Long-term variability and density dependence in Hudson River Dreissena populations

1. We used a 27-year record of Dreissena populations in the freshwater tidal Hudson River to describe interannual variation in population density, body size, and body condition; estimate long-term variation in recruitment, survivorship, and shell growth; and assess possible controls on the populations.
2. Dreissena populations in the Hudson have been highly variable, with interannual ranges of c. 100-fold in abundance and biomass, and 7-fold in mean body mass. This large interannual variation arises from both long-term trends and 2–5-year cycles.
3. Long-term trends include the 2008 appearance of the quagga mussel (Dreissena rostriformis), which still forms a small part (<10%) of the dreissenid community, and a decline in zebra mussel body size. The decline in body size was caused by a long-term decline in adult survivorship rather than a decline in rates of shell growth. We could detect no long-term trends in adult abundance or spread of Dreissena onto soft sediments in the Hudson.
4. We observed persistent, strong cycles in adult abundance and body size. These were driven by the appearance and decay of eight dominant year classes over the 27 years of our study, and were a result of temporal variation in recruitment rather than temporal variation in survivorship. The observed strongly irregular recruitment appears to arise from strong adult–larval interactions, and is consistent with previous simulation model results showing that interactions between adults and larvae can drive persistent cycling.
5. We found evidence that negative density dependence affects recruitment, somatic growth, and body condition of Dreissena in the Hudson. Warm summers may also cause high adult mortality.
6. We put our results into the context of a conceptual model of Dreissena population dynamics, and argue that neither the dynamics nor the controls of populations of these important invaders is known satisfactorily.

     

Socio-ecology of Bat-eared foxes (Otocyon megalotis)

Bat-eared foxes were studied for eight months at four sites in Kenya and Tanzania with the main study in the Masai Mara Game Reserve. Eleven foxes were caught and four fitted with radio-collars. Foxes in Masai Mara feed mainly on ants and termites. They showed a marked preference for short grass habitat, and their range expanded when wildebeest lowered grass height by grazing and trampling. Foxes lived in groups (= 2.8) with overlapping home ranges, and relations between groups were usually amicable. Foxes gave birth from September-December and all members of the group provided some care to the cubs.
A review of Bat-eared fox ecology from other studies reveals a consistent picture of this aberrant insectivorous canid. The species foraging strategy involves locating social insects by sound in short grass areas. The species is able to find suitable habitat and multiply rapidly. Although usually living in monogamous pairs, reports of polygyny, and non-reproductive males in groups also exist. Living in groups may increase each fox's chance of feeding on insect swarms, and also provides protection from predators.