Eurasian watermilfoil and curly-leaf pondweed are examples of ______ plant species that can form dense mats of vegetation.

Eurasian watermilfoil and curly-leaf pondweed are recognized as exotic species because they are not originally from the regions in which they currently thrive. These aquatic plants were introduced to North America from other parts of the world, particularly through human activities such as ornamental planting and aquaculture.

The term "exotic" specifically refers to organisms that are non-native to an ecosystem, which can lead to significant ecological changes. While both Eurasian watermilfoil and curly-leaf pondweed can form dense mats of vegetation, their ability to spread rapidly and outcompete local flora highlights the challenges posed by exotics in aquatic environments.

In contrast, native species are those that have evolved in a particular region and are well adapted to the local environment. Invasive species, while often used interchangeably with exotic, have an additional characteristic; they not only are non-native but also cause harm to the ecosystem, economy, or human health. Endemic species are those native and restricted to a specific geographic area.

Recognizing these characteristics helps in understanding the potential impact of such plants on local ecosystems, specifically in how they can alter habitats, disrupt food chains, and interfere with recreational activities.