Invasive Mammals of the Pacific

Science Center Objects

The terrestrial biota of the Central Pacific is primarily defined by its degree of isolation.  At the center lies the Hawaiian Archipelago, which is more than 3,200 km from any continental land mass.  After tens of millions of years of evolutionary isolation from all mammals except bats, islands of the Central Pacific were quite suddenly besieged by a number of alien rodents, carnivores and both large and small herbivores. The first mammals were introduced by early canoe voyagers of the Pacific more than 1,000 years ago.  The discovery of the Hawaiian Islands by Cook in 1778, like many other islands of the Pacific, marked the second wave of introductions of many hoofed animals for beasts of burden, milk, hides and meat as well as small predators to keep an assortment of stowaway rodents at bay.

Large feral pig captured on a remote wildlife camera

Large feral pig, photographed at night using an IR wildlife camera. Photo: USGS

Overview:

The terrestrial biota of the Central Pacific is primarily defined by its degree of isolation. At the center lies the Hawaiian Archipelago, which is more than 3,200 km from any continental land mass.  After tens of millions of years of evolutionary isolation from all mammals except bats, islands of the Central Pacific were quite suddenly besieged by a number of alien rodents, carnivores and both large and small herbivores. The first mammals were introduced by early canoe voyagers of the Pacific more than 1,000 years ago. The discovery of the Hawaiian Islands by Cook in 1778, like many other islands of the Pacific, marked the second wave of introductions of many hoofed animals for beasts of burden, milk, hides and meat as well as small predators to keep an assortment of stowaway rodents at bay.

Rapid ecological degradation ensued and whole groups of endemic plants and animals suffered extinctions, including virtually all flightless birds, and roughly nine percent of all Hawaiian flora.  After a century of settlement by westerners, the concept of eradicating non-native species came about as a solution to primarily agricultural, public health, or economic problems, and only more recently to solve ecological problems.  Reversing the devastating effects of alien mammals has proven to be difficult, but limited successes have resulted in the dramatic recovery of native biota.

Mammals General Overview

Mouflon running up a hillside

Herd of mouflon sheep as seen from a helicopter. Photo: S. Hess

Most of the isolated islands of the Pacific existed for tens of millions of years in the absence of terrestrial mammals except for bats, which were able to fly over vast oceans and establish themselves.  The first human colonists of these islands brought with them domestic pigs (Sus scrofa) from island southeast Asia and Polynesian rats (Rattus exulans).  European colonists brought an assortment of other mammals with them nearly 1,000 years later, including domestic goats (Capra hircus), sheep (Ovis aries), cattle (Bos taurus), other types of wild and domestic pigs, cats (Felis catus), rabbits (Oryctolagus cuniculus), rats (R. rattus and R. norvegicus), and mice (Mus musculus).  In the absence of natural predators and competitors, these species became abundant on nearly every island, which caused rapid ecological degradation.  Native plants of the islands were poorly defended against the newly established herbivores, and native wildlife were naïve to tree-climbing predators with teeth.  Species which had never been previously domesticated such as axis deer (Axis axis), European mouflon sheep (O. musimon), and mongooses (Herpestes javanicus) were brought to the islands later and proved to be particularly challenging to manage.  Diseases associated with introduced mammals such as toxoplasmosis (Toxoplasma gondii), bovine tuberculosis (Mycobacterium bovis), and bubonic plague (Yersinia pestis) have all caused major problems for wildlife, livestock, and human health in Hawai‘i.  Invasive mammals have caused the decline and extinction of numerous native plants and animals, but researchers and land managers have developed progressively more effective strategies for managing these species, allowing the recovery and restoration of native species in increasingly larger natural areas. 

Small Mammal/Toxoplasmosis Overview

Image: Hawaiian Monk Seal (Monachus schauinslandi)

Toxoplasmosis carried by invasive mammals is the leading cause of death for the Hawaiian Monk Seal.

(Credit: Randolph Femmer, USGS. Public domain.)

Many factors have contributed to the decline and extinction of birds in Hawai‘i, not the least of which has been introduced mammalian predators like rats, mongooses, and cats.  These predators take eggs, kill nestlings, or nesting adults, but are also sources of diseases that kill birds.  One such disease known as toxoplasmosis can cause severe developmental disabilities and occasional mortality in humans but can also have lethal consequences for marine mammals.  The infectious agent of the disease, Toxoplasma gondii, is a protozoan, a single-celled organism which behaves like an animal.  It is considered a parasite, causing infection directly rather than by producing toxins.  It is a zoonosis because it is a disease of animals that can also be transmitted to humans.  Only cats –regardless of whether they are domestic or wild species– can support sexual reproduction of this parasite, and thereby serve as the definitive host of the disease.  Any other warm-blooded animal, most commonly rodents, can also carry the parasite but can only serve as an intermediate host.  Intermediate hosts of the parasite and may sometimes be consumed by people, leading to infection.  Known intermediate hosts include feral pigs (Sus scrofa) and mouflon sheep (Ovis musimon), both popular game mammals in Hawai‘i.  Birds can also be intermediate hosts, and populations that have been infected and killed by T. gondii include the critically endangered ‘Alalā (Corvus hawaiiensis), the endangered Nēnē (Branta sandvicensis), Red-footed Booby (Sula sula), and Erckel’s Francolin (Pternistis erckelii), a common gamebird.  Flowing water may transport T. gondii oocysts –hardy, thick-walled, environmentally resistant spores– in runoff from land to streams and then into marine environments.  Thus, T. gondii represents a land-based pathogen that can pollute nearshore marine ecosystems and infect a wide variety of animals in this environment including whales, seals dolphins, and sea otters.  The leading cause of death of the endangered Hawaiian monk seal (Monachus schauinslandi) is now known to be T. gondii infection.  Many aspects of T. gondii are poorly understood, but current research may soon lead to more effective management strategies to protect native wildlife from both toxoplasmosis and predation.