ISSN:0012-9658    

DOI:10.2307/1938616

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Several approaches are discussed for statistical analysis of large—scale (and possibly unreplicated) ecological experiments. These include intervention analyses and comparisons of alternative models using Bayes' formula. Such techniques are unfamiliar to many ecologists and are not typically included in graduate curricula in ecology. I argue for increased training in these areas and for collaborations between statisticians and ecologists to develop innovative approaches to the analysis of large—scale perturbations.

ISSN:0012-9658    

DOI:10.2307/1938617

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Some practical techniques are discussed for analyzing time series whose statistical properties are changing with time. We first consider how principal component analysis can reduce the multidimensional nature of certain series and, in particular, apply this technique to the analysis of changing seasonal patterns. Discussions of trend, changes in oscillatory behavior, and unusual events follow. The problem of making inferences regarding causation is briefly considered. We conclude with a call for flexibility in approach.

ISSN:0012-9658    

DOI:10.2307/1938618

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Classical hypothesis testing is founded on a long—run frequency perspective that is the basis for error rates and P values used in classical statistical inference. Thus in ecological studies involving formal hypothesis testing, it is common practice to report the P value as the summary result from the test of a point null hypthesis. However, many important ecological studies concern single, non—replicated events in which the P value has no clear interpretation. For the non—replicated study, Bayesian statistical inference provides an attractive alternative to classical statistical inference, as the results from a Bayesian analysis either may assume a long—run frequency interpretation or may be expressed as a probability of a unique event. An example concerning trends in lake acidification is used to show that the Bayesian approach is more compatible with scientific needs and scientific judgment than is classical hypothesis testing.

ISSN:0012-9658    

DOI:10.2307/1938619

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Even unmanaged ecosystems are characterized by combinations of stability and instability and by unexpected shifts in behavior from both internal and external causes. That is even more true of ecosystems managed for the production of food or fiber. Data are sparse, knowledge of processes limited, and the act of management changes the system being managed. Surprise and change is inevitable. Here we review methods to develop, screen, and evaluate alternatives in a process where management itself becomes partner with science by designing probes that produce updated understanding as well as economic product.

ISSN:0012-9658    

DOI:10.2307/1938620

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

We measured uptake rates of phosphorus, nitrate, calcium, and dissolved organic carbon within two headwater streams, one in the Appalachian mountains of North Carolina and the other in the Cascade range of Oregon. The major physical differences between these two streams are parent geology and local geomorphic structure. Uptake rates were measured following low—level nutrient releases during summer 1987 in 20—m reaches with different geomorphology. The relative importance of biotic vs. abiotic sorption of nitrogen and phosphorus by sediments from the two streams was assessed by laboratory experiments. Nitrate—N uptake rates were high for the western stream (11.9 mg°m—2°min—1) and low for the eastern stream (3.9 mg°m—2°min—1) during the summer. The debris dam reaches in Oregon were the most retentive of N (shortest uptake length of 17 m). Conversely, soluble reactive phosphorus (SRP) uptake rates were higher for the eastern stream (18.6 mg°m—2°min—1), primarily through biotic processes. SRP uptake lengths were short for the cobble (32 m), debris dam (35 m), and rock outcrop (40 m) reaches in the eastern stream. Uptake of SRP in either stream was not related to sediment size fraction but rather to a combination of sediment infiltration rates and quality of organic material. Calcium (Ca) uptake lengths were long in the Oregon stream (1278 m) but short in the Appalachian stream (106 m). Surprisingly, the eastern stream was more retentive of Ca than nitrate (shorter uptake lengths for Ca) during this time period. Debris dams greatly enhanced retention of dissolved organic carbon in both streams (60% of all uptake in the eastern stream and 81% in the western stream), although retention was greater in the eastern stream. The lower uptake rate of phosphorus and higher uptake rate of nitrate in the Oregon stream were expected based on geographic location and parent geology; streams in this area drain catchments of volcanic origin and tend to have low N:P (atomic; 1.8 for the western stream) ratios, indicating potential N limitation. Streams flowing over granitic bedrock, such as the eastern stream, tend to have lower P availability (N:P = 15.5). The combined results of laboratory and field measurements indicate that in the eastern stream, strong biotic control of P uptake coupled with high P demand result in relatively short P uptake lengths and a strong impact of P spiraling on ecosystem dynamics. In the western stream, strong biotic control of N uptake combined with strong N demand result in short N uptake lengths. This is especially true at sites of downed timber that retain both FPOM and CPOM, creating a high N demand (shortening N spirals).

ISSN:0012-9658    

DOI:10.2307/1938621

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

In an earlier study, exploitative intraspecific competition was demonstrated for middle instars of the caddisfly Helicopsyche borealis in Big Sulphur Creek, a stream in northern coastal California. This population is univoltine with early instars first appearing in June, growing into late instars the following spring, and emerging as adults in May. Competition occurs during the summer dry season (June—September), a period of low—flow conditions without storms or increases in stream discharge. Beginning in autumn, however, larval densities decline because of wet—season storms and high discharge. Using multiple regression, we found a strong relationship between spring (i.e., post wet season) density of late—instar H. borealis and various hydrologic parameters of the preceding wet season over a 4—yr period, 1985—1988. Of eight hydrologic parameters used as indices of disturbance, total wet—season rainfall and peak discharge together accounted for 42% (P<.0001) of the variation in H. borealis density. In contrast, densities of middle instars of the next generation in summer were unrelated either to the preceding wet—season hydrologic regime of late—instar densities of the previous generation. Field experiments conducted in enclosures using larval densities of H. borealis that correspond to harsh, moderate, and mild wet—season hydrologic regimes (low, intermediate, and high densities, respectively) indicated that: (1) increases in larval density had a strong negative effect on algal biomass (as chlorophyll a); (2) larval density was negatively correlated with the proportion of larvae that pupated, and with pupal size; and (3) the high density (mild hydrologic regime) treatment produced smaller adult males and females, and lower fecundities than intermediate and low densities (moderate and harsh regimes, respectively). In most years, storms and associated disturbance reduce late—instar H. borealis density below levels of larval competition; this counteracts earlier competition among middle instars that occurs during low—flow conditions. In drought years, however, densities may not be reduced as severely by wet—season storms and competition may continue until emergence. In this case, competition can reduce individual fitness, but because of high adult densities during these mild—disturbance conditions the reproductive capacity of the population is unaffected.

ISSN:0012-9658    

DOI:10.2307/1938622

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

The fire ant Solenopsis invicta Buren invaded southeastern United States from South America>50 yr ago. Urban and agricultural consequences of this invasion are well documented; however, ecological effects are still poorly understood. Increasing frequencies ofpolygyne or multiple—queen fire ants in Texas and other areas of the Southeast are disturbing because nest densities of this new form are often ten times as great as those of the more familiar monogyne form. We studied the ecological impacts of a polygyne fire ant invasion on ants and other surface—active arthropods at a field station in central Texas. Arthropod abundance and species richness were assessed using a combination of baits, pitfall traps, and litter samples. This invasion decimated the indigenous ant fauna. Competitive replacement appears to be the primary mechanism behind this effect. Species richness of ants in infested areas dropped by 70%, while the total number of native individuals dropped by 90%. Of 35 species of ants collected in this study, 23 were either significantly less common or absent from infested sites; only S. invicta was more common at infested sites. The most dramatic effect of the invasion was a 10—30 fold increase in the total number of ants at infested sites–of which>99% were the imported fire ant S. invicta. The impact of this invasion on other surface—active arthropods was less severe, but still substantial. The abundance of isopods, erythraeid mites, and tumblebug scarabs declined significantly, while the abundance of ground crickets, a brachypterous roach, and a symbiotic scarab increased significantly. Overall, the species richness of non—ant arthropods was 30% lower in infested sites, and individual numbers were 75% lower. Total arthropod species richness (including ants) was 40% less at infested sites. These data indicate that polygyne fire ants pose a substantial threat to the biodiversity of native arthropod communities.

ISSN:0012-9658    

DOI:10.2307/1938623

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

We used replicated field enclosures to manipulate population densities of two species of bitrophic predators in a terrestrial old—field community: a mantid (Tenodera sinensis), and a wolf spider (Lycosa rabida). The treatments consisted of adding mantids alone (8 individuals/enclosure), lycosids alone (10 individuals/enclosure), and lycosids and mantids together (8 + 10 individuals). A control consisted of enclosures to which no predators were added. The impact of these predators on numbers and biomass of other arthropods in the community was examined on several levels: overall community, different size (body length) categories, and major taxa. We asked whether the impact of these predators in combination could be predicted from their separate effects. Mantids depressed total numbers (10—15%) and biomass (50%), in the arthropod community over the course of 10 d. This effect was confined to the largest size categories in the community. Lycosids had no measurable effect at this level of resolution. Mantids depressed abundance of acridids in both mantid and mantid/lycosid enclosures, but again lycosids had no impact. Lycosids alone enhanced abundance of gryllids, but not in the presence of mantids. Both mantids and lycosids depressed numbers of small spiders (also members of this guild), but this effect was not additive. Interactions among members of bitrophic generalist predator guilds may contribute to the commonness of nonadditive and higher order effects in manipulative experiments. Depending upon the level of resolution, it may be impractical to predict the impact of the whole guild from summing the individual effects of single predator species on terrestrial arthropod communities.

ISSN:0012-9658    

DOI:10.2307/1938624

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Most plant species are attacked by insects with widely different feeding styles and life histories. As a first step toward understanding how plants adapt to such diverse pressures, we investigated the population responses of 17 insect species to the genotypic variation exhibited by clones of goldenrods, Solidago altissima, that were drawn from the same population and grown in common gardens. A cluster analysis on the genetic correlations between the insects' responses produced four groups, termed "herbivore suites," that are attracted and repelled by similar plant genotypes (and their underlying sets of traits). There was no obvious correspondence of an insect's phylogenetic affinity, host range, or feeding style with its membership in a particular suite. Put another way, insects belonging to the same family or guild respond differently to genetically variable traits in populations of this native, perennial plant. We found no negative genetic correlations among suites and few among pairs of species, which suggests that trade—offs between the plant's resistance traits are relatively unimportant. The existence of suites of herbivores, whose members exhibit correlated patterns of abundance on different host genotypes, has several interesting consequences. First, the genetic variability of the plant population will be reflected in the diversity and composition of the insect community. Second, selection caused by one herbivore species can result in increased resistances to other, rarer species whose responses are correlated with those of the damaging species. Third, selection by suites of herbivores may be more intense and less variable than selection by individual insect species, especially when many species are rare. By recognizing that resistance can evolve as a function of selection by entire suites of species we can develop better estimates of selection and perhaps achieve a greater understanding of the process of diffuse coevolution. The linkage of interactions within suites suggests some unexplored possibilities for explaining the rarity of many insect species.

ISSN:0012-9658    

DOI:10.2307/1938625

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY