Soils and Invasive Species

Invasive species are non-native species that become abundant enough to cause significant negative effects on some native species or the function of native ecosystems. Because

Figure 3 Edaphic islands of serpentine outcrops in the Klamath Siskiyou Mountain range in northern California. The relatively barren serpentine outcrops (bare patches with reduced vegetation cover) are embedded in a mosaic of other geologies more favorable for plant growth. Credit: N. Rajakaruna.

soils are an important factor of the environment of organisms, it is no surprise that soil features can affect the ability of non-native species to become invasive. In many cases, disturbance of native communities (including changes in soils caused by disturbance) provides inroads for invasive species. Some studies have contrasted the susceptibility to invasion of unusual soils (such as serpentine soils) and more normal soils. The general conclusion is that the features of the unusual soils that make them challenging for plant growth often inhibit the invasiveness of non-native species. Anthropogenic activities can directly influence soil chemistry of some unusual edaphic habitats making such habitats conducive for colonization by invasive species. This appears to be the case for atmospheric nitrogen deposition on serpentine sites in California. Recent studies suggest that vehicle emissions along major highways in California may have increased the nitrogen content in serpentine soils. Non-native species, previously excluded from such soils due to nitrogen limitation, could potentially invade these unique habitats.

It is common for invasive species (particularly plants) to impact soils and, through changes in soil characteristics, affect other organisms in those communities. For example, in northwestern US, diffuse knapweed (Centaurea diffusa; Asteraceae) has a direct soil-mediated impact on competing native plants. This invasive plant produces 8-hydroxyquinoline, a chemical that builds up in soils occupied by C. diffusa and poisons native plants growing in those soils. Invasive animal species have also been shown to alter soil features that then impact many other organisms in a habitat. For example, earthworms are not native to the forests of Minnesota but have been introduced in many locations. By consuming soil litter and accelerating its breakdown, these animals increase soil compaction, decrease water penetration, and change the nature of the litter layer habitat in ways that reduce its suitability for some native animals, herbaceous plants, and tree seedlings.

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