Plant invasions--the role of mutualisms.

Many introduced plant species rely on mutualisms in their new habitats to overcome barriers to establishment and to become naturalized and, in some cases, invasive. Mutualisms involving animal-mediated pollination and seed dispersal, and symbioses between plant roots and microbiota often facilitate invasions. The spread of many alien plants, particularly woody ones, depends on pollinator mutualisms. Most alien plants are well served by generalist pollinators (insects and birds), and pollinator limitation does not appear to be a major barrier for the spread of introduced plants (special conditions relating to Ficus and orchids are described). Seeds of many of the most notorious plant invaders are dispersed by animals, mainly birds and mammals. Our review supports the view that tightly coevolved, plant-vertebrate seed dispersal systems are extremely rare. Vertebrate-dispersed plants are generally not limited reproductively by the lack of dispersers. Most mycorrhizal plants form associations with arbuscular mycorrhizal fungi which, because of their low specificity, do not seem to play a major role in facilitating or hindering plant invasions (except possibly on remote islands such as the Galapagos which are poor in arbuscular mycorrhizal fungi). The lack of symbionts has, however, been a major barrier for many ectomycorrhizal plants, notably for Pinus spp. in parts of the southern hemisphere. The roles of nitrogen-fixing associations between legumes and rhizobia and between actinorhizal plants and Frankia spp. in promoting or hindering invasions have been virtually ignored in the invasions literature. Symbionts required to induce nitrogen fixation in many plants are extremely widespread, but intentional introductions of symbionts have altered the invasibility of many, if not most, systems. Some of the world's worst invasive alien species only invaded after the introduction of symbionts. Mutualisms in the new environment sometimes re-unite the same species that form partnerships in the native range of the plant. Very often, however, different species are involved, emphasizing the diffuse nature of many (most) mutualisms. Mutualisms in new habitats usually duplicate functions or strategies that exist in the natural range of the plant. Occasionally, mutualisms forge totally novel combinations, with profound implications for the behaviour of the introduced plant in the new environment (examples are seed dispersal mutualisms involving wind-dispersed pines and cockatoos in Australia; and mycorrhizal associations involving plant roots and fungi). Many ecosystems are becoming more susceptible to invasion by introduced plants because: (a) they contain an increasing array of potential mutualistic partners (e.g. generalist frugivores and pollinators, mycorrhizal fungi with wide host ranges, rhizobia strains with infectivity across genera); and (b) conditions conductive for the establishment of various alien/alien synergisms are becoming more abundant. Incorporating perspectives on mutualisms in screening protocols will improve (but not perfect) our ability to predict whether a given plant species could invade a particular habitat.

Preferred room temperature of young vs aged males: the influence of thermal sensation, thermal comfort, and affect.

BACKGROUND: While research has demonstrated that aged persons may show impaired thermoregulatory control, we do not know whether dysthermia in the aged results from altered behavioral or autonomic responses. Consequently, we investigated age-related differences concerning the ability to regulate room temperature. METHODS: Two groups of matched healthy males (22.9 yr and 66.9 yr) were subjects of this study. After equilibration at 24 degrees C (rh 50%), chamber temperature controllers were set into cooling mode. Using a dual position switch, subjects adjusted this temperature when air temperature moved outside their preferred range. Switch operation resulted in maximal cooling or heating, without a steady state. Subjective ratings of thermal sensation, discomfort, and affect were provided at each activation. RESULTS: Both groups controlled temperature equivalently: 24.9 degrees C (+/- 1.3, young) and 24.5 degrees C (+/- 1.5, elderly; p > .05). At cold-induced change points, the skin temperatures of the calf, thigh, chest, and hand were significantly lower in the elderly subjects. During the heat-induced changes, chest, hand, upper arm, and mean skin temperatures were also lower (p < .05). At cold-induced change points, the elderly group felt colder (p < .05), were less uncomfortable (p < .05), and felt better than the young subjects (p < .05). During heat-induced changes, thermal sensation was equivalent, the elderly were more comfortable (p < .05), and felt better (p < .05). CONCLUSIONS: Assuming thermal discomforture drives behavior, it is possible that elderly people may require a more intense thermal stimulus to elicit the appropriate behavioral responses in the home. It is also possible that such stimuli will result in a greater heat flow, elevating the risk of dysthermia in the aged.

Density dependent interactions between VA mycorrhizal fungi and even-aged seedlings of two perennial Fabaceae species.

The interaction of density and mycorrhizal effects on the growth, mineral nutrition and size distribution of seedlings of two perennial members of the Fabaceae was investigated in pot culture. Seedlings of Otholobium hirtum and Aspalathus linearis were grown at densities of 1, 4, 8 and 16 plants per 13-cm pot with or without vesicular-arbuscular (VA) mycorrhizal inoculum for 120 days. Plant mass, relative growth rates, height and leaf number all decreased with increasing plant density. This was ascribed to the decreasing availability of phosphorus per plant as density increased. O. hirtum was highly dependent on mycorrhizas for P uptake but both mycorrhizal and non-mycorrhizal A. linearis seedlings were able to extract soil P with equal ease. Plant size distribution as measured by the coefficient of variation (CV) of shoot mass was greater at higher densities. CVs of mycorrhizal O. hirtum plants were higher than those of non-mycorrhizal plants. CVs of the facultatively mycorrhizal A. linearis were similar for both mycorrhizal and non-mycorrhizal plants. Higher CVs are attributed to resource preemption by larger individuals. Individuals in populations with high CVs will probably survive stress which would result in the extinction of populations with low CVs. Mass of mycorrhizal plants of both species decreased more rapidly with increasing density than did non-mycorrhizal plant mass. It is concluded that the cost of being mycorrhizal increases as plant density increases, while the benefit decreases. The results suggest that mycorrhizas will influence density-dependent population processes of faculative and obligate mycorrhizal species.