Habitat-mediated, density-dependent dispersal strategies affecting spatial dynamics of populations in an anthropogenically-modified landscape.

A major challenge in managing natural populations in ecosystems is understanding and predicting the complexity and consequences of population dispersal. Although many studies have documented the importance of conspecific density and habitat quality in the dispersal process, we lack an understanding of how to integrate these factors in determining the spatial dynamics of populations or how habitat quality can mediate density-dependent dispersal. In this study, we propose a Habitat-mediated, Density-dependent, Spatial Population Dynamics model (HD-SPDM), in which we combined a Habitat Suitability Index (HSI) with a migration function, to explore the emergent effects of habitat mediated, density-dependent dispersal strategies on the spatial dynamics of a population. Our results show that habitat condition (based on HSI score) can influence ranges in conspecific density (which in turn can alter spatial patterns of populations distributed in homogeneous patches). We tested this model using the spatial distribution of Chinese mitten crab in the Yangtze River Estuary, which has been subjected to excessive sea reclamations over time, this allowed us to obtain insight into spatial distribution of population by determining how habitat-mediated, density-dependent dispersal at a small scale interacts with habitat heterogeneity and fragmentation at a landscape scale. We found that each progressive sea reclamation reduced suitable habitat area and habitat connectivity in the estuary. However, the model predicts that intermediate intensities of habitat compression and fragmentation could improve habitat utilization somewhat by facilitating population dispersal. Our model could be used to improve resource management of populations being increasingly impacted by anthropogenic alterations.

Competitive ability, stress tolerance and plant interactions along stress gradients.

Exceptions to the generality of the stress-gradient hypothesis (SGH) may be reconciled by considering species-specific traits and stress tolerance strategies. Studies have tested stress tolerance and competitive ability in mediating interaction outcomes, but few have incorporated this to predict how species interactions shift between competition and facilitation along stress gradients. We used field surveys, salt tolerance and competition experiments to develop a predictive model interspecific interaction shifts across salinity stress gradients. Field survey and greenhouse tolerance tests revealed tradeoffs between stress tolerance and competitive ability. Modeling showed that along salinity gradients, (1) plant interactions shifted from competition to facilitation at high salinities within the physiological limits of salt-intolerant plants, (2) facilitation collapsed when salinity stress exceeded the physiological tolerance of salt-intolerant plants, and (3) neighbor removal experiments overestimate interspecific facilitation by including intraspecific effects. A community-level field experiment, suggested that (1) species interactions are competitive in benign and, facilitative in harsh condition, but fuzzy under medium environmental stress due to niche differences of species and weak stress amelioration, and (2) the SGH works on strong but not weak stress gradients, so SGH confusion arises when it is applied across questionable stress gradients. Our study clarifies how species interactions vary along stress gradients. Moving forward, focusing on SGH applications rather than exceptions on weak or nonexistent gradients would be most productive.

Heavy metal spatial variation, bioaccumulation, and risk assessment of Zostera japonica habitat in the Yellow River Estuary, China.

Globally, seagrass habitats are decreasing due to both natural and environmental contaminations by human activities, including heavy metal pollution. To expand the global seagrass detection network, this study reports the spatial distributions of Zostera japonica seagrass habitats in the Yellow River Estuary, China. In addition, heavy metal concentrations of Z. japonica tissue, sediment, and surface seawater were analyzed to determine the bioaccumulation and consequent ecological risk to Z. japonica habitats due to the effects of heavy metals. It was found that concentrations of heavy metals were 1.00-2.03 times higher in seagrass-rooted sediment than in adjacent non-seagrass sediment, except for Mn (with a factor of 0.99). Pb and Hg concentrations in sediments exceeded background values more than the other heavy metals, by factors of 1.74 and 1.24, respectively. Metal concentrations in the surrounding seawater were 2.60-4.63 times higher at seagrass sites than at non-seagrass sites, except for Hg (factor of 0.97). Metal concentrations were much higher in seagrass tissues than in the sediment (e.g., bioconcentration factor of Cd is 30.95). Pb concentrations in water may cause the greatest adverse reactions among aquatic organisms, while As, Cr, Hg, Mn and Cu in sediments may occasionally cause negative ecological effects. Z. japonica showed higher bioaccumulation of Cd and Pb in the above-ground tissues. Among other recent studies of seagrasses from other parts of the world, Cd concentrations are similar to the results of the present study, but Pb concentration in present study is higher than in other studies. In conclusion, Pb and As in the surrounding environment present potential risks to the seagrass habitats of the Yellow River Estuary, China.