Remembrance of things past: modelling the relationship between species' abundances in living communities and death assemblages

Accumulations of dead skeletal material are a valuable archive of past ecological conditions. However, such assemblages are not equivalent to living communities because they mix the remains of multiple generations and are altered by post-mortem processes. The abundance of a species in a death assemblage can be quantitatively modelled by successively integrating the product of an influx time series and a post-mortem loss function (a decay function with a constant half-life). In such a model, temporal mixing increases expected absolute dead abundance relative to average influx as a linear function of half-life and increases variation in absolute dead abundance values as a square-root function of half-life. Because typical abundance distributions of ecological communities are logarithmically distributed, species' differences in preservational half-life would have to be very large to substantially alter species' abundance ranks (i.e. make rare species common or vice-versa). In addition, expected dead abundances increase at a faster rate than their range of variation with increased time averaging, predicting greater consistency in the relative abundance structure of death assemblages than their parent living community.     

Influence of transportation and time-averaging in fossil assemblages from the Pennsylvanian of Oklahoma

The relative effects of shell destruction, transportation, and time-averaging have been assessed in seventeen soft-bottom, benthic fossil collections from the Pennsylvanian Wewoka Formation in Hughes County, Oklahoma. Invertebrates were identified to species level, and each was subject to a taphonomic evaluation to determine whether it was in place. Species that were not deemed parautochthonous (living at the location of collection shortly before final burial) were eliminated from the faunal lists to reduce the effects of time-averaging and transportation in the fossil assemblages. Both reciprocal averaging and non-metric multidimensional scaling ordinations revealed two biofacies. These were distinguished using both presence/absence data and numerical counts of species despite an overwhelming number of taxa occurring in both biofacies. Comparison between ordinations based on entire faunal lists with those based on parautochthonous faunal lists indicates that the two biofacies faithfully reproduce differences in the living distributions of member taxa but that the two biofacies underwent different degrees of time-averaging and transportation. Time-averaging leads to accumulation of taxa over time, thereby making presence/absence composition of collections more similar. Transportation, on the other hand, homogenizes originally patchy population distributions and makes relative abundances more uniform. Based on differences in abundances and presence/absence patterns, the stratigraphically lower biofacies underwent more transportation and more time-averaging than the upper biofacies.     

Differential taphonomy of modern brachiopods (San Juan Islands, Washington State): effect of intrinsic factors on damage and community-level abundance

The differences in shell structure and population turnover between organic-poor, impunctate (Hemithiris) and organic-rich, punctate brachiopod (Terebratalia) in a mixed-bottom, siliciclastic setting (San Juan Islands, WA) lead to different taphonomic damage and fidelity with respect to community-level abundance in death assemblages. In comparing shell interiors of similar-sized specimens, Terebratalia is predominantly affected by fibre detachment and shows almost no microbioerosion at the SEM scale; whereas, Hemithiris shows less marked fibre detachment at the SEM scale and is more intensely affected by microbioerosion both at SEM and light microscope (LM) scales. Fibre detachment related to rapid, microbially-induced organic matter decay appears to be the main destructive process acting on Terebratalia. Higher bioerosion levels in Hemithiris at SEM and LM scales are probably related to a combination of a low maceration rate and a preferential settlement by borers. From their vastly different abundances in life assemblages it can be deduced that Terebratalia produces dead shells at a much higher rate than Hemithiris. Therefore, the proportion of altered Terebratalia, relative to Hemithiris, is expected to be decreased due to its higher production of recently dead cohorts. That Terebratalia is also characterized by high damage levels shows that differential population turnover alone is not responsible for the differences in taphonomic damage. This shows that organic-rich and organic-poor shells are characterized by differential post-mortem durability. Although very few Hemithiris are present in the life assemblages, high durability ensures its relative over-representation in death assemblages. Terebratalia is not strongly under-represented in death assemblages, despite its high destruction rate, because of large production of recently dead shells. Even with the biasing effect of differential durability, the good fidelity reported in previous live-dead studies can be enhanced by higher population turnover of numerically dominant taxa, leading to constant input of recently dead shells into death assemblages.     

Ecological fidelity of open marine molluscan death assemblages: effects of post-mortem transportation, shelf health, and taphonomic inertia

Based on 38 molluscan datasets from modern open shelf settings, disturbance from human activities – especially anthropogenic eutrophication (AE) – has the strongest negative effect on the fidelity of death assemblages to local living communities, suggesting that the composition of the death assemblage has lagged behind changes in the living community (taphonomic inertia). Fidelity is poorest where shelves are both AE and narrow (≤ 50 km from shore to the 200-m isobath), suggesting that cross-shelf post-mortem transportation might contribute bias, but this does not dominate and shelf width does not emerge as significant among non-AE shelves. Clear signatures of post-mortem transportation are present only in four shoreface datasets, all from wide shelves, that receive abundant allochthonous specimens from adjacent estuaries or rocky intertidal zones.
Shelves experiencing minimal human impact yield fidelity estimates that are most relevant for evaluating (paleo)ecological trends. There, death assemblages are on average 25% richer than a single census of the living molluscan community and show high similarity in taxonomic composition and species relative abundance that, based on a very limited number of studies, is comparable to or better than the agreement found among successive live censuses. Molluscan death assemblages on open shelves are thus generally good samplers of living community diversity and composition under natural conditions, and where the community is undergoing anthropogenic modification, retain a strong record of the precursor community. Taphonomic inertia will be strongest where the change in the community has been especially strong (outside the normal range of natural variability) and/or where the 'new' community has lower net rates of shell input than its precursor, so that input only slowly dilutes the time-averaged skeletal remains of the 'old' community.     

Linking taphonomy to community-level abundance: Insights into compositional fidelity of the Upper Triassic shell concentrations (Eastern Alps)

Although actualistic live/dead comparisons lead to robust estimates of fidelity of modern death assemblages, quantitative evaluation of fidelity of fossil assemblage remains uncertain. In this paper, effects of storm reworking on compositional fidelity of the Upper Triassic shell concentrations (Eastern Alps, Austria) are evaluated. An exploratory approach is based on comparison of reworked and non-reworked assemblages in ordination analyses. Non-reworked assemblages of one or more communities provide a baseline for evaluation of fidelity of reworked assemblages. In siliciclastic-rich intervals of the Kössen Formation, shell concentrations are represented by (1) packstones with small, shallow infaunal bivalves, (2) floatstones and pavements with large semi-infaunal bivalves, and (3) bioclastic marlstones. In carbonate-rich intervals, bioclastic floatstones with bivalves and brachiopods occur. Analyzing all shell concentrations, eight sample groups sharing similar species composition are discriminated. Limited effect of storm reworking on composition of shell concentrations is indicated by (1) a general persistence of six sample groups when only non-reworked assemblages are analyzed, (2) similarity in composition between reworked and non-reworked assemblages within sample groups, and (3) compositional segregation between non-reworked assemblages of distinctive sample groups, mostly without any reworked assemblages of intermediate composition.Depth-related variations in dead-shell production, shell destruction and body size governed preservation and distribution of the shell concentrations along onshore-offshore gradient in the Kössen Basin. First, at times when environmental conditions were unfavorable for shell producers, coupled with high background shell destruction rates, limestone beds formed during storm events were shell-poor. Second, less common shell concentrations in upper than in lower parts of siliciclastic intervals can be related to higher environmental stress in shallower habitats. Third, the difference between shell concentrations dominated by small and large bivalves is driven by between-habitat differences in body size and is not due to a differential sorting of small and large shells. Combining community analysis based on species abundances with taphonomic analysis can thus be helpful in tracking fidelity of fossil assemblages.     

Factors controlling upper Jurassic ammonite assemblages in north-central Mexico

Lower Kimmeridgian to Lower Tithonian (Upper Jurassic) sections studied at Sierra de Palotes (Durango) and Sierra de Catorce (San Luis Potosí), Mexico, show low-energy deposits in which the composition of fossil macroinvertebrate assemblages, including megabenthos, reflects biostratinomic control. Monotonous siltstones provide continuous records of ammonite assemblages and reflect dominant deposition of shells in living areas; meanwhile, discontinues records were forced by episodic post-mortem transportation of shells, which was especially accentuated under storm influence. Rhythmic marly-silty limestones and marls illustrate a fossil record probably determined by minor transgressive-regressive pulses. The major changes in lithofacies are reflected by condensed silty and phosphatic mudstones deposited during significant floodings affecting areas under dominant terrigenous sedimentation. These changes determined more or less significant variations in the composition of fossil assemblages according to their relation to changing ecological conditions. However, shifting ecospaces exhibit no direct relationship to changes in lithofacies. Post-mortem transportation, operating in relation to both marine floodings and changes in the pattern of upper-water currents, was the main biostratinomic factor affecting the areal distribution of ammonite populations. Shell transportation and sedimentation rate controlled preservation and ultimately influenced diversity in recorded ammonite assemblages. The post-mortem behaviour (interpreted from shell structure and preservation), and therefore distribution, of ammonite shells points to shallow-water environments during the Kimmeridgian - Early Tithonian in areas (such as SE Durango and San Luís Potosí) close to the changing boundary between dominant carbonate and terrigenous sedimentation. No reworking affecting ammonite biostratigraphy has been identified in the sections studied.     

The saproxylic beetle assemblage associated with different host trees in Southwest China

Dead wood is a habitat for many insects and other small animals, some of which may be rare or endangered and in need of effective protection. In this paper, saproxylic beetle assemblages associated with different host trees in the subtropical forests in southwestern China were investigated. A total of 277 species (1 439 specimens) in 36 beetle families were collected from 117 dead wood samples, of which 101 samples were identified and respectively belonged to 12 tree genera. The number of saproxylic beetle species varied greatly among logs of different tree genera, with the highest diversity on logs of Juglans. Generally, broad‐leaved trees had a higher richness and abundance of saproxylic species than coniferous trees. Cluster analysis revealed that assemblages from broad‐leaved tree genera were generally similar (except for Betula) and assemblages from coniferous trees formed another distinct cluster. The subsequent indicator analysis proposed that there are different characteristic species for different cluster groups of host tree genera. In our study, log diameter has no positive influence on beetle species density. Conversely, comparisons of individual‐based rarefaction curves suggested that beetle species richness was highest in the small diameter class both in coniferous and broad‐leaved tree genera. With increased wood decay, proportion of habitat specialists (saproxylic beetles living on one tree genus) decreased, whereas proportion of habitat generalists (living on more than three tree genera) increased. The beetle species density was found to be higher in early stages, and decreased in later stages as well. A negative influence of altitude on saproxylic beetle species richness and abundance was detected. It was indicated that different tree genera and altitudes possibly display cross effects in modulating the altitudinal distribution and host preference of the beetles.     

Anthropogenic effect recorded in the live-dead compositional fidelity of land snail assemblages from San Salvador Island, Bahamas

Terrestrial malacofaunas that inhabit islands are vulnerable to human activities. Habitat destruction, introduction of exotic species, predators, etc. are distorting the composition and distribution of indigenous snail communities. Specifically, the taxonomic discordance between live and dead assemblages may be the consequence of anthropogenic disturbances rather than natural post-mortem processes. Live-dead fidelity may hence reflect the degree of human alteration in a given locality. This approach was used to study the relative abundance of live and dead land snails from pristine and urbanized localities around San Salvador Island (Bahamas). Thirteen species were encountered from 64 samples containing 5,343 specimens. Taxonomic composition was significantly different between areas of negligible anthropogenic disturbance and those impacted by humans. Although which species of land snails are endemic to San Salvador is unknown, all identified species are native to the Caribbean region and invasive species were not encountered. However, some synanthropic species, found exclusively in urban-developed areas, have not been recovered from the local fossil record, pointing to their relatively recent introduction possibly from nearby islands. Non-metric MDS, Jaccard-Chao index, and Spearman correlation analyses indicated that dead assemblages displayed a good compositional correspondence to live communities at pristine sites, whereas considerable disparity was sometimes observed at human-modified areas. Urbanized areas probably favored the proliferation of synanthropic species whereas dead assemblages may contain anthropophobic taxa that lived there prior to recent human modification. These findings are consistent with previous live-dead fidelity studies of marine molluscan and terrestrial small-mammal assemblages, and suggest that the taxonomic discordance between live and dead assemblages of land snails may be indicative of recent anthropogenic alteration.     

Taphonomy in the light of intrinsic shell properties and life habits: Marine bivalves from the Eemian of northern Russia

A taphonomic analysis of the bivalvesArctica islandica (Arcticidae),Astarte borealis (Astartidae),Mytilus edulis (Mytilidae), andSpisula elliptica (Mactridae) from shallow-marine last interglacial Sediments exposed along the Pyoza river, Arkhangelsk region, shows that they differ in preservation, probably because of differences in shell shape, shell structures, and life habits. The Shells indicate that the temporal sequence of taphonomic processes was as follows: (1) pre-mortem bioerosion and dissolution; (2) post-mortem bioerosion; (3) abrasion, disarticulation, and fragmentation; and (4) dissolution. A new graphic Illustration, the taphonomic constituent diagram (TCD), is proposed to illustrate the sequences of taphonomic processes. It is inspired by the ichnofabric constituent diagram used in ichnology and integrates the shell surface coverage of different taphonomic features by graphically plotting them against relative time. The taphonomic constituent diagram may display differences in the Chronologie order of paleoenvironmental processes. Also, the diagram enables a combination of both microscopic and macroscopic taphonomic features, and eases comparative studies of fossil assemblages.      

Live and dead benthic foraminiferal assemblages from coastal environments of the Aegean Sea (Greece): Distribution and diversity

Benthic foraminiferal composition assemblages and their temporal changes, ecological indices and foraminiferal densities are used to compare three coastal environments with different physicogeographical features in the Aegean Sea (coastal environment of Avdira–Vistonikos Gulf and Kitros–Thermaikos Gulf and open lagoonal environment of Vravron–South Evoikos Gulf). Three main foraminiferal assemblages have been recognized: a) “Assemblage A”; high degree of similarity between living and dead foraminiferal species, dominated by Ammonia beccarii, Elphidium spp. and relatively abundant and diverse miliolids, b) “Assemblage B1”; intermediate degree of similarity between live and dead assemblages, characterized by highly-abundant and well-diversified foraminiferal assemblages including the algal symbiont bearing Peneroplis pertusus together with Ammonia tepida and several small epiphytic rotaliids and miliolids, and c) “Assemblage B2”; absence of living individuals, strongly dominated by the opportunistic species A. tepida. Our results suggest a good comparison between living and dead assemblages from different coastal environments in the Aegean Sea, however the prevailing environmental conditions (vegetation cover, hydrodynamics, fresh water influx) have a strong impact on the taphonomic processes.     

Differences in leaf‐litter invertebrate assemblages between radiata pine plantations and neighbouring native eucalypt woodland

Abstract We investigated the structure, composition and environmental correlates of leaf‐litter invertebrate assemblages in Pinus radiata plantations and in neighbouring native eucalypt woodland in the Jenolan Caves Karst Conservation Reserve, south‐east Australia. Invertebrate assemblages of plantations were compared with remnant eucalypt woodland located well away from the influence of plantations to determine the direct effects of plantations as a result of habitat‐replacement with a non‐native plantation species. We also included in our comparisons edge habitat of eucalypt woodland located immediately adjacent to plantations. This unique edge habitat is exposed to the intrusion of large volumes of pine leaf‐litter from plantations, which has the potential to affect indirectly invertebrate assemblages of surrounding woodland. We found that species richness of invertebrates was significantly lower in pine plantations compared with remnant eucalypt woodland. There was a complete absence of species from 12 invertebrate orders that were found in surrounding eucalypt woodland. A rich and abundant native plant understorey that provides increased habitat heterogeneity is the most likely explanation for the richer invertebrate assemblage found in remnant eucalypt woodland. The total abundance of all invertebrate taxa in pine plantations in winter was significantly higher than in remnant eucalypt woodland, pine‐litter edges and pine‐free edges. Plantations were characterized by particularly high abundances of species in two orders, Acari and Collembola. High abundances of acarine and collembolan species in plantations were associated with a decompositional environment represented by comparatively higher moisture contents and higher C : N ratios of both leaf‐litter and soil, higher soil conductivity and lower soil pH. We suggest that implementation of The Plantation Biodiversity Benefits Score will be a fruitful way forward to assess the environmental benefits that can be gained from pine plantations in this region of south‐eastern Australia.     

Fish Assemblages Associated with Natural and Anthropogenically-Modified Habitats in a Marine Embayment: Comparison of Baited Videos and Opera-House Traps

Marine embayments and estuaries play an important role in the ecology and life history of many fish species. Cockburn Sound is one of a relative paucity of marine embayments on the west coast of Australia. Its sheltered waters and close proximity to a capital city have resulted in anthropogenic intrusion and extensive seascape modification. This study aimed to compare the sampling efficiencies of baited videos and fish traps in determining the relative abundance and diversity of temperate demersal fish species associated with naturally occurring (seagrass, limestone outcrops and soft sediment) and modified (rockwall and dredge channel) habitats in Cockburn Sound. Baited videos sampled a greater range of species in higher total and mean abundances than fish traps. This larger amount of data collected by baited videos allowed for greater discrimination of fish assemblages between habitats. The markedly higher diversity and abundances of fish associated with seagrass and limestone outcrops, and the fact that these habitats are very limited within Cockburn Sound, suggests they play an important role in the fish ecology of this embayment. Fish assemblages associated with modified habitats comprised a subset of species in lower abundances when compared to natural habitats with similar physical characteristics. This suggests modified habitats may not have provided the necessary resource requirements (e.g. shelter and/or diet) for some species, resulting in alterations to the natural trophic structure and interspecific interactions. Baited videos provided a more efficient and non-extractive method for comparing fish assemblages and habitat associations of smaller bodied species and juveniles in a turbid environment.     

Species composition of dabbling duck assemblages: ecomorphological patterns compared with null models

Ecomorphological patterns of breeding dabbling duck (Anas spp.) assemblages were studied in six regions in northern Europe. Observed spacings among species in terms of bill lamellar density and body length were compared with expected spacings based on null models incorporating different levels of constraints (regional species pools, species relative abundances, lake size and habitat requirements of species). Deviations of observed spacings from expected ones were compared with prey abundance and prey size diversity in the lakes. Observed spacings in terms of body length, but not in terms of bill lamellar density, were greater than expected on the basis of null models. The most abundant species were generally relatively more different than less abundant species in terms of body length but not in terms of bill lamellar density. Deviations between observed and expected spacings in terms of body length were more like those predicted by the competition hypothesis in lakes with low food abundance than in lakes with high food abundance. Patterns in bill lamellar spacings were not related to food abundance nor to food size diversity. In general, patterns in body length spacings were consistent with the competition hypothesis whether the null model used in comparisons was constrained or not.     

Experimental dissolution of a fossil foraminiferal assemblage (Paleocene–Eocene Thermal Maximum,Dababiya, Egypt): Implications for paleoenvironmental reconstructions

Dissolution experiments were carried out on a foraminiferal assemblage from the Paleocene–Eocene Thermal Maximum (PETM) at Dababiya, Egypt, in order to: 1) reveal the effects of differential dissolution on the composition of the foraminiferal assemblage and 2) develop objective criteria for the evaluation of dissolution in foraminiferal assemblages used in early Paleogene paleoenvironmental reconstructions, particularly with respect to neritic Midway-type assemblages from the Paleocene/Eocene transition. Our results confirm two general observations on modern foraminifera: 1) planktic foraminifera are much more vulnerable to dissolution than benthic foraminifera, leading to depressed P/B ratios and 2) dissolution susceptibility differs between size fractions, with the smaller specimens dissolving more rapidly than the bigger ones, leading to a larger average size of the remaining assemblage. Within a size fraction, wall structure and thickness are considered to be the main factors controlling differential dissolution susceptibility. We propose a ranking scheme for taxa with respect to dissolution resistance. Among the benthic taxa, Lenticulina is most resistant, followed by the agglutinated Gaudryina cf. ellisorae and Alabamina midwayensis. Biserial and triserial hyaline taxa and the porcelaneous Spiroloculina sp. are most susceptible to dissolution, whereas rotaliines, such as Cibicidoides and Anomalinoides have an intermediate susceptibility. This implies that mild dissolution of a Midway-type benthic assemblage leads to a relative enrichment in Lenticulina, Gaudryina and rotaliines. Amongst planktic foraminifera, the muricate taxa Acarinina and Morozovella are most resistant, followed by the cancellate Subbotina. The smooth and generally small Globanomalina and Zeauvigerina are least resistant to dissolution. Our data enable to objectively evaluate various degrees of dissolution in benthic and planktic foraminiferal assemblages retrieved from the lower Paleogene Tethyan outcrops. In this way taphonomic artifacts can be readily distinguished from paleoenvironmental signals affecting the primary composition of the assemblages. More generally, we propose that the combined use of foraminiferal numbers, P/B ratio and relative abundances of non-calcareous agglutinated taxa and Lenticulina may provide a powerful proxy for assessing dissolution in hemipelagic assemblages from Cenozoic and upper Cretaceous continental margins. In order to achieve more robust pre-Quaternary paleoenvironmental reconstructions based on quantitative foraminiferal data, application of dissolution proxies, like proposed here, or in slightly modified form, should become a more widely used micropaleontologic procedure. Particularly continental margin studies dealing with major biotic events (e.g. PETM) or employing P/B ratios for sea-level reconstructions should benefit from such an approach.     

Fidelity of molluscan life and death assemblages on sublittoral hard substrata around granitic islands of the Seychelles

This study quantified the degree of coincidence between living and dead molluscan faunas in a shallow-water coral reef environment in the Indian Ocean. The results were compared with those from a similar life:death study in the northern Red Sea, and with those published for reef corals and soft substrata molluscs. The proportions of quantitatively important taxa are robust to sampling intensity, but fidelity indices and rank-order correlations are strongly influenced by quantitatively unimportant taxa. Distinct differences between life and death assemblages were recognized, which are due to distinct biases in the death assemblage. Bivalves that lived in close contact with living corals are preferentially overgrown after death and should provide considerable temporal and ecological information in a potential fossil record, as they will be preserved within a rapidly growing reef framework. Some gastropod taxa are preferentially transported into surrounding soft substrata postmortem. Here they will be affected by time-averaging and taphonomic disintegration typically occurring in sediments resulting in the associated loss of much temporal information. Most gastropod shells, however, are inhabited by hermit crabs postmortem, which may strongly alter the fossil gastropod community structure. The results are similar to a comparable study in the northern Red Sea, with a major exception being the strong dominance of hermit crab-inhabited gastropod shells in the death assemblage of the Seychelles. Comparison of life:death assemblages between hard and soft substrata, in keeping with the northern Red Sea study, showed the strong dominance of dead shells in the soft substrata with the converse on the hard substrata. This results from different accumulation conditions for dead shells in soft substrata. Fidelity indices are well suited to demonstrate that sedimentary death assemblages are typically remarkably robust reflections of local community composition but they do not record the strong biases in death assemblages of coral reef associated hard substrata molluscs and are therefore unsuitable for comparisons of life and death assemblages in reef environments.     

Fish assemblages across a complex,tropical freshwater/marine ecotone

Synopsis Riverine fish assemblages in the temperate zone generally show strong longitudinal patterns of faunal turnover and increases in species richness with increasing stream order. We examined the composition and structure of tropical fish assemblages across a complex freshwater/marine ecotone in Tortuguero National Park on the Caribbean coast of Central America. Species turnover was high between four characteristic habitats that largely corresponded with a longitudinal gradient of stream order over distances of less than 30 km. Suites of common fish species characterized each habitat: creeks, rivers, lagoons, and the sea. In addition to the habitat endemics, several species spanned two habitat types, but only three species were collected in more than two habitats. Multivariate gradient analysis of fish assemblages reflected a gradient of habitats that to some extent corresponded to fluvial distances. Due to the unusual configuration of coastal lagoons lying parallel to the coast, the ordination gradient showed little correlation with linear distance to the coast. Environmental variables related to habitat size and salinity showed greatest correspondence with the fish assemblage ordination gradient. Invertebrate-feeding fishes were the predominant trophic group in 15 of 16 fish assemblages, and inland creek sites contained a greater proportion of herbivores and omnivores than other sites. The relative fraction of herbivorous and detritivorous fishes showed a monotonic decline along the longitudinal habitat gradient from inland to coast. Patterns of species composition and richness at Tortuguero Park appeared to agree well with earlier models of factors influencing temperate zone stream fishes. Headwaters have low aquatic primary productivity and contain small colonizing fish species subject to large fluctuations in local densities and intermittent competition. Lagoons contain both large and small species, the latter being restricted largely to shallow edge habitats by predation. Lagoons exhibit more lentic environmental conditions, experience relatively fewer periodic disturbances than headwaters, and their assemblages are inferred to be under relatively greater influence of biotic factors. Fish assemblages of rivers and caños (swampy side channels and braids) appear to be under less abiotic control than headwaters and influenced less by biotic factors than lagoons.     

The relationship between the classification of Scottish ground beetle assemblages (Coleoptera, Carabidae) and the National Vegetation Classification of British plant communities

A sufficiently large body of knowledge on British ground beetle (Carabidae) communities now exists to allow investigation of whether habitats may be classified or described on the basis of their ground beetle communities, in the same way that the National Vegetation Classification (NVC) describes British plant communities. A data set of ground beetle abundances from pitfall traps at 481 sites in a range of natural, semi-natural and agricultural habitats throughout Scotland was available for analysis. Multivariate analysis (detrended correspondence analysis and fuzzy cluster analysis) was carried out on proportional catch data of 156 species of ground beetle from 444 of these sites and the results related to the NVC of the sites.
Initial analysis classified the sites into five broad categories: 1) peatlands, 2) calcifugous, 3) mesotrophic, 4) dry river sediments and 5) damp river sediments. Further analysis identified 15 ground beetle assemblages, each corresponding to a relatively well defined vegetation type within one of these broad categories. The major environmental factors appearing to determine the distribution of ground beetle assemblages were substratum type, disturbance and soil moisture, all of which are also important determinants of the distribution of plant communities. The presence and absence of relatively stenotopic species were important discriminants of certain habitats such as wetlands and river sediments but the relative abundances within assemblages of more eurytopic species provided good indications of a relationship between ground beetle assemblages and NVC categories.     

Climate‐driven substitution of habitat‐forming species leads to reduced biodiversity within a temperate marine community

Aim

In marine ecosystems, habitat‐forming species (HFS) such as reef‐building corals and canopy‐forming macroalgae alter local environmental conditions and can promote biodiversity by providing biogenic living space for a vast array of associated organisms. We examined community‐level impacts of observed climate‐driven shifts in the relative abundances of two superficially similar HFS, the warm‐water kelp Laminaria ochroleuca and the cool‐water kelp Laminaria hyperborea.

Location

Western English Channel, north‐east Atlantic

Methods

We compared algal and invertebrate assemblages associated with kelp stipes and holdfasts, across multiple sites and sampling events. Significant differences were recorded in the structure of assemblages between the host kelp species at each site and event.

Results

Assemblages associated with stipes of the cool‐water HFS were, on average, >12 times more diverse and supported >3600 times more biomass compared with the warm‐water HFS. Holdfast assemblages also differed significantly between species, although to a lesser extent than those associated with stipes. Overall, assemblages associated with the warm‐water HFS were markedly impoverished and comprised far fewer rare or unique taxa.

Main conclusions

While previous research has shown how climate‐driven loss of HFS can cause biodiversity loss, our study demonstrates that climate‐driven substitutions of HFS can also lead to impoverished assemblages. The indirect effects of climate change remain poorly resolved, but shifts in the distributions and abundances of HFS may invoke widespread ecological change, especially in marine ecosystems where facilitative interactions are particularly strong.     

Surf zone fish community and species response to a wave energy gradient

Analysis of seine netting data for Algoa Bay, South Africa reveals that species richness and relative abundances of surf zone fishes differ at three beach sites, despite their close proximity. A gradient in species compositional and abundance response to spatially varying wave exposure is proposed as the principle cause. Implications are considered in the context of trophic interaction and impact of individuals and species assemblages upon energy and other resources of the habitat.     

Interactions of components of habitats alter composition and variability of assemblages

1. The nature and resources supplied by different components of habitats influence species, creating variability from place to place within a habitat. 2. Experiments were done to investigate the effects of altering components of habitats on the variability of assemblages of numerous species of intertidal gastropods. 3. Artificial habitats with three levels of structure, combining different types of turf (i.e. different densities and height of fronds) were sampled 8 weeks after deployment in the intertidal. They were rapidly colonized by up to 66 species of gastropods. 4. Independently of the types of turf combined to form different habitats, there were differences in assemblages where there was more than one type of component present. Multivariate dissimilarities among units making up each habitat were also greater where there were more than one type of unit, but there was no such difference in the variance of numbers of species per unit. 5. Altering the relative abundances of different types of components made little change to the assemblages, nor their multivariate variability among units of habitat and the variance in number of species per unit in each habitat. 6. Differences in assemblages due to the different structure of habitat are complex to interpret and simple characterizations of structure of habitat are inadequate. Comparing different habitats requires appropriate experimentation to ensure that variability within habitats is also investigated.