To compete or not to compete: an experimental study of interactions between plant species with contrasting root behaviour

Game-theoretic models predict that plants with root systems that avoid belowground competition will be displaced by plants that overproduce roots in substrate shared with competitors. Despite this, both types of root response to neighbours have been documented. We used two co-occurring clonal species (Glechoma hederacea and Fragaria vesca) with contrasting root responses to neighbours (avoidance of competition and contesting of resources, respectively) to examine whether functional variation in other traits affected the success of each rooting strategy, leading to a different outcome from that predicted on the basis of root behaviour alone. Vegetative propagation rates, morphology and biomass allocation patterns were examined when each species was challenged with competition from physically separate ramets with either the same rooting strategy (intraclonal competition) or the contrasting rooting strategy (interspecific competition). Contrary to the predictions of game-theoretic models, the species that exhibits avoidance of root competition (Glechoma) was not competitively inferior to the species that does not (Fragaria). Glechoma achieved greater total mass in the interspecific treatment than in the intraclonal treatment. However, Fragaria did not experience more intense competition from Glechoma than it did in the intraclonal treatment. Strong interference between the two species appeared to be avoided because Glechoma invested preferentially in rapid exploitation of unoccupied space, whereas Fragaria invested in increasing the competitive ability and local persistence of established ramets. Our results suggest that interspecific trade-offs between traits related to competitive ability and resource exploitation can allow coexistence of species with contrasting rooting behaviours. Full assessment of the adaptive value of different root responses to neighbours therefore requires concurrent consideration of the combined effects of a wide array of functional traits.     

The distribution and abundance of boring species of polychaetes and sipunculans in coral substrates in French Polynesia

The distribution and abundance of the dominant initial macroborers of dead coral substrate, sipunculans and polychaetes were investigated over time at seven sites within French Polynesia. Sites were located in the lagoon of high islands and atolls, and varied from highly eutrophic to oligotrophic. Significant differences occurred between sites and patterns of recruitment varied over time and between sites. With increasing exposure, the densities of polychaetes increased but not the number of species present, whereas both the densities and number of species of sipunculans increased. The atoll sites tended to be dominated by suspension feeding polychaetes and the high island sites by deposit feeding polychaetes. Sipunculans tended to dominate the high island sites in comparison to the atoll sites and they all fed by scraping algae and detritus from the substrate. We suggest that this distribution of feeding types is related to water quality and to land run off. In the atolls, the lagoonal waters are oligotrophic and little land run off occurs, whereas at the high island sites, high rates of land run off occur during the wet season with high levels of suspended material in the water column.These variations in densities of boring species, affect rates of bioerosion and have the potential to alter the equilibrium between reef growth and reef destruction. We suggest that it is critical for reef managers to try to maintain water quality and limit land-based terrestrial run off and associated nutrients into coastal waters. This is especially important if the reefs have been affected by bleaching events or Crown of Thorns plagues, resulting in extensive death of coral colonies and with it, the potential for a massive increase in the rate of bioerosion. The long-term maintenance of the reef structure is critical if coral recruitment and recovery of the reef are to occur.     

A model of copepod population dynamics in the southern Benguela upwelling region

A simple population dynamics model was constructed to simulate temporal variability in the biomass of a dominant copepod Calanoides carinatus (Copepoda: Calanoida) along the West Coast region of South Africa. Calanoides carinatus is extensively preyed upon by the commercially important anchovy Engraulis capensis, thus variability in zooplankton production may serve as a useful predictor of variability in anchovy recruitment levels. The model developed here circumvents the need to include a large number of parameters because it uses satellite-derived estimates of chlorophyll a concentration and sea surface temperature as primary inputs. Abundance estimates necessary to initialize the model are readily obtainable from biannual research cruises. The model successfully simulates observed features of a copepod population's response to pulses of upwelling and is robust with respect to most of its parameters because minor changes in their values result in predictable changes in model output. The model showed greatest sensitivity to parameters that are difficult to determine empirically, such as predator-induced mortality rates. Gaps in our present understanding of the nature and scale of processes affecting copepod egg abundance, survival and viability in the southern Benguela system were identified as the dominant impediment to simulating copepod population dynamics in the region.