Of mice and oaks : conditional outcomes in a seed-dispersal mutualism

PhD Thesis (defence scheduled for 10 October 2014, 4pm) – abstracts for library Of Mice and Oaks: conditional outcomes in a seed-dispersal mutualism Lennart Suselbeek Abstract UK Rodents like to wood mouse store acorns to overcome winter, but some of the stored acorns are never retrieved by the rodents. Those acorns that are not retrieved have a chance to germinate and establish into a new tree. Thus, the interaction between mice and oaks is potentially mutualistic. However, the interaction is only mutualistic when the acorns are placed in locations where germination is likely to take place. The aim of this study was to investigate whether and how the strategy of hoarding (i.e. storing the seeds) depended on the presence of wild boar (who are very fond of acorns too!), de abundance of acorns and the abundance of mice in the area. Wild boar did not seem to be very capable to detect the acorns that had been hidden in the soil by the rodents, and as a consequence they seem to have little effect on the hoarding strategy of rodents. The abundance of rodents and of acorns does affect the hoarding patterns of rodents. With more competition (i.e. more mice or fewer acorns), seeds are hidden more quickly and are being scattered more widely. The acorns benefit from this, as their chance to survive and successfully establish as a new tree increases with dispersal distance and seed spacing. Abstract NL Muizen leggen een wintervoorraad aan van eikels, maar een deel van deze voorraad wordt vaak niet opgegeten door de muizen. De niet opgegeten eikels hebben een kans om zich te vestigen als nieuwe eik, en daarmee is de interactie tussen muizen en eiken in potentie een mutualisme. Echter, er is alleen sprake van een mutualisme als de eikels op goede kiemplaatsen terecht komen. Doel van deze studie was te onderzoeken of de verstopstrategie van muizen afhankelijk was de aanwezigheid van wilde zwijnen (die ook dol op eikels zijn!), van het aanbod aan eikels, en van het aantal muizen in het gebied. De wilde zwijnen lijken niet goed in staat te zijn om de door muizen in de grond verstopte eikels terug te vinden, en daardoor hebben zij weinig invloed op het hamstergedrag van de muizen. Het aantal eikels en muizen heeft wel invloed op de verstopstrategie. Hoe meer concurrentie (dus, meer muizen of juist minder eikels), hoe sneller en meer verspreid de zaden verstopt worden. De eikels profiteren daar van, want hoe beter ze verspreid worden des te groter hun overlevingskans.</p

Tracking rodent-dispersed large seeds with Passive Integrated Transponder (PIT) tags

1.Seed dispersal, a critical phase in the life history of many plants, is poorly understood due to the difficulty of tracking and monitoring dispersing seeds until they reach their ultimate fate. Scatter-hoarding rodents play a substantial part in the seed dispersal process of many plant species, however, existing tracking methods do not allow seed monitoring without risk of influencing the hoarding process and seed fate. 2.Here, we describe and test the use of Passive Integrated Transponders (PIT) tags inserted into seeds for the tracking and monitoring of large seeds dispersed by rodents. Unlike other tagging methods, PIT tagging combines the advantages of leaving no external cues and being readable without disturbance of caches. Rodents cannot remove these tags. 3.We evaluated the performance of PIT tagging through a series of trials with Quercus acorns dispersed by rodents, both in North America and in Europe, with equipment from different manufacturers. We quantified effects of tagging on seed removal and caching, cache pilferage and seed germination, by comparison between PIT-tagged and untagged acorns. We evaluated the detectability of buried tags to researchers. 4.Minimal effects of PIT tagging on seed removal, caching, pilferage and germination were found. Buried PIT tags were retrieved with high reliability by naïve researchers, even at burial depths up to 30 cm. Identification codes could be read even when multiple tags were buried at a single location, as in larder hoarding. 5.The method was successfully applied in two field studies of dispersal of Quercus palustris and Q. rubra acorns by Eastern grey squirrels Sciurus carolinensis in North America, and Q. robur acorns by Wood mice Apodemus sylvaticus in the Netherlands. The proportion of seeds recovered was comparable to that in studies using traditional thread tags. 6.We conclude that PIT tagging is a particularly suitable method for tracking and monitoring of seeds dispersed by scatter-hoarding rodents. PIT tagging solves most of the main problems generally encountered when following the fate of rodent-dispersed seeds over time

Scatter hoarding and cache pilferage by superior competitors: an experiment with wild boar, Sus scrofa

Food-hoarding patterns range between larder hoarding (a few large caches) and scatter hoarding (many small caches), and are, in essence, the outcome of a hoard size–number trade-off in pilferage risk. Animals that scatter hoard are believed to do so, despite higher costs, to reduce loss of cached food to competitors against which they cannot defend their food reserves (henceforth: superior competitors). We tested the underlying assumption that the cost of having more caches under scatter hoarding, thus increasing the likelihood of cache encounter by superior competitors, is outweighed by the benefit of having small caches that are less likely to be detected upon encounter by superior competitors. We carried out a controlled experiment in which we distributed a fixed number of acorns over a fixed number of patches within a fixed area, varying cache size and cache depth, thus mimicking alternative hoarding patterns. We then recorded cache pilferage by a fixed number of wild boar, a well-known pilferer of acorn caches. The time wild boar needed to pilfer the first cache was shortest for scatter hoarding, but the time needed to pilfer all caches was slightly longer for scatter hoarding than for larder hoarding. Overall, however, the rate of pilferage did not differ between scatter hoarding and larder hoarding, and was not affected by cache depth. We conclude that the effects of alternative hoarding patterns on reducing cache pilferage by wild boar were smaller than expected, and that superior competitors may thus not be important drivers of scatter hoarding. Instead, other factors, such as conspecific pilferage or the risk of cross-contamination of food items in large caches, which can also cause catastrophic loss of food reserves, may be more important drivers of scatter hoarding