Interplanetary species

This article is based on canon information, but the article's title is conjectural and/or uses descriptions as the title, as there is currently no official name for the subject.

Concept art for multiple varieties of Spadehorn, an exotic creature native to Reach that resembles the bighorn sheep of Earth.

As space-faring sentient species began colonizing star systems throughout the Milky Way, many planets were discovered with thriving populations of identical or near-identical plant and animal species, some recognizable from the colonists' respective homeworlds. In many cases, this was a result of the Forerunner Conservation Measure and manipulation by the Lifeworker Rate. In other cases, the cause was not as clear.

On planets where native species were not identical, they largely tended to follow standard morphological baselines to those observed on Earth. Fish looked like fish, insects looked like insects. Amphibians, reptiles, avians, and mammals could all largely be grouped into the same class as their Earth counterparts, even if local environmental pressures caused them to evolve along alternative biological paths. For example, the Jiralhanae homeworld Doisac had a species of cattle which were used by the Jiralhanae similarly to Earth cattle, and the Kig-Yar homeworld Eayn had species of hawks and vultures. Some of the more exotic life forms could also be found distributed across multiple worlds.

Invasive ecology

"Atmospheric processors could nudge a generally suitable planet toward sustainability, but they couldn’t remake worlds."

— Halo: Contact Harvest

Agricultural livestock in a kiva on Reach.

In the process of terraforming worlds, humanity employed various methods to nudge local environments towards Earth-normal and sustain a growing population of colonists. Many of these involved the introduction of human agriculture and livestock, as well as forms of wild plant and animal life to balance ecosystems and atmospheric levels for human habitability. Some species were also likely introduced by accident after they stowed away aboard ships.

The following is a list of known Earth species that made their way onto alien planets through human action.

Flora Plant Locations introduced Agricultural crop Boundary, Harvest, Mamore, Reach Amaryllis Meridian Canna lillies Meridian Magnolias Harvest

Fauna Animal Locations introduced Agricultural livestock Boundary, Harvest, Reach, unidentified human colony Bats Harvest Crows Bliss Gophers Harvest Honeybees Harvest Mourning warblers Sedra Owls Reach Starlings Harvest White sturgeon Sedra Wolves Reach, Dormire III

Earth-like ecology

The Earth-like forests of Circinius IV.

On other worlds, Earth-native species (or species which appeared visually identical) were discovered prior to modern human interference. On many planets, it was common to find terrestrial grasses, ferns, lichen, and algae. On others, more notable species were observed, sometimes dwelling alongside exotic, alien flora, other times part of ecosystems nearly identical to those of Earth.

The following is a list of known Earth species that exist on alien planets not as a result of modern human action.

Flora Plant Locations found Acacias Onyx Ash tree Sedra Banana plant Arcadia, Meridian Banyan trees Meridian, Onyx Beech tree Sedra Birch tree Sedra Blackberry Sedra Black cottonwood Circinius IV Cashew tree Meridian Cedar tree Alpha Halo, The Ark, Delta Halo, Harvest, Reach, Sedra Coconut tree Meridian Conifer trees The Ark, Delta Halo, Eridanus II, Heian, Nysa, Reach, Sedra, Trove, Warial, Zeta Halo Date palm Arcadia Dandelion Sedra Douglas fir Reach, Sedra Eastern white pine Trove Elderberry Sedra Elephant ear Arcadia Golden rain tree Meridian Hawthorn Circinius IV Hazelnut Circinius IV Huckleberry Circinius IV Juniper Concord Kapok tree Meridian Lamb's ear Arcadia, Trove Larch tree Sedra Maple tree Sedra Moonflower vine Meridian Oak tree Harvest, Sedra Pacific dogwood Circinius IV Peppermint High Charity Red alder Circinius IV Rhododendron Sedra Rubber tree Meridian Sage Concord Spruce tree Circinius IV, Sedra Tobacco plant High Charity Vine maple Circinius IV Water lillies High Charity Western hemlock Circinius IV Western red cedar Circinius IV Willow tree Sedra Yew tree Sedra

Fauna Animal Locations found Bluegill Alpha Halo Butterflies Monarch butterfly The Ark, Reach, Trove, Zeta Halo Alpha Halo Fireflies Alpha Halo, Reach Seagulls Alpha Halo

Exotic ecology

"There were these big damn plants, like tentacles. I swear, it was like the planet was fighting us, too."

— Lieutenant Caitlin de Sauveterre

Exotic plant life on Draetheus V.

Some forms of life were distinct from those found in Earth ecosystems, despite sharing (in most cases) morphological similarities. A number of these exotic organisms had similar ubiquity across multiple star systems. In some cases, life forms were discovered which resembled prehistoric fauna from Earth's Pleistocene epoch, and some may have actually been transplanted as a result of the Forerunner Conservation Measure, continuing to evolve long after they went extinct on Earth. Some exotic species, while first discovered on alien worlds, were transplanted to other planets as a result of interstellar trade.