INVADING SPECIES AS AGENTS OF EXTINCTION

NRE 220 Lecture 7

Corresponding Readings in Primack, Richard B. Essentials of Conservation Biology.
Chapter 10: pages 259-276

Introduction

Wherever humans have colonized, other species have been introduced, either purposefully or as fellow travelers. These alien species have often caused severe damage and the extinction of native species, especially on oceanic islands. This is because the isolation of islands permits the evolution of species often found nowhere else, and thus often unable to withstand the alien onslaught. Such events have occurred in earth history, as in the great faunal interchange between North and South America. In recent history, invasions have been greatly facilitated by the travels of our own species. Extinction of species and homogenization of the world’s biota are principal consequences. We will first see how many recent extinctions can be attributed to this cause, look at some case studies, and then proceed to discuss the ways that invaders vanquish resident species

One Example

The African Great Lakes – Victoria, Malawi and Tanganyika – are famous for their great diversity of endemic species, termed "species flocks", of cichlid fishes. A single exotic species, the Nile Perch, has become established and may cause the extinction of most of the native species, by simply eating them all. It was a purposeful introduction for subsistence and sports fishing, and a great disaster.

The Extent of the Problem

The extent of the spread of species is hard to document, but it involves many thousands of species. Table of estimated numbers in USA. Many pests are non-indigenous species, and attempt to locate and introduce natural enemies is a primary tool of biological control. Non-indigenous species (NIS) cause significant dollar losses and contribute to species endangerment, although NIS have both beneficial and harmful effects (and sometime no effect, or the effect is unknown). Maps of the distribution of NIS, and their rate of spread, also help us recognize the seriousness of the problem.

Invasive Species, Invadable Communities

We don’t really know what makes a species a good invader, or what makes a community more invadable, although the following list gives some likely explanations.

NIS and Extinctions

Of documented extinctions since 1600, introduced species appear to have played a role in at least half. The clue is the disproportionate number of species lost from islands: 93% of 30 sp and sub-sp of herps, 93% of 176 sp and sub-sp of land and freshwater birds, but only 27% of 114 sp and subsp of mammals. Islands have many endemic species that have evolved with few competitors and predators, hence are extremely vulnerable to NIS. Mammals have disappeared from continents due mainly to over-hunting. Of course, there are many more islands than continents, islands contain many unique species, and islands have proven especially vulnerable to invasion. Increased human travel, and the strategic value of island chains during WWII, increased the rate of spread of NIS greatly during this century.

The Worst Actors

About 50 species of mammals and >100 bird species have been introduced to continents. On islands the numbers are greater -- over 80 species of mammals and 300 of birds. The rate of introduction increased steadily from the 1700's until WWII, when the establishment of military bases resulted in a peak of introductions, particularly of rats and cats. Despite the large number of species involved, most of the damage can be attributed to only nine alien species: goat, rabbit, pig, cat, three species of rat, mongooses, and ourselves

Mechanisms Of Extinction By Invading Species

  1. predation: species with no natural enemies may act naive and tame. Eg, Pleistocene humans, many rat/cat examples.
  2. browsing and grazing: reduces plant populations directly or by trampling, and may give a competitive edge to plant species better defended against herbivory. Eg, goats, rabbits
  3. hybridization: genetic identity may be lost if an invading species interbreeds with a native. possible examples include the red wolf (interbreeds with coyote) and the cutthroat trout (rainbow).
  4. competition: NIS may have competitive or reproductive advantage over native species. Introduced carp now constitutes most of biomass in area rivers. May be less common. In a fair fight, most native faunas will hold their own against invaders. However, competitive exclusion often does take place when conditions have been altered to make the environment less than optimal for the native.
  5. Disease/infectious organisms: a blight disease from China has nearly wiped out the American Chestnut tree. Dutch elm disease, chestnut blight

Transparencies: 1. Estimated numbers of non-indigenous species in the US 2. Estimated cumulative $$ losses from harmful NIS 3. Contributionsd of NIS to endangered species listings 4. Beneficial vs harmful effects of NIS 5. State-by-state distributions of some NIS 6. State-by-state spread of NIS 7. Characteristics of invasive species and invadable communities 8. Worldwide extinctions of several groups of vertebrates 9. Introductions to islands and continents since 1700

 

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