invasive species

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

• Lizards in Florida: 48 spp. invasive, 15 spp. native

• 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

Cryptogenic

applies to species whose status - native or nonnative - cannot readily be determined; commonly used for species whose cosmopolitan distribution or unclear taxonomy make their geographic origins uncertian

indigenous

synonymous to native

introduced

refers to a species that has been released outside of its native range; synonymous with nonnative, nonindigenous

introduction

the release or escape of a nonnative species

• some say that its only invasive if it has negative impacts; in this class it is anything nonnative no matter what

invasion

the establishment and spread of an introduced species

invader

an introduced species

Native

Describe species that evolved in a region

nonindigenous

synonymous with introduced, nonnative

reintroduced

refers to intentionally released individuals of a native species that was locally endangered or extinct

what are the impacts of invasive species

There are 2 major considerations:

1. Direct and indirect effects of the invader on native species & communities.

1. Effects of multiple invaders

Note: 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

Direct impacts

immediate interactions with another species thru competition, predation, parasitism, disease, or toxins. Indirect: trophic cascades, habitat modification

indirect impacts

trophic cascades, habitat modification

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)

Predators - Nile Perch

• Nile perch (Lates niloticus) were introduced into Lake Victoria in the 1950’s by British Colonists to boost human food resources; native to Lake Chad, Nile River & Zaire River

• The lake was home to ~400 species of cichlid fish, which are much smaller

• Over the 1980’s, Nile Perch went from 1% to 97% of the total human harvest; at the same time the natives went from 92 to 0%!

• 200 native species disappeared!!

• Today, larger fish are preferentially harvested to manage the fishery

Predators - European Red Fox

• native to England

• Introduced to Australia in 1855 for sport hunting

• Linked to extinctions & declines in small mammals and ground nesting birds

• rock wallabies vulnerable to these foxes

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

Brown tree snake

• native to Australia, New Guinea, Solomon Islands

• Accidentally introduced into Guam following World War II

• Bird fauna on Guam evolved in the absence of snakes

• Spread across the island causing the extinction & declines of many bird species (at least 7 extinctions)

competition - Zebra Mussels

– 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 declines in goannas, snakes, crocodiles & quolls

Invasional meltdown

• Two or more invasive species causing a synergism of impacts

• We know that cane toads are causing declines & extirpations, and feral cats have done the same

• It now appears that cats can fill the niches left open by monitor lizards after toads arrive

• We don’t know the mechanism: predatory release, competitive release, both?

Measuring impacts on invasive 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., m2), A = its abundance over that range (e.g., numbers of biomass per m2), 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

• Best method = ‘before and after’ studies on absolute or relative abundance of the invader and the native species

Challenges to measuring impacts

• Per capita effect difficult to determine

• Lack of pre-invasion data (funding, interest, early knowledge of invasion)

• Long-term nature of impacts

• Difficulty in counting impacted species

What factors determine whether an invasion succeeds or fails?

Which species are the most successful invaders? Which communities are the most invasible?

1. Invasion pathway with a sufficient propagule pressure (number, frequency and quality of invading spp.) must exist

2. Invader’s physiological tolerances & life history must match the new environment well enough for survival and reproduction

3. Interactions between invader and native species must be sufficient for survival and reproduction

Note: the above can all interact

propagule pressure

number, frequency and quality of invading species

context of a filter

Propagule pressure

• By definition, all invasions require a pathway that can transport species beyond their native range

• The particular invasion pathway will determine the propagule pressure (quality, quantity and frequency of arriving organisms)

• Two commonly recurring predictors of a successful invader are:

  1. an association with human activity

  2. a previous history of successful invasion

• (due in part to a larger pool of source populations)

Two commonly recurring predictors of a successful invader are:

1. an association with human activity

2. a previous history of successful invasion

The higher the propagule pressure

the larger and more frequent the initial populations - the higher the chances of a successful invasion

propagule pressure is not constant

invasion pathways grow and shift, particularly due to human trade and transport

propagule pressure and small populations

Most invasions begin as small populations; they must tolerate abiotic and biotic conditions of a new environment and face problems unique to small populations, including:

Allee effect

Demographic stochasticity

Environmental stochasticity

Inbreeding depression: reduced survival and fertility of offspring of highly-related individuals

Allee effect

positive correlation between population density & individual fitness.

Mechanisms: mate finding, group defense, sex ratios, inbreeding depression

Demographic stochasticity

variability in population growth rate with population density; due to random differences among individuals in survival and reproduction

Environmental stochasticity

unpredictable variation in environmental conditions in space and/or time

inbreeding depression

reduced survival and fertility of offspring of highly-related individuals

Which species are good invaders?

Characters that enhance invasibility

1. High fecundity

2. ability to spread vegetatively

3. parthenogenetic or hermaphroditic reproduction

4. broad physiological tolerance and diet

some successful invaders will not have these traits, and some organisms with these traits may not be successful invaders

an invasion pathway may favor different characteristics than does the establishment process

invasion steps

introduction, establishment, and spread

category characteristics - successful

faster growth, wider temperature tolerance, wider salinity range

category characteristics - quickly spreading

slower growth, wider temperature tolerance, lower survival at high temperature

category characteristics - nuisance

better survival at low temperature, wider salinity tolerance, smaller eggs

quirk of fate ex Argentine ants

• Argentine ants in their native range exhibit high inter-colonial aggression

• Invasive populations, which are a major threat to native species, are much more cooperative among colonies, but maintain their aggression towards other species

• This altered social behavior appears to have a genetic basis, and is found only in the introduced populations

Success as a “quirk of fate”

Enemy release

Introduced populations are likely to acquire new parasites and diseases in the new environment, BUT a sufficient delay may enhance its initial prospect of success

enemy release

the few introduced individuals that found a new population happen to be free of a parasite or disease

invaded community characteristics

• every community is invasable given the right introduced species

• if some communities are more easily invaded than others, conservation efforts could be prioritized to protect vulnerable communities

• there are 2 primary features

  • First, the climate and habitat must be hospitable to potential invaders (e.g., the Sahara Desert may not be hospitable to most species)

  • Second, the species richness, interaction strength, and trophic structure of the community, must be able to accommodate new species (e.g., biotic resistance hypothesis)

biotic resistance hypothesis

species rich systems are more stable and therefore less susceptible to species invasions

Invasive tree experiment on Christmas Island

• Green et al. (2004) placed seedling of 4 different tree spp. in forests and forest edges on Christmas island

• caged half the seedlings to prevent predation by herbivorous crabs

• in two species there was no survival in the forest interior

• indirect evidence for species rich natives helping combat invasives

disturbance hypothesis for species invasions

• Disturbance - either natural or anthropogenic - makes a community more invasible

• this can be due to several reasons:

  • disturbance may make resources more available, allowing invaders to get a toehold

  • new forms of disturbance may disfavor native species that are not adapted to it

  • Invaders themselves can be agents of disturbance, altering natural disturbance regimes (fire, hydrology) by causing physical disturbance themselves, and by simply consuming other species

Resource availability and susceptibility to invasion

Increased resource availability increases a plant community’s susceptibility to invasion

The resource availability at a given time (A) could increase due to an increase in resources (B), a decline in resource use by a resident species (C), or both (D) (Davis et al., 2000)

          

Unintentional pathways

species can hitchhike on any type of human transport from airplanes to cars and especially ships (ballast)

ship ballasts

originally dirt was used and introduced invasives

By the late 1800’s wooden ships with solid ballasts were replaced by metal ships with water ballasts

Water is taken in through a grate, allowing plankton and even larger organisms to enter

Any that survive the voyage are released alive upon arrival

intentional pathways

Agriculture, aquaculture, recreation, erosion control, biological control & ornamental purposes

• europe brought different species while colonizing

How do we manage species invasions

Eradication is usually not possible (invasive species islands are often an exception)

Control is next option

1. Physical control (trapping, hunting, digging, pulling, collecting)

2. Chemical control (pesticides, herbicides, medication)

3. Biological control (introduction of nonnative predator, parasites or diseases)

4. Multiple approaches

pitfalls of biological control

1. We may introduce a new species of pest

2. The biocontrol agent may ‘jump’ into other species

3. The biocontrol agent may spread into other countries

Invasive species control: conservation vs. ethics

If we have a proven effective control method, conservation trumps animal ethics; that is, (ethically) killing individual animals is warranted for the sake of the invaded community

If however, our control method is not effective in at least partially reversing the damage, then conservation does NOT trump animal ethics; that is, we are killing for no reason, and can be guilty of animal cruelty