Chapter 9: Invasive Species

Chapter 9: Invasive Species

Consider Hawaiian plants: more than 1/3 are introduced; American Plains about 11%. San Francisco Bay: About 150 introduced species in 150 years; increase is exponential. Similar situation for Laurentian lakes.

9.1 Accumulation of established estuarine and marine invaders in San Francisco Bay

Case 9.2 (A) Reported invasion rate for invasive species in the Laurentian Great Lakes

Disease-causing organisms may also be introduced; this includes HIV, smallpox etc. (i.e., not just non-human parasites).

Case 9.4 (A) Invasive species are often those that escape their herbivores from their native habitat

Brown tree snake in Guam: several species of birds extinct; similar for giant African land snail in Hawaii for plants. Partly a case of “novelty”: the introduced species fills an empty niche (e.g., predator).

9.3 Map of Guam showing the spread of Boiga and the loss of forest bird species

9.2 Annual electrical outages caused by the invading brown tree snake on Guam

Giant land snail: brought in predatory snail, but it preferred other species (and was resistant to a native parasite). Drove 75% of native snails extinct; spread to other Polynesian islands and wiped out over 80% of species of one snail genus. This also directly affected the economy based on decorative snail shells. U.S.: a predatory snail is marketed that switches to plants when prey run out.

Sometimes, multiple introductions may bring things back to “normal” -- but hard to predict. e.g., jellyfish that eats fish eggs and larvae (Black Sea), then jellyfish that eats jellyfish seems (?) to have helped solve the problem.

9.6 Change in the abundance of (A) fish eggs and (B) fish larvae following the bloom of thectenophore Mnemiopsis leidyiin 1988, then arrival of Beroe (which eats Mnemiopsis)

Often easiest to see effects of predators, but may be other effects too: zebra mussel from Europe is problematic in many ways. One is that they colonize shells of native unionid mussels; this is one reason (besides pollution, habitat alteration etc. that most species are now extinct).

9.4 Native unionid mussels recently dead vs. weight of introduced mussels attached to their shells

Introductions can cause complex ecological effects: Gulf of Maine: huge beds of kelp and various algae until 1970s. Then: Overfishing of sea urchin predators; urchins ate the kelp. Sea urchin harvesting began in 1990s, but kelp didn’t bounce back.

Because: A bryozoan and an alga had been introduced; bryozoan grows on kelp and chokes off light etc; the other alga takes over.

9.5 Impacts of introduced Asian alga and European bryozoan on the survival of native subtidal kelp

Introduction of marsh cordgrass to W. coast of North America: grows fast, takes over; more sediment deposited; water level rises. System changes from mudflat to saltmarsh. Native invertebrates, fishes etc. affected; even migratory birds.

African tree (Myrica) introduced to Hawaii: established on volcanic soils. The tree has nitrogen-fixing bacteria; enriches soil with N. But -- native plants are adapted to low-N soils; poisoned by high levels. Another case: Introduced plants may be more attractive to pollinators than native.

9.7 Bumblebee visits to (A) and seed set of (B) the native marsh woundwort Stachys palustris

Genetic effects: Introduced mallard ducks interbreed with native species; “dilute” gene pool. Salmon farming: escapees hybridize with native strains/species. Introductions can change the evolutionary fates of native species in unpredictable ways.

Checkerspot butterfly in W. North America prefers to lay eggs on an introduced plant species. This has a genetic basis: selection occurs for butterflies with heritable preference for non-native plant. Butterflies rely more and more on disturbed habitat with introduced species; native ecosystem changes.

9.8 Percent of Euphydryas editha preferring to oviposit on the native plant Collinsia parviflora vs. the introduced Plantago lanceolata

Susceptibility of communities to invasions: Elton (1958): Biotic Resistance Hypothesis: In species-rich communities, most niches are already occupied, situation is stable so less susceptible to invaders. Some experiments support this; some show the opposite.

Maybe introduced species just do well where many other species do too (e.g., more nutrients) -- opposes hypothesis. Consider three introduced plant species: do well on a broad scale in species-rich plant communities, but seeds germinate poorly when introduced to some species-rich communities. As usual, no general rule, and may vary with different aspects of life cycle etc.

9.12 Invader incidence and successful germinations vs. species richness (Part 1)

Disturbance may increase susceptibility of a community to invasion: reduces number of native species; resources now available to invaders; new types of disturbance (e.g., human induced) may affect native species that aren’t adapted to this. Invaders may even make conditions better for themselves: change nutrient availability, fire cycles, soil stability, moisture, shade, etc.

Connections between the Biotic Resistance Hypothesis and the Intermediate Disturbance Hypothesis: Suppose the community is species-rich; lots of competition. Disturbance knocks back some species: opens up opportunity for invader. e.g., Hawaiian N-fixer: not good for native plants (a disturbance), but creates opportunity for non-natives.

9.14 The conceptual theory that increased resource availability increases a plant community’s susceptibility to invasion

And -- humans may also increase resource availability -- e.g., nutrient runoff, reduced shade, etc.

Introduced Pacific sea squirts: larvae settle and grow; fewer individuals the more native species are present. Mainly a consequence of less physical space; also analogous to “niche space”.

9.15 (A) Survival of introduced Pacific ascidians vs. number of resident species

9.15 (B) Available free space vs. number of resident species

Species invade in many ways; may be physically carried by humans, or get in naturally (e.g., seeds, windborne insects), but human activities allow them to become established. And -- may be deliberate (especially historically), either for specific purpose or just because people like them (e.g., American Acclimatization Society (1800s) tried to introduce all birds mentioned by Shakespeare (e.g., starlings).

Many approaches to control/removal; those that involve introducing predators, diseases, etc. can be very risky -- create yet another problem species. Or disease: e.g., myxomatosis and Australian introduced rabbits: initially successful, then virus evolved to become attenuated (weaker), and resistance evolved in rabbits; still works as a control if mortality due to predation or removal is also high.

A safer approach: release sterile males (e.g. mosquitoes, medfly, screwworm fly): can be very effective. Or: RIDL (Release of Insects with Dominant Lethals): introduce dominant lethal allele that has antidote in lab but not nature. Another recombinant DNA method: daughterless carp.

9.17 The process faced by policymakers who must decide which nonindigenous species to eradicate or prevent

Chapter 10: Climate Change

Broad consensus that climate change is real AND due to human activities. Mainly the result of “greenhouse” gases: carbon dioxide, methane, nitrous oxide, etc. Since early 20th Century, average temperature of Earth has risen 0.6o C, the most in at least 10,000 years.

Trend is accelerating; likely to affect the entire Earth and almost all ecosystems. Solar energy enters Earth’s atmosphere as UV and visible light; transformed to infrared radiation on contact with surface: heat generated. Trapped by gases, plus water vapor: normally keeps surface temperatures stable.

Without greenhouse gases (at normal levels), Earth would be about 15o C cooler. But as more gases added to atmosphere (especially carbon dioxide and methane), creates “enhanced greenhouse effect”. Amounts of these gases have increased about 30% since start of Industrial Revolution.

Chapter 9: Invasive Species