The targets of proximate sensing

The first important question one has to answer when designing a proximate sensor is “What is your target”? Below, you’ll find an overview of the species groups that might require monitoring.

Mammals

Mammals, in the class Mammalia, are a diverse group of vertebrates characterised by features such as mammary glands, hair or fur (except for whales). They can be found in marine, freshwater and terrestrial environments. There are 104 native mammal species in Germany, of which 39% are endangered or considered extinct. A further 12% are on the early warning list and 6% are extremely rare (Red List Centre/RL 2009). All 25 bat species found in Germany are listed in Annex IV of the Habitats Directive. The territorial behaviour of mammals varies considerably between species, often correlating with body size and ecological requirements. Smaller mammals such as rodents occupy smaller areas, mainly for foraging and nesting, although in the case of bats, even small mammals can move very large distances in a night. Larger mammals tend to have larger territories, reflecting their need for resources to support their greater metabolic needs. For example, predators such as lions and wolves have large territories. The mobility of many mammals makes them extremely difficult to track in person. Traditional methods of monitoring mammal populations include track and sign surveys, which rely on the identification and interpretation of animal tracks, droppings and other signs of presence; radio telemetry, which involves attaching radio transmitters to animals to track their movements and behaviour; and, arguably the oldest proximate sensors for wildlife detection, camera trapping using trail cameras for larger mammals and audio recording for bats. Smaller terrestrial mammals are often trapped using sherman traps.

Birds

Birds are warm-blooded vertebrates characterised by feathers, beaks and the ability to lay eggs. They inhabit virtually every ecosystem on Earth, from polar regions to tropical rainforests, from deserts to oceans, and play important roles such as seed dispersal, nutrient cycling and insect population control. In temperate regions, many birds migrate, allowing them to exploit seasonal resources and avoid harsh environmental conditions. Mating behaviour can include elaborate courtship displays, vocalisations and territorial defence. There are 300 species of breeding birds in Germany¹. All wild birds in Germany enjoy at least special protection. Bird watching, or ornithology, has a long history and is practised by a relatively large number of biologists. Traditional methods of monitoring bird populations include bird banding, point counts, nest surveys and breeding bird surveys. Bird banding involves capturing birds, attaching uniquely numbered bands to their legs and releasing them to track movements and population dynamics. Nest surveys involve monitoring bird nests to assess reproductive success and population trends, either by locating nests or by providing artificial nest boxes. Point counts involve stationary observers recording all birds seen or heard within a defined area over a specified period of time. Breeding bird surveys involve standardised protocols for counting and mapping birds within defined survey areas. These traditional methods can be supplemented by radio telemetry or GPS tags, which allow researchers to monitor bird movements, migration routes and habitat use over large spatial scales.

Reptiles & Amphibians

Reptiles are a diverse group of ectothermic vertebrates characterised by scales, lungs and typically laying amniotic eggs. Reptiles exhibit thermoregulatory behaviour, basking in the sun to raise their body temperature or seeking shade to cool down. All 13 reptile species in Germany are at least specially protected. Amphibians make up the second group of ectothermic vertebrates. There are around 7,000 known species of amphibians, including frogs, toads, salamanders and caecilians. In Germany, however, there are only 21 species (²). The larval stage, which is typically aquatic, is very different from the adult stage (metamorphosis). Despite their dependence on water, they have managed to inhabit a wide range of terrestrial and aquatic ecosystems worldwide, from tropical rainforests to arid deserts - like this little fellow here³. Amphibians are also known for their sensitivity to environmental changes, burrowing or seeking refuge to avoid extreme temperatures or desiccation. Other deadly challenges include being killed by cars on their way to and from spawning grounds, illegal filling of spawning grounds and the chytrid fungus. Traditional methods of monitoring reptile populations include visual surveys and drift fences, where animals move along a fence and are directed into a bucket, or by checking man-made shelters made from pond liner under which reptiles crawl to warm themselves. As with reptiles, traditional methods of monitoring reptile populations include visual surveys, particularly of spawning sites, and drift fences. As most species congregate at spawning sites, i.e. in water, other options include call and tadpole surveys. Call surveys rely on identifying and recording species-specific vocalisations during the breeding season to estimate populations.

Insects

Insects are the most diverse group of animals on Earth, with over a million known species and potentially millions more to be discovered. They inhabit almost every ecosystem and play an essential role in ecological processes such as pollination, decomposition and nutrient cycling. Insects show remarkable diversity in size, shape, colour, behaviour and ecology. Their diversity and small body size make it incredibly difficult to collect detailed information on more than a fraction of the species. Their different lifestyles also make it difficult to generalise. There are 32 orders of insects: for now, we’ll focus on just a fraction of them.

Orthoptera

Orthoptera is an order of insects that includes grasshoppers, crickets and katydids. Over 28,000 species have been identified worldwide, but only ~82 species occur in Germany, 40% of which are endangered. Orthoptera species often use acoustic signalling for communication, territorial defence, mate attraction and courtship rituals. Some species are known for their complex songs produced by stridulation, which serve as a means of species recognition and mate selection. Traditional monitoring of Orthoptera populations often involves field surveys, including visual observations, acoustic sampling of their calls and capture methods such as sweep nets.

Hemiptera

The order Hemiptera includes species such as aphids, cicadas, leafhoppers and true bugs. Many feed on plant sap, but there are others that are predatory or parasitic. They can be identified by their piercing sucking mouthparts, which form a proboscis or rostrum. Some species are important agricultural pests that damage crops, while others play important roles in ecosystems as pollinators, decomposers or prey for other organisms. There are over 80,000 described species worldwide, of which 2520 are found in Germany. Of the 875 bug species assessed, only 50% are considered ‘not threatened’. Similarly, 37% of the 635 native cicada species in Germany are endangered or already extinct. Some species use acoustic signalling or chemical communication to attract mates or defend territories ⁴. Traditional methods of monitoring Hemiptera populations include sweep netting or suction sampling. Malaise traps and flight interception traps and, to a limited extent, pitfall traps are also commonly used.

Coleoptera

Coleoptera, commonly known as beetles, are the largest order of insects with more than 350,000 described species, representing about 40% of all known insect species. In Germany there are about 7000 (!) species ⁵. Their forewings are modified into hardened, protective structures called elytra, which cover the membranous hindwings. They are holometabolous, meaning that the adult stage (“imagos”) differs in appearance and behaviour from the juvenile stages. Beetles play an incredibly important role in ecosystems as decomposers, herbivores, predators and pollinators. They contribute to nutrient recycling by breaking down organic matter, regulate insect populations through predation and act as pollinators for many plant species. Some beetles are also important pests of agricultural crops and stored products. Beetles live in terrestrial, freshwater and marine environments, occupying niches ranging from leaf litter and soil to tree canopies and underground burrows, and as their high diversity suggests, morphology and behaviour vary widely between species, with adaptations tailored to their ecological niche and life history. Traditional monitoring methods for beetles include a variety of sampling techniques, including visual surveys, pitfall traps and emergence traps. These methods are used to assess the diversity, abundance and distribution of beetles in different habitats. Monitoring efforts often target specific groups of beetles, for example epigaic beetles such as Carabidae when pitfall traps are used.

Hymenoptera

Hymenoptera is an order of insects that includes bees, wasps, ants, and sawflies. Bees, encompassing not only the honey bee but numerous other species, are indispensable pollinators of flowering plants, thus holding both ecological and economic significance. Wasps and parasitic wasps often act as natural pest controllers, preying on agricultural pests or regulating insect populations in natural habitats. The order comprises over 150,000 described species worldwide. In Germany, there are approximately 600 bee species, 116 ant species, 500 wasp species, and 754 plant wasp species. The best-known hymenopteran species often live in colonies and construct elaborate nests consisting of wax, cellulose combs, or needles. However, there are also numerous solitary species where the female constructs her own nest, typically in pre-existing cavities such as hollow plant stems, beetle tunnels in wood, or underground burrows. Solitary wasps, such as mud daubers and potter wasps, craft individual nests for their offspring using materials like mud, clay, or plant fibers, often shaped like tubes or pots. To monitor Hymenoptera populations, various methods are employed. Visual surveys are conducted to detect larger colonies, especially in the case of ants. Other monitoring techniques include sweep netting and various trap systems designed to catch flying insects. Pan traps are commonly used to attract and capture hymenopterans, while in the case of species nesting in wood, artificial nests are provided and monitored for breeding activity.

Lepidoptera

Lepidoptera, which includes butterflies and moths, is the order with the second largest number of species after Coleoptera. They can be recognized by their scaled wings, which are covered with tiny overlapping scales that give them their characteristic colors and patterns. In addition, most Lepidoptera species have a proboscis, a long, coiled mouthpart used to feed on nectar and other liquids. However, in some families of moths, such as the Saturniidae (giant silk moths), the proboscis may be reduced or absent in adults. They are holometabolous, going through four distinct life stages: egg, larva (caterpillar), pupa (chrysalis or cocoon) and adult. While adult Lepidoptera feed on nectar, tree sap, rotten fruit or nothing at all, the larval stage is predominantly herbivorous. Caterpillars feed on a wide range of plant material, including leaves, flowers, fruits and stems, and can be significant pests of agriculture and forestry, sometimes devastating entire forests. However, there are also species of Lepidoptera larvae that have specialized feeding habits, such as feeding on lichens or on decaying plant matter such as rotten wood. Butterflies are mostly monitored by transect walking and sweep netting. As butterflies tend to be very colorful compared to their mostly nocturnal counterparts, they are often a target for citizen science⁶. Moths, on the other hand, are mostly sampled using light - either with lethal or non-lethal bucket traps or by hand at a light tower, though this is not suitable for all species (Note, that only this year, a study⁷ found the potential reason for that!). That’s why one often also spreads bait (rotten fruits, wine-applesauce mixtures).

Plants

Being both sessile and fundamental to human sustenance, plants are a prime target for monitoring. They can be systematically mapped and monitored as needed, particularly those utilized in the context of food production. This is where initial developments in proximate sensing often emerge, aimed at monitoring plant health and potential damage caused by pests.

In Germany, there is indeed a regular national inventory of forest conditions. The native vascular plants of Germany, numbering around 4100 species, are typically surveyed within the framework of biotope mapping. Detailed floristic mapping, where all plants in a given area are mapped, is more restricted to scientific surveys or driven by volunteers. However, there has been systematic data integration into the BfN FlorKart database since the late 1980s.

With the emergence of artificial intelligence and tools like Flora Incognita⁸ in combination with citizen science, more data points have been collected, albeit in a less systematic manner. Nevertheless, it remains crucial to conduct repeat site visits to account for plant phenology.