摘要:

AbstractPathogens are potent selective forces whose importance in shaping the size and structure of individual plant populations and whole communities has been underestimated. Even in situations where host and pathogen have been associated over long periods of time, pathogens regularly affect host fitness by reducing fecundity and increasing mortality either directly or indirectly through reductions in competitive ability.The genetic consequences of such disease‐induced reductions in fitness are profound. On a broad geographic scale, race‐specific resistance generally occurs more frequently in regions characterized by environments favourable for disease development. Within such areas, however, the distribution of resistant plant genotypes is often very patchy. This probably reflects the importance of extinction and colonization events in the continuing co‐evolutionary dynamics of host‐pathogen associations.At a demographic level, pathogen‐induced reductions in host fitness may lead to changes in the size of populations. In turn, this may lead to changes in the relative diversity of whole communities. Documentation of this scale of interaction is poor, but the devastating consequences of the introduction of pathogens into alien environments provides a salutary reminder of their power to change plant communities

ISSN:0307-692X    

DOI:10.1111/j.1442-9993.1991.tb01072.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractTrophic interactions involving plants and animals in tropical mangrove forests have important controlling influences on several population, community and ecosystem‐level processes. Insect herbivores remove up to 35% of leaf area from some mangrove tree species and can cause the death of seedlings. Leaf chemistry and toughness and soil nutrient status all appear to be important in explaining the between‐ and among‐species variance in leaf damage. Insects also attack and damage, mainly by boring, a large proportion of mangrove seeds. Shadehouse experiments have shown that such post‐dispersal predation can have a significant effect on seedling survival, growth and biomass allocation to leaves, stems and roots. Sesarmid crabs are also responsible for severe post‐dispersal seed predation. In field trials, crabs consumed more than 70% of the seeds of five tree species. For four of these five species there was an inverse relationship between seed predation rate and the dominance of conspecific adult trees, while the within‐site distribution pattern of one tree species appears to be partially controlled by crabs. The same crab species also consume 30–80% (depending on forest type and intertidal elevation), of the annual litter fall in mangrove forests and, thus, have an important role in controlling the rate of remineralization of detritus within forests and the export of particulate matter from the forests to other nearshore habitats. The other major component of litter in the forests is wood, which is broken down relatively rapidly by teredinid molluscs (shipworms). More than 90% of the weight loss from decomposing trunks ofRhizophoraspecies during the first four years of decay is through ingestion by teredinids. The annual turnover of dead wood mass inRhizophoraforests is equivalent to that of the processing of leaf detritus by crabs. Because of the relatively low species richness of trees and consumers in tropical mangrove forests, they are likely to serve as productive sites for further investigations of the influence of plant‐animal interactions on the dynamics of t

ISSN:0307-692X    

DOI:10.1111/j.1442-9993.1991.tb01073.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractMyrmecochores are plant species that bear seeds with food‐bodies adapting them for dispersal by ants. Within Australia and South Africa, myrmecochores account for a large percentage of the flora in sclerophyll vegetation on very infertile soils. On fertile soils, there are few myrmecochores and more species with fleshy fruits adapted for dispersal by vertebrates. This effect of very infertile soils may also account for the high incidence of myrmecochores in the floras of Australia and South Africa compared with other continents. The taxonomic distribution of myrmecochory shows it has evolved many times within the Australian flora.Evidence has been collected in relation to eight hypotheses that might account for the prevalence of myrmecochory on very infertile soils:(1) Myrmecochore seeds do not appear to be relocated to nutrient‐enriched microsites. Seed‐removing ant species relocate their nest entrances frequently; active ant nests are not nutrient‐enriched; and seedlings emerging after fire are not located in nutrient‐enriched soil.(2) Traffic of seed‐removing ants is not greater on infertile than on fertile soils.(3) Burial to avoid predation by small mammals is not a significant factor for seeds in Australian vegetation.(4) Experiments in which diaspore traits are manipulated support the idea that a larger food body increases the likelihood that a diaspore will be taken by a seed‐disperser ant species that will not eat the embryo, rather than by a seed‐predator ant. However, evidence so far does not support the hypothesis that, on very infertile soils, seed‐disperser ants are consistently more available than seed‐predator ants.(5) Removal rates of fruits adapted for dispersal by vertebrates were, on average, two and a half times greater in vegetation on fertile soils. Birds were the main removers. Our current opinion is that the fruit‐removing bird assemblage is more likely to be a consequence of the high incidence of species with vertebrate‐adapted fruits, rather than the reverse.(6) Fleshy fruits are more expensive in potassium than food bodies for ants, but not in nitrogen or phosphorus. This might be a reason why fleshy fruits are uneconomic on very infertile soils.(7) Species with smaller embryo‐plus‐endosperm weight are relatively more likely to be adapted for dispersal by ants rather than vertebrates. However, differences in the frequency distribution of embryo‐plus‐endosperm weights between infertile and fertile soils are moderate in the Sydney area. An indirect association between soil fertility and dispersal mode via embryo‐plus‐endosperm weight cannot account for much of the difference in dispersal spectrum.(8) Species growing to less than 2 m tall are relatively more likely to be adapted for dispersal by ants rather than vertebrates, and also account for more of the flora on infertile than on fertile soils. This indirect correlation is capable of accounting for as much as 80% of the higher incidence

ISSN:0307-692X    

DOI:10.1111/j.1442-9993.1991.tb01074.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractSmall frugivorous birds that feed largely on the fruits of stem‐parasitic mistletoes have independently evolved in various parts of the world. Local populations of mistletoes may be dispersed almost exclusively by these birds. Four attributes of mistletoe dispersal systems may have enhanced the evolution of reciprocal dependence between mistletoes and specialized dispersers:(1) Safe sites for mistletoe seeds (i.e. the young branches of a compatible host) are precisely defined in space and time.(2) The viscidity of mistletoe seeds induces smaller dispersers to deposit seeds in safe sites.(3) Frugivores differ markedly in the efficiency with which they disperse mistletoe seeds to safe sites.(4) Relatively large viscid fruits and adaptive fruiting displays exclude or deter most members of the potential disperser guild.Some birds have anatomical adaptations as a result of dietary specialization on mistletoe fruit, and some mistletoes have fruiting displays that target specialized birds or a narrow disperser spectrum. Coevolution between guilds of mistletoes and specialized dispersers is therefore probable. The uncoupled selective pressures that might have driven their coevolution are the mistletoes’ provision of fruit crops that are unavailable to more generalized frugivores, in return for seed dispersal to the small stems most suitable for infection. As in other mutualistic seed dispersal systems, phylogenetic, ecological and life history factors constrain the evolution of monophyletic interdependence, resulting in varying degrees and patterns of reciprocal specificity between mistletoes and dispers

ISSN:0307-692X    

DOI:10.1111/j.1442-9993.1991.tb01075.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractAlthough there has been much recent interest in ant‐plant mutualisms, few data are available on the effects of foraging ants on herbivore numbers and levels of herbivory on plants that do not offer specific inducements to attract ant visitation. In forestry plantations and tropical crops, ants have erratic but sometimes dramatic effects on the numbers of insect herbivores but, in more natural habitats, their effects on levels of herbivory appear to be largely unknown. In Australia, where ants andEucalyptuswoodlands are ubiquitous and abundant and where considerable debate has occurred regarding levels of herbivory inEucalyptusforests, very little work has been done to examine the effects of ants on densities of insect herbivores on eucalypts.In this study, ants were experimentally excluded from mature and immature foliage of saplings of the malleeEucalyptus incrassatain South Australia, and herbivore numbers and levels of leaf herbivory were assessed during the next 6 months. No significant differences in herbivory were found between ant‐access and ant‐exclusion treatments. In spring and early summer, ants were found in associations with aggregations of eurymelid bugs on young foliage, and the effects of ants on bug densities were experimentally investigated. Bug densities decreased rapidly in ant‐exclusion treatments compared with ant‐access controls. Ants also quickly removed seeds ofE. incrassatafrom experimental caches.The potential of ants to limit the numbers of insect herbivores on eucalypts seems limited given their tendency to form mutualistic associations with sap‐feeding Homopterans and because of a lack of other herbivores that are particularly vulnerable to an

ISSN:0307-692X    

DOI:10.1111/j.1442-9993.1991.tb01076.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractMean sizes of duck hunters’ bags on the opening days of annual waterfowl seasons at selected wetlands in southeastern Australia, between 1972 and 1990, are correlated with an index of the Southern Oscillation (SOI). Simple correlations exist between bag sizes at the various sites, and the mean bag size in Victoria shows a positive relationship with that obtained at Barrenbox Swamp, New South Wales. Bag sizes are most significantly correlated with monthly SOI some 25–28 months before the eventual start of an open season. Interannual variations in rainfall in eastern Australia are generated by the El Niño–Southern Oscillation, which is monitored by the SOI, and the waterfowl populations respond to changing water and wetland availability. Accepting that annual bag sizes represent an index of waterfowl abundance, the regional population size may be related to climatic events associated with changes in SOI values. Further consideration of the SOI and indices of waterfowl abundance may allow variations in legislative or management options well in advance of individual open s

ISSN:0307-692X    

DOI:10.1111/j.1442-9993.1991.tb01077.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractYellow‐bellied gliders on occasion extensively incise the trunks of eucalypts to feed on sap but in doing so show strong preference for the species and number of trees utilized. Three hypotheses that may account for aspects of this behaviour were examined. They were: (i) sap feeding is related to the incidence of rain; (ii) sap feeding is influenced by the sap flow idiosyncracies of trees; and (iii) sap feeding is influenced by the availability of alternative food resources. Sap feeding at two sites occurred independent of ‘wet’ or ‘dry’ nights. Sap flow (measured as an index) in two species of eucalypt was highly variable among trees and among sample periods. This is the only hypothesis that accounts for the selectivity of the trees incised. Sap feeding at one site coincided with periods of high sap flow. However, at another site, sap feeding did not occur on measured trees despite their often high sap flow. At such times, flowering trees were abundant and gliders fed extensively on nectar. Thus, elevated sap flow may provide the necessary precondition conducive for sap feeding but may be ignored when other food types are

ISSN:0307-692X    

DOI:10.1111/j.1442-9993.1991.tb01078.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractInduced resistance of cultivatedGossypiumto its exotic, agricultural pests is well studied but little is known about whether native cottons respond to damage by endemic herbivore populations. This study examined induced responses ofGossypium australeto its most abundant folivore, Bucculatrix gossypii.Prior damage did not affect the number of new mines initiated. Survival of miners on damaged, young leaves and cotyledons was reduced compared with survival on young leaves and cotyledons of undamaged plants. However, the induced resistance was not systemic; survival of miners on older, undamaged leaves of damaged seedlings was not different from survival on older leaves of undamaged controls. This localized induced resistance did not produce an overdispersed distribution of either mines or successful mines. On the contrary, the distributions tended towards clumped, although they were not statistically distinguishable from random. Although a localized induced response affected miner survival, no effects on behaviour were observed.

ISSN:0307-692X    

DOI:10.1111/j.1442-9993.1991.tb01079.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe temporal stability of estimates of demographic behaviour for a sample population of the tropical rainforest gymnospermAraucaria cunninghamiiwas analysed using a stage‐classified (Lefkovitch) matrix model. Previously published life‐table analyses, which used only 2 years of held data (1975–77), were compared with life‐table analysis based on a further 5 years of field data for growth and survival (1977–82). The new analysis calculated the finite rate of natural increase, λ, as 1.009 ± 0.077. This value is closer to the theoretical value of 1.0 for a stable population than is the value calculated on the basis of only 2 years of field data (1.021 ± 0.093). However, the two estimates are not significantly different from each other or from a value of 1.0.Despite the similarity of estimates for λ, the life‐table analysis based on 5 years of data showed some marked differences from the earlier analysis. Revised growth and survival rates imply a slower progression to reproductive maturity and shorter reproductive life for A. cunninghamii.The predicted stable stage distribution(SSD) showed a better fit to the initial stage distribution (ISD), and bulges in the frequency distribution for large trees can be explained by the varying rates at which individuals grow from one stage to the next.The relative contributions of survival, growth and fecundity to the observed population growth rate (elasticity) remained similar for both analyses. Survival rate contributes most significantly to the achieved population growth rate, and this appears typical of long‐lived plant species. However, the pattern of elasticities across size‐classes changed for the new analysis and identified different critical life history stages from those recorded previously.Elasticity is proposed as a measure for classifying species using the competitive (C), stress‐tolerant (S) and ruderal (R) mode

ISSN:0307-692X    

DOI:10.1111/j.1442-9993.1991.tb01080.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractExtensive areas ofAcacia harpophylla(brigalow) woodlands have been cleared for pasture production in Queensland. The woody regrowth of A.harpophyllainfluences pasture production and composition following initial development of these woodlands.Biomass component regressions were developed using tree basal area as the predictor variable and used to estimate component yields of regrowing A.harpophyllacommunities. Leaf biomass ofA. harpophyllareached a maximum of 12 t ha‐1with a leaf area index of 2.5 when regrowing plants were 2.5 m high and about 10 years old. Pasture production, pasture basal area and the proportion of sown pasture species were lower at higher tree basal area of A.harpophylla.The greatest decrease was seen between 0 and 2 m2ha‐2tree basal area. Annual grasses and broadleaf herbaceous species represented a greater proportion of pasture biomass at high tree basal area.The poor control achieved by herbicide sprays on A.harpophyllaplants that are 2–2.5 m tall would be partly due to poor coverage of the herbicide spray, as leaf area index is at a maximum at this stage. A density of 1000 plants ha‐1is a threshold value above which regrowth control would be necessary at some stage to maintain acceptable pasture production and composition.The non‐linear regression relationship between pasture production and tree basal area was similar to that observed inEucalyptusspp. However, in the equation that relates pasture yield (Y) to tree basal area (X), Y = A + B*e(‐kX)the value of kforA. harpophyllacommunities was generally higher than observed inEucalyptusspp. communities with the same potential pasture production in the absence of trees. Within even‐aged stands, the use of tree leaf biomass to predict pasture production showed no advantage over the use of tree basal area as the predictor. However, there are advantages in using tree leaf biomass as a predictor when comparing communities with different size‐ or age

ISSN:0307-692X    

DOI:10.1111/j.1442-9993.1991.tb01081.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY