Habitat loss and degradation
Loss and degradation of terrestrial and aquatic habitats are the primary causes of species' rarity in Minnesota. Habitat degradation includes loss of diversity, fragmentation, disruption of critical processes such as fire, and water quality degradation due to pollutant chemicals, nutrient input, or sedimentation/siltation.
The prairie vole was once a common species in the Great Plains, including the tall grass prairie in southern and western Minnesota. Today, it is uncommon in the state due almost exclusively to the destruction of its prairie habitat through plowing and over-grazing.
Invasive species and competition
Nonnative invasive species and native species outside of their natural range contribute to the decline and elimination of native species by carrying disease, altering and degrading habitat, and competing with or preying on native species.
The viability of remaining Higgins eye mussel populations in Minnesota is jeopardized by the infestation of nonnative zebra mussels (Dreissena polymorpha) in the Mississippi River and its tributaries. Zebra mussels can attach themselves in large numbers to the shells of native mussels, eventually causing death by suffocation.
Pollution
Contaminants such as pesticides, herbicides, heavy metals, and automobile and industrial emissions, as well as sedimentation and siltation in aquatic systems, continue to play a role in the decline of many species in Minnesota. Species can be affected directly by the toxicity of pollutants, or indirectly by the influence that pollutants have on their food resources.
Lead poisoning is one of the greatest threats to trumpeter swans in Minnesota and responsible for about 40% of the swan fatalities in the state. Trumpeter swans ingest lead fishing sinkers and lead shots when they eat grit and forage for tubers and roots from marsh and lake bottoms.
Habitat specificity
Some species are rare because they depend on rare, declining, or vulnerable habitats in Minnesota.
The southern brook lamprey requires different microhabitats during different portions of its life history. Adults need very clear, small to large streams and rivers, with swift, permanent flow over sand and gravel riffles. Larvae need fine sediment or organic debris in areas with embedded woody debris for burrowing, usually in lower gradient stream segments.
Persecution and exploitation
Some species in Minnesota are negatively impacted by recreational or commercial overexploitation (e.g., reptiles, orchids, medicinal herbs) or by indiscriminate killing due to some perceived threat or undesirable quality.
The major cause of the timber rattlesnake's decline in Minnesota is its vulnerability to systematic and willful destruction by humans. Snakes at den sites are particularly vulnerable to disturbance and poaching.
Peripheral populations
Some species are considered rare in Minnesota because of where they exist in relation to their entire range. As species reach the far edges of their natural range, they may become less common, and may be considered rare if their populations are low in numbers or geographically limited.
The Laurentian tiger beetle is known only from Ontario, Manitoba, and Minnesota, and is apparently found only north of the Laurentian Divide in Minnesota.
Vulnerable characteristics
Some species are rare in Minnesota because they have certain characteristics that make them vulnerable. Some species require large home ranges or habitat patch sizes, some have low dispersal abilities or low reproductive rates, some are highly localized or have a restricted distribution (endemic species), and some concentrate their populations during some time of the year (such as bats clustering in hibernacula).
The dwarf trout lily is endemic to Minnesota, occurring worldwide only in Rice, Goodhue, and Steele counties. Because of its inefficient system of reproduction and dispersal, the dwarf trout lily has expanded its range very little and remains restricted to small portions of certain drainage systems in just these three counties.
Relict populations
Relict species were probably once more common in Minnesota when the climate was different than it is today. As the climate changed, most populations of these species were eliminated, but small, isolated populations survived in appropriate habitats.
The bluff vertigo is a rare landsnail found in a small number of sites in southeastern Minnesota. This species was more widespread during the Wisconsin glacial period (12,000 years ago), but it now survives only as small relict populations with very specific habitat requirements.
Disease
Introduced diseases and native diseases outside of their natural range can contribute to the decline of native species either through direct mortality or by increasing species' vulnerability to other limiting factors.
The emergence of white-nose syndrome, a fungal disease that is decimating hibernating bat populations in the eastern United States, poses a potential threat to northern myotis populations in Minnesota.
Global climate change
The complexities of global climate change present uncertainty in anticipating its impacts on rare species in Minnesota. It is reasonable to expect that climate changes that result in warmer, dryer weather will alter critical habitats that support many rare species. These changes in habitat may ultimately lead to extirpation of populations of rare species.
Textured lungwort grows on trees and mossy rocks in moist areas. Any environmental changes that disrupt the humidity of its habitat, such as global warming, will likely have a negative effect on this rare lichen.