Data from: Picky hitch-hikers: vector choice leads to directed dispersal and fat-tailed kernels in a passively dispersing mite

Dispersal is a central life-history trait for most animals and plants: it allows to colonize new habitats, escape from competition or avoid inbreeding. Yet, not all species are mobile enough to perform sufficient dispersal. Such passive dispersers may use more mobile animals as dispersal vectors. If multiple potential vectors are available, an active choice can allow to optimize the dispersal process and to determine the distribution of dispersal distances, i.e. an optimal dispersal kernel.

We explore dispersal and vector choice in the neotropical flower mite Spadiseius calyptrogynae using a dual approach which combines experiments with an individual-based simulation model. Spadiseius calyptrogynae is found in lowland rainforests in Costa Rica. It inhabits inflorescences of the understorey palm Calyptrogyne ghiesbreghtiana and is phoretic on a number of flower visitors including bats, beetles and stingless bees. We hypothesised that the mites should optimise their dispersal kernel by actively choosing a specific mix of potential phoretic vectors.

In a simple olfactometer setup we showed that the flower mites do indeed discriminate between potential vectors. Subsequently we used an individual-based model to analyse the evolutionary forces responsible for the observed patterns of vector choice. The mites combine vectors exhibiting long-distance dispersal with those allowing for more localized dispersal. This results in a fat-tailed dispersal kernel that guarantees the occasional colonization of new host plant patches (long distance) while optimizing the exploitation of clumped resources (local dispersal). Additionally, kin competition results in a preference for small vectors that transport only few individuals at a time. At the same time, these vectors lead to directed dispersal towards suitable habitat, which increases the stability of this very specialized interaction. Our findings can be applied to other phoretic systems but also to vector-based seed dispersal, for example.

扩散（dispersal）是绝大多数动植物核心的生活史特征：可助力物种拓殖新生境、规避种间竞争并避免近交衰退。然而，并非所有物种都拥有足够的运动能力以完成有效扩散。此类被动扩散物种可借助活动性更强的动物作为扩散媒介（dispersal vector）。若存在多种潜在扩散媒介，主动选择可优化扩散流程，并确定扩散距离的分布模式，即最优扩散核（dispersal kernel）。

本研究采用实验与基于个体的模拟模型（individual-based simulation model）相结合的双路径方法，对新热带界花螨*Spadiseius calyptrogynae*的扩散行为与媒介选择机制展开探究。该螨类分布于哥斯达黎加的低地热带雨林中，栖息于下层棕榈植物*Calyptrogyne ghiesbreghtiana*的花序内，并以携播（phoretic）方式依附于多种访花动物，包括蝙蝠、甲虫与无刺蜂。我们提出假说：该螨类可通过主动选择特定组合的潜在携播媒介，优化其扩散核的分布。

借助简易嗅觉仪（olfactometer）实验装置，我们证实该花螨确实能够区分不同的潜在扩散媒介。随后，我们利用基于个体的模拟模型，解析了驱动其媒介选择模式演化的进化动力。该螨类会兼顾兼具长距离扩散能力的媒介与更利于局域扩散的媒介，由此形成肥尾扩散核（fat-tailed dispersal kernel）：既保障了偶发的新生境拓殖（长距离扩散），又优化了对聚集型资源的利用效率（局域扩散）。此外，亲缘竞争会促使螨类偏好单次仅携带少量个体的小型媒介；与此同时，这类媒介可引导螨类定向扩散至适宜生境，进而强化这一高度特化的共生关系的稳定性。本研究结论不仅可推广至其他携播共生系统，例如也可应用于基于媒介的种子扩散体系。