Abstract: Both natural and anthropogenic disturbances affect ecosystem integrity and biodiversity. Nearly all natural disturbances in arctic regions and elsewhere are directly or indirectly driven by climate (Walker and Walker 1991). Human disturbances also operate over large spatial and temporal scales and produce immediate direct effects and numerous indirect effects (Harte et al. 1992). The effects of these two types of disturbances on ecosystem function and biodiversity may be synergistic. Each disturbance reinforces the other, so that there is mutual amplification of their respective effects (Myers 1992). Organisms often respond nonlinearly to environmental variability (DeAngelis 1992). The responses are linked, to positive and negative feedback processes which involve both abiotic and biotic components of the environment. The environmental changes associated with these feedbacks, which are not easily predictable, may lead to a loss of ecosystem resilience and biodiversity.

Abstract: This study is a comprehensive and experimental approach to understand the dynamics producing structure in an intertidal algal community. Both biological factors (competition and herbivory) and physical factors (disturbance and physical stress) were investigated through field manipulative experiments, non-manipulative monitoring and laboratory experiments for the last 3 years in Barkley Sound, Vancouver Island. The community studied contains three dominant perennial macroalgae, Mazzaella cornucopiae (Rhodophyta), Fucus distichus and Pelvetiopsis limitata (Phaeophyta), and some ephemeral algae as well as coexisting invertebrates such as barnacles, limpets and snails. Biological interactions and their mechanisms for all these organisms turned out to be complicated and non-hierarchical interaction networks which include reversal of dominance and indirect interaction. There was evidence for both negative and positive interactions existing between the same pair of species. Competitive dominance was changed depending on the developmental stages of the competitors and due to differences in morphology, and also the outcomes of interactions were modified by physical stresses (i.e., desiccation and wave action). Snails preferentially grazed the red alga, and limpets reduced the abundance of the ephemeral algae (the early colonizers) which otherwise inhibited the settlement of the later successional species such as the fucoids. Barnacles facilitated the colonization of Fucus and ephemeral algae after disturbance. Responses of the three dominant algae and ephemerals to the different sizes and time of disturbance were species-specific and depended on the alga’s life history and reproductive characteristics. The study shows that both the biological and physical factors influenced the structure of this community, and that the non-hierarchical interactions among the major species and their well-balanced responses to disturbance support a high likelihood of maintaining the current diversity in this algal community.

Abstract: The far-reaching changes in the number and composition of plant and animal species which normally occur in conjunction with the liming of lakes and watercourses involve major changes in the structure and function of communities of organisms and the complexity of food webs. This is primarily attributable to changes in water chemistry, but interaction within and among species is of crucial importance to the post-liming development of a community. The short-term effects of liming often differ from those of the long-term effects. Normally, individual species expand rapidly for a short time, since competition and predation are reduced and there is a surplus of nutrients. In certain cases, this may have undesirable effects such as blooms of certain phytoplankton species, sharp increases in macrophytes or abnormal expansion of fish populations. The long-term impact, on the other hand, is to a great extent determined by biotic factors such as competition and predation within the new post-liming abiotic limits which emerge.

Abstract: Abiotic factors, principally rainfall, drive the population dynamics of the Australian plague locust, Chortoicetes terminifera (Walker), in semi-arid source areas. However, with the possible exception of the low summer rainfall southern invasion areas, biotic factors-especially scelionid, sarcophagid and nemestrinid insect parasitoids and mermithid nematode parasitoids- become increasingly important in reducing survival in successive generations following outbreaks, while the abiotic factors assume a more indirect role through their influence on biotic factors. Parasitoids of acridids have a relatively stable population of acridid hosts, principally eastern plague grasshopper, Oedaleus australis Saussure, on the western plains and wingless grasshopper, Phaulacridium vittatum (Sj6stedt), on the western slopes and tablelands, because of the high summer rainfall in these invasion areas. The host-parasitoid relationship of each parasitoid is distinctive and exhibits temporal variation in relation to the outbreak phase of C. terminifera and alternative host abundance. Typically the unlimited availability of hosts during outbreaks allows full realization of the reproductive capacity of the parasitoids. In the future parasitoid induced recessions may be initiated at an earlier stage in the outbreak cycle because of the increasingly efficient control of C. terminifera in semi-arid source areas and consequent reduction in the density of populations invading the Southeast. However, parasitoid-accommodating control strategies, including the demarcation of districts in which a "do nothing" strategy would be appropriate in some seasons, need to be adopted so that the supplementary control of C. terminifera afforded by parasitoids can be maximized.