How can habitat alteration impact freshwater biodiversity?

Habitat alteration significantly impacts freshwater biodiversity primarily by leading to the loss of species and changes in community structure. When freshwater habitats are altered—due to human actions like urbanization, agriculture, or dam construction—there can be profound effects on the environment. These changes disrupt the natural balance of ecosystems, often rendering them unsuitable for many aquatic species that depend on specific conditions for survival.

For example, alterations might include changes in water flow patterns, water quality degradation, or the destruction of critical habitats like wetlands and riparian zones. Such transformations can result in the decline or extinction of sensitive species that cannot adapt to the new conditions. Additionally, as native species decline, invasive species might take their place, thereby altering community structure and dynamics. This can lead to decreased overall biodiversity and impair the ecosystem's ability to function effectively, thereby reducing community resilience to environmental stresses.

The other options incorrectly suggest that habitat alteration has positive effects, that it has no effect at all, or that its effects are limited to specific groups like larger fish species. However, the complexity of freshwater ecosystems means that changes often result in cascading effects affecting a wide range of organisms and ecological interactions.