I’ll start with lichens. Uncommon or rare species of lichens can indicate many things. Lichens are sensitive to air pollution, especially sulphur dioxide. So lichens can indicate good air quality (in areas such as the north shore of Lake Superior, or even a few hundred meters away from a roadway on a wooded trail). Lichens are also very slow growing so they can indicate the age of a forest and show the level of foot traffic an area receives (as many lichens grow on the ground or on bare rock). A brief note – even in younger forests, remains of older wood can be found – stumps or downed branches.
Many lichens grow directly on rock and some lichens are like calcareous rock or not. Some may like siliceous rock or other kinds of rocks as well. So lichens can indicate in a small degree the geology of an area.
Plants in the blueberry family (Ericaceae) generally prefer an acidic soil pH. Carnivorous plants are adapted for nutrient poor, often wet soils. Is that what you’re thinking about?
Leeches are really well known for being picky about their water quality, oxygen levels, acidity, etc.
When I intereviewed malacologist Jann Vendetti, she pointed out that the presence of endemic snails is often an indicator that the habitat is pretty healthy and likely hasn’t changed much in a while.
Can’t remember which - but there are plants which show the ‘Romans were here’ in English hedgerows.
Various taxa are used to indicate age of habitat, without being too precise about how old. The size of yellow meadow ant nests (Lasius flavus) has been used to date pasture, since they get slightly bigger each year, they don’t thrive in meadows and don’t survive ploughing. They grow at different rates in different situations so you have to calibrate against similar pastures of known age.
Waxcap fungi and their associates also indicate old undisturbed unfertilised grassland and are reckoned to reach maximum species-richness after about a century. Pignut Conopodium majus also indicates old undisturbed grassland, though it will persist in woodland that develops where grassland is abandoned.
There is a suite of plants which are considered ancient woodland indicators here in UK, ancient in this case meaning not put to any other land-use since before 1600. Also saproxylic beetles have been assessed for their association with sites having long continuity of dead wood habitat so can indicate not just ancient woodland but ancient woodland which has not been clear-felled. Similarly, flightless water beetles are used to distinguish ancient wetlands from recent. With these invertebrate indicators, the presence of several indicator species are needed to suggest great age.
With birds, the abundance or absence of migrants can indicate what weather systems are doing. With the right system you might get a good fallout of birds, perhaps with some rarities or accidentals in the mix.
Also, an abundance of birds at a bird feeder can indicate a lack of natural food in the wild or exceptionally cold temperatures (as birds must eat more to stay warm). So if there are no birds at your feeders it could indicate there is still lots of food in the forest during winter.
Thirdly, where winter finches are in Canada and the US can indicate what conifers trees are doing to ensure they reproduce.
Actual population trends of birds can’t be determined by casual observations. Only massive data sets such as eBird, breeding bird atlases and other projects can determine that.
For butterflies to be abundant in an area might require a mix of good natural habitat with a bit of disturbance. An example would be Sandy Lake Road in Ontario. It has lots of fen habitat that borders the dirt road.
Good natural habitat doesn’t mean things are easy for the flowers that provide the nectar for the butterflies. Alvars are difficult areas for plants to grow, and so plants produce more flowers to ensure they can reproduce properly.
The presence of Walking Fern (Asplenium rhizophyllum), or particular other ferns, generally indicates the occurrence of alkaline minerals such as calcite or dolomite in the local rock and soil.
See observations of Walking Fern (Asplenium rhizophyllum).