Three Quite Different Challenges in Insect Conservation: Spotlights on Odonata, Guests of Ants, and Soil Insects

Abstract

Important insect localities may easily be overlooked in ordinary conservation plans. Odonata, ant guests, and soil insects illustrate three different approaches to their conservation. Odonata diversity can be limited by access to specific wetland or pond habitats, but their habitat demands can sometimes be restored. Ant guests depend fully on the long-term survival of their ant host species, which again depends on the preservation of sufficient habitat area. Soil insects may depend on a combination of specific soil types, vegetation, and climate for their larval development. Entomologists have a responsibility to identify critical ecological parameters for threatened insect species and to suggest tailored rescue plans.

1. Introduction

As explained in the introduction, this is a spotlight book with advice. We have already gone into some depth regarding the large functional group of dead wood-depending insects (the so-called saproxylic insects). In that section, we presented a case study of Nordic saproxylic beetles, their problems, and possible measures to conserve them. We have also treated pollinating insects, ground-dwelling carabid beetles, and the need for a general awareness for including insects in the general nature conservation work. Here we add three new spotlights, to further illustrate the diversity in insect conservation challenges: Odonata depending on a diversity of non-polluted freshwater habitats, the fascinating guests of ants, completely depending on the nests of their host species, and insects depending on suitable soil qualities, at least in part of their life cycle. Three ways of living, three types of vulnerability, and three examples on the overriding challenge of bringing insect diversity safe into the future.

2. The vulnerable Odonata (dragonflies and damselflies)

See (Figure 1).

Figure 1.

Escaped from water life: A newly hatched dragonfly clings to a straw above its empty pupa holster, slowly drying its wings. Photo: Jørn Bøhmer Olsen.

The insect order Odonata consists of two groups: dragonflies (Anisoptera) and damselflies (Zygoptera). At rest, dragonflies keep their wings spread out or up, while damselflies keep their wings folded together along their body. As adults, both groups are predators, hunting other insects with great acrobatic maneuvering in the air. Even larvae are predators, but living in water, often in ponds or small brooks and rivers. On a world basis, Odonata comprises over six thousand species, with approximately equal species numbers in the two suborders dragonflies and damselflies (Figure 2).

Figure 2.

Different colors, but one species: The so-called variable damselfly (Coenagrion pulchellum) in copulation. This European species is rare and declining in England but has stable populations for instance along the coast in Norway. The larva lives in vegetated ditches, canals, and ponds. Photo: Ove Bergersen.

2.1 Threats

Species are often sufficiently known to be evaluated as candidates for red listing. A press release from the international “IUCN Dragonfly Specialist Group” in December 2021 concluded that sixteen percent of the world’s Odonata are red listed. More than half of these, eleven percent, belong to the three highest categories: critically threatened (CR), endangered (EN), or vulnerable (VU) [1]. IUCN concluded that:

“The destruction of wetlands is driving the decline of dragonflies worldwide, according to the first global assessment of these species. Their decline is symptomatic of the widespread loss of the marshes, swamps, and free-flowing rivers they breed in, mostly driven by the expansion of unsustainable agriculture and urbanization around the world”.

The IUCN report pointed to water pollutants, eutrophication, and pesticides, as well as climate change, as current threats. However, positive changes have occurred. Due to improved water quality and restoring natural river morphology in Central Europe, many riverine species have recovered after heavy declines [2]. Such restoration success gives hope for the future.

2.2 The valuable but vulnerable ponds

While freshwater research, policy, and conservation up till now have focused mainly on larger water bodies, Hill et al. [3] stressed the need for taking care of small water bodies. Globally, ponds are among the most biodiverse and ecologically important freshwater habitats. The authors identified a number of knowledge gaps and research questions. Among them were anthropogenic stressors, pond monitoring, conservation management and policy, and the need for long-term monitoring of species communities and habitats. To halt the decline in freshwater biodiversity, we need to include pond ecosystems, and so-called pondscapes, in environment policy. A pondscape may be a pond surrounded by vegetation at a farm or in a garden, creating a beautiful and pleasant local world (Figure 3). On a landscape scale, connectivity is important, that is, ponds must be situated close enough to exchange species and individuals.

Figure 3.

Habitat diversity: This cultural landscape contains many different insect habitats: a pond, open fields, forests of different character and age, and belts of special vegetation between main habitats. The picture illustrates the potential of restoring habitats and landscape qualities. The pond has been artificially created, and the whole landscape may be managed to maintain habitat diversity. Photo: Ove Bergersen.

2.3 Odonata and climate change

A rising temperature in water bodies where Odonata larvae develop, or in the air where adults fly, can affect the developmental rate, phenology, and trophic interactions. Depending on dispersal ability and the availability of suitable habitats in the landscape, species may change their geographical distributions. A rapid environmental change will initiate a strong selective pressure and a possible loss of populations and species. However, many species can benefit from the warming by expanding their range poleward. Physiological experiments may help to predict reactions to increasing temperature. The thermal sensitivity of Odonata may be used as a barometer for environmental change [4]. Following up on this idea, Cerini et al. [5] studied changes in the Odonata fauna during the last five decades in three countries (Tunisia, Mauritania, and Sweden). Whereas generalist species were often advantaged by warming due to their ability to colonize new habitats toward the north, specialists were more likely to go toward extinction. Species inhabiting lentic waters were more prone to show species turnover than species typical for standing waters. The authors called for more detailed long-term field studies of changes in local faunal composition.

Cadena et al. [6] predicted that dragonflies and damselflies in west and central Asia will undergo strong changes in diversity and distribution due to climate change. They estimated that the combined effect of anthropogenic forces and climate change will lead to near extinction of some species by 2100. In Europe, Cancellario et al. [7] forecasted widespread latitudinal and altitudinal rearrangements in Odonata community composition and traits due to climate change.

2.4 Conservation efforts and restauration possibilities

In an extensive study, Zhao et al. [8] studied the diversity and conservation needs of North American damselflies. Based on more than a hundred thousand georeferenced occurrence records of 296 damselfly species, species diversity, and environmental variables were mapped in a 100 x 100-km grid size. The most important parameters for species richness and endemism were water availability and temperature conditions. Five percent of the grids were selected as hotspots due to their species richness and weighted endemism. A considerable conservation gap was identified: Two-thirds of the hotspot grids were not a part of existing protected areas.

The diversity and conservation needs of European dragonflies and damselflies were studied by Kalkman et al. [9]. The central and western-central parts of Europe had the highest general species richness and of the predominantly lentic species. Concerning strictly lotic species, their diversity center was situated in southwest France and parts of the Iberian Peninsula. Endemic species were mainly found in the latter two areas as well as in the Balkan Peninsula. A strong mismatch was identified between species protected by the EU Habitats Directive, and today’s conservation needs in the Mediterranean area. While water and habitat quality has improved since the 1990s in several central European countries due to early EU measures, the pressure on aquatic habitats has increased markedly around the Mediterranean. In Morocco, for instance, the marshes in the Smir area represent a rich Odonata locality but are today influenced by the tourism industry. A special study has been performed on the Odonata fauna in these wetlands and measures for protection have been considered [10].

Odonata may thrive in urban environments, even in artificial ponds, as documented in the capital of Oslo [11]. For instance, in the botanical garden, an ornamental pond has fostered enthusiasm for dragonflies and concern for their conservation.

Figure 4.

Extinction avoided: In England, the White-faced Darter (Leucorrhinia dubia) is a specialist of lowland bogs. However, most such habitats have been destroyed. By restoring bog habitats and translocating eggs and larvae to new localities, the species has now been rescued. Photo: Ove Bergersen.

Figure 5.

The vulnerable bogs: This bog in southern Norway, with an open pond in the central part, has offered a good living place for Odonata. Now the habitat is drained and destroyed, and the excavated turf is sold as a component of garden soil. Photo: Jørn Bøhmer Olsen.

The British Dragonfly Society [12] is very active, editing a Newsletter, mapping the distribution of species and arranging contacts, meetings, and excursions. They also run several conservation projects. One successful project has been to introduce the rare White-faced Darter (Leucorrhinia dubia) to Foulshaw Moss (Figure 4). The species is a specialist in lowland bogs, but peatland destruction (as in Figure 5) has limited the species to a few sites. In 2008, eggs and larvae were introduced into deep bog pools with floating Sphagnum moss in the Foulshaw Moss reserve. This translocation has been a great success. Drumburgh Moss is the next site into which the species will be introduced. Prior to the introduction, a number of bog pools have been created. Another endangered species in the United Kingdom is the Southern Damselfly (Coenagrion mercuriale), which has gone extinct, or is close to extinction, in seven European countries. Together with several other organizations, the British Dragonfly Society is working to save this beautiful blue species. A third project has been to conserve the rich dragonfly locality with several ponds in the Bramshill common, within Thames Basin Special Protection Areas, in North Hampshire. By modification, reshaping, and replanting the system of ponds, this work has dramatically improved the biodiversity of this pond landscape, both for dragonflies, other freshwater organisms, and rare plants. Figure 6 illustrates a restored Norwegian pond with several functions.

Figure 6.

Successful restauration: This artificial pond in Norway has several functions. It absorbs organic pollution from agricultural fields, it is a rather good habitat for dragonflies, damselflies, and various plants, and it is a pleasant landscape element for humans. Photo: Ove Bergersen.

3. Guests of ants: a vulnerable diversity in a modern world

3.1 Biology of ant guests

See (Figures 7 and 8).

Figure 7.

The nest of a host species: An ant mound of a Formica species, constructed mainly by spruce needles. Many ant guests live in such nests, which can be maintained for decades. Photo: S. Hågvar.

Figure 8.

The host: Busy Formica ants on the surface of an ant mound. Sometimes, especially in spring, some of the ant guests may appear on the surface of the nest. Others stay deep in the mound and must be excavated to be recorded. The ants show great ability to repair a damaged mound. Photo: S. Hågvar.

Ants are social insects that may form large and durable communities. These colonies, or nests, are defended against predators, which may be other invertebrates, amphibians, reptiles, birds, or mammals. However, certain specialized animals, mainly among insects, have adapted to live within ant nests, taking favor of both the ants’ defending ability and various habitats and food resources within the nest. These species have, during millions of years, evolved mechanisms to break through the ant’ defense system. They may smell like ants, behave like ants, or excrete sugar or other chemicals that please the ants. Some even look like ants. Such “ant guests” represent many different insect groups, for instance, beetles (Coleoptera), flies (Diptera), butterflies (Lepidoptera), crickets (Orthoptera), spiders (Aranea), and other wasps (Hymenoptera). In Scandinavia, a total of 369 species of beetles are found in association with ants, of which 73 species may be characterized as myrmecophile [13]. Ødegaard et al. [14], list 123 species of myrmecophile arthropods found in Norway. Some of them live with little direct contact with the ants, for instance in the refuse heaps of the colony. Other guests are allowed to enter the innermost rooms, where they predate on ant eggs, larvae, or pupae. In certain cases, they are even fed by the ants. Among today’s ant guests, we see various degrees of adaptations that may mirror the evolution process that the most sophisticated species have gone through (Figure 9).

Figure 9.

A very specialized beetle: The small and blind Claviger testaceus (Staphylinidae, Pselaphinae) is fully dependent on ants as a host. The beetle is allowed to feed on eggs and larvae in the brood chamber of Lasius or Myrmica ants. It pleases the ants by excreting sweet compounds from tuffs of hair [15]. Photo: Oddvar Hanssen.

The content of the present subchapter is mainly picked from a recent, fabulous book: The guests of ants. How myrmecophiles interact with their hosts [15]. The book goes deeply into recent research results, for instance how the ant world is to a high degree regulated by odors, and how certain guests produce chemicals that make them accepted by the ants. Among Staphylinidae beetles, for instance, there are species that not only smell like ants but may excrete chemicals that are very attractive to the ants, calming them down (Figure 10). If being attacked, certain beetles release chemicals that effectively repel the attacking ant.

Figure 10.

The staphylinid beetle Lomechusa emarginata depends on two different ants: It lives together with Formica ants during summer and with Myrmica ants during winter. From glands at the base of special abdominal hairs, the beetle secrets compounds that please the ants. The beetle is allowed to enter the ants’ breeding rooms to deposit its eggs, and beetle larvae prey on eggs and larvae of the ant. Adults and larvae of the beetle are even fed by the ants. This relationship between ants and beetles is the result of a long evolution [15]. Photo: Oddvar Hanssen.

A high level of adaptation to ant colonies is shown by larvae of certain species of blue butterflies of the family Lycaenidae. In early life, these larvae are plant eaters, but at a certain age, they drop to the ground. There, they are picked up by ants and carried into the nest. The trick used by the larva is to excrete a drop of sugar from a special gland on its back. The “valuable,” sugar-producing caterpillar is carried to the brood chamber of the colony where it acts as a predator on ant larvae and eggs. The ants even feed them with regurgitated food, in the same way as they feed their own larvae. In fact, the butterfly larva both smells like an ant larva and behaves like one. The relationship between the butterfly larva and the ant can be regarded as mutualistic since the larva provides sweet secretions for the ants. Pupation occurs within the ant nest, but the adult butterfly is not so welcome and leaves the colony rapidly.

3.2 Conservation aspects

In 1979, “the large blue butterfly,” Maculinea arion, (also called Phengaris arion) went extinct in southern England. Attempts were made to reintroduce the species from mainland Europe, but without success. It was not until the detailed dependence on both a specific food plant, a specific ant species, and a specific microclimate that the reintroduction became a success. The food for the young larva was a Thymus plant, the host ant for the larger larva was Myrmica sabuleti, and the necessary management was to keep the vegetation short enough (for instance by grazing or periodic burning) to heat the soil sufficiently to make the actual ant species thrive. The direct cause of the decline of the butterfly during the 1970s was probably a disease (myxomatosis) that killed off the rabbits that had kept the grass short in the hillsides where M. sabuleti lived. In addition, the weather was unfavorable for the butterfly during some years, either too wet or too dry. A curious element in this story is that during the decline of the butterfly, fences had been set up to stop collectors. These fences toward lepidopterists also excluded grazing animals, worsening the ground climate for the ant. Finally, meticulous detective work over several years by scientists solved the complex riddle. This is now a classic rescue operation and has been described by Thomas et al. [16].

This case illustrates how difficult it may be to get a lost species back, but that detailed ecological insight can make it possible. Obviously, the best is to protect intact nature or to proceed with the usual management practice for species that depend on our cultural landscape.

While the referred book is a fascinating journey into the diversity and ecology of ant guests, it also illustrates their vulnerability in our modern world. These specialized guest species, being products of long evolutionary lines, fully depend on the continuous presence of their ant hosts. It is easy to foresee a massive loss of ant guests in our time, through loss of ant diversity by habitat destruction, or even climate change. There may, however, be situations where human habitat changes are favorable, for instance, for ant species that need open and sunny sites. The book opens and ends with a story told by Bert Hölldobler, about an abandoned limestone quarry in Germany, where he as a child learned a lot from his father about ants and their guests. Today, this species-rich quarry has been totally destroyed, being used as a landfill. The marvelous insect world that once inspired him has gone.

There is today a need to be conscious about the long-term conservation of ant species, either their habitats are open, sunny sites, wetlands and bogs, or various types of forest. Besides conserving intact and original nature, there are options to restore ant habitats, to create them artificially as in the mentioned limestone quarry which was an open, human-made habitat rich in surface stones under which several ant species thrived. Measures that preserve the diversity of ants, even conserve the highly specialized and fascinating guests. Without hosts, no guests.

From the book of Hölldobler and Kwapich [15], we learn that several guest species follow the migrating column of army ants, which are social hunters. Even certain specialized bird species take favor of the invertebrates scared up by the aggressive ants. Each colony of army ants needs a certain area of rainforest. Furthermore, the actual birds may follow different ant species during different periods of the year. For the long-term preservation of the ecological combination of ants, their guests, as well as the specialized bird fauna, a key question is: How large forest area is needed? A protected area must contain a minimum number of colonies of army ants as well as a certain number of army ant species. Skillful entomologists and ecologists are needed in such conservation work.

The future fate of insect diversity will to a high degree be a fight about area. This is especially evident when insect survival depends on the continuous presence of other species, either it may be a question of host plants or animal hosts. For ant guests, the host is not only a species of ant but also a whole community, or nest, with its complicated structure and function, microhabitats, and food sources.

We should add that even the book “The Ants” by Hölldobler and Wilson [17] has an interesting chapter about ant guests, listing the many invertebrates that have adapted to—and are dependent on—the continuous existence of ant colonies.

4. Soil insects: neglected diversity under our feet

See (Figure 11).

Figure 11.

From soil to the air: Many Diptera species have soil-living larvae and pupae. This hatching crane fly is freeing its long legs from the pupa, which has just wiggled its way to the soil surface. Photo: Jørn Bøhmer Olsen.

A “forgotten” topic in nature conservation is the rich fauna living in various soil types [18, 19, 20, 21, 22, 23]. While insects that are active on the soil surface often attain attention, for example, the diversity of carabid beetles, the organisms under our feet are easily overlooked, although soil is teeming with life. Nowhere else in nature are species and specimens so densely packed as in soil. Here we find a mixture of insect larvae, springtails, mites, nematodes, enchytraeides, earthworms, and various other organisms. Among insects, especially Coleoptera and Diptera contain many species whose larvae are soil-living, either as root feeders, decomposers, or predators [19]. Sandy soils are also important nesting places for many species of Aculeate Hymenoptera, such as bees. Moreover, cracks in the soil or narrow spaces beneath stones may represent important hiding places, for instance for ground beetles during inactive periods. A related family to carabid beetles is tiger beetles (Cicindelidae) (Figure 12). The larvae live buried vertically in sandy soil, using their head to close the opening of their tunnel, but rapidly snapping a passing prey. Such open, sandy, and sunny habitats are vulnerable to human activity. Some tiger beetles are on the IUCN red list but most species are listed as data deficient.

Figure 12.

Tiger beetles (Cicindelidae) are famous for their running speed, their efficiency as predators, and their beauty. Larvae are sitting vertically buried in sandy soil. This is the common green tiger beetle (Cicindela campestris). Photo: Ove Bergersen.

Larvae of click beetles (Elateridae) are typically soil-living and often easy to identify due to their stiff body and yellow color. Several species of scarab beetles (Scarabaeidae) have their large, U-formed larvae in soil. For instance, larvae of the European cockchafer Melolontha melolontha (Figure 13) need three to four years for their development and feeding on roots. The scarab beetles belong also to a large number of dung beetles (often defined as a separate family, Geotrupidae). They are earth-boring beetles that transport animal excrements into soil, where the larvae develop (Figure 14). Furthermore, a large number of beetles from several families (Silphidae, Staphylinidae, Hydrophilidae, and others) contribute to bury or decompose dead birds or mammals lying on the soil surface. Most famous are the large burying beetles of the genus Nicrophorus within Silphidae. Beetles of the family Hydrophilidae also contribute in decomposing mammal excrements, a process that improves soil fertility (Figure 14).

Figure 13.

A first flight: The forest cockchafer Melolontha hippocastani is soil-living as larva, feeding on plant roots and using 3–4 years for its development. Photo: Ove Bergersen.

Figure 14.

The Dung Beetle, or Dor Beetle (Geotrupes stercorarius), is a strong digger, transporting dung into the soil, as food for the larvae. Photo: S. Hågvar.

Several groups of Diptera have soil-living larvae. The large, long-legged crane flies (Tipuloidea) are well-known among these. According to Frouz [24], soil-living Diptera larvae can be used as bioindicators, based on their ecological requirements and response to disturbance. Larvae of various Diptera groups occur in both natural and agricultural soil types. They can number several thousand per square meter and take part in many ecological processes. Furthermore, they respond to various stress factors. Some specialized species among wingless Sciaridae and Cecidomyiidae dwell in soil during their entire life span.

Two other large insect groups are more or less soil-dwellers: ants (Formicoidea) and termites (Isoptera). Both groups may contribute significantly to the energy flow in the ecosystem, and to create a porous soil structure. An overview over termite biology, diversity, and sustainable management was given by Khan and Ahmad [25]. An opus magnus about ants is the book “The Ants” by Hölldobler and Wilson [17]. The many fascinating insects that are ant guests and depend on host ants for their existence, were described by Hölldobler and Kwapich [15]. Moreover, several insects need suitable soil for pupation or overwintering. For instance, adult larvae of many butterfly species dig down into soil for safe pupation, and/or overwintering.

Soil-dependent insects, and their habitat needs, are easily overlooked in insect conservation work. The soil fauna can have a rather local character depending on soil types, vegetation, and climate, and it is highly relevant to include different soil types as a parameter in biodiversity surveys. Intact soil profiles are vulnerable to changes in vegetation, trampling, or compression by heavy machines. Tilling changes the life conditions of soil animals dramatically. Primack [26] pointed to the “option value” of soil animals for practical use, for instance as decomposers, food, environmental indicators, or in science. Future insect diversity depends on our awareness of shielding a diversity of soils from human destruction, preserving their natural vegetation, and continuous litter production. In the long term, climate change represents a threat even to soil communities [19].