Taxonomic homogenization of woodland plant communities over 70 years Keith, Sally A.; Newton, Adrian C.; Morecroft, Michael D. ...
Proceedings - Royal Society. Biological sciences/Proceedings - Royal Society. Biological Sciences,
10/2009, Volume:
276, Issue:
1672
Journal Article
Peer reviewed
Open access
Taxonomic homogenization (TH) is the increasing similarity of the species composition of ecological communities over time. Such homogenization represents a form of biodiversity loss and can result ...from local species turnover. Evidence for TH is limited, reflecting a lack of suitable historical datasets, and previous analyses have generated contrasting conclusions. We present an analysis of woodland patches across a southern English county (Dorset) in which we quantified 70 years of change in the composition of vascular plant communities. We tested the hypotheses that over this time patches decreased in species richness, homogenized, or shifted towards novel communities. Although mean species richness at the patch scale did not change, we found increased similarity in species composition among woodlands over time. We concluded that the woodlands have undergone TH without experiencing declines in local diversity or shifts towards novel communities. Analysis of species characteristics suggested that these changes were not driven by non-native species invasions or climate change, but instead reflected reorganization of the native plant communities in response to eutrophication and increasingly shaded conditions. These analyses provide, to our knowledge, the first direct evidence of TH in the UK and highlight the potential importance of this phenomenon as a contributor to biodiversity loss.
Plant communities are often assumed to be stable on a year-to-year basis. We present evidence that species composition in permanent quadrats changes considerably between years at a range of ...Environmental Change Network sites across the UK. The extent of this variability varies with habitat type. Communities associated with low disturbance levels and low agricultural inputs, particularly moorland (upland grass and heath) and bog communities, are most stable.
Inter-annual variability should, therefore, be considered in designing monitoring schemes to ensure that frequency of recording is sufficient to avoid short-term fluctuations obscuring long-term trends.
More diverse communities were more stable, with less species turnover between years. However, diverse communities also tended to be dominated by slow-growing, slow-reproducing plants, adapted to low nutrient conditions, identified as ‘stress tolerators’ in the Grime CSR scheme and low Ellenberg N values. Species compositional stability was more strongly correlated with these indices of plant functional types than species richness. Nevertheless, a significant effect of species richness could be identified, even after other causes of variation were accounted for.
More stable communities in our study are likely to be resilient to low levels of environmental change, although they may still change, and possibly change dramatically if critical ‘tipping points’ are reached.
The area of heathland has declined substantially in southern England since the eighteenth century but recent incentives and financial encouragement through lottery grants and agri-environment schemes ...have promoted restoration projects.
A restoration programme on three contrasting habitats was carried out within Burnham Beeches NNR in southern England, using managed vs control areas to aid evaluation of the effectiveness of the management.
Annual surveys of the botanical composition within restored and control areas up to 15 years post-management was used to evaluate the responses of (i) overall species richness, (ii) number and cover of target/indicator species, and (iii) movement of community floristic composition towards target communities defined using a simple method based on the National Vegetation Classification.
Both wet heath/mire sites showed similar outcomes with both showing increases in species richness and positive indicators but with different species or groups in the latter case. The open acid grassland/heath site showed a delayed response to restoration management until grazing was introduced when both species richness and positive indicators increased. The grass-heath-scrub sites showed different responses with one showing relatively little change compared with the control but the other showing a rapid increase in both species richness and positive indicators, particularly following the introduction of grazing.
Communities approaching the targets have been reached in two areas but for the other three, after initial movement towards the targets, the communities subsequently drifted away. Most reached the target ‘score’ along axis 1 of the ordination, indicating that the most important target core species had been approximated but that subtle differences in community composition persisted.
Restoration techniques such as grazing and bracken and scrub control are discussed along with the impact of visitor pressure. Grazing is beneficial in diversifying communities, but over-grazing coupled with high visitor pressure may be the reason why one plot has moved away from the target. Ongoing management is required to control woody species. Setting targets helped to critically evaluate the results of the restoration work which was achieved by removal of trees without any ground preparation such as seeding.
Palaeoecological and current ecological evidence suggests that species will respond individualistically to future climate change. This is likely to lead to the formation of Non-Analogous Communities ...(NACs), which may be defined as communities that are different in species composition from any communities that can be recognised at a selected reference point in time. We explore the process of NAC formation, with reference to the key processes of immigration and extinction and the potential influence of landscape pattern, in the context of a metacommunity framework. NAC formation has considerable implications for the development and implementation of conservation policies, which frequently refer to the maintenance of current communities. The achievement of such an objective represents a substantial challenge in an era of rapid environmental change, and fails to accept the dynamic nature of communities. We suggest that conservation policies should identify potential responses to community change based on an understanding of the processes of NAC formation.
Calcareous grasslands are of high conservation importance and have been maintained by livestock grazing over many centuries. Following the reduction in traditional livestock grazing (cattle and ...sheep) of semi-natural grassland during the twentieth century, conservation-based grazing management was introduced in order to conserve remaining grasslands. In UK semi-natural grasslands, grazing pressure has fluctuated over time due to changes in conservation funding and stock availability, whilst the time of year at which stock graze has also shifted. There is little quantitative data available to demonstrate the details of changed grazing patterns, or their long-term effects on the vegetation, thus the full impacts of these changes are not well understood. This study examines the response of a diverse calcareous grassland community to changes in grazing management between 1979 and 2010 at Martin Down National Nature Reserve, southern England, using historical vegetation surveys and grazing records collected and recorded by reserve staff. Historical data showed a shift in grazing regime, whereby grazing occurred across all seasons in the early period, but switched to predominately the autumn and winter in the present century. Despite this significant shift in management over time, the vegetation community did not change dramatically over this period, although small increases in species richness and diversity were detected. The richness of calcareous grassland indicator species remained largely consistent, suggesting the conservation value of the grassland persisted, and the community types also stayed fairly constant over the 31 years. Furthermore, weak evidence was found for impacts of grazing in particular seasons on vegetation community measures; species richness, species diversity, indicator richness, indicator abundance, grass abundance and forb abundance. This study suggests that these vegetation communities are robust to changes in grazing seasonality providing that sufficient grazing pressure is provided within the year. The compartmentalised grazing employed at Martin Down may be a useful method for ensuring this outcome. However more research is required, preferably using controlled field experiments before more reliable recommendations can be prescribed.
The numbers and breeding success of the Stone Curlew Burhinus oedicnemus, a rare bird in the UK, were studied on chalk downland at Porton Down, Wiltshire, from 1987 to 1996. The number of pairs ...varied annually between nine and 19 and was positively correlated with spring counts of Rabbits in the current and previous years. This correlation probably results from a previously documented preference for very short vegetation strewn with small stones created by grazing and soil disturbance by Rabbits. The average number of young fledged per pair was adequate to maintain a stable population, but varied by an order of magnitude among years. Annual variation in chick survival had the largest influence on variation in overall productivity, because breeding failures at the egg stage were usually replaced. Differences in chick survival among years were not correlated with the mean rate of growth of chicks. Nest success and overall breeding success were positively correlated with a spring Rabbit count and, for nest success, there was also a negative effect of the number of Foxes culled per unit effort, which may be an index of Fox numbers. The effect of Rabbit abundance might occur because predators only seek out Stone Curlew eggs and chicks when Rabbits are scarce. It is recommended that the high Rabbit population is maintained as this appears to be beneficial to Stone Curlews breeding on seminatural grassland.
The majority of Galliformes are ground dwelling, many live in forests, and c. 25% are on the IUCN Red List. The Djibouti francolin Francolinus ochropectus is a Critically Endangered galliform endemic ...to only two areas of relict Juniperus procera forest in Djibouti. This study assessed population status and habitat condition in the species' stronghold in the Forêt du Day during the post-breeding season. Line transect distance sampling was used to survey the francolin, recording visual encounters and calls. Canopy and understorey vegetation were sampled across the study area at 150-m intervals using 400-m2 quadrats. Interviews were conducted in all adjacent villages to obtain information about francolin sightings, forest use and capacity for community-based conservation. Distance was used to generate francolin population density estimates. A geographical information system and generalized linear modelling were used to determine predictors of francolin presence and juniper condition. The Distance model estimated francolin density to be 38–94 km-2. Within the Forêt du Day this is equivalent to a population of 285–705 individuals. The presence of juveniles in the samples suggests that the effective population size may be lower and therefore, although this is the first estimate of Djibouti francolin density using standard survey methodology, it should be interpreted cautiously. Juniper condition in the Forêt du Day is poor. The healthiest forest is 50% dead. Francolins are more abundant where tree cover is high. This cover now mostly consists of Buxus hildebrandtii, which appears to have mostly replaced the original juniper. In areas of high tree cover, grazing intensity is significantly negatively correlated with francolin presence. Anthropogenic influences on juniper health and francolin decline are mediated through the large number of cows grazing in the forest. We recommend an ecosystem approach to conservation of the forest, with additional species-specific protection measures for the francolin and juniper.
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