Valerie K. Brown

Insect herbivory below ground.

Publisher Summary Insect herbivores can attack both the vegetative and reproductive structures of plants in a variety of ways such as defoliating, sap feeding, stem boring, leaf mining, gall forming, flower and seed feeding, and root feeding. This chapter discusses insect herbivory below ground. The organisms feeding on below-ground plant structures are either different species, or different stages in the life-history of species, to those feeding above ground. Below-ground herbivory is mainly found in the rodents, nematodes, molluscs, and insects. Of these groups, herbivory by nematodes is most widely documented. Nematodes exhibit very similar adaptations to insects for life in the soil and there are parallels among them in terms of their effects on plant physiology and growth. The paucity of ecological studies of below-ground herbivory most likely stems from difficulties with sampling and taxonomy of the causal agents and in assessing the extent of damage. Below-ground herbivory can influence the full complement of parameters affected by the feeding activities of herbivores above ground.

Plant species diversity as a driver of early succession in abandoned fields: a multi-site approach

Abstract Succession is one of the most studied processes in ecology and succession theory provides strong predictability. However, few attempts have been made to influence the course of succession thereby testing the hypothesis that passing through one stage is essential before entering the next one. At each stage of succession ecosystem processes may be affected by the diversity of species present, but there is little empirical evidence showing that plant species diversity may affect succession. On ex-arable land, a major constraint of vegetation succession is the dominance of perennial early-successional (arable weed) species. Our aim was to change the initial vegetation succession by the direct sowing of later-successional plant species. The hypothesis was tested that a diverse plant species mixture would be more successful in weed suppression than species-poor mixtures. In order to provide a robust test including a wide range of environmental conditions and plant species, experiments were carried out at five sites across Europe. At each site, an identical experiment was set up, albeit that the plant species composition of the sown mixtures differed from site to site. Results of the 2-year study showed that diverse plant species mixtures were more effective at reducing the number of natural colonisers (mainly weeds from the seed bank) than the average low-diversity treatment. However, the effect of the low-diversity treatment depended on the composition of the species mixture. Thus, the effect of enhanced species diversity strongly depended on the species composition of the low-diversity treatments used for comparison. The effects of high-diversity plant species mixtures on weed suppression differed between sites. Low-productivity sites gave the weakest response to the diversity treatments. These differences among sites did not change the general pattern. The present results have implications for understanding biological invasions. It has been hypothesised that alien species are more likely to invade species-poor communities than communities with high diversity. However, our results show that the identity of the local species matters. This may explain, at least partly, controversial results of studies on the relation between local diversity and the probability of being invaded by aliens.

Vesicular-Arbuscular Mycorrhizal Fungi: A Determinant of Plant Community Structure in Early Succession

1. Infection of plants by vesicular-arbuscular mycorrhizas (VAM) was reduced by application of the contact fungicide iprodione in two early successional plant communities. 2. One community was initiated in 1988 and monitored for four years, the other started in 1990 and monitored for 18 months. 3. In both communities reduction in fungal infection resulted in a lower plant species richness, with a number of species occurring in untreated (control) plots but absent in fungicide-treated plots. These were mostly perennial forbs. 4. It is suggested that VAM fungi are important in structuring early successional communities and that this is brought about by enhanced seedling establishment of forbs when fungi are present

Differential Effects of Above- and Below-Ground Insect Herbivory during Early Plant Succession

The effects of aboveand below-ground insect herbivory on a natural plant community colonising bare ground were determined by manipulative field experiments. A foliar insecticide (Dimethoate-40) and a soil insecticide (Dursban 5G) were applied separately and in combination in a factorial experimental design over a two-year period. Characteristics of the developing vegetation were assessed in insecticidetreated and control plots. The insecticides had no independent effect on plant growth. Plant species richness and diversity were increased by the application of soil insecticide and, by the second season, depressed by foliar insecticide. Vegetation frequency and particularly cover abundance were enhanced by both compounds, with the soil insecticide having a greater effect in the second year. Vegetation height, assessed by a weighted mean height index, was only increased by the application of soil insecticide in the first year. Three major life-history groupings (annual and perennial herbs and perennial grasses) responded differently to herbivory. Annual herbs were promoted by foliar and soil insecticides in the first season, but only by the latter in the second. Within this general pattern, common species varied in response to the two compounds. Perennial grasses were strongly promoted by foliar insecticide and although few perennial herbs were present these increased in number of species and cover when below-ground herbivory was reduced. The different responses of life-history groupings and individual species to the two types of herbivory have a considerable effect on the pattern of early succession. The role of herbivory in relation to competition and seedling mortality in the developing plant community are discussed. The latter is relevant to the creation of species-rich communities of potential conservation value.

Effects of root herbivory by an insect on a foliar-feeding species, mediated through changes in the host plant

SummaryThe effects of root herbivory by larvae of the scarabaeid, Phyllopertha horticola, on the growth of Capsella bursa-pastoris were examined. Individuals of Aphis fabae were reared on the leaves to determine what effect, if any, root feeding has on the performance of this insect. The experiment was conducted under two watering regimes (‘low’ and ‘high’). Low watering and root feeding caused water stress in the plants and this was reflected in a reduction in vegetative biomass and an increase in the proportion of material allocated to reproduction. Supplying plants with ample water in the ‘high’ treatment enabled the water stress caused by root herbivory to be offset, but not completely overcome. Low watering and root feeding caused an increase in aphid weight and growth rate, while root feeding also increased fecundity and adult longevity. These effects are attributed to an improvement in food quality, measured by total soluble nitrogen, and caused by amino acid mobilization due to the water stress. The implications of these results in agricultural and ecological situations are discussed.

Effects of global changes on above- and belowground biodiversity in terrestrial ecosystems: implications for ecosystem functioning.

bove- and belowground organisms are criticalforthe biogeochemical cycles that sustain the Earth,butthere is limited knowledge on the extent to which the biotabelow ground and the functions they perform are dependenton the biota above ground,and vice versa.Hooper et al.(2000) provide a synthesis ofthe patterns and mechanismslinking above- and belowground biodiversity.The close re-lationship between vegetation change and soil carbon (C)dynamics (Jobbagy and Jackson 2000) suggests that anydisruption ofthe coupling between plants and soil organ-isms as a result ofglobal change may have deleterious con-sequences for functioning ofterrestrial ecosystems.However,most ofthe scientific evidence supporting this hypothesiscomes from correlative approaches.The complexity ofthenumerous interactions between various environmental

Secondary plant succession: how is it modified by insect herbivory?

The effects of foliar- and root-feeding insects on the dynamics of an early successional plant community, representing the first four years of colonisation, were examined. Subterranean insect herbivores were found to increase in density with increasing successional age of the plant community. In early succession, chewing insects mainly Coleoptera (Scarabaeidae) and Diptera (Tipulidae) were dominant. This was in direct contrast to the foliar-feeding insects, which were dominated by sap-feeders (mainly Auchenorrhynchan Hemiptera). Reduction of both foliar- and root-feeding insects with appropriate insecticides had different, but dramatic, consequences for the plant community. Reducing foliar herbivory resulted in large increases in perennial grass growth, with plant species richness being reduced as the grasses outcompeted the forbs. Reducing subterranean herbivory prolonged the persistence of annual forbs, greatly increased perennial forb colonisation and, as a consequence, plant species richness. Foliar-feeding insects thus act to delay succession by slowing grass colonisation. In contrast, root-feeding insects accelerate succession by reducing forb persistence and colonisation. The structure of early successional plant communities is therefore modified by the two modes of herbivory.

Effects of habitat fragmentation on Amazonian termite communities

Intuitively, termites would seem to be a very suitable group to illustrate effects of ecosystem fragmentation. Being detritivores, they do not control directly the rate at which their resources are available, nor do they restrict the ability of the resources to regenerate. Consequently, termites do not mask the ecosystem depletion caused by fragmentation. With this in mind, we compared the communities of termites in undisturbed Amazonian forest with those of two isolated fragments nearby, aiming to show that the differences observed may have resulted from habitat fragmentation. Dissimilarities between communities in the undisturbed forest suggest natural patchiness in their distribution, which could lead to misinterpretation of the effects of fragmenta- tion. Continuous forest had higher species richness and fewer rare species than the fragments. Guild structure in the forest was biased towards soil-feeding termites, which are subterranean and soft bodied, and therefore more sensitive to variation in microclimate. In the fragments, litter- feeders and species intermediate between soil-feeding and wood-feeding types were numerically more important. Habitats in the forest were more equally used than in the fragments, suggesting habitat unsuitability increased with fragmentation. It is suggested that the community composition of the fragments is a result of the intrinsic patchiness of the original forest and deterministic and stochastic extinctions caused by fragmentation. The need for, manipulative experiments to test such ideas is discussed. RESUMO: Cupins sao intuitivamente bastante adequados para o estudo da fragmentasao de ecossistemas. Por serem detritivoros, eles nio controlam diretamente a taxa sob a qual seus recursos tornam-se disponiveis, nem prejudicam a capacidade de regeneracao de tais recursos. Portanto, cupins nao mascaram a depauperasao do ecossistema causada pela fragmentacao. Mantendo tal considerasao em mente, comparamos as comunidades de cupins em floresta Amazonica na- perturbada com aquelas de duas reservas isoladas vizinhas, objetivando mostrar que as diferencas observadas poderiam ter sido causadas pela fragmentasao do habitat. Dissimilaridades entre com- unidades dentro da floresta sugerem uma variegasao natural do ambiente, a qual poderia causar erros na interpretasao dos efeitos da fragmentacao. A floresta continua teve maior riqueza em especies e menor proporsao de especies raras do que os fragmentos. Cupins ge6fagos - subterraneos e de corpo pouco esclerotizado, portmanto mais sensiveis a variacoes microclimaticas - dominaram a guilda da floresta. Nos fragmentos, os comedores de serrapilheira e os de habito alimentar intermediario entre geofagia e xilofagia foram numericamente mais importantes. Os habitats na floresta foram usados mais equitativamente que nos fragmentos, o que sugere uma inadequabilid- ade de habitat crescente com a fragmentacao. Sugere-se que a composisao das comunidades dos fragments e resultado da variegacao intrinseca da floresta e de extincoes causadas pela frag- mentasao. E discutida a necessidade de experimentos manipulativos para se testar tais ideias.

Seed Size of Woody Plants in Relation to Disturbance, Dispersal, Soil Type in Wet Neotropical Forests

Fresh and dry masses of seeds of woody species collected at Mabura Hill, Guyana were evaluated according to dispersal mode, gap size preference, and soil type. Data published for woody species from Manu, Peru and Barro Colorado Island (BCI), Panama were analyzed by dispersal mode and gap size preference for comparison. Larger seeds at the three sites were significantly associated with canopy or emergent trees or lianas that are dispersed by mammals or gravity and recruit best in small gaps. Variation in seed size, after accounting for the effects of plant height and life-form, could be explained by dispersal mode (Mabura Hill : 22%, Manu : 14%, BCI : 26%) and gap size preference (Mabura Hill : 9%, Manu : 7%, BCI : 11%). Soil type preference, analyzed only for Mabura Hill, was not associated with seed size. Mean seed size at Mabura Hill was significantly larger than at Manu or BCI. Short-term successional processes may explain the variation of seed size within sites ; dispersal mode is particularly important in defining seed size selection during the successional process. However, short-term successional processes cannot explain the variation of seed size among sites. It is suggested that this variation may be attributed to differences in the effects of larger-scale variations in soils and disturbance on seed size among the three sites.

Insect herbivores and plant succession

Insect-plant interactions at the community level are discussed in terms of the life-cycle strategies of the insects at different stages in plant succession and the effects of early colonising insect herbivores on the development of the plant community. Insect life-cycle strategies are influenced by habitat permanence, habitat complexity and resource availability and diversity. These factors increase as plant succession proceeds. The life-cycle strategies of insects also change during plant succession with differences in generation time, migratory ability, overwintering stage and reproductive potential. The degree of niche specialisation and the size of the organisms also vary. The characteristic life-cycle strategies of an early colonising species are discussed and the impact of phytophagous species on early successional plant communities found to be considerable. Single species studies (e.g. Raphanus raphanistrum) enable this effect to be assessed more fully. Insect grazing affects the survival, growth and reproductive potential of a plant species. At a community level insect grazing has an effect comparable to that of vertebrates, namely a reduction in the rate of plant succession.