Parasites and Communities; Empirical and Theoretical Scaling

寄生虫引起生态学家的兴趣是因为，通过使动物生病，它们改变了这些动物与自然群落中其他物种的互动方式。我们研究了两种本地虾和入侵虾的寄生虫，发现寄生虫改变了一种虾捕食另一种虾的频率（这个过程被称为野生捕食），以及它们选择捕食的其他类型的猎物。此外，虾可能会对被捕食者（鱼）吃掉的威胁做出反应，变得不那么活跃，因此可能不太可能吃其他动物。现在的挑战是研究这些影响对更广泛的群落的影响：寄生虫和捕食者如何影响我们在群落中发现的物种，以及它们有多常见。我们将使用实验室和实地实验，以及数学来研究这个问题：首先，建立一幅寄生虫和捕食者如何影响两到三个物种之间相互作用的图景；其次，研究这些影响如何改变群落中其他物种之间的关系。1. 我们将研究寄生虫如何影响捕食，例如，使用实验室实验来测量在存在和不存在捕食者线索的情况下，被感染的虾与未感染的虾的功能反应（捕食者对猎物的消耗率与猎物密度之间的关系）。这将帮助我们建立数学模型，其中寄生虫改变宿主的功能反应，这样我们就可以研究行为改变如何影响种群动态（种群规模随时间的变化）。2. 我们将使用稳定同位素分析（通过测量化学变化来告诉我们动物的饮食）来分析天然虾种群，看看寄生虫是否会导致宿主饮食的改变。我们还将使用实验室实验来研究这个想法。我们将建立一个数学模型，将寄生、捕食和种群内捕食的相互作用联系在一起，并利用这个模型来研究相关物种的种群动态，并观察我们期望在虾群落中发现的寄生虫引起的功能反应和饮食变化是否可以改变群落结构。我们的发现将对其他关注寄生虫对物种之间相互作用的影响以及它们对更广泛的群落和环境的影响的科学家感兴趣。它也会引起管理自然栖息地、多样性和入侵的机构的兴趣（环境署；北爱尔兰环境署，农业和农村事务部，北爱尔兰）。由于许多原因，研究寄生虫很重要。首先，由于它们通常是群落的“隐藏”成员，并且不经常杀死它们的宿主，与物种之间的其他相互作用（如捕食和竞争）相比，它们的影响被忽视了。然而，它们可以对群落产生巨大的影响，改变竞争物种之间的平衡，有时允许两个物种共存，如果没有寄生虫，一个物种就会灭绝（这使得寄生虫成为“关键物种”）。其次，寄生虫经常参与外来物种进入新环境的生物入侵；在许多情况下，入侵物种造成的破坏更大，因为它没有被该环境中的寄生虫严重感染，或者它引入了导致本地物种生病/死亡的新寄生虫。事实上，在我们的研究系统中，其中一只虾是入侵物种，我们渴望了解推动其成功的力量，以便我们更好地保护本地物种。第三，寄生虫具有重要的公共卫生和经济后果：突发疾病（跨越物种边界感染新物种的寄生虫）对人类健康、家畜和野生动物构成越来越大的威胁。该项目将通过提高我们对自然群落中寄生虫影响的了解，帮助理解这些重要问题。

Parasites interest ecologists because, by making animals ill, they alter how those animals interact with other species in natural communities. We have studied the parasites of two species of native and invading shrimps and found that parasites alter how often the shrimp of one species eat the other shrimp species (a process called intraguild predation), and the other types of prey that they select to eat. In addition, shrimp may respond to the threat of being eaten by predators (fish) by becoming less active and so potentially less likely to eat other animals. The challenge now is to study the consequences of these effects for the wider community: how parasites and predators influence which species we find in the community and how common they are. We will use lab and field experiments, and mathematics to look at this problem, to: firstly, build up a picture of how parasites and predators affect interactions between two or three species; secondly, examine how these effects alter the relationships between other species in the community. 1. We will examine how parasites affect predation, for example using laboratory experiments to measure the functional response (the relationship between a predator's consumption rate of prey and the density of the prey) for infected shrimp compared with uninfected shrimp in the presence and absence of predator cues. This will help us build mathematical models in which parasites alter their hosts' functional responses so that we can investigate how behaviour modification affects population dynamics (changes in population size over time). 2. We will use stable isotope analysis (which measures chemical variation to tell us about an animal's diet) of natural shrimp populations to see whether parasites cause a shift in host diets. We will also investigate this idea using laboratory experiments. We will build a mathematical model that links together the interactions of parasitism, predation, and intraguild predation, and use this to investigate the population dynamics of the species involved and see whether the parasite-induced changes in functional responses and diet that we expect to find in the shrimp community can change community structure. Our findings will be of interest to other scientists concerned with the impact of parasites on interactions between species, and their impact in the broader community and environment. It will also interest agencies who manage natural habitats, diversity and invasions (Environment Agency; N. Ireland Environment Agency, Department of Agriculture and Rural Affairs, N. Ireland). It is important to study parasites for many reasons. Firstly, as they are often 'hidden' members of the community, and do not often kill their hosts, their effects have been neglected compared to other interactions between species (such as predation and competition). Nevertheless, they can have dramatic impacts on communities, altering the balance between competing species, sometimes allowing two species to coexist where one would go extinct if parasites were not present (this makes the parasite a 'keystone species'). Secondly, parasites are often involved in biological invasions by foreign species into a new environment; in many cases an invading species does more damage because it is not as badly infected by the parasites in that environment, or it introduces new parasites that cause illness/death of the native species. Indeed, in our study system, one of the shrimps is an invading species and we are keen to understand the forces driving its success so that we can better protect the native species. Thirdly, parasites have important public health and economic consequences: emergent diseases (parasites that cross the species boundary to infect a new species) are an increasing threat to human health, domestic animals and wildlife. This project will help to understand these important problems by improving what we know about the effects of parasites in natural communities.