1. Body size differences among consumers often lead to asymmetric interactions, with larger individuals typically being stronger competitors and/or predators on small individuals. These types of interaction are particularly exemplified in freshwater pond communities, where substantial size variation exists both within and among species of top consumers. We investigated whether density dependence can modify the outcome of size-structured interactions between larval stages of two pond-breeding salamanders, Ambystoma annulatum and Ambystoma opacum. Size structure exists in populations of these species due to variation in the timing of breeding, which we hypothesised would amplify predation rates and competitive asymmetries from the early-arriving species (A. annulatum) on the later-arriving species (A. opacum).
2. We manipulated the relative densities of both A. annulatum and A. opacum in outdoor mesocosms. We maintained the experiment through metamorphosis, and analysed size at metamorphosis, larval period length and survival of each species.
3. Ambystoma annulatum imparted a strong density-dependent effect on A. opacum through a combination of predation and competition. Survival of A. opacum was negatively related to the density of A. annulatum. For the A. opacum that survived, body size was reduced and larval period lengthened at higher A. annulatum densities, indicative of interspecific competition that was partly explained by resource pre-emption. In contrast, A. annulatum was only affected by intraspecific density-dependent competition.
4. Our results suggest that density-dependent effects reinforce asymmetric interactions among larval salamanders. However, the intensity of the asymmetric interactions is mediated by the arrival time and size of conspecifics. Specifically, earlier-arriving species can negatively affect the later-arriving species via size-mediated predation and competition.
5. The interactive effects of density dependence and arrival time of community members are probably a common mechanism generating size variability in ecological communities. Yet, most studies only evaluate one mechanism or the other. By interweaving these two processes, our work displays the importance of understanding context-dependence in species interactions.

中文翻译：

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幼体蝾螈之间不对称的密度依赖竞争和捕食

1. 消费者之间的体型差异通常会导致不对称的互动，较大的个体通常是更强大的竞争者和/或小个体的掠夺者。这些类型的相互作用在淡水池塘群落中尤为突出，在这些群落中，顶级消费者的物种内部和物种之间都存在巨大的大小差异。我们研究了密度依赖性是否可以改变两种池塘养殖蝾螈、Ambystoma annulatum和Ambystoma opacum幼体阶段之间大小结构相互作用的结果。由于繁殖时间的变化，这些物种的种群中存在大小结构，我们假设这会放大早期到达物种的捕食率和竞争不对称性（A . annulatum ) 在后来到达的物种 ( A .  opacum ) 上。
2. 我们操纵了A. annulatum和A的相对密度。 户外中观中的不透明。我们通过变态维持实验，并分析了变态时的大小，幼虫期长度和每个物种的存活率。
3. Ambystoma annulatum对A产生强烈的密度依赖性影响。 opacum通过捕食和竞争的结合。A 的生存。 opacum与A的密度呈负相关。 花环。对于A . opacum存活，体型减小，幼虫期在较高的A延长。 年轮密度，表明种间竞争，部分原因是资源抢占。相比之下，A . 环形仅受种内密度依赖性竞争的影响。
4. 我们的研究结果表明，密度依赖效应加强了蝾螈幼体之间的不对称相互作用。然而，不对称相互作用的强度由同种的到达时间和大小介导。具体而言，较早到达的物种可以通过大小介导的捕食和竞争对较晚到达的物种产生负面影响。
5. 群落成员的密度依赖和到达时间的交互作用可能是导致生态群落大小变异的常见机制。然而，大多数研究只评估一种机制。通过将这两个过程交织在一起，我们的工作显示了理解物种相互作用中上下文依赖性的重要性。