Patterns and processes influencing helminth parasites of Arctic coastal communities during climate change

Sympatry in a nightingale contact zone has no effect on host-specific blood parasite prevalence and lineage diversity

Parasites are a key driving force behind many ecological and evolutionary processes. Prevalence and diversity of parasites, as well as their effects on hosts, are not uniform across host species. As such, the potential parasite spillover between species can significantly influence outcomes of interspecific interactions. We screened two species of Luscinia nightingales for haemosporidian blood parasites (Plasmodium, Leucocytozoon and Haemoproteus) along an approximately 3000 km transect in Europe, incorporating areas of host distant allopatry, close allopatry and sympatry. We found significant differences in infection rates between the two host species, with common nightingales having much lower parasite prevalence than thrush nightingales (36.7% versus 83.8%). This disparity was mostly driven by Haemoproteus prevalence, which was significantly higher in thrush nightingales while common nightingales had a small, but significantly higher, Plasmodium prevalence. Furthermore, we found no effect of proximity to the contact zone on infection rate in either host species. Despite having lower infection prevalence, common nightingales were infected with a significantly higher diversity of parasite lineages than thrush nightingales, and lineage assemblages differed considerably between the two species, even in sympatry. This pattern was mostly driven by the large diversity of comparatively rare lineages, while the most abundant lineages were shared between the two host species. This suggests that, despite the close evolutionary relationships between the two nightingales, there are significant differences in parasite prevalence and diversity, regardless of the distance from the contact zone. This suggests that spillover of haemosporidian blood parasites is unlikely to contribute towards interspecific interactions in this system.

Littorina snails and Microphallus trematodes: Diverse consequences of the trematode-induced metabolic shifts

The intricate relationships between parasites and hosts encompass a wide range of levels, from molecular interactions to population dynamics. Parasites influence not only the physiological processes in the host organism, but also the entire ecosystem, affecting mortality of individuals, the number of offspring through parasitic castration, and matter and energy cycles. Understanding the molecular mechanisms that govern host-parasite relationships and their impact on host physiology and environment remains challenging. In this study, we analyzed how infection with Microphallus trematodes affects the metabolome of two Littorina snail species inhabiting different intertidal zone shore levels. We applied non-targeted GC-MS-based metabolomics to analyze biochemical shifts induced by trematode infection in a host organism. We have identified changes in energy, amino acid, sugar, and lipid metabolism. In particular, we observed intensified amino acid catabolism and nitrogenous catabolites (glutamine, urea) production. These changes primarily correlated with infection and interspecies differences of the hosts rather than shore level. The changes detected in the host metabolism indicate that other aspects of life may have been affected, both within the host organism and at a supra-organismal level. Therefore, we explored changes in microbiota composition, deviations in the host molluscs behavior, and acetylcholinesterase activity (ACE, an enzyme involved in neuromuscular transmission) in relation to infection. Infected snails displayed changes in their microbiome composition. Decreased ACE activity in snails was associated with reduced mobility, but whether it is associated with trematode infection remains unclear. The authors suggest a connection between the identified biochemical changes and the deformation of the shell of molluscs, changes in their behavior, and the associated microbiome. The role of parasitic systems formed by microphallid trematodes and Littorina snails in the nitrogen cycle at the ecosystem level is also assumed.

Impact of anthropogenous environmental factors on the marine ecosystem of trophically transmitted helminths and hosting seabirds: Focus on North Atlantic, North Sea, Baltic and the Arctic seas

Alongside natural factors, human activities have a major impact on the marine environment and thus influence processes in vulnerable ecosystems. The major purpose of this review is to summarise the current understanding as to how manmade factors influence the marine biocenosis of helminths, their intermediate hosts as well as seabirds as their final hosts. Moreover, it highlights current knowledge gaps regarding this ecosystem, which should be closed in order to gain a more complete understanding of these interactions. This work is primarily focused on helminths parasitizing seabirds of the North Atlantic and the Arctic Ocean. The complex life cycles of seabird helminths may be impacted by fishing and aquaculture, as they interfere with the abundance of fish and seabird species, while the latter also affects the geographical distribution of intermediate hosts (marine bivalve and fish species), and may therefore alter the intertwined marine ecosystem. Increasing temperatures and seawater acidification as well as environmental pollutants may have negative or positive effects on different parts of this interactive ecosystem and may entail shifts in the abundance or regional distribution of parasites and/or intermediate and final hosts. Organic pollutants and trace elements may weaken the immune system of the hosting seabirds and hence affect the final host's ability to control the endoparasites. On the other hand, in some cases helminths seem to function as a sink for trace elements resulting in decreased concentrations of heavy metals in birds' tissues. Furthermore, this article also describes the role of helminths in mass mortality events amongst seabird populations, which beside natural causes (weather, viral and bacterial infections) have anthropogenous origin as well (e.g. oil spills, climate change, overfishing and environmental pollution).

Extrinsic and intrinsic drivers of parasite prevalence and parasite species richness in a marine bivalve

Parasite species richness is influenced by a range of drivers including host related factors (e.g. host size) and environmental factors (e.g. seawater temperature). However, identification of modulators of parasite species richness remains one of the great unanswered questions in ecology. The common cockle Cerastoderma edule is renowned for its diversity and abundance of parasites, yet drivers of parasite species richness in cockles have not been examined to investigate the association of both macro and microparasite communities. Using cockles as a model species, some of the key drivers of parasite prevalence and parasite species richness were investigated. Objectives of this 19-month survey were to determine the influence of the environment, host-parasite dynamics and parasite associations on parasite species richness and prevalence at two different geographic latitudes, chosen based on environmental differences. The highest parasite species richness was recorded in the northern sites, and this was potentially influenced by a range of interactions between the host, the pathogens and the environment. Parasite prevalence increased with host size and age, and parasite species richness increased with reduced salinity. A number of interactions between parasites, and between parasites and pathologies may be influencing parasite infection dynamics. New and concerning information is also presented regarding interactions between parasites and their environment. A number of parasites and potential pathogens (bacteria, Trichodina ciliates, metacercariae, trematode sporocysts) may be advantaged under climate change conditions (warming seas, increased precipitation), increasing disease incidence, which may prove detrimental not just for cockles, but for other bivalve species in the future.

Contrasting temperature responses in seasonal timing of cercariae shedding by Rhipidocotyle trematodes

Global warming is likely to lengthen the seasonal duration of larval release by parasites. We exposed freshwater mussel hosts, Anodonta anatina, from 2 high-latitude populations to high, intermediate and low temperatures throughout the annual cercarial shedding period of the sympatric trematodes Rhipidocotyle fennica and R. campanula, sharing the same transmission pathway. At the individual host level, under warmer conditions, the timing of the cercarial release in both parasite species shifted towards seasonally earlier period while its duration did not change. At the host population level, evidence for the lengthening of larvae shedding period with warming was found for R. fennica. R. campanula started the cercarial release seasonally clearly earlier, and at a lower temperature, than R. fennica. Furthermore, the proportion of mussels shedding cercariae increased, while day-degrees required to start the cercariae shedding decreased in high-temperature treatment in R. fennica. In R. campanula these effects were not found, suggesting that warming can benefit more R. fennica. These results do not completely support the view that climate warming would invariably increase the seasonal duration of larval shedding by parasites, but emphasizes species-specific differences in temperature-dependence and in seasonality of cercarial release.

Cercariae of a Bird Schistosome Follow a Similar Emergence Pattern under Different Subarctic Conditions: First Experimental Study

The emergence of cercariae from infected mollusks is considered one of the most important adaptive strategies for maintaining the trematode life cycle. Short transmission opportunities of cercariae are often compensated by periodic daily rhythms in the cercarial release. However, there are virtually no data on the cercarial emergence of bird schistosomes from freshwater ecosystems in northern latitudes. We investigated the daily cercarial emergence rhythms of the bird schistosome Trichobilharzia sp. “peregra” from the snail host Radix balthica in a subarctic lake under both natural and laboratory seasonal conditions. We demonstrated a circadian rhythm with the highest emergence during the morning hours, being seasonally independent of the photo- and thermo-period regimes of subarctic summer and autumn, as well as relatively high production of cercariae at low temperatures typical of northern environments. These patterns were consistent under both field and laboratory conditions. While light intensity triggered and prolonged cercarial emergence, the temperature had little effect on cercarial rhythms but regulated seasonal output rates. This suggests an adaptive strategy of bird schistosomes to compensate for the narrow transmission window. Our results fill a gap in our knowledge of the transmission dynamics and success of bird schistosomes under high latitude conditions that may serve as a basis for elucidating future potential risks and implementing control measures related to the spread of cercarial dermatitis due to global warming.

Dynamics of helminth fauna of black-legged kittiwake in the Russian Arctic in the context of climate changes

We present the results of our studies of the helminth fauna and the diet of the black-legged kittiwake (Rissa tridactyla Linnaeus, 1758) in the Gorodetskiy bird colonies on the Rybachiy Peninsula (Murman coast of the Barents Sea) carried out in 2006-2008 and in 2018-2020. We did not find any noticeable changes in the species diversity of the helminth fauna of the kittiwakes, the proportion of the dominant parasite species and the values of most quantitative infection indices between the two study periods. At the same time, there was a marked decrease in the mean abundance of the dominant cestode species (Alcataenia larina Krabbe, 1869 and Tetrabothrius erostris Loennberg, 1889) in 2018-2020 as compared to 2006-2008. The changes in parasitology of birds found in our study appear to be largely determined by fluctuations of abiotic conditions (increased water and air temperature) and the state of the food supply (size structure of the zooplankton) in the study area.

Animal migration to northern latitudes: environmental changes and increasing threats

Every year, many wild animals undertake long-distance migration to breed in the north, taking advantage of seasonally high pulses in food supply, fewer parasites, and lower predation pressure in comparison with equatorial latitudes. Growing evidence suggests that climate-change-induced phenological mismatches have reduced food availability. Furthermore, novel pathogens and parasites are spreading northwards, and nest or offspring predation has increased at many Arctic and northern temperate locations. Altered trophic interactions have decreased the reproductive success and survival of migratory animals. Reduced advantages for long-distance migration have potentially serious consequences for community structure and ecosystem function. Changes in the benefits of migration need to be integrated into projections of population and ecosystem dynamics and targeted by innovative conservation actions.

A month for the mission: using a sentinel approach to determine the transmission window of digenean cercariae in the subarctic White Sea

In the digenean life cycle the cercaria ensures an important transmission stage, from the first intermediate host to the second or the definitive host. In regions with pronounced seasonality, this process occurs within a certain interval, the transmission window. In high latitudes, the size of transmission window has previously been determined only by comparing data on seasonal dynamics of infection level in various categories of hosts or extrapolating the results of laboratory experiments on cercarial biology to natural conditions. In this study, we evaluated the dynamics of infection of the second intermediate hosts (mussels Mytilus edulis) with cercariae of two digenean species, Himasthla elongata (Himasthlidae) and Cercaria parvicaudata (Renicolidae), at a littoral site at the White Sea by exposing cages with uninfected mussels during the warm season. This is the first such study in a subarctic sea. Mussel infection was observed from May to mid-September, but its intensity was the greatest only for approximately a month, from some moment after 10 July to mid-August, when water temperature was within the optimal range for cercarial emergence in both studied species (15‒20°C). During this time, the mussels accumulated 66.3 ± 6.2% metacercariae of H. elongata and 79.7 ± 5.3% metacercariae of C. parvicaudata out of the total number accumulated during the experimental period. We suggest that climate warming at high latitudes may prolong the period when the water temperatures are optimal for cercariae emergence, thereby intensifying digenean transmission in coastal ecosystems.

Coming home - Boreal ecosystem claims Atlantic sector of the Arctic

The Atlantification of the European Arctic has been an increasingly discussed topic in polar science over the past two decades. The alteration of local marine ecosystems towards a more temperate state and the appearance/range expansion of subarctic-boreal species at higher latitudes is a complex phenomenon induced mainly by the changing properties of Atlantic water (AW) transported from the south. Areas under the direct influence of AW experience biological Atlantification of their communities on all trophic levels, resulting in the growing complexity of arctic food webs. Here, besides summarising the main documented messages of biological Atlantification, we take a critical view on the threat posed on Arctic marine communities. We take into account the formation of the Arctic marine fauna, as well as the nature of (re)colonisation of Arctic sites by boreal organisms when evaluating the extent of the issue. We take a look at the history of Arctic colonisations by boreal organisms in an attempt to identify 'neonative taxa returning home'. We also highlight the role of floating plastic debris as an 'instrument from the toolbox of the Anthropocene' aiding the distribution of marine taxa.

Life cycle truncation in Digenea, a case study of Neophasis spp. (Acanthocolpidae)

Truncated life cycles may emerge in digeneans if the second intermediate host is eliminated, and the first intermediate host, the mollusc, takes up its role. To understand the causes of this type of life cycle truncation, we analyzed closely related species of the genus Neophasis (Acanthocolpidae) with three-host and two-host life cycles. The life cycle of Neophasis anarrhichae involves two hosts: wolffishes of the genus Anarhichas as the definitive host and the common whelk Buccinum undatum as the intermediate host. Neophasis oculata, a closely related species with a three-host life cycle, would be a suitable candidate for the comparison, but some previous data on its life cycle seem to be erroneous. In this study, we aimed to redescribe the life cycle of N. oculata and to verify the life cycle of N. anarrhichae using molecular and morphological methods. Putative life cycle stages of these two species from intermediate hosts were linked with adult worms from definitive hosts using ribosomal molecular data: 18S, ITS1, 5.8S-ITS2, 28S. These markers did not differ within the species and were only slightly different between them. Intra- and interspecific variability was also estimated using mitochondrial COI gene. In the constructed phylogeny Neophasis spp. formed a common clade with two other genera of the Acanthocolpidae, Tormopsolus and Pleorchis. We demonstrated that the first intermediate hosts of N. oculata were gastropods Neptunea despecta and B. undatum (Buccinoidea). Shorthorn sculpins Myoxocephalus scorpius were shown to act as the second intermediate and definitive hosts of N. oculata. The previous reconstruction of the two-host life cycle of N. anarrhichae was reaffirmed. We suggest that life cycle truncation in N. anarrhichae was initiated by an acquisition of continuous morphogenesis in the hermaphroditic generation and supported by a strong prey-predator relationship between A. lupus and B. undatum.

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Life cycles of digeneans Neophasis oculata and N. anarrhichae were redescribed.

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Phylogenetic position of genus Neophasis within family Acanthocolpidae was established.

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Causes of life cycle truncation within Neophasis are discussed.

Parasitic flatworms infecting thorny skate, Amblyraja radiata: Infection by the monogeneans Acanthocotyle verrilli and Rajonchocotyle emarginata in Svalbard

Parasite diversity above the Arctic circle remains understudied even for commercially valuable host taxa. Thorny skate, Amblyraja radiata, is a common bycatch species with a growing commercial value. Its natural range covers both sides of the North Atlantic including the Arctic zone. Svalbard is a Norwegian archipelago located on the northwest corner of the Barents Shelf which sustains a spectacular species diversity. So far, several monogenean species have been reported infecting thorny skate across the Atlantic Ocean. In the present study, we intend to fill in the knowledge gap on monogenean parasites infecting thorny skate in the northern part of its range and thus indirectly assess the connectivity between the thorny skate populations off the Svalbard coast and from previously studied locations. 46 monogenean individuals were recovered from 11 specimens of thorny skate. Following morphological and molecular assessment, two species of monogeneans, Acanthocotyle verrilli and Rajonchocotyle emarginata, were identified. The results serve as the northernmost record for both parasite genera and the first record of monogenean species off Svalbard. Detailed morphometric evaluation revealed a relatively high level of morphological variation in A. verrilli compared to its congeners. Phylogenetic reconstruction placed A. verrilli in a well-supported clade with A. imo. Our study also suggests high diagnostic significance of sclerotised structures in the identification of Rajonchocotyle. Even though the occurrence of two directly transmitted parasite species supports the previously suggested long-distance migration of A. radiata, future studies employing highly variable genetic markers are needed to assess the ongoing and historical migration patterns.

Short communication: New data support phylogeographic patterns in a marine parasite Tristriata anatis (Digenea: Notocotylidae)

Intraspecific diversity in parasites with heteroxenous life cycles is guided by reproduction mode, host vagility and dispersal, transmission features and many other factors. Studies of these factors in Digenea have highlighted several important patterns. However, little is known about intraspecific variation for digeneans in the marine Arctic ecosystems. Here we analyse an extended dataset of partial cox1 and nadh1 sequences for Tristriata anatis (Notocotylidae) and confirm the preliminary findings on its distribution across Eurasia. Haplotypes are not shared between Europe and the North Pacific, suggesting a lack of current connection between these populations. Periwinkle distribution and anatid migration routes are consistent with such a structure of haplotype network. The North Pacific population appears ancestral, with later expansion of T. anatis to the North Atlantic. Here the parasite circulates widely, but the direction of haplotype transfer from the north-east to the south-west is more likely than the opposite. In the eastern Barents Sea, the local transmission hotspot is favoured.

Infection with behaviour-manipulating parasites enhances bioturbation by key aquatic detritivores

The ecological ubiquity of parasites and their potential impacts on host behaviour have led to the suggestion that parasites can act as ecosystem engineers, structuring their environment and physical habitats. Potential modification of the relationship between parasites and their hosts by climate change has important implications for how hosts interact with both their biotic and abiotic environment. Here, we show that warming and parasitic infection independently increase rates of bioturbation by a key detritivore in aquatic ecosystems (Gammarus). These findings have important implications for ecosystem structure and functioning in a warming world, as alterations to rates of bioturbation could significantly modify oxygenation penetration and nutrient cycling in benthic sediments of rivers and lakes. Our results demonstrate a need for future ecosystem management strategies to account for parasitic infection when predicting the impacts of a warming climate.

Helminths in common eiders (Somateria mollissima): Sex, age, and migration have differential effects on parasite loads

In birds, parasites cause detrimental effects to the individual host, including reduced survival and reproductive output. The level of parasitic infection can vary with a range of factors, including migratory status, body size, sex, and age of hosts, or season. Understanding this baseline variation is important in order to identify the effects of external changes such as climate change on the parasitic load and potential impacts to individuals and populations. In this study, we compared the infection level (prevalence, intensity, and abundance) of gastrointestinal parasites in a total of 457 common eiders (Somateria mollissima) from four different sampling locations (Belcher Islands, Cape Dorset, West Greenland and Newfoundland), and explored the effects of migration, sex and age on levels of parasitism. Across all samples, eiders were infected with one nematode genus, two acanthocephalan genera, three genera of cestodes, and three trematode genera. Migratory phase and status alone did not explain the observed variation in infection levels; the expectation that post-migratory eiders would be more parasitized than pre-migratory eiders, due to the energetic cost of migration, did not fit our results. No effect of age was detected, whereas effects of sex and body size were only detected for certain parasitic taxa and was inconsistent with location. Since gastrointestinal helminths are trophically-transmitted, future studies of the regional and temporal variation in the diet of eiders and the associated variation and infestation level of intermediate hosts might further explain the observed variation of the parasitic load in eiders in different regions.

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Parasitic infections are understudied for avian populations in the Arctic.

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The helminth infection level in eiders varied greatly between sites and subspecies.

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Migratory phase and status alone did not explain observed helminth variation.

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Inconsistent effects of sex were detected for a few helminth taxa; no effect of age.

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Future studies investigating associations between diet and parasites are needed.

Parasites of seabirds: A survey of effects and ecological implications

Parasites are ubiquitous in the environment, and can cause negative effects in their host species. Importantly, seabirds can be long-lived and cross multiple continents within a single annual cycle, thus their exposure to parasites may be greater than other taxa. With changing climatic conditions expected to influence parasite distribution and abundance, understanding current level of infection, transmission pathways and population-level impacts are integral aspects for predicting ecosystem changes, and how climate change will affect seabird species. In particular, a range of micro- and macro-parasites can affect seabird species, including ticks, mites, helminths, viruses and bacteria in gulls, terns, skimmers, skuas, auks and selected phalaropes (Charadriiformes), tropicbirds (Phaethontiformes), penguins (Sphenisciformes), tubenoses (Procellariiformes), cormorants, frigatebirds, boobies, gannets (Suliformes), and pelicans (Pelecaniformes) and marine seaducks and loons (Anseriformes and Gaviiformes). We found that the seabird orders of Charadriiformes and Procellariiformes were most represented in the parasite-seabird literature. While negative effects were reported in seabirds associated with all the parasite groups, most effects have been studied in adults with less information known about how parasites may affect chicks and fledglings. We found studies most often reported on negative effects in seabird hosts during the breeding season, although this is also the time when most seabird research occurs. Many studies report that external factors such as condition of the host, pollution, and environmental conditions can influence the effects of parasites, thus cumulative effects likely play a large role in how parasites influence seabirds at both the individual and population level. With an increased understanding of parasite-host dynamics it is clear that major environmental changes, often those associated with human activities, can directly or indirectly affect the distribution, abundance, or virulence of parasites and pathogens.

Microphallus ochotensis sp. nov. (Digenea, Microphallidae) and relative merits of two-host microphallid life cycles

A new digenean species, Microphallus ochotensis sp. nov., was described from the intestine of Pacific eiders (Somateria mollissima v-nigrum) from the north of the Sea of Okhotsk. It differs from other microphallids in the structure of the metraterm, which consists of two distinct parts: a sac with spicule-like structures and a short muscular duct opening into the genital atrium. Mi. ochotensis forms a monophyletic clade together with other congeneric species in phylograms derived from the 28S and ITS2 rRNA gene. Its dixenous life cycle was elucidated with the use of the same molecular markers. Encysted metacercariae infective for birds develop inside sporocysts in the first intermediate host, an intertidal mollusc Falsicingula kurilensis. The morphology of metacercariae and adults was described with an emphasis on the structure of terminal genitalia. Considering that Falsicingula occurs at the Pacific coast of North America and that the Pacific eider is capable of trans-continental flights, the distribution of Mi. ochotensis might span the Pacific coast of Alaska and Canada. The range of its final hosts may presumably include other benthos-feeding marine ducks as well as shorebirds. We suggest that a broad occurrence of two-host life cycles in microphallids is associated with parasitism in birds migrating along sea coasts. The chances that migrating birds would stop at a site where both first and second intermediate hosts occur are relatively low. The presence of a single molluscan host in the life cycle increases the probability of transmission.

Habitat foraging niche of a High Arctic zooplanktivorous seabird in a changing environment

Here, we model current and future distribution of a foraging Arctic endemic species, the little auk (Alle alle), a small zooplanktivorous Arctic seabird. We characterized environmental conditions [sea depth, sea surface temperature (SST), marginal sea ice zone (MIZ)] at foraging positions of GPS-tracked individuals from three breeding colonies in Svalbard: one located at the southern rim of the Arctic zone (hereafter ‘boreo-Arctic’) and two in the high-Arctic zone on Spitsbergen (‘high-Arctic’). The birds from one ‘high-Arctic’ colony, influenced by cold Arctic water, foraged in the shallow shelf zone near the colony. The birds from remaining colonies foraged in a wider range of depths, in a higher SST zone (‘boreo-Arctic’) or in the productive but distant MIZ (second ‘high-Arctic’ colony). Given this flexible foraging behaviour, little auks may be temporarily resilient to moderate climate changes. However, our fuzzy logic models of future distribution under scenarios of 1 °C and 2 °C SST increase predict losses of suitable foraging habitat for the majority of little auk colonies studied. Over longer time scales negative consequences of global warming are inevitable. The actual response of little auks to future environmental conditions will depend on the range of their plasticity and pace of ecosystem changes.

Arctic systems in the Quaternary: ecological collision, faunal mosaics and the consequences of a wobbling climate

Climate oscillations and episodic processes interact with evolution, ecology and biogeography to determine the structure and complex mosaic that is the biosphere. Parasites and parasite–host assemblages are key components in a general explanatory paradigm for global biodiversity. We explore faunal assembly in the context of Quaternary time frames of the past 2.6 million years, a period dominated by episodic shifts in climate. Climate drivers cross a continuum from geological to contemporary timescales and serve to determine the structure and distribution of complex biotas. Cycles within cycles are apparent, with drivers that are layered, multifactorial and complex. These cycles influence the dynamics and duration of shifts in environmental structure on varying temporal and spatial scales. An understanding of the dynamics of high-latitude systems, the history of the Beringian nexus (the intermittent land connection linking Eurasia and North America) and downstream patterns of diversity depend on teasing apart the complexity of biotic assembly and persistence. Although climate oscillations have dominated the Quaternary, contemporary dynamics are driven by tipping points and shifting balances emerging from anthropogenic forces that are disrupting ecological structure. Climate change driven by anthropogenic forcing has supplanted a history of episodic variation and is eliminating ecological barriers and constraints on development and distribution for pathogen transmission. A framework to explore interactions of episodic processes on faunal structure and assembly is the Stockholm Paradigm, which appropriately shifts the focus from cospeciation to complexity and contingency in explanations of diversity.