Download Terrestrial and Freshwater Invertebrates chapter chapter 7
TERRESTRIAL AND FRESHWATER INVERTEBRATES (Chapter 7)
Lead Author: Ian D. Hodkinson
Contributing Authors: Anatoly Babenko, Valerie Behan-Pelletier, Jens Böcher, Geoffrey Boxshall, Fenja Brodo, Stephen J. Coulson, Willem De Smet, Klára Dózsa-Farkas, Scott Elias, Arne Fjellberg, Romolo Fochetti, Robert Foottit, Dag Hessen, Anders Hobaek, Martin Holmstrup, Seppo Koponen, Andrew Liston, Olga Makarova, Yuri M. Marusik, Verner Michelsen, Kauri Mikkola, Tero Mustonen, Adrian Pont, Anais Renaud, Leopoldo M. Rueda, Jade Savage, Humphrey Smith, Larysa Samchyshyna, Gaute Velle, Finn Viehberg,Veli Vikberg, Diana H. Wall, Lawrence J. Weider, Sebastian Wetterich, Qing Yu and Alexy Zinovjev
Consulting Authors: Richard Bellerby, Howard Browman, Tore Furevik, Jacqueline M.Grebmeier, Eystein Jansen, Steingrimur Jónsson, Lis Lindal Jørgensen, Svend-Aage Malmberg, Svein Østerhus, Geir Ottersen and Koji Shimada
The known terrestrial and freshwater invertebrate faunas of the Arctic comprise several thousand described species, representing over 16 major phyla. Many other species remain to be discovered and/or described. Arctic endemic species occur in many invertebrate groups. A significant proportion of Arctic species have circumpolar distributions. By comparison with better known groups such as vertebrates and plants, the invertebrates exhibit much higher biodiversity at all taxonomic levels and attain greater population densities in favorable habitats. Springtail (Collembola) numbers, for example, sometimes exceed 0.5 × 106/m2 and eelworm (Nematoda) populations reach over 7.0 × 106/m2 in areas of Taimyr.
Little is know about the detailed distribution and biology of most species, and good long-term population data on individual species, sufficient to indicate population trends, are almost entirely lacking. Predictions of how Arctic invertebrate communities may respond to climate change are, of necessity, based on extrapolations from experimental and/or distributional studies based on a few selected species or species groups in a restricted range of habitats.
This chapter brings together, and highlights for the first time, baseline information on the biodiversity of all Arctic terrestrial and freshwater invertebrates. It evaluates the importance of habitat diversity, climatic severity and biogeography, particularly historic patterns of glaciations, as determinants of invertebrate biodiversity. The significance of the Beringia refugium for biodiversity in several groups is stressed. Invertebrates are key players in a range of ecosystem services within the Arctic, including herbivory, decomposition, nutrient cycling, pollination, parasitism and predation.
Big new insects have appeared, beetles that fly. [American buring beetle] Jolene Nanouk Katchatag an Inupiaq from Unalakleet, Alaska; Mustonen & Mustonen 2009.
Changes in invertebrate communities, perhaps involving new invasive species, may have important impacts on several of these processes, particularly through interactions with other groups of organisms. The key environmental factors (drivers) determining species success in an era of climate warming are likely to be mean summer and winter temperatures, soil-moisture availability, length of growing season and the frequency of freeze/ thaw events that may disrupt preparation for and emergence from the overwintering state.Several recommendations for future action are listed. Highest priority should be given to establishing an inventory of Arctic invertebrate species, including their distribution, habitat preference and ecological function. This list should be used to identify true Arctic endemic taxa, classify species according to IUCN Red Book criteria and identify the vulnerability of species and their habitats. Key indicator species that are responsive to habitat change should be identified and monitored. For a group as diverse as the invertebrates, conservation action should focus on the maintenance of habitat diversity coupled with the selection of ecologically important flagship species that can provide a focus for raising the profile of invertebrates as a whole.
The observations by indigenous peoples given on the title page of this chapter, often made in association with traditional activities such as reindeer herding, hunting and fishing, clearly suggest that profound changes are occurring in the invertebrate faunas of the Arctic regions. This chapter attempts to set a baseline for invertebrate biodiversity within the Arctic, to document the scientific evidence for such change and to provide a prognosis and recommendations for the future.
Even within the scientific community, the biodiversity of invertebrates inside the Arctic is poorly understood by non-specialists and is thus frequently underplayed or sometimes ignored. The CAFF Habitat Conservation Report No.4 (Principles and Guidelines), for example, states that “invertebrate fauna in the Arctic is scarce” (CAFF 1996), a statement far removed from reality. Collectively, the number of Arctic invertebrate species greatly exceeds that of all other non-microbial eukaryotic species groups combined, including the plants and the vertebrates. Furthermore, invertebrates are often found at densities of several hundred thousand, and occasionally several million, per square meter. Arctic invertebrate faunas are thus far from simple, but their complexity is less overwhelming than for many tropical ecosystems, and their diversity is perhaps more readily understandable (Danks 1990, Vernon et al. 1998).
The mistaken idea of an overly ‘simple’ Arctic invertebrate food web almost certainly owes its origin to a summarizing diagram of the nutrient flow pathways through the ecological community of Bjornøya, Svalbard, published by Charles Elton in 1923 (Hodkinson & Coulson 2004). This diagram, erroneously interpreted as a ‘simple’ food web, still holds sway in several modern ecology textbooks. In such diagrams, it is assumed that individual species within related invertebrate groups are ecologically interchangeable, performing similar ecological functions or responding in similar ways to environmental change. They are in consequence usually consigned together, for example to a ‘box’ labeled ‘ciliates’ or ‘Collembola’. This assumption of species equivalence is mistaken, and important components of biodiversity become hidden when species are aggregated and compartmentalized in this way. Take for example the unicellular ciliates, a group whose biodiversity is poorly known within much of the Arctic. Despite their relatively simple body form, the freshwater ciliates of Svalbard fall into eight different trophic groups, each feeding on different microscopic prey categories representing various trophic levels and with individual species performing different ecological roles (Petz 2003). Similarly, species within several of the larger groups of Arctic invertebrates such as eelworms (Nematoda), springtails (Collembola), mites (Acari), flies (Diptera) and ground beetles (Coleop tera), to name but a few, display a similarly wide range of multi-trophic feeding specializations and adpatations (Chernov 1996, Rusek 1998, Chernov 2002, Makarova & Böcher 2009, Peneva et al. 2009). Trophic, behavioral and physiological divergence among related species is thus an important yet frequently overlooked component of invertebrate biodiversity within the Arctic.
Now the black flies appear before the mosquitoes, this is something new. Komi Irina Kaneva from the Krasnochelye wilderness village on the Kola Peninsula; Mustonen 2011.
Many invertebrate species are endemic to the Arctic and display highly restricted distributions. However, being small and lacking the charisma of their vertebrate and floral counterparts, few have received special conservation status, despite their vulnerability to climate change. A notable exception is the round spine tadpole shrimp Lepidurus couesii found in the American Arctic and listed as ‘endangered’ in the IUCN Red Data List. By contrast, many other Arctic invertebrate species are broadly distributed across a wide circumpolar range and display unusually wide within-species genetic diversity, or differences in their methods of reproduction, throughout their geographical range (Hobaek & Weider 1999, Reiss et al. 1999, Hessen et al. 2004, Wheat et al. 2005). Because of their small size and mobility, terrestrial and freshwater invertebrates are well-adapted to the multiplicity of different microhabitats generated by macro- and microtopographic variations in the landscape, interacting with climatic differences and the contrasting biotic environments created by different plant species and communities (Coulson 2000). Many species show strict fidelity to particular restricted microhabitat types, whereas others are more generally distributed across a range of habitats. Such variation in habitat occupancy is an important facet of biodiversity within the Arctic.
This chapter seeks to present a balanced assessment of invertebrate biodiversity and population trends within the Arctic regions. The quantitative data presented represent the best estimates available, but it should be recognized from the outset that our knowledge of Arctic invertebrates is far from complete, especially for many of the microscopic soil-dwelling forms. Our current understanding of their biodiversity rests on the extent and quality of available data and the reliability of the methods used to obtain those data. For many invertebrate groups, our knowledge of their distribution is based on a few samples taken from selected habitats at a few wellstudied sites. Often these inadequacies are compounded by taxonomic problems, particularly a lack of critical comparison of species across different regions of the Arctic. Furthermore, large areas of the Arctic remain under-sampled for many invertebrate groups. Current sampling methods may also fail to record all species present, as evidenced by divergence between studies of soil fauna using traditional extraction techniques coupled with morphological taxonomy versus those based on the direct extraction of animal DNA from soil (Wu et al. 2009). Among ciliates and testate amoebae, for example, the number of described species may represent only a fraction of the total number of species present (Foissner et al. 2008, Smith et al. 2008). Even in relatively well-known groups such as the springtails, molecular techniques are also beginning to reveal the presence of sibling species not discernible by traditional taxonomy based on morphology (Hogg & Hebert 2004).
Species abundance distributions for invertebrate communities normally follow patterns in which the community is dominated by a few common species supported by a long tail of less common species, as for example in the Arctic testate amoebae on Richards Island, Canada (Dallimore et al. 2000). From a biodiversity perspective, this tail is highly significant but is rarely adequately sampled. The Arctic can also still produce surprises, as evidenced by the relatively recent discovery of Limnognathia maerski, a representative of an entirely new Class of animal, the Micrognathozoa, in a cold spring on Disko Island, W Greenland (Kristensen & Funch 2000). This species has subsequently been found on the sub-Antarctic Crozet Islands and is probably much more widely distributed than is currently recorded (De Smet 2002).
Population density estimates exist for many terrestrial and freshwater Arctic invertebrates in a variety of habitats (e.g. Hammer 1944, Coulson 2000, Sorensen et al. 2006), but these are often spot estimates, and ther are few if any data sets that reliably indicate population trends over extended recent time periods. Even the more detailed population studies, with repeated sampling, rarely extend for periods greater than 3-5 years (e.g. Addison 1977, Hodkinson et al. 1998, Søvik 2004). Frequently such population estimates have been made for taxonomic groups combined, such as for the total springtails or oribatid mites, rather than for individual species It is thus difficult to identify shorter term trends in individual species populations associated with environmental change, and it is here that manipulation experiments are important. Such experiments, measuring experimentally the response of invertebrate populations to climate manipulation and ideally linked to laboratory-based physiological studies, probably give us the best clues as to the direction of potential future change (Hodkinson et al. 1998). The woolybear caterpillar Gynaephora groenlandica in Canada provides a good example of such a study (Kukal & Dawson 1989, Morewood & Ring 1998, Bennett et al. 1999). However, where a vertebrate ecologist might regard a drop of 25% in a species population density as significant, invertebrate ecologists struggle to estimate mean population densities of even the commoner species with an associated statistical error of less than 25%. Furthermore, invertebrate populations are often highly aggregated and frequently display wide natural fluctuation over short time scales and across topographically diverse landscapes (e.g. Høye & Forchhammer 2008). Their densities and the associated fluctuations are thus normally expressed on the logarithmic rather than the more sensitive linear scale. Invertebrates are also capable, within limits, of shifting their population center to more suitable habitat in response to deteriorating conditions. Several species of springtails, for example, track optimum soil moisture status across a drying landscape within a given season, confusing population estimates at any one fixed point (Hayward et al. 2001).Despite the limitations listed above, the stratigraphy of subfossil remains of invertebrate groups within the Arctic such as beetles, chironomid midge (Chironomidae) larvae, testate amoebae and ostracod crustaceans (Ostracoda) have successfully been used to indicate past climatic conditions and the way these conditions have changed over time (e.g. Bobrov et al. 2004, Wetterich et al. 2005, Zinovjev 2006, Thomas et al. 2008, Porinchu et al. 2009, Elias 2000a, 2000b, 2009a, 2009b). Comparison of the species composition of these subfossil assemblages with the known distribution and environmental preferences of the same species today indicates the likely conditions that prevailed when the subfossil invertebrates were deposited. Examination of the different temporal assemblages in successive strata permits the reconstruction of changing palaeoclimatic conditions at a given locality over historical time.
Judging by the last year there are almost no mosquitoes left in Lovozero [Luujavre]. It can be real evidence that climate is changing. Even some species of southern bugs and spiders appeared in tundra. Vladimir Galkin, a member of the Sámi community Piras on Lovozero Lake in the Murmansk region of Russia; Mustonen & Zavalko 2004
Large areas of the Arctic are occupied by mesic and wet tundra, grading into shallow pools, ponds and lakes where the transition between terrestrial and aquatic habitats becomes blurred. Several important groups of organisms, notably ciliates, testate amoebae, rotifers (wheel animals), tardigrades (water bears), nematodes (eelworms) and enchytraeid worms, are commonly found in both terrestrial and aquatic habitat types and several nominally terrestrial arthropod species are typical of the marine littoral zone. Some Arctic taxa, usually thought of as aquatic, such as chironomid midge larvae, contain terrestrial species, as in the genus Smittia. Similarly, the predominantly ‘terrestial’ springtails contain ‘aquatic’ species such as Heterosminthurus aquaticus, Podura aquatica and Sminthurides aquaticus (Babenko & Fjellberg 2006, Deharveng et al. 2008). For these reasons the non-marine Arctic invertebrates are considered here as an integrated whole rather than split artificially into terrestrial and aquatic groups. Invertebrates that are endoparasites of other terrestrial, freshwater and marine animals are considered by Hoberg & Kutz, Chapter 9.
Emphasis within this chapter is, of necessity, placed on documenting, essentially for the first time, the true biodiversity and abundance of the entire terrestrial Arctic invertebrate fauna and the driving factors that determine that diversity. Available knowledge of these organisms is sparse, precluding prediction of future population trends for the majority of species. Nevertheless, potentially important indicator groups are highlighted wherever possible and recommendations for future action are given.
CONCLUSIONS AND RECOMMENDATIONS
Sensitive areas and hotspots
In addition to the known major biodiversity hotspots within the Arctic, e.g. Beringia, there are many smaller biodiversity hotspots or oases with features favorable to invertebrates. Such sites may, for example, have a particularly favorable microclimate, habitat diversity or nutrient status. These sites are more likely to attract new colonizing species and to harbor source populations from which species may spread as conditions become more favorable in the surrounding areas. Several thermally favorable ‘oases’ are sheltered south or west facing sites, often with a reflective body of water in front and cliff behind (Mikkola 1992). Consequently, such sites occur most frequently at the sheltered heads of fjords or adjacent to sea coasts where climate is ameliorated by a warmer ocean current.
Examples of oases for invertebrates in the Canadian Arctic include Lake Hazen and Alexandra Fjord on Ellesmere Island and Truelove Lowland on Devon Island (Bliss 1987, Svoboda et al. 1994, Ring 2001). Greenland sites include low Arctic Disko Island with its homothermal springs, the sub-Arctic inner fjord region around Narsarsuaq on the west coast, and the high Arctic Zackenberg adjacent to Young Sund on the northeast coast (Høye & Forchammer 2008). These sites, because of their perceived diversity, have frequently been the subject of the most intensive investigations. On Svalbard, Ossinsarsfjellet oasis at the head of Kongsfjord in NW Spitsbergen supports a relatively rich flora and fauna. The moth Pyla fusca, a more typical denizen of temperate regions, is persistently found here. This is an excellent example of a species that has managed to establish a toehold within a Svalbard oasis, albeit at a single favorable site (Coulson et al. 2003c). Wrangel Island is an important biodiversity hotspot within the Russian high Arctic.
The areas on, below and in front of nesting seabird cliffs that receive high subsidies of nutrients from bird droppings, and allochthonous detritus often have greater diversity of invertebrates such as beetles. These areas may also support atypically high population densities for several invertebrate species. High total populations of mites and springtails, however, are often associated with lowered species diversity within these groups.
There is a danger that because diversity hotspots often coincide with areas of climatic favorability or historic glacial refugia, any conservation focus on such areas may result in the cold-adapted, true Arctic species with wide ranging distributions being ignored.
Key knowledge gaps and recommendations
Our fragmentary knowledge of the biodiversity of many Arctic invertebrate taxa and the lack of good long-term data on population trends suggests the following important priorities for Arctic invertebrate diversity research:
- There is a pressing need for an increased recognition within CAFF that the invertebrates play a significant and essential role in the functioning of Arctic ecosystems. Given their dominant contribution to Arctic biodiversity and their role in providing key ecosystem services such as energy flow, decomposition, nutrient cycling and pollination (e.g. Wall et al. 2008), it is surprising how little attention has been paid to them in previous syntheses on the impact of climate change on the Arctic biota. For example, the Arctic Climate Impact Assessment barely touches on their biodiversity and makes few suggestions as to how they might respond to changing climate (Callaghan et al. 2004, 2005). Furthermore, their interaction with other organism groups through pollination (higher plants), ecto- and endo-parasitism (birds, mammals and other invertebrates) and their role as food for tundra-nesting birds or fish species at critical stages of their life cycle further emphasizes their importance to the functional health of Arctic ecosystems.
- A comprehensive inventory should be compiled for invertebrate species within the Arctic, listing their known distribution, abundance, habitat preference and functional role within the ecosystem. Traditional knowledge and expertise should be incorporated wherever feasible. Initially this inventory should be based on existing literature. It is recognized that this will be fraught with difficulties and will require the resolution of many taxonomic and nomenclature problems. This latter issue might be tackled by utilizing and further developing molecular methodologies such as the DNA Barcode of Life (BOL) initiative at the University of Guelph, Canada.
- There is a pressing need for further field survey work throughout previously neglected areas of the Arctic to ensure that the species inventory is as complete as possible and to establish more clearly the distribution patterns of species, particularly among the neglected invertebrate groups such as the eelworms and most lower invertebrates. Potential sites for long-term monitoring should be identified within these areas.
- The inventory should be used to identify and list the number and distribution patterns of the true Arctic endemic species, spread across many higher taxa, which are most likely to be most affected by a warming climate. All species, where possible, should be classified using the IUCN Red List Categories and Criteria. The inventory should also be used to identify or confirm areas of high diversity and endemism at various taxonomic levels across the invertebrates.
- There is an urgent need to establish a longer-term program monitoring population trends for selected indicator species that are likely to show both adverse and positive reactions to changing climate. It is essential that both above-ground and soil-dwelling species are included as they are likely to respond to climate change at different rates. Lake/pond dwelling species may similarly exhibit a buffered response to temperature changes. Compared with vertebrates and plants, many species/communities of invertebrates posses the attributes to act as highly sensitive indicators of changing climate. Their often effective powers of dispersal, coupled with rapid development rates leading to short generation times, ensure that they are able to rapidly shift location and re-establish populations as conditions permit (Hodkinson & Bird 1998). The potential exists to identify key indicator species/communities that may be used, through changes in phenology and distribution, to track climate changes and their impacts over time. Such changes may have cascading effects within ecosystems. Indicator species could include generalist, temperature-limited predators/scavengers such as ground and rove beetles and cold-adapted spiders including the genus Erigone (dwarf spiders), or species of host-specific herbivorous insect, such as psyllids (jumping plant lice) or leaf beetles, which currently do not occupy the full range of their host plant. The former group would be particularly easy to monitor as baseline data on their distribution along northsouth transects already exist, and their common and widespread host-plants are easy to locate. Monitoring should also examine longer term population/genetic trends in indicator species/communities at fixed locations. The indicator species should include both Arctic endemics and widespread Arctic species across a range of sites. Candidate species/groups might include chironomid midges and water beetles in lakes, herbivorous terrestrial species such as the aphid Acyrthosiphonsvalbardicum on Svalbard and the woolybear caterpillar Gynaephora in Canada, and certain widespread springtail species such as Folsomia quadrioculata and Hypogastrura tulbergi, soil-dwelling and surface-active species respectively. Inclusion of species with a long continuous history within the Arctic, such as the Beringean pill beetle Morychus viridis, could provide the longerterm context for change.
- Community change in the Arctic is likely to be driven in part by newly arrived incomer species. It would be instructive to set up a sampling program to analyze the species composition and abundance of the aerial invertebrate plankton that is carried into the Arctic from farther south by northwards-moving weather systems. These are the potential colonizing species. An inventory of newly establishing species should be developed and the extent of human mediated introductions of species into the Arctic assessed.
- The effects of climate change on economically significant biting fly populations should be evaluated throughout the Arctic in relation to alterations in the hydrology of habitats and rising temperatures. This is particularly important for the indigenous peoples of the Arctic, especially with respect to reindeer herding and other traditional activities. It also has implications for the tourism industry. Assessment should be made of the potential spread of important arthropod vectorborne diseases of humans, other mammals and birds into the Arctic.
Recommended conservation actions
Because of the sheer number of species, it is impractical to take a species-based approach to conservation of Arctic terrestrial and freshwater invertebrates. Conservation actions should focus on the maintenance of habitat diversity and protection. Nevertheless, invertebrate conservation in the Arctic has suffered from a lack of focal species that can be used to highlight the problems of conservation. Focal species, however, must be chosen for their uniqueness or for their importance in ecological processes rather than for their aesthetic appeal. Examples of the former might include the flightless aphid Sitobion calvulus with its highly restricted distribution on Svalbard or chrysomelid beetles on the high Arctic islands. Examples of the latter could include a typical widely-distributed, surface-active springtail such as Hypogastrura tullbergi or widespread Arctic species of enchytraeid worms.
Other key messages
Our knowledge of the invertebrates as a group lags far behind that of higher plants, mammals and birds, yet the invertebrates represent the dominant group in terms of species-based biodiversity. This deficiency is reflected in the paucity of data concerning numerical trends, drivers and stressors presented in the preceding sections. Invertebrates are small and, to many, aesthetically unappealing, but they are almost invariably the numerically dominant group of organisms (excluding microorganisms) at sites in the Arctic, where they serve a wide variety of ecological functions and are key players in important ecosystem processes. There is danger in overstating the importance of larger, more charismatic vertebrate species with conservation appeal at the expense of those lesser invertebrates with greater functional significance for the well being of Arctic ecosystems.