Locomotion during digestion changes current estimates of seed dispersal kernels by fish
Dispersal of seeds by animals is an important mechanism regulating plant diversity, range expansions and invasions. Many birds, mammals, fish and reptiles regularly ingest, transport and excrete viable seeds (known as endozoochory).The effectiveness of endozoochory is modelled in dispersal kernels: functions that describe seed shadows in the landscape by combining movement of animals with experimentally obtained seed retention times and survival. Currently, dispersal kernels use experimental data from resting animals, yet only moving animals disperse seeds. Although physical activity is known to affect digestive processes, little is known on how and to what extent this may influence current estimates of endozoochory. Activity may either prolong seed retention in the animal's gut (locomotion-priority mode hypothesis) or may not affect seed excretion rate (digestion-priority mode hypothesis), and may affect seed survival and germination positively or negatively. We tested how activity alters dispersal estimates in fish. We compared the seed dispersal potential of two riparian plant species (Carex acuta and Carex riparia) by the common carp (Cyprinus carpio) subjected to three different activity levels: low (basal metabolic rate, BMR), medium (2 × BMR) or high activity (3 × BMR). Physical activity of the fish did not affect the number of intact retrieved seeds over 15 h of activity, but significantly affected seed retrieval patterns over time for both seed species. More active fish started seed excretion about 1 h later and kept excreting seeds at least 2 h longer. Effects of gut passage on germination could only be tested for C. acuta, where it reduced the percentage of germinating seeds by 22%, independent of the activity level. Seeds ingested by the fish germinated on average 3·5 days later than non-ingested control seeds. Seed retention times did not affect the timing of germination. Our results support the locomotion-priority mode hypothesis and show that modelling dispersal kernels using parameters from inactive fish may underestimate potential dispersal distances. Because a trade-off between physical activity and digestive physiology is likely common in animals, it should be taken into account in future modelling of endozoochorous seed dispersal kernels.