Lecture 15. The Ecological Effects of Invasive Species Ecological effects can occur at many different levels. Some exotic species, even if widespread, appear to have little effect on native species (the introduced species may fill a vacant niche) or on ecosystems (the invader replaces a similar native species and fills most or all of its function). The effects of invaders on diversity are often hard to quantify. Few species extinctions have been directly and solely attributable to an invasive species. Viewed simply, many invaded ecosystems contain more species than they did prior to invasion (a diversity increase!!). IUCN Red List records for 680 extinct animals. ◦ 170 (25%) species have causes listed for their extinction ◦ of these, 91 (54%) included effects of invasive species ◦ In 34 extinctions of those with causes assigned, invasive species were the only cited cause of extinction. However, often effects on diversity difficult to measure ◦ Competition, Hybridization/Introgression ◦ E.g., few plants have gone extinct due to invasive species although many have declined in population density ◦ Extinction debt? ◦ time delays between impacts on a species, such as habitat loss, and the species' ultimate disappearance. We need to be careful though, as we are looking at only a short snapshot of time and we don’t know what the future outcome of these novel interactions might be. For example, the three most abundant lady beetles in North America are all introduced. We know that many native species have decreased as the invasive species have become common but we don’t know if the invasive generalists are filling the roles that a larger number of native specialists used to do. We also do not know the mechanism behind the declines in native species (i.e., is it a direct relationship, an indirect relationship mediated through food or another resource, an indirect interaction between the invasive species and other environmental stressors, etc). Generally, the invasions that have the greatest impact are those that alter the way an ecosystem works. These are rare but account for largest ecological and / or economic harm. The main types of invasions that affect ecosystems are those that: • (1) Alter trophic structure • E.g. novel top predator (nile perch, brown tree snake in Guam – see text book), or a new type of plankton feeder (Asian carp). • By altering food web structure, they change the flow of energy in the ecosystem. • (2) Alter abiotic factors (some examples) • Resource availability • Nutrients (e.g., nitrogen fixers such as the Morella faya tree in Hawaii) • Water (e.g., plants with high evapotranspiration or dense growth in a water limited environment e.g., Tamarisk in American southwest) • Light (e.g., change light reaching forest floor – invasive woody shrubs in temperate hardwood forests) • Disturbance frequency / intensity • Fire (increase or decrease in fire return interval and amplify or decrease the intensity of fire) e.g., cheat grass creates greater intensity (hotter) and more frequent (shorter fire return interval). • In New Zealand, invasion of tussock grassland by forbs, reduced the flammability of this fire dependent system, increasing return interval and lowering intensity. • Change physical environment • Water flow eg., invasive beavers in Chile create impoundments), nutria remove salt march grasses allowing wave action to create larger open areas and facilitate salt water intrusion into coastal marshes. • Sedimentation. Dense invasive plant growth can change flow regimes in watersheds, leading to increased sediment deposition (tamarisk, phragmites, loosestrife) • Earthworms (especially the large lumbricoid species) change the structure and porosity of soil columns thus alter nutrient and water properties. .