Conservation Science

Protecting Pennsylvania’s Plants and Animals

Indicator Species

What do carnivorous pitcher plants, elusive hellbenders, singing woodthrush and humming mayflies have in common? They’re all important species known as indicators. An indicator species is any species (plant, animal, bird, aquatic, insect or even bacteria) that reflects the condition of its environment. Indicator species are often the first in their ecosystem to be affected by environmental change. 

Their presence can signify many things, including type or quality of habitat, health of an ecosystem or even land conservation value, just to name a few. 

Some indicator species occur commonly. For example, turkey and white-tailed deer indicate early successional habitat, woodthrush can indicate interior forest habitat, and mayflies can indicate good water quality on a stream. Other indicators are rare, such as freshwater mussels, which, like mayflies and other aquatic macroinvertebrates, indicate good water quality. The plant species found at a site reflect the site’s environmental conditions and degree of human disturbance, also called “ecological integrity.”  

The following is a small selection of indicator species found in Western Pennsylvania. Discover what they indicate, how these species inform the Conservancy’s work, and their conservation status in the 2015-2025 Pennsylvania Wildlife Action Plan 

Native Brook Trout (Salvelinus fontinalis)

Brook trout assessment efforts. Brook trout in hand, about 10 inches long with a body that’s wide in the middle and tapers at each end. Its back is olive green and its sides are tinged blue with yellow and red spots.

Pennsylvania’s state fish, brook trout grow 9 to 12 inches long with a body that’s wide in the middle and tapers at each end. Its olive green back has pale, worm-like markings, and its sides are tinged blue with yellow and red spots.

Indicates: High-quality cold-water stream, because reproducing brook trout require cold, exceptionally clean water 

Found In: Mountain (high gradient) streams and rivers, especially those with riffles and shade


Conservancy watershed scientists can note the presence of brook trout as an indicator of a successful project. For example, if a dam or culvert is removed to improve aquatic organism passage, the presence of breeding trout in the stream indicates that the project was successful because the trout can now move freely upstream to breed in a larger gene pool. We locate populations through our partnership in the Unassessed Waters Program with Pennsylvania Fish and Boat Commission, and improve habitat improvements through in-stream and riparian restoration. 

Conservation Status

Species of Greatest Conservation Need

Freshwater mussels (67 species in Pennsylvania) 

A variety of freshwater closed mussel shells, including fatmuckets and spikes, of various sizes. They vary in color from light brown to dark brown . Some have white markings on their shells.

Freshwater mussels have unconventional names such as snuffbox and wartyback, and can be mistaken for stones on a creek’s sandy bottom. But these hard-shelled, long-lived bivalve mollusks (also known as nature’s filters, lungs of the water and workhorses of the water) remove silt and reduce pollutants, filtering up to 15 gallons of fresh water daily.  

Indicates: Long-term stream health and good aquatic habitat 

Found in: Variety of aquatic ecosystems including streams, rivers and lakes 


Watershed conservation and PNHP teams conduct freshwater mussel surveys and have introduced juvenile mussels to streams to assess which streams are candidates for restocking freshwater mussels.  

Conservation Status

Of PA’s 67 freshwater mussel species, 12 are state or federally endangered or threatened and another 13 have not been observed in PA waterways for decades. The U.S. Fish and Wildlife Service has proposed salamander mussels be listed as endangered.  

Eastern Hellbender (Cryptobranchus alleganiensis alleganiensis) 

Eastern hellbender salamander has a stout body, wide flattened head, small eyes, and four short legs. It appears to be smiling. It's skin is grayish brown.

The eastern hellbender, Pennsylvania's state amphibian and North America's largest salamander, can weigh more than four pounds and grow as long as two feet. These reclusive nocturnal creatures live almost exclusively under large cover objects, such as flat rocks the size of a kitchen table. 

Indicates: Long-term ecosystem health 

Found in: Streams and rivers with good flow, and large substrate for nest rock locations 


Watershed conservation staff has studied the species since 2007 to document as many populations as possible. They use a variety of techniques to find this elusive animal, including: lift and turn surveys, SCUBA surveys and eDNA (environmental deoxyribonucleic acid).

Conservation Status

Species of Concern 

Scarlett Tanager, Wood Thrush, Evening Grosbeak and Other Forest-interior Birds 

Male Scarlet Tanager songbird sits on an oak twig. Its body feathers are bright red. It has black wings and tail and a thick beak.

Forest-interior birds, including scarlet tanagers and evening grosbeaks, are a particular group of forest birds that require large, mature forests to maintain populations. A variety of species rely on these habitats, including warblers, thrushes, vireos, flycatchers and tanagers. Of the nearly 110 species of forest-dependent breeding birds in Pennsylvania, approximately 40 require interior forest to maintain healthy populations. 

Indicates: High quality habitat especially with the presence of other forest interior birds 

Found in: Mature interior forests 


PNHP: Our Evening Grosbeak conservation Project  to join the Road to Recovery Initiative as one of their pilot projects for  On-Alert and Tipping Point Species in need of strong and immediate action to pinpoint causes of declines and to support efforts to recovering their populations. 

Conservation Status

Scarlet tanager is Species of Greatest Conservation Need; Evening grosbeak is at risk; other species vary.

Since 1970 we have lost nearly 3 billion birds across North America – almost a 30% population decline across all bird species.

Birds face unprecedented, widespread threats including habitat loss. Forest birds in particular thrive best in large tracts of contiguous forest, making them very vulnerable to the impacts of fragmentation. 

Frosted Elfin Butterfly (Callophrys irus) 

Callophrys-irus-Frosted Elfin Butterfly on a flower. The upperside of its wings is a uniform dark gray-brown, and the underside is variegated with pale scales on the hindwing.

The frosted elfin is a small brownish-gray butterfly with tiny tails and silvery scales on the hindwings that give it a frosted appearance. The relationship between a frosted elfin butterfly and its host plants, wild indigo and wild lupine, is an excellent example of how a host plant indicates a habitat for a specific pollinator. It is important to know how to make the habitat more suitable for the host plant, which is better for the butterfly. If the plant is not there, the butterfly won’t be there.  

Indicates: Barrens and open, dry habitats that support their host species 

Found in: Pine barrens, a rare habitat type characterized by fire-dependent conifers, scrub oak and grassy openings that support wild indigo and wild blue lupine 


PNHP scientists investigate the distribution and extent of the butterflies and their caterpillar food plants in PA, evaluate the condition of their habitats, and identify potential population stressors such as habitat succession, development, invasive plants, deer herbivory, and incompatible vegetation management. This survey information aids in evaluating the status of the butterflies in Pennsylvania, and helps us protect and manage their remaining populations.   

Conservation Status

Critically imperiled. Habitat loss has caused a scarcity of host plants, which the butterflies need to lay eggs and feed. 

Pitcher Plants (Sarracenia purpurea) 

A small clump of Pitcher Plants with purple-red flowers and flaring, juglike leaves with downward-pointing bristles where it traps prey

These carnivorous Pennsylvania native plants, found in bogs and mossy swamps, have leaves shaped like pitchers or tubes, edged with a nectar to attract insects. Insects land on the edge of the tube and fall into the trap, where hairs inside prevent escape. The insect is digested by enzymes in the plant. 

Indicates: High quality habitats. Peatlands and bogs, where pitcher plants grow, regulate streamflow and groundwater recharge, protecting water quality. Home to many rare plants, bird and insects, they store large amounts of carbon, which mitigate the effects of climate change. 

Found in: Sphagnum bogs and other acidic wetlands 


PNHP has been characterizing and sampling peatlands across the state to better understand these systems, add to the natural community classification of Pennsylvania and set up monitoring to evaluate the impacts of climate change over time. 

Conservation Status

Apparently secure; common 

Fairy Shrimp (Eubranchipus ssp) 

fairy shrimp (Eubranchipus ssp), a vernal pool species with 10 pairs of legs and long stalks with eyes

Growing to half inch to two inches long, fairy shrimp have 11 pairs of leg-like appendages called phyllopods used for swimming belly-side up. Wood frogs and salamanders such as spotted, Jefferson and marbled, are all vernal pool indicators, as are the delicate fairy shrimp.  

Various insects, crustaceans, amphibians and other invertebrates that need water early in their lifecycles live in vernal pools, free from predation from fish, which could not survive in vernal pool conditions. Vernal pool species can survive low oxygen levels and drying out completely in summer, and fare well in stagnant water – all conditions that fish cannot tolerate.  

Indicates: Vernal pool habitat 

Found in: Vernal pools, which are shallow depressions on land, usually surrounded by trees, that temporarily fill with snow melt and rainwater and only appear in the spring and fall.  


Heritage Program scientists have been working with partners from PA DCNR State Parks and PA Fish and Boat Commission to catalogue, map, describe and restore vernal pools across Pennsylvania.

Fairy shrimp are a primary source of food for many species of amphibians found in vernal pools.  

Conservation Status

No PA status; secure 

For More Information:

Western Pennsylvania Conservancy
Natural Heritage Program
800 Waterfront Drive
Pittsburgh, PA 15222


Pennsylvania Natural Heritage Program
Rachel Carson State Office Building
5th Floor
400 Market Street
Harrisburg, PA 17105