Mangrove trees trap fine sediment brought in by rivers and the tide. This sediment is the home of various bacteria, algae, protozoa, and invertebrates (such as marine worms) that cope well with the challenges of an intertidal mudflat, including the risk of desiccation, overheating, oxygen deficiency, and regular exposure to air and hydrogen sulphide. These organisms modify their environment and interact with other organisms by exchanging materials in the form of food, waste material, and respiratory gases. However, some larger species are especially important in the ecosystem. Going beyond these relatively straightforward transactions, they alter the nature of the sediment in ways that affect organisms other than their direct competitors, predators, or prey. Ecologists call such species 'ecosystem engineers' because they create new habitats and change the availability of nutrients to other species.