M. Dorenbosch

Homing and daytime tidal movements of juvenile snappers (Lutjanidae) between shallow-water nursery habitats in Zanzibar, Western Indian Ocean

We studied daily tidal movements of tagged juvenile Lutjanus fulviflamma and Lutjanus ehrenbergii between two adjacent habitats, a subtidal channel and shallow tidal notches in the fossil reef terrace, in a shallow marine bay on Zanzibar Island (Tanzania). Due to a large tidal range, the notches were dry at low-tide and were only accessible to the snappers at high-tide. Of the resighted individuals, 48% showed clear movement between the two habitats, orientated in a direction perpendicular to the tidal currents. Individuals resighted more than once showed site fidelity, indicating homing in both the channel and the notches. We suggest that a significant part of this population of juvenile snappers may move from a low-tide resting habitat to a high-tide resting habitat during the daytime, perhaps to avoid predation by larger predators that may enter the channel at high-tide.

Mangrove habitat use by juvenile reef fish: meta-analysis reveals that tidal regime matters more than biogeographic region

Identification of critical life-stage habitats is key to successful conservation efforts. Juveniles of some species show great flexibility in habitat use while other species rely heavily on a restricted number of juvenile habitats for protection and food. Considering the rapid degradation of coastal marine habitats worldwide, it is important to evaluate which species are more susceptible to loss of juvenile nursery habitats and how this differs across large biogeographic regions. Here we used a meta-analysis approach to investigate habitat use by juvenile reef fish species in tropical coastal ecosystems across the globe. Densities of juvenile fish species were compared among mangrove, seagrass and coral reef habitats. In the Caribbean, the majority of species showed significantly higher juvenile densities in mangroves as compared to seagrass beds and coral reefs, while for the Indo-Pacific region seagrass beds harbored the highest overall densities. Further analysis indicated that differences in tidal amplitude, irrespective of biogeographic region, appeared to be the major driver for this phenomenon. In addition, juvenile reef fish use of mangroves increased with increasing water salinity. In the Caribbean, species of specific families (e.g. Lutjanidae, Haemulidae) showed a higher reliance on mangroves or seagrass beds as juvenile habitats than other species, whereas in the Indo-Pacific family-specific trends of juvenile habitat utilization were less apparent. The findings of this study highlight the importance of incorporating region-specific tidal inundation regimes into marine spatial conservation planning and ecosystem based management. Furthermore, the significant role of water salinity and tidal access as drivers of mangrove fish habitat use implies that changes in seawater level and rainfall due to climate change may have important effects on how juvenile reef fish use nearshore seascapes in the future.

Invasive Crayfish Threaten the Development of Submerged Macrophytes in Lake Restoration

Submerged macrophytes enhance water transparency and aquatic biodiversity in shallow water ecosystems. Therefore, the return of submerged macrophytes is the target of many lake restoration projects. However, at present, north-western European aquatic ecosystems are increasingly invaded by omnivorous exotic crayfish. We hypothesize that invasive crayfish pose a novel constraint on the regeneration of submerged macrophytes in restored lakes and may jeopardize restoration efforts. We experimentally investigated whether the invasive crayfish (Procambarus clarkii Girard) affects submerged macrophyte development in a Dutch peat lake where these crayfish are expanding rapidly. Seemingly favourable abiotic conditions for macrophyte growth existed in two 0.5 ha lake enclosures, which provided shelter and reduced turbidity, and in one lake enclosure iron was added to reduce internal nutrient loading, but macrophytes did not emerge. We transplanted three submerged macrophyte species in a full factorial exclosure experiment, where we separated the effect of crayfish from large vertebrates using different mesh sizes combined with a caging treatment stocked with crayfish only. The three transplanted macrophytes grew rapidly when protected from grazing in both lake enclosures, demonstrating that abiotic conditions for growth were suitable. Crayfish strongly reduced biomass and survival of all three macrophyte species while waterfowl and fish had no additive effects. Gut contents showed that crayfish were mostly carnivorous, but also consumed macrophytes. We show that P. clarkii strongly inhibit macrophyte development once favourable abiotic conditions for macrophyte growth are restored. Therefore, expansion of invasive crayfish poses a novel threat to the restoration of shallow water bodies in north-western Europe. Prevention of introduction and spread of crayfish is urgent, as management of invasive crayfish populations is very difficult.

Human Disturbance affects a Deciduous Forest Bird Community in the Andean Foothills of Central Bolivia

Neotropical deciduous forests are threatened by anthropogenic activities and only a small area is protected against exploitation. In contrast to other Neotropical forest types, not much is known about the effect of habitat disturbance on deciduous forest bird communities. In the present study, we examine the effects of human disturbance on a deciduous forest bird community in the Andean foothills of Bolivia. Bird community composition was determined by means of point count surveys in three forest types, i.e. undisturbed deciduous and semi-deciduous forest, and disturbed forest habitat. Habitat disturbance was estimated indirectly by measuring vegetation structure variables in these forests. The effect of habitat disturbance was subsequently examined at bird community level (species richness, species abundance and diversity indices) and functional group level (habitat preference categories and foraging guild). Differences in community composition and reduced species abundance could clearly be related to habitat disturbance. However, results also indicated that sensitivity to disturbance varied among functional groups. Humid forest specialists showed the strongest response to habitat disturbance, predominantly bark-gleaning insectivores, canopy insectivores, understorey insectivores and ground-dwelling species, whereas dry forest specialists were not affected by disturbance. The present study suggests that anthropogenic disturbance negatively influenced the forest bird community in the investigated deciduous forest in Bolivia. This may indicate that conservation of both deciduous and semi-deciduous forest remnants is relevant for a diverse montane bird community in South America. Because various bird species typical of deciduous forests may also be able to survive in partly disturbed forests, conservation strategies should not only be focussed on the protection of pristine remnants but should also include secondary forest edge zones.