摘要:

Four mechanisms of coexistence are considered that may contribute to the diversity of desert granivorous rodent communities. In the first, bush/open microhabitat selection, coexistence is possible if there is a trade—off between foraging efficiency in the bush and open microhabitats. In the second, temporal variation in resource abundances, coexistence is possible if there is a trade—off between foraging efficiency and maintenance efficiency. The first species can forage profitably on low resource abundances while the second uses dormancy to travel inexpensively in time between periods of high resource abundances. In the third, spatial variation in resource abundance, coexistence is possible if there is a trade—off between foraging efficiency and the cost of travel. The first species forages patches to a lower giving—up density, (the density of resource at which a forager ceases foraging), while the second can inexpensively travel between patches with high resource abundances. In the fourth, seasonal rotation in foraging efficiencies, coexistence is possible if there is a trade—off between the costs of foraging during different seasons. The species that is the more efficient forager changes seasonally. The first mechanism of coexistence has received much empirical attention and support. The other three have not previously been considered with desert rodents. In a community of four granivorous rodent species, I used artificial seed patches to measure species— and habitat—specific foraging efficiencies and live—trapping to measure population sizes and mean distances between recaptures. Of the four, the fourth mechanism of coexistence best explained the presence of Perognathus amplus, Dipodomys merriami, and Spermophilus tereticaudus in the community. Each species enjoyed a period of the year during which it was the most efficient forager. Furthermore, the annual population densities of these three species fluctuated out of phase. Seasonal changes in species—specific predation risks and body size—dependent metabolic costs may have accounted for these results. The third mechanism of coexistence best explained the presence of Ammospermophilus harrisii in the community. This species preferred to forage a large number of widely spaced patches to a high giving—up density rather than forage a few patches to a low giving—up density.

ISSN:0012-9615    

DOI:10.2307/2937289

出版商:Ecological Society of America    

年代:1989

数据来源: WILEY

摘要:

The detritus standing crop, microbial respiration, and macroinvertebrate biomass were examined in monthly samples from the riffle sections of a first—order woodland stream. Total detritus was remarkably constant; the average (with 95% CL) ash—free dry mass standing crop was 426.4 ± 85.9 g/m2over the 14 mo of the study. Throughout the year benthic detritus was dominated by fine particulate detritus (<1 mm), which made up 68.9% of the total ash—free dry mass. Woody debris made up 8%, whole leaves 3.5%, and leaf fragments and other coarse particulate detritus accounted for 19.7% of the total standing crop. Decreases in standing crop were attributable to microbial respiration, macroinvertebrate assimilation, and downstream export. Microbial respiration annually removed 150% of the average standing crop, with the major effect on the smallest particle size category. Macroinvertebrate assimilation, defined as the sum of respiration and growth, removed 11.6% of the detritus standing crop annually. Shredders accounted for 20% of total animal assimilation, with the remaining 80% attributable to collectors and grazers. Based on monthly changes, it appears that total detritus standing crop is the result of the past discharge regime, which determines the overall amount of detritus present, and the rate of biological (microbial and invertebrate) processes, which determine the size and quality of the detritus particles. This suggests that detritus in streams, while strongly affected by both biotic and abiotic factors, may be in equilibrium within physical and biological constraints such that an annual steady—state system exists, similar to that for soil systems.

ISSN:0012-9615    

DOI:10.2307/2937290

出版商:Ecological Society of America    

年代:1989

数据来源: WILEY

摘要:

We used descriptive and experimental approaches on Gould Creek, a first—order tributary of the Mississippi River near Lake Itasca, Minnesota to assess the influence of (1) flow regime on the colonization dynamics and abundance of invertebrates and cyprinids, and (2) cyprinid predation on invertebrates and fishes. Stream flow varied annually during the 3—yr (1984—1986) period. One dry year (1984), with few periods of elevated flow, was followed by two wet years (1985—1986), with prolonged elevated (nonscouring) discharge. The density of benthic and drifting invertebrates increased dramatically with elevated flow. Benthic riffle invertebrates in particular increased, from a maximum of 9000 individuals/m2in 1984 to 91 000 individuals/m2in 1985 and 51 000 individuals/m2in 1986. Larval Hydropsychidae and Simuliidae were the primary groups increasing in abundance during elevated flow. To assess directly the influence of flow regime on benthic insect densities, flow was manipulated in six subsections of a riffle and colonization of natural rock substrates monitored. Total insect abundance was higher under elevated (nonscouring) vs. low flow within 6—8 d; after 24 d insect abundance was three times as high under elevated flow. Insect families responded differently to elevated flow, with the largest increase resulting from a pulse of colonization by larval Hydropsychidae in 6—8 d. Cyprinid density in Gould Creek also increased with elevated flow from 0.1—0.3 cyprinids/m2in 1984 to 1—2 cyprinids/m2in 1985 and 1986. The increased fish density was most pronounced during spawning periods in spring (May—June) and consisted primarily of older individuals. All cyprinids, except the creek chub (Semotilus atromaculatus), decreased in abundance 4—5 wk after spring colonization, regardless of flow conditions and invertebrate abundance. All cyprinids in Gould Creek selected pool habitats. Experiments in an artificial stream on Gould Creek indicated cyprinids influenced invertebrate abundance, but the effect of cyprinid predation was variable among habitats. Invertebrate abundance decreased most in structurally complex pools but exhibited little response to cyprinid predation in shallow riffle and raceway habitats. Because predation intensity varied among habitats, pool—dwelling invertebrates such as Chironomidae and Crustacea decreased more in the presence of cyprinid predation than riffle—dwelling Hydropsychidae and Simuliidae. However, if Simuliidae occurred in pool habitats, they were strongly selected by cyprinids, resulting in a significant depression in prey size in pools. Experiments in the artificial stream indicated creek chubs preyed on adult cyprinids, but larger species (adults 70—80 mm) were less susceptible to predation than smaller species (adults 50—60 mm). However, even taxa with small adult size were preyed on at a low rate, and all cyprinids strongly selected pools, with creek chubs having minimal effect on habitat use. These results suggest that (1) the hydrologic regime has broad and pronounced effects on the colonization dynamics and abundance of invertebrates and fishes in headwater streams, and (2) cyprinid predation has weaker but variable effects on the abundance of stream organisms. Predation intensity varies (a) over short temporal scales, because of the dynamic nature of flow regime and the rapid colonizing ability but short post—spawning persistence of cyprinids, (b) over small spatial scales, because of increased abundance of cyprinids in pool vs. riffle habitats, (c) between invertebrate and vertebrate trophic levels because the creek chub is a relatively ineffective piscivore, and (d) between small and large fish because many minnows have a size refuge from creek chubs.

ISSN:0012-9615    

DOI:10.2307/2937291

出版商:Ecological Society of America    

年代:1989

数据来源: WILEY