Biology 12

494 MHR • Unit 5 Population Dynamics

Biology Magazine TECHNOLOGY • SOCIETY • ENVIRONMENT

Alien Invaders

What do rice, dogs, gypsy moths, and Ophiostoma ulmi

(a fungus that causes Dutch elm disease) have in

common? All originated in other parts of the world and

were introduced to North America by people. They are

non-indigenous, or alien, species. Many species were

transported deliberately to provide food and other

materials, or for purposes of sport, landscaping, or

biological control. Some were introduced to this continent

to be pets. Many others, however, arrived as unwanted

stowaways — hidden passengers on ships and aircraft.

The rate of transportation of species from one part of the

world to another has increased greatly over the past

40 years, largely due to an increase in international trade

and travel. Some alien species are not suited to their new

environment and do not survive. Others thrive and rapidly

expand their populations and range. When uncontrolled,

these invasive organisms produce catastrophic changes

in local ecosystems. They compete with and prey on

many indigenous species. They have become a major

cause of species extinction and a loss of biodiversity.

An estimated 50 000 non-indigenous species live in

North America today, and their number is increasing

yearly. Over 40 percent of the plants and animals on the

threatened or endangered species lists in North America

(and as many as 80 percent of endangered species in

other regions of the world) are at risk as a result of

pressures created by alien invaders.

The Alien Advantage

Why do some introduced species undergo a rapid

population growth? In their original home, their numbers

are limited by the availability of resources (such as food,

soil nutrients, and water) and by various predators,

diseases, or competitors. When transported to another

environment, they potentially have a double advantage.

Their new home may lack some or all of the predators,

diseases, and other factors that kept their numbers in

check, and it may have more available resources

because of less competition.

The process of natural selection ensures that species

have continually adapted to their surroundings over time.

Moving organisms from the ecosystem in which they

evolved to a different ecosystem that has had no

previous experience with them suddenly creates a new

selective factor that may put native organisms at a

disadvantage. For example, many species living on

islands evolved without the predators and competitors

faced by their relatives on the mainland. As a result, the

island populations commonly lack important defences.

Why put energy and materials into structures and

behaviours that are not needed? Many island birds

cannot fly and many island plants do not have the thorns

or bitter tastes and poisons that deter herbivores. When

predators such as rats, snakes, and cats are introduced

to islands, flightless birds are helpless and are quickly

reduced in numbers or exterminated. When herbivores

such as goats and cattle are taken to islands, the native

vegetation is overgrazed and destroyed.

Invading species have dramatically altered aquatic

ecosystems as well as terrestrial ones. In the introduction

to this chapter, you learned about the zebra mussel. It

colonizes on surfaces such as docks, boat hulls,

commercial fishing nets, water intake pipes and valves,

native molluscs, and other zebra mussels. A rapid

breeder, zebra mussels have been known to reach a

density of 700 000/m^2 in some locations. These large

mussel populations have completely covered and

displaced native mussels, clams, and snails, and have

reduced food and oxygen for native fauna. Their only

known predators are some diving ducks, fresh-water

drum, carp, and sturgeon, but these predators are not

numerous enough to have a significant effect on them.

As well, zebra mussels have caused billions of dollars in

For a particular population, ecologists often

summarize the relationship of survivorship (and

its opposite, probability of death, which is often

referred to as mortality) and fecundity to age in

what is called a life table. The first column of a life

table typically shows the age of individuals in the

cohort being studied. In Table 14.2 on page 496, age

is expressed in years, but in life tables for short-

lived species, it may be in months or even days.

Life tables also vary with respect to the amount

of detail they include. Although ecologists

sometimes build tables with many columns

expressing survivorship and fecundity in different

ways, many look just like the one shown in

Table 14.2 on page 496. The third column of the

table records age-specific survivorship — the

proportion of the cohort that survives to the start of

the age class. For example, of the 530 grey squirrels

forming this cohort, only 0.253 or 25.3 percent

were alive one year later. Put another way, when

one of these squirrels was born, it had a

25.3 percent chance of surviving for one year.

Understanding survivorship is important for

making accurate estimates of future population size.