A Multi-Faceted Approach to Understand How Resource Diversity Can Mediate the Coexistence of Cryptic Marine Nematode Species

Based on the principle of competitive exclusion, species occupying the same ecological niche cannot stably coexist due to strong interspecific competition for resources. Niche diversification, for instance through resource partitioning, may alleviate competition. Here, we investigate the effects of resource diversity on foraging behavior, fitness and interspecific interactions of four cryptic bacterivorous nematode species (Pm I–IV) of the Litoditis marina species complex with sympatric field distributions. Three resource (bacteria) diversity levels (low, medium, high) were used as food treatments and compared to a treatment with only Escherichia coli as food. Differences in taxis to food existed between the cryptic species and between bacterial mixtures of different diversity: all the cryptic species except Pm I showed higher attraction toward medium-diversity food. Furthermore, the cryptic species of L. marina generally exhibited higher fitness on a more diverse food resource. Resource diversity also impacted the interspecific interactions between the cryptic species. Our results show that resource diversity can alter the interspecific interactions among the cryptic species of L. marina, indicating that competitive equilibria between species are very context-dependent. Although a considerable body of evidence supports the hypotheses (e.g., “variance-in-edibility” hypothesis and the “dilution hypothesis” or “resource concentration hypothesis”) which predict a negative impact on consumers when resource diversity is increased, the benefits of a diverse resource can outweigh these disadvantages by offering a more complete and/or complementary range of nutritional resources as suggested by the “balanced diet” hypothesis.

Habitat Structure, Resources and Natural Enemies: Their Influence on Population Fluctuations of the Kowhai Moth Uresiphita Polygonalis Maorialis (Felder)

<p><b>The importance of habitat structure has been historically discussed in terms of its influence on diversity, distribution and abundance of living organisms. In this regard, the population fluctuations of any particular species, particularly outbreaking insect species, can be expected to be profoundly influenced by the structure of the habitat. A set of ecological hypotheses, such as, the associational resistance, plant decoy, habitat heterogeneity and resource concentration have implicitly included the structure of the habitat determined by the structure (size, density, physical location) of the host plant and other surrounding plant species. Moreover, type, quality and availability of resources, in addition to the presence of other interacting organisms, e.g. competitors, predators and parasites, have also been considered determining factors in the population fluctuation of outbreaking species. The aim of this thesis is to contribute to the understanding of how the outbreaks of the kowhai moth, U. polygonalis maorialis, relate to the physical structure of the habitat, the availability of resources, specific host plants and to natural enemies.</b></p> <p>In the first experimental chapter of my thesis I studied the fluctuations of the U. polygonalis maorialis larvae and their impacts on the defoliation levels of Sophora spp. plants. I carried out a survey in urban and suburban areas of Wellington city. I examined levels of defoliation of the host plants and population fluctuations in terms of a set of biotic and abiotic variables. These variables were selected in order to cover a range of measures of habitat structure, resource availability and invertebrate community. I modelled such responses to find which variables better explained the observed defoliation and larval population fluctuations. The best fitted model showed that levels of observed defoliation were explained by the structure of the vegetation surrounding the main host plant (vertical and horizontal) and the species of host plant. Population fluctuations of the kowhai moth were explained by the following predicting variables: density of natural enemies, structure of the vegetation surrounding the main host plant (vertical and horizontal), host plant size, level of habitat disturbance, type of habitat (urban/suburban) and the Sophora spp.</p> <p>In my second experimental chapter, I focused on the importance of availability of resources to explain observed densities of U. polygonalis maorialis and phytophagous insects. In my observational experiment I tested the resource concentration hypothesis and the natural enemies hypothesis, by studying the fluctuations of U. polygonalis maorialis larvae on individuals of Sophora microphylla plants located in gardens across Wellington city. Larval densities were found to be higher on smaller plants than large plants, whereas natural enemies did not show specific responses to plant size. In my manipulative experiment I originally aimed for the establishment of U. polygonalis maorialis in the experimental plots. Unfortunately, these were not colonised by U. polygonalis maorialis, instead I studied phytophagous insects that colonised the plots. I found no differences among the S. microphylla treatments for the levels of establishment of phytophagous invertebrates. On the contrary, the amount of nil records was high and there was an overall high variability among treatments and low rate of establishment throughout the sampling season. Nevertheless, natural enemies were found to occur more often at higher densities in plots with lower plant density in only two specific dates.</p> <p>Uresiphita polygonalis maorialis is the main defoliator of Sophora spp in New Zealand. In this context I studied the feeding and oviposition preferences of the moth for the three most commonly found species of Sophora plants in Wellington city. Sophora tetraptera was the preferred species chosen by the female moth. The same species was also the most palatable and preferred when confronted to S. microphylla and S. prostrata. These patterns observed in controlled conditions are coincident with observations made in the field throughout the study.</p> <p>Within the set of variables determined by the invertebrate community, the influence of natural enemies on an herbivorous population is one of the most important in terms of population regulation. In my last experimental chapter I found a positive correlation among the parasitism by M. pulchricornis and U. polygonalis maorialis larval densities, which opens the possibilities for future research to explore the potential existence of population regulation mechanisms between these two taxa.</p> <p>Overall, the results of my thesis highlight the importance of understanding the influence of the structure of the habitat, types of resources provided by plants and natural enemies in determining the fluctuations of outbreaking insect species.</p>

Habitat Structure, Resources and Natural Enemies: Their Influence on Population Fluctuations of the Kowhai Moth Uresiphita Polygonalis Maorialis (Felder)

<p><b>The importance of habitat structure has been historically discussed in terms of its influence on diversity, distribution and abundance of living organisms. In this regard, the population fluctuations of any particular species, particularly outbreaking insect species, can be expected to be profoundly influenced by the structure of the habitat. A set of ecological hypotheses, such as, the associational resistance, plant decoy, habitat heterogeneity and resource concentration have implicitly included the structure of the habitat determined by the structure (size, density, physical location) of the host plant and other surrounding plant species. Moreover, type, quality and availability of resources, in addition to the presence of other interacting organisms, e.g. competitors, predators and parasites, have also been considered determining factors in the population fluctuation of outbreaking species. The aim of this thesis is to contribute to the understanding of how the outbreaks of the kowhai moth, U. polygonalis maorialis, relate to the physical structure of the habitat, the availability of resources, specific host plants and to natural enemies.</b></p> <p>In the first experimental chapter of my thesis I studied the fluctuations of the U. polygonalis maorialis larvae and their impacts on the defoliation levels of Sophora spp. plants. I carried out a survey in urban and suburban areas of Wellington city. I examined levels of defoliation of the host plants and population fluctuations in terms of a set of biotic and abiotic variables. These variables were selected in order to cover a range of measures of habitat structure, resource availability and invertebrate community. I modelled such responses to find which variables better explained the observed defoliation and larval population fluctuations. The best fitted model showed that levels of observed defoliation were explained by the structure of the vegetation surrounding the main host plant (vertical and horizontal) and the species of host plant. Population fluctuations of the kowhai moth were explained by the following predicting variables: density of natural enemies, structure of the vegetation surrounding the main host plant (vertical and horizontal), host plant size, level of habitat disturbance, type of habitat (urban/suburban) and the Sophora spp.</p> <p>In my second experimental chapter, I focused on the importance of availability of resources to explain observed densities of U. polygonalis maorialis and phytophagous insects. In my observational experiment I tested the resource concentration hypothesis and the natural enemies hypothesis, by studying the fluctuations of U. polygonalis maorialis larvae on individuals of Sophora microphylla plants located in gardens across Wellington city. Larval densities were found to be higher on smaller plants than large plants, whereas natural enemies did not show specific responses to plant size. In my manipulative experiment I originally aimed for the establishment of U. polygonalis maorialis in the experimental plots. Unfortunately, these were not colonised by U. polygonalis maorialis, instead I studied phytophagous insects that colonised the plots. I found no differences among the S. microphylla treatments for the levels of establishment of phytophagous invertebrates. On the contrary, the amount of nil records was high and there was an overall high variability among treatments and low rate of establishment throughout the sampling season. Nevertheless, natural enemies were found to occur more often at higher densities in plots with lower plant density in only two specific dates.</p> <p>Uresiphita polygonalis maorialis is the main defoliator of Sophora spp in New Zealand. In this context I studied the feeding and oviposition preferences of the moth for the three most commonly found species of Sophora plants in Wellington city. Sophora tetraptera was the preferred species chosen by the female moth. The same species was also the most palatable and preferred when confronted to S. microphylla and S. prostrata. These patterns observed in controlled conditions are coincident with observations made in the field throughout the study.</p> <p>Within the set of variables determined by the invertebrate community, the influence of natural enemies on an herbivorous population is one of the most important in terms of population regulation. In my last experimental chapter I found a positive correlation among the parasitism by M. pulchricornis and U. polygonalis maorialis larval densities, which opens the possibilities for future research to explore the potential existence of population regulation mechanisms between these two taxa.</p> <p>Overall, the results of my thesis highlight the importance of understanding the influence of the structure of the habitat, types of resources provided by plants and natural enemies in determining the fluctuations of outbreaking insect species.</p>

Simulation of the effects of movement patterns and resource density on the egg distributions of Pieris rapae (Lepidoptera) at multiple spatial scales.

<p>This thesis presents a spatially explicit, agent based simulation, used to explore the ovipositing behaviour of the Small Cabbage White butterfly, Pieris rapae (Lepidoptera). The study concerns the effects of host-plant (Cabbage, Brassica oleracae) density upon P. rapae egg distribution patterns, at multiple scales. A general review of the literature is provided which covers the ecology of animal movement, methods of quantifying movement, models of movement, ecological theory of herbivore responses to plant density (Resource Concentration Hypothesis) and the biology of P.rapae. The construction of the simulation is described in detail and the source code plus an executable version of the software are available as a companion CD. A number of simulation experiments are reported which demonstrate the basic behaviour of the simulation over a simplified resource layout. The framework is then used to explore more complex layouts which are compared to field experiments conducted as part of a separate PhD thesis (Hasenbank, in prep). A Correlated Random Walk simulated a negative relationship between forager egg distributions and resource densities, observed at all scales. Including a diffuse attraction to resources (e.g. olfaction), simulated a negative relationship between egg distributions and resource densities at smaller scales, and a positive relationship at larger scales. This work builds on a large body of previous simulation studies and attempts to produce a useful framework for subsequent researchers to explore the effects of animal movement through the use of random walks. It demonstrates the use of the framework with a specific example concerning the egg distributions of P. rapae and the effect of scale. It provides some useful insights into both the analysis of results from a complex spatial experimental layout, and potential responses which may be observed. It demonstrates that a simple model can, in the case of P rapae be used to obtain relatively realistic egg distributions.</p>

Simulation of the effects of movement patterns and resource density on the egg distributions of Pieris rapae (Lepidoptera) at multiple spatial scales.

<p>This thesis presents a spatially explicit, agent based simulation, used to explore the ovipositing behaviour of the Small Cabbage White butterfly, Pieris rapae (Lepidoptera). The study concerns the effects of host-plant (Cabbage, Brassica oleracae) density upon P. rapae egg distribution patterns, at multiple scales. A general review of the literature is provided which covers the ecology of animal movement, methods of quantifying movement, models of movement, ecological theory of herbivore responses to plant density (Resource Concentration Hypothesis) and the biology of P.rapae. The construction of the simulation is described in detail and the source code plus an executable version of the software are available as a companion CD. A number of simulation experiments are reported which demonstrate the basic behaviour of the simulation over a simplified resource layout. The framework is then used to explore more complex layouts which are compared to field experiments conducted as part of a separate PhD thesis (Hasenbank, in prep). A Correlated Random Walk simulated a negative relationship between forager egg distributions and resource densities, observed at all scales. Including a diffuse attraction to resources (e.g. olfaction), simulated a negative relationship between egg distributions and resource densities at smaller scales, and a positive relationship at larger scales. This work builds on a large body of previous simulation studies and attempts to produce a useful framework for subsequent researchers to explore the effects of animal movement through the use of random walks. It demonstrates the use of the framework with a specific example concerning the egg distributions of P. rapae and the effect of scale. It provides some useful insights into both the analysis of results from a complex spatial experimental layout, and potential responses which may be observed. It demonstrates that a simple model can, in the case of P rapae be used to obtain relatively realistic egg distributions.</p>

Latitude dictates plant diversity effects on instream decomposition

Running waters contribute substantially to global carbon fluxes through decomposition of terrestrial plant litter by aquatic microorganisms and detritivores. Diversity of this litter may influence instream decomposition globally in ways that are not yet understood. We investigated latitudinal differences in decomposition of litter mixtures of low and high functional diversity in 40 streams on 6 continents and spanning 113° of latitude. Despite important variability in our dataset, we found latitudinal differences in the effect of litter functional diversity on decomposition, which we explained as evolutionary adaptations of litter-consuming detritivores to resource availability. Specifically, a balanced diet effect appears to operate at lower latitudes versus a resource concentration effect at higher latitudes. The latitudinal pattern indicates that loss of plant functional diversity will have different consequences on carbon fluxes across the globe, with greater repercussions likely at low latitudes.

Were Neoarchean atmospheric methane hazes and early Paleoproterozoic glaciations driven by the rise of oxygen in surface environments?

Insofar as methane was the predominant greenhouse gas of the Archean and early Proterozoic eons, its wax and wane in Earth’s atmosphere would have contributed to climate change and the relative flux of harmful UV radiation to surface environments. If correct, understanding the first-order environmental controls (e.g., O2 or resource concentration) of the biological methane cycle might shed light on the repetition of biological, atmospheric and climatic events preserved in the sedimentary rock record between ~2.8 and 2.0 billion years ago. Environmental controls on the dynamics of methane cycling may further explain other repetitious events in deep time, as well as the present-day increase in the methane flux to the atmosphere from wetland environments. In this study, we developed an ecological interaction model to predict the conditions in which methane is preferentially released to the atmosphere, and found that the interplay of resource and O2 availability can cause complex cyclic patterns in methane dynamics that are unrelated to the size and efficiency of any of the microbial communities, to initial conditions, or to other model constraints. Based on these model results, we propose that the cyclicity of methane haze events and glacial episodes in the late Archean and early Proterozoic may have been linked to the progressive increase in oceanic and atmospheric O2 through the interval.

Effects of tributary size on the resource supply and physical habitat at tributary junctions along two mainstem rivers

Tributary junctions are regarded as ecologically important due to unique habitat present; however, there is limited understanding of the drivers of habitat attributes at these locations. Using six sites across two mainstem rivers, we tested whether tributary size relative to main stem governs the strength and direction of response of substrate size, stream temperature, and nutrient and coarse particulate organic matter (CPOM) concentration. We found that only phosphorus and CPOM concentration showed a significant relationship with relative tributary size. Small tributaries contributed high concentrations, whereas concentrations in larger tributaries resembled the main stem. Often, tributary exports were enough to increase the resource concentration in the main stem by 40%. Substrate coarsened by ∼60% downstream of tributaries. Temperature asynchrony was observed, where tributaries contributed water between 2.8 °C cooler to 1.9 °C warmer than the main stem within one diel period. Our results highlight the importance of small tributaries for whole network functioning. However, large spatiotemporal variability revealed how habitat attributes are highly context-dependent in these locations and may be difficult to predict in both scientific and management settings.