Two varieties of empirical information have been collected for guilds of animals: first, information related to functional organization as revealed by field manipulations; second, descriptive data that are used to characterize guilds by measuring such quantities as niche breadth, niche segregation, niche overlap, etc. Only functional data can be used to test proximate theoretical models because these data can be organized such that specific hypotheses can be falsified. Descriptive data can only be consistent in the predictions of hypotheses, although inconsistencies clearly imply falsification of such hypotheses. A three—phase procedure that can be used to specify the influence of interspecific competition on guild structure is presented. These phases are: (1) measurement of overlap of resource usage (° descriptive data); (2) establishment of interaction, which entails the experimental demonstration that the presence of other species leads to altered patterns of resource usage; and (3) demonstration of depressive competition, in which the interaction is shown to be responsible for altered characteristics of populations (e.g., population density, age—structure, etc.) or individuals within the populations (e.g., fecundity, mortality, relative fitness). The potential drawbacks associated with the experimental demonstration of interaction or depressive competition are listed. These include the relative strengths of interspecific and intraspecific density effects, choices of times and locations for perturbations, possible effects of competitive mutualism, frequency—dependent predation, and choice of variables with which to assess competition. Depressive competition has been adequately shown to occur in a number of studies, particularly those that concentrate on small guilds (less than six species). Functional organization will be more difficult to show or delineate in guilds consisting of larger numbers of species because such guilds are more likely to be influenced by alternative processes.
Many species of Eucalyptus in Australia provide copious amounts of nectar during their reproductive seasons. The nectar is used by many animal species but especially by birds, insects and some bats, which act as pollinators. One of the major features of eucalypt flowering in southern Australia is the patchy, asynchronous flowering of different species, which appears to drive mass nomadism of nectarivorous birds among regions and among habitats. Here we explore whether flowering asynchrony or climate is primarily responsible for the influxes and effluxes of vast numbers of nectarivorous birds in central Victoria, Australia. By using a structured sampling program, we show that winter flowering by red ironbark Eucalyptus tricarpa is the most likely agent controlling avian‐nectarivore dynamics rather than climatic differences among regions. Densities and species richness of nectarivores, and numbers of nectarivory events, are all closely related to measures of flowering intensity. However, non‐ nectarivores, such as insectivores and granivores, show no relationships with either habitat or region. We discuss how dependence on a patchily distributed but highly rewarding resource such as nectar influences population densities and community structure in birds.
There has been debate as lo whether the composition of avifaunas is determined primarily by responses of speeies to the Jloristie composition of habitats or to the struetural features of habitats (viz. physiognomy). Some evidence suggests ihat the effeets of geographie seale may be the souree of this dispute. The present analysis demonstrated that the influence of both floristics and physiognomy could be detected al the regional level for avifaunas of south-eastern Australian woodlands. The impact of florist ies often may be obscured by patchiness and relatively rapid rates of spatial change in the phytosociology of habitats relative lo those of avian species, even at the local seale (say «£ 7(9 km). Such potential occlusion of the importance of floristics to avifaunas may be overeome by using Jloristie similarity indiees that are based on plant taxonomie representations at both the specific and subfamilial levels. Both floristics and physiognomy are likely to affect avifaunal composition, although the pereeived significance of each faetor may vary with the scale of observation.
We evaluated the status of coarse woody debris (CWD, fallen wood) on floodplains of the southern Murray-Darling basin of southeastern Australia. The floodplains are dominated floristically by the river red gum Eucalyptus camaldulensis. Aerial survey techniques were used to estimate the amounts of woody debris within 200 m of the channels along 2,442 km of 11 rivers of the system, including the Murray and Darling Rivers and the Darling Anabranch. Aerially based indices were converted into wood volumes by using ground-truthing at a selection of sites; there was a strong correlation between index values and measured wood volume densities. For thickly forested sites such as Barmah, Gunbower Island, and the Ovens floodplains, the aerial method was not useful, so ground measurements at randomly positioned sites within the forests were used. Volumes were translated into mass by using conversion factors drawn from the literature. We estimated that total tonnage on approximately 221,000 ha of floodplain forests was 4.175 Ϯ 0.579 ϫ 10 6 tonne. In the larger forested blocks ( Ͼ 7,000 ha), mean wood densities ranged between approximately 12 tonne/ha on the lower Goulburn up to approximately 24 tonne/ha at Barmah State Forest. The area-weighted mean for the entire area was approximately 19 tonne/ha. A main purpose of the research was to place these figures into an historical perspective to evaluate implications for restoration. A thorough search of historical documentation revealed that there are no extant data upon which to estimate pre-European settlement levels. We used information from an apparently undisturbed "unmanaged" site in the Millewa forests of southern New South Wales as a basis. Wood density there corresponded to a mean figure of 125 tonne/ha wood-mass density. By using this figure we estimate that CWD levels on the southern Murray-Darling basin may be of the order of 15% of pre-European settlement levels. Full restoration of the 221,000 ha surveyed would require 23.5 Ϯ 0.579 ϫ 10 6 tonne, which is equivalent to about 600,000 mature (1 m diameter at breast height) river red gum trees or the amount of timber derived from clear felling about 115,000 ha of river red gum forest at current stocking levels. We discuss the implications of this massive deficit and possible short-and long-term solutions.
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