Human impacts on ecosystems go beyond species loss

Natural ecosystems are likely to be a lot more sensitive to change, and especially to human activities, than previously thought, researchers in Texas say. A study at Rice University focused on human influences on the biosphere and how altering the dynamics of a population -- for example, by removing larger members of a species through overfishing -- can have measurable consequences, a university release reported Tuesday. "Natural communities are increasingly altered through human impact, and ecologists have long striven to determine how these changes influence communities," Rice ecologist Volker Rudolf said. The complete disappearance of a species is the most extreme result, he said, but is not the only cause of biodiversity loss. "That's the last thing that happens after you mess up the entire ecosystem for a long period of time," he said. But long before that, he said, changes forced upon the structure of a population -- such as the ratio of smaller young animals to larger older examples in a species -- already have an impact up and down the food chain. Altering the population specifics of any species also affects the structure of the entire community and ultimately how the whole ecosystem functions, the researchers found. For example, Rudolf said, the study could yield insights into why size-selective harvesting can alter the structure of entire food webs in some ocean systems even when no species had gone extinct and the total biomass of the targeted fish remained the same. The findings "challenge classical assumptions and studies that say we can make predictions by assuming that all individuals of a species are the same." "While these changes would be hard to predict by the classical approach, our results suggests such changes are expected when human activities alter the population structure of keystone species in an ecosystem," Rudolf said. "Thus, natural ecosystems are likely to be much more fragile then we previously thought."