Primack: Chapter 10.
1. What are invasive
species?
A)
Definitions
i)
Native –
applies to a species that is found within its natural range.
ii)
Introduced –
applies to species that have been moved to a new area (usually by humans).
iii)
Exotic or alien – applies to species when found
outside of their natural range (i.e., after being introduced).
iv)
The terms
“non-native”, “introduced”, “exotic” and “alien” are often used
interchangeably.
v)
Invasive –
species that spread rapidly, and undergo explosive population increases, such
that they dominate a community. Usually
things that are called “invasive” cause some problem (from the perspective of
humans). Usually invasive species are
also introduced, but sometimes the term is used for species that are native to
an area.
B)
Why are introductions/invasives a problem?
i)
Because they
alter species interactions, often to the detriment of native species.
ii)
For example,
invasive species can affect other species by preying upon them, by
out-competing them, by causing disease, etc.
They also can have effects indirectly, e.g., by altering the way in
which the ecosystem functions. For
example, beavers have been introduced into southern Chile – what kind of
consequences do you think this would have?
iii)
Note, that the
interactions among native species can change dramatically as a result of human
activities, creating effects that are similar to those of introduced
species. Examples, to think about
include: removing a key predator and allowing its prey to increase dramatically
(e.g., wiping out wolves resulted in a big increase in coyotes and other medium
sized predators); increasing the food available for some species (e.g., waste
grain in the farm-belt has caused massive increases in various geese
populations); altering the habitat (e.g., cowbirds, discussed in a previous
lecture, have increased because of forest fragmentation and increased cattle
populations).
C)
But, introductions are not always a problem
i)
In some cases,
introductions can be beneficial, for example:
–
Some very rare
species have been introduced to new areas where they do not face the threats
that exist in their native range.
–
Sometimes
introduced species provide valuable habitat for threatened species.
–
Some species are
introduced for very specific purposes that are considered valuable (maybe even
necessary) for humans – e.g., nearly all agricultural crops are
introduced.
ii)
Most
introductions fail, and only a very small proportion of all introduced species
become invasive (but note that a small proportion of a big number of
introductions can still result in a lot of invasive species).
iii)
For a species to
become invasive it must overcome three (big) challenges: (a) it must get
somewhere (which means somehow being transported there), (b) it must become
established once it has arrived (which requires conditions conducive to
avoiding rapid extinction), and (c) it must undergo explosive population
growth. At each of these steps, most
species fail.
D)
Can we predict which species will be invasive?
i)
In a very gross
sense we can determine which species are likely to become invasive. Successful invaders have a host of typical
characteristics:
|
Successful invaders |
Unsuccessful |
|
abundant in original range polyphagous (eats lots of things) short generation times much genetic variability fertilized female able to colonize larger than most relatives associated with humans habitat / environmental
generalist disturbed habitat users |
Rare monophagous long little not smaller not specialist not |
ii)
Not all invasives have all of these features, but most probably have
at least a few. It would be a good idea
for you to think about why each of the things in this table help species to
become invasive.
iii)
Even though there
are a host of biological characteristics that are good predictors of invasion
success, there are many exceptions, and many invasions that cannot be explained
as easily as we would like.
iv) Some of the things that influence invasion success have nothing to do with the biological characteristics of the organisms. For example, one of the best studies of what it is that allows a species to become established after being introduced looked at introduced birds in New Zealand. This study was good because there were very detailed records about the earliest stage of the invasion (something we typically know little about). In this study, none of the biological factors considered helped to predict which species would become established. But, two other variables did correlate with the success of a species. Species for which a lot of individuals were released, or for which there had been multiple attempts at introduction were most likely to become established.
2. Why do people move
species to new areas?
A)
Hunting/fishing
i)
Example: The lakes of the Rift Valley in East Africa have
incredibly high fish diversity. This
small area contains as many fish species as all of Europe and North America
combined. The Nile perch, however, was
introduced to provide food for the people who lived in the area. This species is a voracious predator and has
been blamed for the widespread loss of many fish species. In Lake Victoria there were 300 species of
cichlid fish before Nile perch was introduced.
Now there are fewer than 100 species.
ii)
Example: European rabbits were introduced to Australia as a
source of food. But, the population
rapidly took off (they bred like …. OK you get it), and within a few decades
they had spread across the entire continent and numbered in the hundreds of
millions.
B)
Aesthetic reasons
i)
Example: Eurasian starlings (and many other species) have
been introduced into North America and various other places around the
world. Originally, these birds were
introduced by organized groups that wanted to populate the New World with
familiar species from home – some of these societies had the major goal of
trying to introduce all the birds mentioned in Shakespeare’s plays. In many places, starlings have become very
common and now compete with native species for nest sites (starlings nest in
tree cavities, which are usually occur in limited numbers) and maybe also for
food. In North America about 100 birds
were initially released in New York City.
Now there are estimated to be well over 100 million, and they occur
across the entire continent.
ii)
Example: Species brought in as pets can also wreck havoc on
native populations. In one especially
dramatic example, a single cat (belonging to the local lighthouse keeper) wiped
out an entire species: the Stephen’s Island wren, which was found only on a
small island near New Zealand.
C)
Horticulture/agriculture
i)
Many (probably
most) invasive plants were initially introduced as plantings in peoples’
gardens (or other ornamental settings).
Most species grown in gardens have stayed in gardens, but many have not.
ii)
A few examples of
invasive plants include: Kudzu, which is now found in the southeastern US and
which completely overgrows everything it encounters. Tamarisk (also known as salt cedar), which is
introduced into the deserts of the southwest and helps to displace native trees,
while also lowering the water table and making it harder for native species to
survive.
iii)
Finally, purple
loosestrife is a species that you can easily find around Storrs (even on
campus). It is a very attractive plant
and is consequently favored as a garden ornamental, but it tends to completely
take over wetlands out-competing all other species.
D)
Accidental transport
i)
Example: The brown tree snake is an Australasian species that
was introduced to the island of Guam (in the Pacific) after World War II. It is thought to have got there accidentally,
probably by stowing away on military aircraft.
After a period of rarity, the population suddenly exploded with the
result that there were snakes everywhere.
Coincident with this population increase, all species of forest birds on
the island (including some found nowhere else in the world) began to
decline. Now they are all gone from the
island; predation by the snakes is thought to be the primary reason.
ii)
One major way in
which aquatic species get transported around the world is in the ballast water
that large ships take on-board. This
water may be taken onto a ship in one part of the world, and then discharged
(along with all the critters it contains) thousands of miles away. It is estimated that 40,000 gallons of
foreign ballast water is discharged into US waters every minute.
iii)
Many other routes
for accidental transport exist. These
include: via the mail, on airplanes (see above), in the dirt on someone’s boots
(seeds, fungal spores), in shipped products (e.g., various forest insect pests
probably get around in wood shipments), etc. etc.
E)
Incidental invasions due to human activites
i)
Explosive
population growth of “white” geese :
Most species that are termed invasive are introduced. But sometimes the term is used for native
species that have expanded their ranges (e.g., like brown-headed cowbirds,
which we discussed in a previous lecture), or that have simply undergone
massive population growth. An example of
the latter, is two species of North American geese – snow and Ross’s
geese. These birds breed in the Arctic
and winter in the southern US (in California, and along the Gulf and Atlantic
coasts). Their populations have steadily
increased in recent decades, largely because of increased food availability at
wintering/migration sites, which tend to be agricultural areas where there is
now a lot of spilled grain available after harvest. Predation during these periods is also higher
than it was historically (both because there is much less human hunting, but
also because many natural predators have declined). A consequence of these population increases
is widespread damage to Arctic breeding grounds, where the geese overgraze the
tundra. For instance, it has been
estimated that 65% of the inter-tidal habitat in southern and western Hudson
Bay has been badly damaged by these geese.
In addition to having harmful effects on the plant species that live in
these areas, this grazing affects many other species of animal that live in
this habitat.
F)
Biological control
i)
Another reason
for introducing a species to an area is to try to undo the results of a
previous introduction. Frequently,
attempts have been made to control invasive species using biology. A common argument is that a key thing that
allows a species to become invasive is that it is “released” from the effects
of key predators or parasites that keep its population in check in its native
range. So, the argument goes, bringing
in the predator will help control the invasive.
ii)
The history of
biological control is very checkered – with many complete failures. For example, mongoose were introduced to
Hawaii (and elsewhere) to help control the (introduced) rats that had become
very abundant in sugar cane (also introduced) fields. Unfortunately, mongoose are diurnal while
rats are nocturnal – so it didn’t work.
Not only did the mongoose not control the rats, but they started to prey
on native species that they could find during the day instead – these included
several native wetland birds and their young.
Today mongoose are the subject of major predator control programs
(designed to protect native species) in may parts of Hawaii.
i)
Biological
control is not always a disaster. In
some cases it has worked very well – e.g. prickly pear cactus, which became
invasive after being introduced to Australia, was effectively controlled by the
introduction of Cactoblastis
moths which feed on the cactus. In
general, biological control is most likely to work (and to not cause other
problems) when the species brought in has a very specific effect on the species
that needs to be controlled. Extensive
testing prior to release is consequently an important part of many biological
control programs today.