Robert B. O’Hara

A multispecies perspective on ecological impacts of climatic forcing

1. In the prevailing context of concerns over climate change and its potential impacts on ecosystems, evaluating ecological consequences of climatic forcing has become a critical issue. 2. Historical data on the abundance of organisms have been extensively used to characterize the ecological effects of climatic forcing through specific weather and/or climatic variables, with most of the studies confined to single population models. 3. However, population responses to environmental fluctuations typically depend upon positive and negative feedbacks induced by interactions with other species. It is therefore important to integrate the insights gained from single population approaches into a multispecies perspective. 4. Here we combine the hierarchical Bayesian modelling approach with the state-space formulation to extend the scope of previously proposed models of population dynamics under climatic forcing to multi-species systems. 5. We use our model to analyse long-term macro-moth (Lepidoptera) community data from the Rothamsted Insect Survey network in the UK, using winter rainfall and winter temperature as environmental covariates. 6. The effects of the two weather variables were consistent across species, being negative for winter rainfall and positive for winter temperature. The two weather variables jointly explained 15-40% of the total environmental variation affecting the dynamics of individual species, and could explain up to 90% of covariances in species dynamics. 7. The contribution of interspecific interactions to community-level variation was found to be weak compared to the contributions of environmental forcing and intraspecific interactions.

Relatedness and spatial proximity as determinants of host-parasite interactions in the brood parasitic Barrow's goldeneye (Bucephala islandica)

Recent studies, which have found evidence for kin‐biased egg donation, have sparked interest in re‐assessing the parasitic nature of conspecific brood parasitism (CBP). Since host–parasite kinship is essential for mutual benefits to arise from CBP, we explored the role of relatedness in determining the behaviour of conspecific nest parasites and their hosts in nesting female Barrows goldeneyes (Bucephala islandica), a duck in which CBP is common. The results revealed that the amount of parasitism increased with host–parasite relatedness, the effect of which was independent of geographical proximity of host and parasite nests. Proximity per se was also positively associated with the amount of parasitism. Furthermore, while hosts appeared to reduce their clutch size as a response to the presence of parasitic eggs, the magnitude of host clutch reduction also tended to increase with increasing relatedness to the parasite. Hence, our results indicate that both relatedness and spatial proximity are important determinants of CBP, and that host clutch reduction may be an adaptation to nest parasitism, modulated by host–parasite relatedness. Taken together, the results provide a demonstration that relatedness influences host and parasite behaviour in Barrows goldeneyes, resulting in kin‐biased egg donation.

Effects of habitat edges and trampling on the distribution of ground beetles (Coleoptera, Carabidae) in urban forests

Urban forest patches are generally small with highly contrasting edges bordering non-forest habitat, landscape features that increase with urbanisation. These forest patches are also subject to high human foot traffic resulting in trampling and other user disturbances that affect their quality as habitat for invertebrates. We studied the effects of these factors on carabid beetles in urban forests in the cities of Helsinki (Finland) and Edmonton (Canada). In both cities, the structure of carabid assemblages was affected by trampling intensity and distance to the forest edge. Moderate intensity of trail use was associated with increased beetle captures, especially in Edmonton. The effects on individual species were apparent in Edmonton but harder to demonstrate in Helsinki where forest specialist species may have been largely extirpated already. We suggest that these differences result because there has been a long history of fragmentation of urban forest patches in Helsinki, species loss from such patches is gradual, and understorey vegetation structure constrains human foot traffic in forest patches more in Edmonton than it does in Helsinki.

Quantifying the effects of trampling and habitat edges on forest understory vegetation – A field experiment

We investigated the effects of human trampling on boreal forest understory vegetation on, and off paths from suburban forest edges towards the interiors and on the likelihood of trampling-aided dispersal into the forests for three years by carrying out a trampling experiment. We showed that the vegetation was highly sensitive to trampling. Even low levels of trampling considerably decreased covers of the most abundant species on the paths. Cover decreased between 10 and 30% on paths which had been trampled 35 times, and at least by 50% on those trampled 70-270 times. On-path vegetation cover decreased similarly at forest edges and in the interiors. However, some open habitat plant species that occurred outside the forest patches and at forest edges dispersed into the forests, possibly through the action of trampling. A higher cover percentage of an open habitat species at the forest edge line increased its probability to disperse into the forest interior. The vegetation community on, next to, and away from lightly trampled paths remained the same throughout the trampling experiment. For heavily trampled paths, the community changed drastically on the paths, but stayed relatively similar next to and away from the paths. As boreal vegetation is highly sensitive to the effects of trampling, overall ease of access throughout the forest floor should be restricted to avoid the excessive creation of spontaneous paths. To minimize the effects of trampling, recreational use could be guided to the maintained path network in heavily used areas.

Integrating the niche and neutral perspectives on community structure and dynamics.

Elucidating the mechanisms underlying the assembly and dynamics of ecological communities is a fundamental goal of ecology. Two conceptual approaches have emerged in this respect: the niche-assembly view and the neutral perspective. The debate as to which approach best explains the biodiversity patterns observed in nature is becoming outdated, as ecologists increasingly agree on the existence of a niche-neutral continuum of community dynamical behaviors. However, attempts to make the continuum idea operational and measurable remain sparse. Here, we propose a model-based approach to achieving this. The proposed methodology consists of separating out fluctuations in species abundances into niche-mediated and stochastic factors, linking the niche configuration to community dynamics through competition, and adding demographic stochasticity. This results in a comprehensive framework including neutrality and strict niche segregation as extreme cases. We develop an index of departure from neutral drift as a surrogate for community position on the niche-neutral continuum. We evaluate the performance of our modeling approach with simulated data, and subsequently use the model to analyze rodent web-trapping data from a real-world system. The model fitting is carried out with a Bayesian approach using Markov chain Monte Carlo simulation methods.

Dealing with varying detection probability, unequal sample sizes and clumped distributions in count data

Temporal variation in the detectability of a species can bias estimates of relative abundance if not handled correctly. For example, when effort varies in space and/or time it becomes necessary to take variation in detectability into account when data are analyzed. We demonstrate the importance of incorporating seasonality into the analysis of data with unequal sample sizes due to lost traps at a particular density of a species. A case study of count data was simulated using a spring-active carabid beetle. Traps were ‘lost’ randomly during high beetle activity in high abundance sites and during low beetle activity in low abundance sites. Five different models were fitted to datasets with different levels of loss. If sample sizes were unequal and a seasonality variable was not included in models that assumed the number of individuals was log-normally distributed, the models severely under- or overestimated the true effect size. Results did not improve when seasonality and number of trapping days were included in these models as offset terms, but only performed well when the response variable was specified as following a negative binomial distribution. Finally, if seasonal variation of a species is unknown, which is often the case, seasonality can be added as a free factor, resulting in well-performing negative binomial models. Based on these results we recommend (a) add sampling effort (number of trapping days in our example) to the models as an offset term, (b) if precise information is available on seasonal variation in detectability of a study object, add seasonality to the models as an offset term; (c) if information on seasonal variation in detectability is inadequate, add seasonality as a free factor; and (d) specify the response variable of count data as following a negative binomial or over-dispersed Poisson distribution.

Heritability of Asymmetry and Lateral Plate Number in the Threespine Stickleback

The estimation of individual fitness and quality are important elements of evolutionary ecological research. Over the past six decades, there has been great interest in using fluctuating asymmetry (FA) to represent individual quality, yet, serious technical problems have hampered efforts to estimate the heritability of FA, which, in turn, has limited progress in the investigation of FA from an evolutionary perspective. Here we estimate the heritability of number of lateral plates, their FA and directional asymmetry (DA) in threespine stickleback, Gasterosteus aculeatus. By (i) using a meristic trait and (ii) basing our calculations on a large half-sib design experiment involving 2,079 offspring from 84 families, we overcame many of the difficulties faced by earlier FA studies. Both lateral plate number and FA in lateral plates were heritable (h2 = 0.46 and 0.21, respectively), even after controlling for marker genotypes linked to EDA (the major locus influencing plate number). Likewise, DA in lateral plates was heritable h2 = 0.23). The additive genetic component of FA in lateral plates makes it a prime candidate for further investigation into the evolutionary implications of FA and the genetic underpinnings of developmental instability. This discovery in an evolutionary model species holds the possibility to invigorate the study of FA from an evolutionary perspective.

Shifts from native to invasive small mammals across gradients from tropical forest to urban habitat in Borneo

AbstractUrbanization has paved the way for the spread of commensal rodents at global scale. However, it is largely unknown how these species use tropical anthropogenic landscapes originally covered with forests and inhabited by diverse small mammal assemblages. We surveyed non-flying small mammals in various urban and suburban habitat types and adjacent forest in the tropical town of Kota Kinabalu in Borneo. We used occupancy and polynomial regression models to determine variation in species occurrences along gradients of land-use intensity. Müller’s sundamys (Sundamys muelleri) was the only native small mammal species found in urban and suburban landscapes with a continuous decrease in occurrence probability from forests to urban habitats. The invasive Asian black rat (Rattus rattus species complex) and the invasive Asian house shrew (Suncus murinus) had the highest occurrence probabilities in habitats of intermediate land-use intensity, but Asian black rats are also likely to occasionally invade forested habitats and occupied urban habitats in sympatry with the Norway rat (Rattus norvegicus). In urban and suburban habitats, fallow land possibly favoured the occurrence of S. muelleri and S. murinus. Other native small mammal species (Muridae, Sciuridae, Tupaiidae) were found only in forested areas. Our study shows that native small mammals found in forest are largely replaced by invasive species in urban and suburban habitats. Due to their occurrence in habitats of various land use intensities, S. muelleri and R. rattus comprise central links between forest wildlife and urban species, an association that is important to consider in studies of parasite and disease transmission dynamics.

Cross-realm assessment of climate change impacts on species’ abundance trends

Climate change, land-use change, pollution and exploitation are among the main drivers of species’ population trends; however, their relative importance is much debated. We used a unique collection of over 1,000 local population time series in 22 communities across terrestrial, freshwater and marine realms within central Europe to compare the impacts of long-term temperature change and other environmental drivers from 1980 onwards. To disentangle different drivers, we related species’ population trends to species- and driver-specific attributes, such as temperature and habitat preference or pollution tolerance. We found a consistent impact of temperature change on the local abundances of terrestrial species. Populations of warm-dwelling species increased more than those of cold-dwelling species. In contrast, impacts of temperature change on aquatic species’ abundances were variable. Effects of temperature preference were more consistent in terrestrial communities than effects of habitat preference, suggesting that the impacts of temperature change have become widespread for recent changes in abundance within many terrestrial communities of central Europe.

Parasites as Biological Tags for Stock Discrimination of Beaked Redfish (Sebastes mentella): Parasite Infra-Communities vs. Limited Resolution of Cytochrome Markers

The use of parasites as biological tags for discrimination of fish stocks has become a commonly used approach in fisheries management. Metazoan parasite community analysis and anisakid nematode population genetics based on a mitochondrial cytochrome marker were applied in order to assess the usefulness of the two parasitological methods for stock discrimination of beaked redfish Sebastes mentella of three fishing grounds in the North East Atlantic. Multivariate, model-based approaches demonstrated that the metazoan parasite fauna of beaked redfish from East Greenland differed from Tampen, northern North Sea, and Bear Island, Barents Sea. A joint model (latent variable model) was used to estimate the effects of covariates on parasite species and identified four parasite species as main source of differences among fishing grounds; namely Chondracanthus nodosus, Anisakis simplex s.s., Hysterothylacium aduncum, and Bothriocephalus scorpii. Due to its high abundance and differences between fishing grounds, Anisakis simplex s.s. was considered as a major biological tag for host stock differentiation. Whilst the sole examination of Anisakis simplex s.s. on a population genetic level is only of limited use, anisakid nematodes (in particular, A. simplex s.s.) can serve as biological tags on a parasite community level. This study confirmed the use of multivariate analyses as a tool to evaluate parasite infra-communities and to identify parasite species that might serve as biological tags. The present study suggests that S. mentella in the northern North Sea and Barents Sea is not sub-structured.