摘要:

"Switching" in predators which attack several prey species potentially can stabilize the numbers in prey populations. In switching, the number of attacks upon a species is disproportionately large when the species is abundant relative to other prey, and disproportionately small when the species is relatively rare. The null case for two prey species can be written: P1/P2= cN1/N2, where P1/P2is the ratio of the two prey expected in the diet, N1/N2is the ratio given and c is a proportionality constant. Predators were sea—shore snails and prey were mussels and barnacles. Experiments in the laboratory modelled aspects of various natural situations. When the predator had a strong preference (c) between prey the data and the "null case" model were in good agreement. Preference could not altered by subjecting predators to training regimens. When preference was weak the data did not fit the model replicates were variable. Predators could be trained easily to one or other prey species. From a number of experiments it was concluded that in the weak—preference case no switch would occur in nature except where there is an opportunity for predators to become trained to the abundant species. A patchy distribution of the abundant prey could provide this opportunity. Given one prey species, snails caused a decreasing percentage mortality as prey numbers increased. This occurred also with 2 prey species present when preference was strong. When preference was weak the form of the response was unclear. When switching occurred the percentage prey mortality increased with prey density, giving potentially stabilizing mortality. The consequences of these conclusions for prey population regulation and for diversity are discussed.

ISSN:0012-9615    

DOI:10.2307/1942352

出版商:Ecological Society of America    

年代:1969

数据来源: WILEY

摘要:

Decomposition and effects of fire on microclimate and plant production of a Tall Grass Prairie were studied in east—central Illinois. After two years simulated litter samples in contact with soil lost 64% of their initial weight. Burning caused a 2—3 fold increase in production and a ten fold increase in flowering. Application of mulch, lowered flowering rate; response was proportional to thickness of mulch and length of time it was in place, Artificial shading of burned areas under green—house frames lowered flowering rate. Burning or clear cutting vegetation increased percent N and P in May foliage. A May application of N fertilizer increased flowering and dry matter production. Nutrient release from ash had no effect on flowering or production. Root production was similar on burned and Burn 3 areas; a turnover rate of 0.45 was calculated. Calcium content of roots in May increased after fire and N content decreased. Temperature from —5 to +50 cm and light intensity from 0 to +50 cm were higher on burned than Burn 3 areas. Maximum soil surface temperatures occurred three months earlier on Burn 0 than Burn 3 areas. Soil moisture was never severely limiting to plant growth. Growing season microclimate measurements at 50 and 200 cm were combined into a microclimatic index, which allowed comparison of the microenvironment of three burning regimes. Total flowering was proportional to duration and degree of favourable microclimatic conditions from May to mid July.

ISSN:0012-9615    

DOI:10.2307/1942353

出版商:Ecological Society of America    

年代:1969

数据来源: WILEY

摘要:

Ninety—four species of Orthoptera (s. lat.) occur along an altitudinal transect from 5,000 ft (1,530 m) to above 14,000 ft (4,265 m) in the Front Range of the Rocky Mountains of Colorado, near 40° N. Lat. These were sampled, with observations on life histories in relation to altitude, at a series of altitudinally spaced stations during the seasons of 1949, 1958, 1959, and 1960, the stations being selected for maximum variety of Orthoptera at each altitude. Other observations of the past 35 years were added. Sampling was by a sweeping technique designed to be qualitatively exhaustive of species and age groups; it was thus quasiquantitative. Times of hatching, times of different juvenile stages, and times of maturing were determined for species at the different altitudes at which they occurred. These data are summarized by species, the 94 species being in the following families: Mantidae, 2; Phasmidae, 1; Tetrigidae, 4; Acrididae, 73; Tettigoniidae, 8; Gryllacrididae, 3; Gryllidae, 3. Numbers of resident species found in 1,000 ft altitudinal bands were as follows: 5—6,000 ft, 71; 6—7,000 ft, 49; 7—8,000 ft, 34; 8—9,000 ft, 25; 9—10,000 ft, 19; 10—11,000 ft, 15; 11—12,000 ft, 9; over 12,000 ft, 3. Larger average numbers of individuals per species occur at higher elevations. Many more species of low frequency occur at low altitudes than high, the total number of species known from a given high altitude station often being present in a single collection, while, at low altitude stations, even the most varied collection lacks species occasionally collected there. The reduction in species numbers with altitude is correlated with a similar reduction with latitude, and the geographic distribution of particular species of Orthoptera is correlated with altitude. Of the 18 species of Acrididae recorded north of 60° N. Lat. in North America nine are common to abundant in the area of study up to 10,000 ft, six of these to 11,000 ft or more. However, three strictly montane species from above 10,000 ft are not known from the far north. The altitudinal distribution of the 36 most common species provided a basis for testing various suggested schemes of altitudinal zonation. A few high plains species that feed only on forbs are apparently restricted in part by the distribution of host plants, and a few grass—feeders do not invade the grassy clearings in montane forest, but the majority of species occurring on the high plains do range into clearings in the lower montane forest up to about 7,000 ft. Pest species with wide altitudinal ranges are essentially indiscriminate plant feeders. The reduction in number of species of Orthoptera with altitude is much greater than the reduction in number of plant species, so variety of plants is not a limiting factor in their altitudinal distribution. A more important factor is apparently the length of growing season–in particular its abbreviation at the end. Evidence for this is the occurrence of early juvenile instars of some species above the altitude at which the species normally completes its life cycle. Evidence was found for the relations between phenology and altitude postulated by A. D. Hopkins in the times of maturing of species with wide altitudinal ranges, with events somewhat earlier than expected at the highest altitudes. Events that occur near the middle of the summer (hatching of eggs of species that overwinter as juveniles) are independent of altitude, but this too is the harmony with the Hopkins law.

ISSN:0012-9615    

DOI:10.2307/1942354

出版商:Ecological Society of America    

年代:1969

数据来源: WILEY

摘要:

An objective technique was used to identify and study related groups of chaetognath species from the Southwest Indian Ocean off the coast of the Republic of South Africa. Groups or community—type data agreed with, corroborated, and extended the scope of data on single species, multiple species and hydrographic conditions. The most representative groups were from the data of the N70V—nets which formed 3 groups. The 1st (or main) group was composed of 9 species and was distributed as a unit almost solely in the upper 200 meters of oceanic waters, and occurred in neritic waters only when>1 to 2 species were absent. This agrees with and corroborates individual species data as each was, according to its distribution and abundance, a resident of the general ocean and each showed, except 3 species found predominantly in deeper waters, a positive correlation with the flow—pattern of the Agulhas current. The 2nd (or associate) group of 3 species was found 61% of the time at mixed—water stations, the best of these being located along a line where mixing between neritic waters and the Agulhas current was most likely to occur. Hydrographic data gave no indication of the latter. The 3rd group was made up of 1 oceanic— species which was distinctive by virtue of its vertical distribution; it was a mesoplanktonic beast, inhabiting water>200 m in depth. Species fecundity correlated quite well with each group and its distribution, in that neritic individuals of only one species of the 1st (main) group contained significantly more eggs than corresponding oceanic individuals whereas neritic individuals of all 3 species of the 2nd (associate) group contained significantly more eggs than corresponding oceanic individuals. This evidence suggests that the species of each group were reacting similarly to their environment and that fecundity is a suitable index to hydrographic conditions. Despite the close similarity of chaetognath species in regard to their habitats and niches considerable overlap existed among the species as a result of differences in their horizontal and vertical distributions, in their abundances, in their size relationships, in food diversity which apparently allows for some specialization in feeding but not in a rigid manner, and finally in structural aspects of the food chain itself as 2 of the larger species appeared to function as 2nd—order carnivores.

ISSN:0012-9615    

DOI:10.2307/1942355

出版商:Ecological Society of America    

年代:1969

数据来源: WILEY

摘要:

The breeding migrations and wandering movements of Notophthalmus viridescens were monitored at several ponds in Dryden, Tompkins County, New York. Breeding migrations occurred in both the spring (April—May) and fall (August—October, November, or December) to all permanent ponds. The spring migrations involved both eft migrants and adult migrants, their relative numbers varying considerably from pond to pond. The fall migrations involved eft migrants almost exclusively. Eft migrants were maturing juveniles returning to the water for the first time; adult migrants were individuals which had bred before and subsequently spent some time, often an entire winter, on land. The sex ratio of eft migrants was 42 to 47% males, that of aquatic adults at least 55% males; male eft migrants had a greater future life expectancy than do female eft migrants. Male migrants arrived earlier in the spring, female migrants arrived earlier in the fall. Breeding migrants tended to use linear depressions (e.g. streambeds, slope junctions) as migration routes; their routes were conspicuously altered by seasonal changed in vegetation. About 57% of N. viridescens migrants, but only 3% of Ambystoma maculatum migrants, reached the ponds during the day; the remainder entered at night. Breeding migrations were more strongly correlated with rainfall and temperature in early spring than in late spring and in late fall than in early fall. For a brief period following their arrival at a pond migrants wandered back and forth between water and land, especially during rainy weather. Fully aquatic adults abandoned temporary ponds as or after these dried up. In July, August, and September some adults left permanent ponds, especially during rainy weather.

ISSN:0012-9615    

DOI:10.2307/1942356

出版商:Ecological Society of America    

年代:1969

数据来源: WILEY