There is a growing threat of habitat change in estuarine and coastal regions, yet there are few models that enable ecologists and resource managers to forecast the response of macrofaunal species to long-term changes in sediment type. This study details a novel strategy that enabled us to rapidly collect data on macrofaunal densities and sediment characteristics by sampling mud-to-sand transition zones in 19 estuaries. Species-specific models that predict probability of occurrence relative to sediment mud content were developed for 13 common macrofaunal species. However, the roles played by many macrofaunal species are influenced by density, not just occurrence. Over broad spatial scales, the constraint an environmental variable places on density can be represented by the upper (or lower) limit on density. Thus, the distribution of maximum density along the gradient from mud to sand was modelled as another indicator of a species' preference. Both the maximum and minimum values for number of taxa, number of individuals, Shannon-Wiener diversity and taxonomic distinctness were also modelled. For most variables, good models (r 2 > 0.6) were developed. The models developed for the different species exhibited a wide variety of functional forms, highlighting the potential variation in response to habitat change even for closely related species with similar natural history characteristics. Probability-of-occurrence models and maximum-density models for a specific species also varied in functional form, emphasising that changes in both occurrence and density need to be considered when predicting likely responses to changes in habitat.

Two hypotheses have been proposed to explain increases in plant nitrogen (N) and phosphorus (P) concentrations with latitude: (i) geochemical limitation to P availability in the tropics and (ii) temperature driven variation in growth rate, where greater growth rates (requiring greater nutrient levels) are needed to complete growth and reproduction within shorter growing seasons in temperate than tropical climates. These two hypotheses were assessed in one forest type, intertidal mangroves, using fertilized plots at sites between latitudes 36 masculine S and 27 masculine N. The N and P concentrations in mangrove leaf tissue increased with latitude, but there were no trends in N : P ratios. Growth rates of trees, adjusted for average minimum temperature showed a significant increase with latitude supporting the Growth Rate Hypothesis. However, support for the Geochemical Hypothesis was also strong; both photosynthetic P use efficiency and nutrient resorption efficiency decreased with increasing latitude, indicating that P was less limiting to metabolism at the higher latitudes. Our study supports the hypothesis that historically low P availability in the tropics has been an important selective pressure shaping the evolution of plant traits.

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Changes in land use and the development of coastal regions around the world have markedly increased rates of sediment input into estuarine and coastal habitats. Field studies looking at the consequences of terrestrial sediment deposition, water‐borne sediment, and long‐term changes in habitats indicate that increasing rates of sediment loading adversely affect the biodiversity and ecological value of estuarine and coastal ecosystems. Managing this threat requires means with which to convey the magnitude of the problem, forecast long‐term trends, and assess the risks associated with changes in land use. Here we focus on approaches for assessing the risks of changes in land use, which include combining biological effect studies with catchment and hydrodynamic modeling, using statistical models that forecast the distribution and abundance of species relative to changes in habitat type, and using sensitive species that play important ecological roles as indicators of change.

Sediment run-off from land has been recognised as a threat to the biodiversity of shallow estuarine and coastal areas. Extreme rainfall events can cause flooding and landslides, which may result in rapid deposition of fine terrigenous sediments and have serious impacts on benthic communities. We designed a field experiment to study the response of intertidal benthos to such depositions of terrigenous clay. The experiment was conducted at 2 contrasting intertidal sites: a sheltered muddy sand habitat and an exposed sand habitat influenced by wind waves. Terrigenous clay (50% water content) was deposited in replicated experimental plots (2 m diameter) at each site in layers 0, 3, 6 or 9 cm thick. The initial response of the resident macrofauna and subsequent recolonisation was monitored over a period of 408 d. Physical and chemical properties of the experimental plots and wave climate at each site were also measured. The experiment demonstrated highly deleterious effects of catastrophic terrigenous clay deposition on estuarine macrobenthic communities. At both sites following clay deposition, the numbers of individuals were reduced by more than 50% after 3 d and by more than 90% after 10 d, irrespective of clay thickness. Mud crabs Helice crassa were the only animals able to emerge through the clay layer. They also exhibited elevated densities in clay treatments over the course of the experiment. After 28 d, a storm occurred (maximum wave height 0.4 m, period = 6 s) which dispersed the clay deposits at the exposed sandy site. Recolonisation of the surficial sediments was rapid at the exposed site following this wind-wave disturbance. However, deeper-dwelling animals such as large bivalves had not recovered to levels observed in the control plots by the end of the experiment. At the more sheltered muddy sand site, the clay deposition resulted in long-lasting habitat change; Although the clay was gradually broken up and re-colonised by small crabs and surface dwelling macrofauna, recovery was still incomplete 408 d after deposition. The results emphasise the role of wind-wave disturbance and transport of sediments and macrofauna with bedload, and the importance of bioturbation by crabs as facilitators of macrobenthic recovery after disturbance.