摘要:

This paper reformulates the notion of density dependence and shows how this notion plays an important role in constructing appropriate models for data analysis. The regulation and persistence of population processes are interpreted as a close resemblance to the behavior of a series of random variables in which the second moments are bounded. On this basis the formal criteria of persistence are deduced. General structural models of population processes are set up and translated into discrete single—variable difference equations, ranging from the simplest linear first—order process to more complex nonlinear second—order processes. The discussion includes the derivation of general conditions for the second—order limit cycles, a reanalysis of the Canadian lynx 10—yr cycle, and models for population outbreaks. Based on the results of the preceding study of models, the notion of density dependence is reformulated. First, the meaning of the word 'dependence' is discussed. In the context of 'density dependence,' the word has two meanings; the causal dependence of a factor on density, and the statistical dependence. Statistical dependence is defined as a converse of statistical independence, the latter being a process in which the rate of change in density has zero correlation with density; this is a very special class of processes and is unlikely to occur in natural population processes. Therefore, the test of density dependence against the null hypothesis of statistical independence will not provide much insight. It is also argued that a deduction from the persistence criteria shows that a negative correlation between density and its rate of change is a necessary outcome of regulation and hence that the notion of 'density—dependent regulation' in statistical dependence is an uninspiring tautology. As opposed to statistical density independence, which necessarily generates an unbounded population process, causal density independence may satisfy the persistence conditions and hence may regulate populations. However, such a causally 'density—independent regulation' tends to be 'fragile' against perturbations by random exogenous factors. It is a particular class of causally density—dependent processes that can ensure regulation more durable against such perturbations. The inference of generating mechanism from observation is discussed. Although regression analysis is an essential method of inference, simple regression analysis will not work unless the observed processes are known to be a simple Markov chain. Statistical inference of generating mechanisms in observed systems depends largely on the choice of appropriate models, and it is in the construction of such models that the notion of causal density dependence plays an important role.

ISSN:0012-9615    

DOI:10.2307/1942222

出版商:Ecological Society of America    

年代:1977

数据来源: WILEY

摘要:

Because of the nature of their substratum, the sessile invertebrate species of the marine epifaunal community living on rocks occur in discrete patterns of distribution. The rocks are finite patches or habitat islands with a limited space for colonization and growth. Such a system is ideal for studying the parameters affecting the distribution of species within a community. Also, because of the small size and immobility of the adults, the system is also ideal for studying the pattern of change in species composition and diversity within a community. This study used multiple series of manipulated experimental plates, which both duplicated natural rock surfaces and could be compared with samples of the rocks, to investigate the developmental and distributional processes of this community. Five major factors were found to be important to both the development of the community and its distribution on the rocks: (1) the selectivity of the metamorphosing larvae as to site of attachment; (2) the seasonal fluctuation in larval abundances; (3) the biological interactions within and between species; (4) the size of rock substrata; and (5) the physical disturbance of the substrata (rock turnover). Initially, the developmental process can be uncoupled from the effects of the substrate size and disturbance. Predation is relatively unimportant as a biological interaction within this community, but the species can be ranked according to their ability to compete for the available space on a substratum. This ranking implies a type of successional sequence in the development of the community; however, the sequence is greatly affected by historical components. The colonization of a substratum is directly dependent upon the abundance of settling larvae, which in turn is a function of seasonality and selectivity. The eventual competitive outcome and development of the community will depend upon which species have immigrated onto the substratum and is thus dependent upon history. The process is, therefore, open ended: colonization will be highly variable and change seasonally and, although one species may eventually dominate the substratum, it may be one of nine different species depending upon the individual history of that area. The frequency with which a substratum is disturbed (with the resultant extinction of its fauna) is a function of wave force and is inversely proportional to both the size of the substratum and the depth at which it occurs. Disturbance will determine when a substratum is initially exposed for colonization and how long it will have for development. In the shallow subtidal (mean low water to —2.5 m), the frequent disturbance of small rocks will cause them to support less than their equilibrium number of species and their fauna will reflect immediate larval abundances. Large rocks will remain stable for long periods of time and will usually be dominated by a single species. Intermediate—sized rocks (1 to 10 dm3) will remain stable long enough to develop an equilibrium number of species but will be disturbed before dominance occurs. They will thus have the highest diversity because of their 'optimal' frequency of disturbance. In deeper water (10 m) smaller rocks will be more stable. More rocks will develop dominance and the smaller size of a rock at the optimum frequency will mean that substrata at this frequency will have a lower equilibrium number of species and thus a lower diversity. The increased stability with depth will mean a lower overall diversity for the community. Lastly, the increased disturbance and lower pool size for rocks in the intertidal will also cause diversity to be lower in this area. There appears, therefore, to be an optimal frequency of disturbance at which diversity is maximized. An increase or decrease in this level causes a reduction in diversity because of a decrease in the number of species present or an increase in dominance. This optimum will vary with the physical environment and the type of community.

ISSN:0012-9615    

DOI:10.2307/1942223

出版商:Ecological Society of America    

年代:1977

数据来源: WILEY

摘要:

Concentrations of elements in streamwater were monitored in 57 watersheds located along elevational and successional gradients in the White Mountains of New Hampshire, USA. Concentrations of C1— and SO4– decreased with increasing elevation as a result of differences in relative evapotranspiration along the elevation gradient. Concentration of Na+, Ca++, and Mg++ also decreased with increasing elevation, but with a steeper slope. Rock weathering/unit of water flux as well as relative evapotranspiration controls concentrations of these cations. Potassium and NO3— concentrations were highly variable, both seasonally within a stream and among streams. Differences among watersheds in terms of successional status are important in controlling NO3— and K+ concentrations. Streams draining old—aged forested ecosystems had higher concentrations of NO3—, K+, and other plant nutrients than did streams draining intermediate—aged successional ecosystems at the same elevation. Nine spruce—fir watersheds which have no record of logging or other extensive human disturbance had streamwater NO3— concentrations averaging 53 microequivalents/litre, while five other spruce—fir watersheds which had been logged 30 yr previously had streamwater NO3— concentrations averaging 8 meq/1. The major factors controlling streamwater concentrations of elements in these watersheds are factors related to supply (precipitation chemistry, relative evapotranspiration, rock weathering) and to plant accumulation of nutrients. Rapidly—growing successional ecosystems can accumulate a large fraction of inputs of nutrient elements, particularly during the growing season. Steady state ecosystems, those with no net biomass accumulation, have nutrient outputs equal to nutrient inputs. The increased losses of nutrient elements from clearcut watersheds in New Hampshire can be explained in these terms. Clearcut or otherwise disturbed ecosystems may have mineralization in excess of plant uptake, leading to element outputs in excess of inputs. Field experiments demonstrated that the prevention of plant uptake by root trenching led to soil NO3— concentrations comparable to those observed in streamwater in the clearcut watershed at Hubbard Brook Experimental Forest.

ISSN:0012-9615    

DOI:10.2307/1942224

出版商:Ecological Society of America    

年代:1977

数据来源: WILEY

摘要:

Clark's Nutcracker (Nucifraga columbiana) is one of four species of corvids that commences harvesting, transporting, and caching the seeds of pinon pine (Pinus edulis) in mid—August. Caching continues until inclement weather intervenes or the seed crop is depleted. Nutcrackers are efficient at collecting seeds from the trees because the birds discriminate edible and aborted seeds, select cones with higher than average number of good seeds and appear to concentrate on those trees that produce cones with large numbers of good seeds. The use of a sublingual pouch allows the nutcracker to carry up to 95 pinon pine seeds per trip (x = 55). A pouch full of seeds may be carried as far as 22 km from the collecting area (the pinon—juniper woodland, elevation ~ 1800 m) to communal caching areas where the seeds are buried in clumps in subterranean caches at a depth of 2 to 3 cm. These caching areas are on steep, south—facing slopes that are usually free of snow by late winter. Although solitary nesters, the birds, during the nonbreeding season, often form flocks when harvesting seeds. A flock of 150 nutcrackers cached between 3.3—5.0 X 106pinon pine seeds in the autumn of a good cone crop. Each bird caches between 2.2 and 3.3 X its needed energetic requirements for the insect—free portion of the year. Some of the unrecovered seeds germinated and grew. The pinon pine reaches its highest density in relatively dry areas below the elevational range of other members of the genus Pinus, but ranges up to 3,100 m into the mixed coniferous forest. This pine has developed a number of traits that enhance its efficiency to be located and dispersed by the birds. The seeds are unusually large, of high energy value, and have relatively thin seed coats. The large size improves the germination potential of the seed but also attracts predators and dispersal agents. The seed coat color is markedly different for edible and aborted seeds, thus labeling them for visually oriented seed harvesters. Most pinon pine cones are oriented outward and upward which increases illumination of the seeds and tends to hold them in the cone. The pinon pine displays the seeds in its cones more readily than wind—dispersed pines by opening the seed—bearing cone scales to a uniform angle. Pinon pine seeds are retained for a long period of time in the cone because they are held in deep depressions on the cone scales by small flanges. This constellation of features indicated an evolutionary strategy on the part of the pinon pine to exploit and encourage members of the seed—caching guild to disperse the seeds. Deposition sites are presumably more favorable for germination than those obtained by simply broadcasting the seeds over a wide area. Individual seedlings potentially experience immediate intraspecific competition because the seeds are cached in clumps. Only one seedling, presumably the strongest, fastest—growing individual, survives. The benefits of this system to the nutcracker are that the bird has a supply of energy for the most unproductive months of the year. The bird also breeds in late winter or early spring, using the stored seeds for reproductive energy.

ISSN:0012-9615    

DOI:10.2307/1942225

出版商:Ecological Society of America    

年代:1977

数据来源: WILEY