Karen I. Bonner

Biodiversity and plant litter : experimental evidence which does not support the view that enhanced species richness improves ecosystem function

There has been a rapidly increasing recent interest in the effects of biological diversity on ecosystem properties, and while some studies have recently concluded that biodiversity improves ecosystem function, these views are based almost entirely on experiments in which species richness of live plants has been varied over all the species diversity treatments. However, most net ecosystem primary productivity eventually enters the decomposition subsystem as plant litter where it has important afterlife effects. Weconducted a field experiment in which litter from 32 plant species (i.e. effects. We conducted a field experiment in which litter from 32 plant species (i.e. eight species of each of four plant functional groups with contrasting litter quality) was collected and placed into litter-bags so that each litter-bag contained between one and eight species; the species which were included in the multiple (>2) species litter-bags were randomly selected. This litter diversity gradient was created within each functional group and across some functional groups. We found large non-additive effects of mixing litter from different species on litter decomposition rates, litter nitrogen contents, rates of nitrogen release from litter and the active microbial biomass present on the litter. The patterns and directions of these non-additive effects were dependent upon both plant functional group and time of harvest, and these effects could be predicted in some instances by the initial litter nitrogen content and the degree of variability of nitrogen content of the component species in the litter-bag. There was no relationship between litter-bag species richness and any of the response variables that we considered, at least between two and eight species. Within plant functional groups our results provide some support for the species redundancy and idiosyncratic hypotheses about how biodiversity alters ecosystem function, but no support for the ecosystem rivet hypothesis or the view that species richness of plant litter is important for ecosystem function. We suggest that increased species diversity of plant litter is less important than that of live plants for determining ecosystem properties (and provide possible reasons for this) and conclude that perceived relationships between biodiversity and ecosystem function may be of diminished significance when the ecological importance of plant litter is fully appreciated.

PLANT REMOVALS IN PERENNIAL GRASSLAND: VEGETATION DYNAMICS, DECOMPOSERS, SOIL BIODIVERSITY, AND ECOSYSTEM PROPERTIES

The consequences of permanent loss of species or species groups from plant communities are poorly understood, although there is increasing evidence that individual species effects are important in modifying ecosystem properties. We conducted a field experiment in a New Zealand perennial grassland ecosystem, creating artificial vegetation gaps and imposing manipulation treatments on the reestablishing vegetation. Treatments consisted of continual removal of different subsets or “functional groups” of the flora. We monitored vegetation and soil biotic and chemical properties over a 3-yr period. Plant competitive effects were clear: removal of the C3 grass Lolium perenne L. enhanced vegetative cover, biomass, and species richness of both the C4 grass and dicotyledonous weed functional groups and had either positive or negative effects on the legume Trifolium repens L., depending on season. Treatments significantly affected total plant cover and biomass; in particular, C4 grass removal reduced total plant biomass in summer, because no other species had appropriate phenology. Removal of C3 grasses reduced total root biomass and drastically enhanced overall shoot-to-root biomass ratios. Aboveground net primary productivity (NPP) was not strongly affected by any treatment, indicating strong compensatory effects between different functional components of the flora. Removing all plants often negatively affected three further trophic levels of the decomposer functional food web: microflora, microbe-feeding nematodes, and predaceous nematodes. However, as long as plants were present, we did not find strong effects of removal treatments, NPP, or plant biomass on these trophic groupings, which instead were most closely related to spatial variation in soil chemical properties across all trophic levels, soil N in particular. Larger decomposer organisms, i.e., Collembola and earthworms, were unresponsive to any factor other than removal of all plants, which reduced their populations. We also considered five functional components of the soil biota at finer taxonomic levels: three decomposer components (microflora, microbe-feeding nematodes, predaceous nematodes) and two herbivore groups (nematodes and arthropods). Taxa within these five groups responded to removal treatments, indicating that plant community composition has multitrophic effects at higher levels of taxonomic resolution. The principal ordination axes summarizing community-level data for different trophic groups in the soil food web were related to each other in several instances, but the plant ordination axes were only significantly related to those of the soil microfloral community. There were time lag effects, with ordination axes of soil-associated herbivorous arthropods and microbial-feeding nematodes being related to ordination axes representing plant community structure at earlier measurement dates. Taxonomic diversity of some soil organism groups was linked to plant removals or to plant diversity. For herbivorous arthropods, removal of C4 grasses enhanced diversity; there were negative correlations between plant and arthropod diversity, presumably because of negative influences of C4 species in the most diverse treatments. There was evidence of lag relationships between diversity of plants and that of the three decomposer groups, indicating multitrophic effects of altering plant diversity. Relatively small effects of plant removal on the decomposer food web were also apparent in soil processes regulated by this food web. Decomposition rates of substrates added to soils showed no relationship with treatment, and rates of CO2 evolution from the soil were only adversely affected when all plants were removed. Few plant functional-group effects on soil nutrient dynamics were identified. Although some treatments affected temporal variability (and thus stability) of soil biotic properties (particularly CO2 release) throughout the experiment, there was no evidence of destabilizing effects of plant removals. Our data provide evidence that permanent exclusion of plant species from the species pool can have important consequences for overall vegetation composition in addition to the direct effects of vegetation removal, and various potential effects on both the above- and belowground subsystems. The nature of many of these effects is driven by which plant species are lost from the system, which depends on the various attributes or traits of these species.

INTRODUCED BROWSING MAMMALS IN NEW ZEALAND NATURAL FORESTS: ABOVEGROUND AND BELOWGROUND CONSEQUENCES

Forest dwelling browsing mammals, notably feral goats and deer, have been introduced to New Zealand over the past 220 years; prior to this such mammals were absent from New Zealand. The New Zealand forested landscape, therefore, presents an almost unique opportunity to determine the impacts of introduction of an entire functional group of alien animals to a habitat from which that group was previously absent. We sampled 30 long-term fenced exclosure plots in indigenous forests throughout New Zealand to evaluate community- and ecosystem-level impacts of introduced browsing mammals, emphasizing the decomposer subsystem. Browsing mammals often significantly altered plant community composition, reducing palatable broad-leaved species and promoting other less palatable types. Vegetation density in the browse layer was also usually reduced. Although there were some small but statis- tically significant effects of browsing on some measures of soil quality across the 30 locations, there were no consistent effects on components of the soil microfood web (com- prising microflora and nematodes, and spanning three consumer trophic levels); while there were clear multitrophic effects of browsing on this food web for several locations, com- parable numbers of locations showed stimulation and inhibition of biomasses or populations of food web components. In contrast, all microarthropod and macrofaunal groups were consistently adversely affected by browsing, irrespective of trophic position. Across the 30 locations, the magnitude of response of the dominant soil biotic groups to browsing mammals (and hence their resistance to browsers) was not correlated with the magnitude of vegetation response to browsing but was often strongly related to a range of other variables, including macroclimatic, soil nutrient, and tree stand properties. There were often strong significant effects of browsing mammals on species composition of the plant community, species composition of leaf litter in the litter layer, and composition of various litter-dwelling faunal groups. Across the 30 locations, the magnitude of browsing mammal effects on faunal community composition was often correlated with browser effects on litter layer leaf species composition but never with browser effects on plant community composition. Browsing mammals usually reduced browse layer plant diversity and often also altered habitat diversity in the litter layer and diversity of various soil faunal groups. Across the 30 locations, the magnitude of browser effects on diversity of only one faunal group, humus-dwelling nematodes, was correlated with browser effects on plant diversity. However, browser effects on diversity of diplopods and gastropods were correlated with browser effects on habitat diversity of the litter layer. Reasons for the lack of unidirectional relationships across locations between effects of browsers on vegetation community attri- butes and on soil invertebrate community attributes are discussed. Browsing mammals generally did not have strong effects on C mineralization but did significantly influence soil C and N storage on an areal basis for several locations. However the direction of these effects was idiosyncratic and presumably reflects different mechanisms by which browsers affect soil processes. While our study did not support hypotheses predicting consistent negative effects of browsing mammals on the decomposer subsystem through promotion of plant species with poorer litter quality, our results still show that the introduction of these mammals to New Zealand has caused far-ranging effects at both the community and ecosystem levels of resolution, with particularly adverse effects for indigenous plant com- munities and populations of most groups of litter-dwelling mesofauna and macrofauna.

Response of soil microbial biomass dynamics, activity and plant litter decomposition to agricultural intensification over a seven-year period

Soil microorganisms and the processes that they govern are essential for long-term sustainability of agricultural systems, but most studies on agricultural effects on the soil microflora are inherently short-term. We investigated the effects of three aspects of agricultural intensification, i.e. cultivation (disturbance), herbicide addition (modification of plant composition) and mulching (resource addition) on soil biological properties such as microbial biomass and activity over 7 yr in annual (maize) and perennial (asparagus) cropping systems. The mulching treatment had strong, usually positive effects on both substrate-induced respiration (SIR) and CO2–C release from chloroform-fumigated soil throughout the study. In the perennial crop, treatments allowing high weed biomass caused large increases in microbial biomass and respiration after yr 3, and in both sites microbial biomass was positively correlated with weed biomass and negatively with crop plant biomass. This latter effect appears due to the high decomposability of weed residues relative to those from crop plants. Microbial biomass was also enhanced in atrazine-treated plots in the annual crop but only during the final year, presumably due to beneficial effects of plot invasion by herbicide-tolerant weeds. Mulching often also enhanced the microbial metabolic quotient (qCO2), the bacteria-to-fungal biomass ratio and within-year temporal variability of the microbial biomass, all of which are indicative of greater turnover and instability of the microbial biomass. Other treatments generally had smaller effects on these properties, although in the perennial crop an intense summer drought in yr 4 caused a large elevation in the metabolic quotient in the herbicide-treated (low weed) plots relative to the other plots, suggesting that high quality weed residues have stabilising effects. Temporal variability across years of both SIR and CO2–C release from fumigated soil was greatest in the herbicide-treated plots in the perennial crop, suggesting that high weed biomass (producing easily degradable organic matter) has stabilising effects. Decomposition rates of added litter were partially consistent with the microbial biomass data, with the highest litter breakdown rates usually occurring in the mulched plots. Our study shows that soil biological properties such as microbial biomass and activity are not necessarily adversely affected by agricultural intensification and that consequences of intensification mainly depend upon practices which alter the quality and quantity of residue inputs. Further, our results underline the need for long,-term field experiments, and several of the effects we identified could only have been detected through an experiment of several years duration.

Effects of agricultural intensification on soil-associated arthropod population dynamics, community structure, diversity and temporal variability over a seven-year period

Abstract While most studies focusing on the effects of agricultural intensification on soil biota are inherently short-term in nature, long-term (multiyear) studies are essential in assessing long-term temporal responses of soil biota to agronomic practices. We investigated the effects of three components of agricultural intensification, i.e. cultivation (disturbance), herbicide addition (modification of floristic composition) and mulching (resource addition) on soil-associated arthropods in an annual (maize) and a perennial (asparagus) cropping system over a 7 yr period. An additional treatment (hand-hoeing of weeds during the crop growing season) was used to represent minimal intensification. Many taxa of arthropods responded positively to mulching and to treatments which allowed high weed biomass in the non crop-production period, e.g. the hand-hoeing and cultivation treatments in the perennial crop. Herbicide treatments also facilitated high numbers of many taxa in the annual crop when this coincided with plot invasion by herbicide-tolerant weeds. Generally, arthropod taxa were positively correlated with weed biomass and negatively with crop plant biomass, probably because of the superior resource (litter) quality produced by the former. Ordination analyses indicated that arthropod community structure was often correlated with weed community structure. Mulching and allowing high weed biomass also promoted a high species richness of soil-associated Coleoptera, but coleopteran diversity was not related to weed species diversity. Analyses of temporal variability (inversely related to stability) of arthropod taxa across years revealed few treatment effects in the annual crop, but showed destabilising effects of weed reduction in the perennial crop. In the perennial crop, temporal variability was also positively correlated with crop biomass and negatively with weed biomass across plots. Our study shows that agricultural intensification is not consistently harmful to the soil fauna, that soil-associated arthropods are most responsive to management practices which affect the nature and quality of resource input, and that long-term experiments are essential for answering questions about how agricultural practices affect soil organisms against the natural backdrop of temporal variation.

Direct and indirect effects of rats: does rat eradication restore ecosystem functioning of New Zealand seabird islands?

Introduced rats (Rattus spp.) can affect island vegetation structure and ecosystem functioning, both directly and indirectly (through the reduction of seabird populations). The extent to which structure and function of islands where rats have been eradicated will converge on uninvaded islands remains unclear. We compared three groups of islands in New Zealand: islands never invaded by rats, islands with rats, and islands on which rats have been controlled. Differences between island groups in soil and leaf chemistry and leaf production were largely explained by burrow densities. Community structure of woody seedlings differed by rat history and burrow density. Plots on islands with high seabird densities had the most non-native plant species. Since most impacts of rats were mediated through seabird density, the removal of rats without seabird recolonization is unlikely to result in a reversal of these processes. Even if seabirds return, a novel plant community may emerge.

Response of soil food-web structure to defoliation of different plant species combinations in an experimental grassland community

We established a greenhouse experiment based on replicated mini-ecosystems to evaluate the effects of defoliation of different plant species combinations on soil food-web structure in grasslands. Plant communities, composed of white clover (Trifolium repens), perennial ryegrass (Lolium perenne) and plantain (Plantago lanceolata), were subjected to the following defoliation treatments: no defoliation of any species (control) and selective trimming of all possible one-, two- and three-way combinations of the species either to 27 cm height (weak defoliation) or to 15 cm height (strong defoliation) above the soil surface three times over a 10-week period. Successive defoliations removed the largest amounts of shoot mass from systems in which T. repens was included among the defoliated species because T. repens dominated aboveground plant biomass. At the final harvest shoot mass was lowest in treatments that included defoliation of T. repens, while total root mass was on average lower in strongly than in weakly defoliated systems and did not differ between the control and defoliation treatments. Total shoot production was not affected by defoliation. Microbial basal respiration and soil NO3-N concentration differed between the combinations of defoliated species; e.g. microbial respiration was on average 32% lower in systems in which only L. perenne was defoliated than in systems in which only T. repens was defoliated. Microbial biomass and soil NH4‐N concentration were not significantly affected by defoliation treatments. Enchytraeid abundance differed significantly between the combinations of defoliated species: in systems in which only L. perenne was defoliated enchytraeid abundance was on average 88% lower than in systems in which all species or only T. repens were defoliated. Enchytraeid abundance was also positively associated with total defoliated shoot mass. Abundances of both bacterial-feeding and fungal-feeding nematodes were affected by the combination of defoliated species; e.g. the abundance of bacterial feeders was on average 52% lower in systems in which only T. repens was defoliated than in systems in which both P. lanceolata and T. repens were defoliated. Fungal-feeding nematodes were also more numerous in strongly than in weakly defoliated systems and positively associated with total defoliated shoot mass. Herbivorous nematode abundance was not significantly affected by defoliation treatments. The results show that the response of soil food webs to defoliation can be affected by which combination of species in a plant community is defoliated. Further, it seems that the role of the combination of species that are defoliated may for some components of the soil biota (e.g. fungal-feeding nematodes) be explicable simply in terms of the total mass of foliage removed. However, for other components of the soil biota (e.g. bacterial-feeding nematodes and enchytraeids) species-specific properties of different plant species in the combination of defoliated species are also clearly important, over and above simple mass removal effects of defoliation. q 2001 Elsevier Science Ltd. All rights reserved.

Plasticity in above‐ and belowground resource acquisition traits in response to single and multiple environmental factors in three tree species

Functional trait plasticity is a major component of plant adjustment to environmental stresses. Here, we explore how multiple local environmental gradients in resources required by plants (light, water, and nutrients) and soil disturbance together influence the direction and amplitude of intraspecific changes in leaf and fine root traits that facilitate capture of these resources. We measured population-level analogous above- and belowground traits related to resource acquisition, i.e. “specific leaf area”–“specific root length” (SLA–SRL), and leaf and root N, P, and dry matter content (DMC), on three dominant understory tree species with contrasting carbon and nutrient economics across 15 plots in a temperate forest influenced by burrowing seabirds. We observed similar responses of the three species to the same single environmental influences, but partially species-specific responses to combinations of influences. The strength of intraspecific above- and belowground trait responses appeared unrelated to species resource acquisition strategy. Finally, most analogous leaf and root traits (SLA vs. SRL, and leaf versus root P and DMC) were controlled by contrasting environmental influences. The decoupled responses of above- and belowground traits to these multiple environmental factors together with partially species-specific adjustments suggest complex responses of plant communities to environmental changes, and potentially contrasting feedbacks of plant traits with ecosystem properties. We demonstrate that despite the growing evidence for broadly consistent resource-acquisition strategies at the whole plant level among species, plants also show partially decoupled, finely tuned strategies between above- and belowground parts at the intraspecific level in response to their environment. This decoupling within species suggests a need for many species-centred ecological theories on how plants respond to their environments (e.g. competitive/stress-tolerant/ruderal and response-effect trait frameworks) to be adapted to account for distinct plant-environment interactions among distinct individuals of the same species and parts of the same individual.

Belowground legacies of Pinus contorta invasion and removal result in multiple mechanisms of invasional meltdown

Invasive plants alter plant communities and transform landscapes aboveground, but also have strong belowground effects that are potentially even more important to ecosystem outcomes. Using management treatments of the widespread invasive tree, Lodgepole Pine, we find that pines and pine removal transform belowground ecosystems, increasing ectomycorrhizal inoculum and driving a change from slow-cycling fungal-dominated soils to fast-cycling bacterial-dominated soils with increased nutrient availability. This results in increased growth of graminoids, particularly exotic grasses, and facilitation of Douglas-fir establishment, hindering ecosystem restoration. The results highlight the importance of considering multiple species interactions in invasion, particularly in terms of belowground legacies.

A megafauna's microfauna: gastrointestinal parasites of New Zealand's extinct moa (Aves: Dinornithiformes).

We perform the first multidisciplinary study of parasites from an extinct megafaunal clade using coprolites from the New Zealand moa (Aves: Dinornithiformes). Ancient DNA and microscopic analyses of 84 coprolites deposited by four moa species (South Island giant moa, Dinornis robustus; little bush moa, Anomalopteryx didiformis; heavy-footed moa, Pachyornis elephantopus; and upland moa, Megalapteryx didinus) reveal an array of gastrointestinal parasites including coccidians (Cryptosporidium and members of the suborder Eimeriorina), nematodes (Heterakoidea, Trichostrongylidae, Trichinellidae) and a trematode (Echinostomida). Parasite eggs were most prevalent and diverse in coprolites from lowland sites, where multiple sympatric moa species occurred and host density was therefore probably higher. Morphological and phylogenetic evidence supports a possible vicariant Gondwanan origin for some of the moa parasites. The discovery of apparently host-specific parasite taxa suggests paleoparasitological studies of megafauna coprolites may provide useful case-studies of coextinction.