1. Oviposition substrata are a crucial resource for many stream-dwelling insects and may have a strong effect on population densities. 2. From February to March 2010, we conducted a large-scale experiment manipulating the density of oviposition substrata available to two taxa, Cheumatopsyche spp. and Ecnomus spp. in Hughes Creek (south-east Australia). These caddisflies oviposit on hard surfaces underwater. Hughes Creek has a sandy bed, and females rely predominantly on bark and wood as oviposition substrata. 3. Bark density was manipulated in 25-m-long sites, creating a range of bark surface areas (SAs) spanning an order of magnitude, with appropriate controls. Estimates of the number of egg masses and bark SAs were obtained 22 and 55 days after the experiment commenced in each site. To characterise taxon-specific oviposition site preferences and test whether preferences were dependent on overall bark availability, the conditions (velocity and emergence) of individual substrata were manipulated within sites. Egg masses were enumerated at approximately weekly intervals for 7 weeks. 4. We found a strong relationship between the SA of bark at sites and the number of masses of Ecnomus spp., demonstrating that oviposition is limited by substrata availability. In contrast, the number of Cheumatopsyche spp. egg masses was not related to the total amount of bark available. Ecnomus spp. showed a weak preference for emergent compared to submerged bark, but did not respond to water velocity. Cheumatopsyche laid eggs predominantly in fast flows, regardless of bark emergence, which may explain the lack of a site-level response because only a small proportion of bark occurred in fast flows even when bark was plentiful. 5. Our results suggest that the strength of oviposition preferences may depend on overall background densities of substrata. Oviposition preferences and substrata availability interact to set the initial distribution (and density) of the next generation. Our study has wide implications for the management of streams with soft sediment beds, where bark and wood inputs from riparian vegetation provide not only food and habitat for larvae but also oviposition substrata that are critical to successful recruitment for some taxa.
Environmental watering is frequently used to achieve specific ecological objectives, such as triggering spawning or seed germination. These short-term objectives are often met, but longer-term objectives, such as population growth, may not be, especially where multiple hydrological and non-hydrological factors influence success. We propose a framework to identify these factors in space and time. Our framework steps users through identifying possible inhibiting (strictures) and supporting (promoters) factors, and placing these factors in their spatial and temporal context. This allows users to identify potential limiting factors that may require additional intervention, or render the original watering action unsustainable. We illustrate the framework with examples of a floodplain tree (black box, Eucalyptus largiflorens), colonial nesting waterbird (royal spoonbill, Platalea regia) and large-bodied migratory fish (golden perch, Macquaria ambigua). The framework explores strictures and promoters for major life-history stages, emphasising the need to support and protect all stages if objectives include population maintenance or growth. In this way, the framework can document existing mental models and can be used as the basis of a risk portfolio, a prioritisation tool or future quantitative models. Thus, the framework enables individual management actions to be better grounded in a broader context, increasing the likelihood of achieving long-term ecological objectives.
Disease can be a powerful driver of population and community dynamics, as well as evolutionary processes. Disease is also emerging at increasing rates, resulting in massive impacts on populations, communities, and ecosystems. However, assessing these impacts requires foundational knowledge of disease agents and hosts, which is often lacking, particularly in aquatic insects. We describe a recent disease outbreak in caddisflies, suggesting potential consequences for population and community dynamics of the host. We use a series of complementary studies to develop a cohesive foundation of information about this disease, including identification using genomic methods, observational prevalence studies, laboratory experiments to establish transmissibility and fitness consequences, and laboratory and field investigations to infer transmission mechanisms. We identified the infection as being caused by the oomycete Saprolegnia—the first time this parasite has been noted in insect eggs. Prevalence surveys found high prevalence (up to 36%) with variation across space, time, and host species. We demonstrated increasing egg mortality with increasing infection within an egg mass (every 10% increase in infection rate doubles odds of mortality), thereby confirming disease. We established transmissibility and show that transmission occurs through both direct contact with infected egg masses and from background sources, which probably interact to create complex patterns of disease. Taken together, our findings show that conditions necessary for population and community consequences are present. Specifically, increased mortality rates almost certainly occurred during the outbreak, yielding lower larval numbers and potentially altering community interactions. Transmission by contact between egg masses combined with observed species‐specific prevalence suggest shifts in the relative performance of different species because of interactions between host and parasite life histories. Outside extreme examples such as chytrid fungus, disease has traditionally received less interest than resource competition or predation in community ecology, although disease ecology is advancing rapidly. One major hurdle is a lack of foundational knowledge characterising disease processes in natural communities, particularly in aquatic insects. Our findings highlight the importance of investigating diseases in insect eggs and provide the foundation for further investigations of how these processes play out at the population and community scale.
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