically severe habitat. Adult populations were sampled in four localities on Bogue Banks, North Carolina, USA, monthly during 1972 and in March, June and September 1973. Size distribution–in spite of some transient differences in the abundance of the larger individuals–was similar at all localities in each month of the year, and underwent dramatic simultaneous changes in spring and summer. The changes in spring resulted from rapid growth of overwintering animals. Changes in summer were produced by the recruitment of new individuals and the death of many large ones. Although size distribution was similar at all localities, significant differences in abundance were found in each month for all sizes except the smallest, suggesting that recruitment was simultaneous and of equal magnitude all along Bogue Banks. In spring, ovigerous females were larger, and carried a greater number of larger eggs. However in summer females were more abundant and produced several clutches. Most recruitment occurred in September as a result of the summer spawning. A greater proportion of females was observed in spring and fall. The spring peak was caused by death of the larger males. The fall peak was caused by a large recruitment of females (63%). When males died in spring, they were about half the size of the largest females observed. No increase in the number of medium—size females was observed in spring. In both sexes the mortality rates were constant, but greater for males, resulting in a shorter life expectancy for the latter. Thus it appears unlikely that sex reversal in males occurs in Emerita talpoida. Survival curves indicate that if sex reversal occurs in this species it would happen at a constant rate in all age intervals. Sex reversal was not observed under laboratory conditions. Life table parameters calculated from data for 1972 indicate that the population has expanded in the recent past, assuming egg production to be independent of density. This result, along with the appearance of larger females in 1973 (which were less abundant in 1972) suggests that in 1972 the population was recovering from mortality probably caused by two hurricanes in the fall of 1971.

ISSN:0012-9615    

DOI:10.2307/1942652

出版商:Ecological Society of America    

年代:1980

数据来源: WILEY

摘要:

During the dry season of 1974, collections of the purple—striped gudgeon Mogurnda mogurnda were taken from the Finniss River of the Northern Territory; during the wet season, this river receives acidic and metallic pollutants from the formerly mined area of Rum Jungle. The fish consumed a great variety of foods, with high abundances of Dytiscidae, Chironomidae, Ceratopogonidae, Trichoptera, Odonata, Copepoda, Decapoda, and Pisces. Within polluted and unpolluted zones of the river; the dietary abundances of some foods varied with the size of the consuming fish and with habitat descriptors. For no food was there a significant regression against habitat descriptors in both of the zones; this emphasizes that pollution effects are more complex than simply depression or raising the abundances of the dietary species. Five foods were more abundant in the diets of the polluted zone, and three in those of the unpolluted zone. The dietary abundances of two foods, both crustacean, increased in the polluted zone, compared to the unpolluted zone, through the dry season. Two categories of foods: (1) those obtaining oxygen directly from the atmosphere or via a film or bubble of air; and (2) the converse set (aquatic respirers), were contrasted for their usefulness in distinguishing between the zones, by both hierarchical agglomerative and discriminant function analyses. Aquatic respirers were the more effective. The polluted collections were more heterogeneous than the unpolluted ones in both the hierarchical and the discriminant analyses. One collection from the polluted zone was reclassified as unpolluted on the basis of its grouping behavior. The relevance for pollution studies of dietary analysis of euryphagous fish, rather than sampling aquatic fauna directly, is discussed in terms of food substitution, switching, strategies for abundance and recovery by the fauna, and possibly effects of the pollutants on the fish as an intervening collector. Downstream patterns of abundance of the invertebrate fauna in this study are broadly similar to those of other studies in which such chemically diverse metals as Pb, Zn, and Cu are the pollutants, but there are differences for some faunal components. Difficulties in the comparison and causal interpretation of results of pollution studies in the field stem from a lack of experimental data on tolerances to pollutants and causes of abundance of stream invertebrates. However, because of the great detail required in such data, more profit for purposes of prediction may derive from experimental multivariate studies on faunal assemblages.

ISSN:0012-9615    

DOI:10.2307/1942653

出版商:Ecological Society of America    

年代:1980

数据来源: WILEY

摘要:

Biological interactions between meiofauna and macroepifauna were studied in a high salt marsh habitat using manipulative techniques. When macroepifauna were excluded from experimental cage sites, meiofauna densities increased significantly compared to open, noncaged areas. Meiobenthic polychaetes and copepods displayed consistent, seasonal increases in abundance inside cages, although their patterns of response were notably different. The dominant meiobenthic copepods, Stenhelia (D.) bifidia, Microarthridion littorale, Enhydrosoma propinquum, and Schizopera knabeni, responded to macroepifauna exclusion by rapidly (within weeks) increasing in density inside cages and, subsequently, crashing to control field levels. Polychaetes, represented mainly by Manayunkia aestuarina, increased more slowly than copepods inside cages, and unlike copepods, showed no signs of density limitation inside macroepifauna—excluded areas. In cage sites reopened to macroepifauna influences, copepod densities decreased to values intermediate between cage and open control areas, while polychaetes were quickly reduced to open field densities. The exclusion of macroepifauna not only produced changes in densities of polychaetes, but also resulted in increased survival of large size classes. My experimental results support the hypothesis that acroepifauna predation/disturbance has an important effect on meiofaunal assemblages, thus providing insight into possible competitive associations between meiofauna taxa. The significance of meiofauna—macrofauna interactions in benthic systems is discussed.

ISSN:0012-9615    

DOI:10.2307/1942654

出版商:Ecological Society of America    

年代:1980

数据来源: WILEY

摘要:

Thoreau's Bog in Concord, Massachusetts, is a floating—mat Sphagnum bog developed in a glacial kettle hole. Low shrub vegetation of the open mat is dominated by Chamaedaphne calyculata; trees include scattered Picea mariana and Larix laricina. Hydrological investigations show the bog to be ombrotrophic, with an annual water input of 1.45 m and an annual runoff of 0.24 m. Corresponding metal inputs are 88, 132, and 54 mg°m—2°yr—1for K, Mg, and Pb, respectively. K and Mg are vertically distributed in the bog profile in accord with the inhomogeneous ion exchange chemistry of peat, while lead is distributed in accord with historical trends in atmospheric lead fallout. Isotopic dating using210Pb is a valuable tool for determining net peat accumulation rate, which is 180 g°m—2°yr—1. Annual storage rates of K, Mg, and Pb in peat amount to 36, 54, and 46 mg°m—2°yr—1, respectively. Bog acidity (pH = 3.8) is maintained by organic acids at concentrations of 10—3eq/L. The effect of cation exchange on bog acidity is modest, while the much larger contribution of "acid rain" is offset by alkalinity increases of the same magnitude resulting from sulfate reduction and nitrate uptake. These latter processes are, in effect, a strong buffer mechanism against acid rain.210Pb dating and historical records suggest that the floating mat is relatively young, perhaps as few as 500 yr old. These data raise the possibility that the bog is not a relict of colder, early postglacial periods, but instead, may have developed under modern climatic conditions.

ISSN:0012-9615    

DOI:10.2307/1942655

出版商:Ecological Society of America    

年代:1980

数据来源: WILEY

摘要:

Plate limpets, Acmaea (Notoacmea) scutum, are common in central California on mid—intertidal vertical faces, where they ignore erect algae and consume primarily two encrusting algae, Petrocelis middendorffii (=P. franciscana) and Hildenbrandia occidentalis. This mid—intertidal system offers a manageable diversity of species, slow—moving consumers, and a two—dimensional, stationary food resource. Such an assemblage of organisms enables detailed studies on food selection within localized sets of available foods, and provides an example of how an animal utilizes a particular, mixed assortment of foods as it forages. Previous theories of diet selection predict that an animal will maintain a fixed hierarchy of preferences; less preferred foods would be consistently passed over if a preferred food is encountered frequently. However, individual Acmaea scutum preferentially consume mixtures of foods, even when each of these foods is readily available. Adjacent individuals tend to select the same mixture of foods while sharing the same foraging area, without intraspecific partitioning of food species. A family of simple graphic models illustrates hypothetical tendencies to select a particular (proportioned) mixture of foods, and predicts how food preferences would differ where relative abundances of the foods differ. Avoiding either too much or too little of a given type of food, relative to other foods, yields a mixed diet. Unlike foraging patterns leading to increased diversity of coexisting foods, maintaining a mixed diet involves seeking out separate components of the diet, particularly when a component is locally rare. Hypothetically, this would drive rare foods toward local extinction, decreasing diversity of available foods within each foraging area. The models of food selection were tested in the natural environment using adult limpets on a series of isolated rocks where percentage cover (probability of random encounter) of each food differed. Feeding of labeled individuals on these rocks was observed repeatedly during successive high tides. Acmaea scutum is seen to fit the mixed diet model with about 60% of its algal diet consisting of Petrocelis and 40% Hildenbrandia; this is true over a wide range of availabilities of these two foods. Maintaining this mixed diet appears to avoid excessive tooth wear associated with eating tough Hildenbrandia, though this food is palatable in limited quantities. Hypothetical consequences on the food resources due to diet mixing were tested by manipulating population densities of limpets on a series of boulders, and determining overall effects of foraging on various sets of algal abundances. However, even unusually high experimental densities of limpets failed to decrease algal abundances detectably. Algal declines on both experimental and control boulders are attributed to physical factors. Simple underwater listing techniques reveal feeding activities of individual adult molluscs. Contrary to visual observations, local molluscs almost always bite part way into the macroscopic algae rather than simply brushing off and consuming microscopic epiphytes. Time lapse photographs of individual limpets indicate that their feeding usually causes negligible damage to these encrusting algae, but the limpets sometimes revisit specific foraging sites and produce visible wounds on the plants. Monthly photographs of damaged plants indicate that slow vegetative reproduction, not new settlement, usually recolonizes patches of rock exposed by feeding. Such maintenance of mixed diets may evolve generally among animals that rarely decreases the abundance of available foods.

ISSN:0012-9615    

DOI:10.2307/1942656

出版商:Ecological Society of America    

年代:1980

数据来源: WILEY