Species Invasions Ch. 9

Invasive species spreading across America ▪ Youtube video

Conservation implications of invasive species ▪ 11% of plants on plains & prairies of North America are invasive; Hawaii = 35% ▪ >3,000 spp. of invasive plants in California alone ▪ Worldwide, 20% of endangered vertebrate spp. are threatened by invasive species (13% on mainlands, 31% on islands) ▪ 77% of fish invasions caused native species to decline ▪ San Francisco Bay: rate of successful new invasions increased from one every 55 weeks in 1851 to one every 14 weeks in 1960 ▪ These statistics are a small example of how invasions have changed the biotic makeup of land, freshwater & marine communities

Terminology

Conservation implications of invasive species ▪ Questions: 1. What are the impacts of invasive species? 2. How do they alter ecological communities and at what spatial and temporal scales? 3. When will impacts demand our attention? 4. What factors determine whether an invasion succeeds or fails? 5. How are species introduced? 6. How can we prevent or control invasions?

What are the impacts of invasive species? ▪ There are 2 major considerations: 1. Direct and indirect effects of the invader on native species & communities. Direct: immediate interactions with another species thru competition, predation, parasitism, disease, or toxins. Indirect: trophic cascades, habitat modification 2. Effects of multiple invaders: Differences in densities & behavior of invader or invaded community, or both, can lead to different impacts. Also, synergistic effects of multiple invaders (invasional meltdown) Caveat: ecological impacts of invasive species are inherently neither bad nor good; these judgements are made only within the context of a value framework, such as endangered species conservation, habitat restoration, or economy

Predators ▪ Nile Perch – native to Africa ▪ Introduced into Lake Victoria in the 1950’s (Kenya, Uganda, Tanzania) ▪ Caused the extinction of many endemic cichlid fish, and many other species ▪ Drastically changed the food web in the lake

Predators ▪ European Red Fox – native to England ▪ Introduced to Australia in 1855 for sport hunting ▪ Linked to declines in mammals and ground nesting birds

Rock wallabies

Predators ▪ Rosy Wolfsnail – native to SE USA ▪ Deliberately introduced into Hawaii to control the African Giant Snail ▪ 15 of 20 endemic snails in Hawaii went extinct due to this voracious carnivore

Predators ▪ Brown Tree Snake – native to Australia, New Guinea, Solomon Islands ▪ Accidentally introduced into Guam following World War II ▪ The island had only one native snake species – tiny blind snakes ▪ Spread across the island causing the extinction of bird species

Competition ▪ Zebra Mussel – native to Eurasia ▪ Introduced into North America in 1988 ▪ They settle at high densities on the shells of native mussels, reducing the natives’ ability to filter water, respire, feed and excrete wastes ▪ 40-75% of native mussels (Unionidae) are endangered, largely due to zebra mussels (but also due to dams, canals, harvest, and habitat loss and degradation

Competition ▪ Himalayan Balsam – native to Himalayan mountains ▪ Introduced into Europe ~100 years ago ▪ Competes with natives for the service of pollination ▪ Nectar produced at a very high rate – bees visit 4X more often then other native flowers

Disease ▪ Malaria – causes sickness and death in humans ▪ Native range shown (tropical) ▪ Climate change (warming) is predicted to cause malaria to spread via mosquitos becoming invasive

Toxins ▪ Cane toad – native to Central & South America ▪ Deliberately introduced into Australia in 1935 to control cane beetle ▪ Causes lethal toxic ingestion to would-be predators ▪ Population-level declines in goannas, snakes, crocodiles & quolls

Measuring impacts ▪ Best method = ‘before and after’ studies on relative abundance of the invader and the native species ▪ Formula for quantifying impacts, from Parker et al (1999): ▪ I = R X A X E ▪ Where I = impact, R = range that the invader occupies (e.g., m 2 ), A = its abundance over that range (e.g., numbers of biomass per m 2 ), and E = its per capita effect (e.g., the change caused by a single individual) ▪ Strongest impacts: those invaders with large range, high abundance an strong per capita effects

Challenges to measuring impacts ▪ Per capita effect difficult to determine ▪ Lack or pre-invasion data (funding, interest, early knowledge of invasion) ▪ Long-term nature of impacts ▪ Difficulty in counting impacted species (e.g., predators)