The Hawaiian archipelago, formed by volcanic activity and isolated by thousands of miles of ocean, developed an ecosystem with unique flora and fauna that evolved largely in the absence of terrestrial mammals. This isolation means that native species lack natural defenses against new predators, making the islands vulnerable to invasive species. Despite this, mice and other small rodents have established widespread populations across the islands.
Identifying the Species of Mice Present in Hawaii
The term “mice” in Hawaii generally refers to the true House Mouse (Mus musculus) but often includes the smaller Polynesian Rat (Rattus exulans) due to its size. The House Mouse is the smallest of the four invasive rodent species found across the Hawaiian Islands, weighing about half an ounce and measuring six to seven inches long from nose to tail tip. This species is found in urban settings, agricultural areas, and low-elevation forests and beaches across the major islands.
The Polynesian Rat, while technically a small rat, is significantly smaller than the Black Rat or Norway Rat, weighing between 1.5 and 3 ounces. It has a slender body, a pointed snout, and a tail roughly the same length as its head and body combined. Both the House Mouse and the Polynesian Rat are widely distributed across all the main Hawaiian islands, thriving in diverse habitats from sea level up to elevations of nearly 10,000 feet.
The Historical Pathways of Introduction
The arrival of rodents in Hawaii occurred in two distinct historical waves, tracing the paths of human migration. The first introduction was the Polynesian Rat, which arrived with the early Polynesian voyagers around 1000 AD. These rodents were stowaways on the double-hulled voyaging canoes, often carried accidentally in food stores.
This initial arrival of Rattus exulans quickly spread throughout the islands, becoming a permanent fixture in the lowland forests and agricultural areas preferred by the early human settlers. Their presence serves as a biological marker, reflecting the extensive reach of the ancient Polynesian seafaring culture.
The second wave of rodent arrivals, which included the House Mouse (Mus musculus), began with the post-European contact era starting in the late 18th century. Whaling ships, cargo vessels, and military transports inadvertently carried these new invaders across the Pacific. The House Mouse, along with the larger Black Rat and Norway Rat, stowed away in the ballast and cargo hold of these modern ships.
The House Mouse is a highly successful commensal species, meaning it lives in close association with humans, which facilitated its global dispersal via shipping networks. The House Mouse’s ability to breed rapidly allowed it to establish populations quickly upon arrival. These later-arriving species were able to colonize areas, including high-elevation forests.
Ecological Impact on Native Hawaiian Biota
The introduction of mice and rats poses a threat to the native biota, which evolved without mammalian predators. The island’s endemic species, such as flightless insects, ground-nesting birds, and land snails, lack evolved defense mechanisms against these threats. This lack of defense means that rodents can cause damage to vulnerable populations.
House Mice and Polynesian Rats are omnivores, and their diet includes a wide variety of native organisms. They actively prey upon the eggs and chicks of ground-nesting seabirds and forest birds, contributing to population declines. Research confirms that these rodents consume native invertebrates, including rare species of Hawaiian land snails.
Beyond direct predation, these invaders significantly affect native flora. Rodents consume the seeds and seedlings of native plants, disrupting the natural regeneration of the forests. The Polynesian Rat has been implicated in the decline of native lowland palm forests, such as the Pritchardia species, by eating their seeds and preventing forest regrowth.