Poole Harbour

50.683827, -2.004393

Study A

Study A

Birds

Oystercatcher, Haematopus ostralegus

Curlew, Numenius arquata

Black-tailed godwit, Limosa limosa

Dunlin, Calidris alpine

Redshank, Tringa tetanus

Sanderling, Calidris alba

Modelling

MORPH

Abstract

Stillman et al. 2005 (Contains public sector information licensed under the Open Government Licence v3.0):

This is the final report for the joint English Nature and Centre for Ecology and Hydrology (CEH) project entitled “Estuary Special Protection Areas – Establishing Baseline Targets for Shorebirds”. The aim of this project was to use individual-based models, comprised of fitness-maximising individuals, to predict how changes in the mortality rate and body condition of wader and wildfowl populations are related to the amount of food and human disturbance within a site. Individual-based models predict population level responses, such as mortality rate, from the combined behavioural responses of the individual animals within a population. An important aspect of the models developed in the project is that the behavioural decisions of model animals are derived from fitness-maximising decision rules (for example, animals attempt to maximise their chances of surviving starvation by feeding in locations in which prey are most abundant, or can be consumed most rapidly). As real animals are thought to use similar decision rules, model animals should behave in similar ways to real animals.

Models were applied to four Special Protection Areas (SPAs), the Humber estuary, the Wash, Poole harbour and the Exe estuary, to predict the food abundances required to maintain low shorebird and wildfowl mortality rates and high body conditions. The major site-specific predictions were as follows.

  • Humber estuary shorebirds. Nine shorebird species were modelled; dunlin, ringed plover, knot, redshank, grey plover, black-tailed godwit, bar-tailed godwit, oystercatcher and curlew. The model predicted that the presence of fields around the estuary provided supplementary feeding areas which increased the survival rates of curlew. With the exception of curlew, shorebird survival rates fell below 90% when autumn, estuary-wide food biomass density was below about 4 g AFDM m-2. Curlew survival rates fell when food biomass density decreased below 8 g AFDM m-2. Prey biomass densities for all species except curlew were above the threshold prey density below which survival rates were predicted to decline. Shorebird survival was most strongly influenced by the biomass densities of annelid worms, and the bivalve molluscs Cerastoderma eduleand Macoma balthica. It was unaffected by the biomass densities of Corophiumand Hydrobia.
  • Wash shorebirds. Eight shorebird species were modelled; dunlin, knot, redshank, grey plover, black-tailed godwit, bar-tailed godwit, oystercatcher and curlew. Shorebird survival rates fell below 100%, i.e. birds began to starve, when autumn, estuary-wide food biomass density was below about 5g AFDM m-2 and fell below 90% at 4g AFDM m-2. Prey biomass densities for all species were above the threshold prey density below which survival rates were predicted to decline. Shorebird survival was most strongly influenced by the biomass densities of Arenicolaand other annelid worms, and the bivalve molluscs Cerastoderma eduleand Macoma balthica. It was unaffected by the biomass densities of Corophiumand Hydrobia. The survival of most species in the model remained high at fewer than 20 disturbances per hour, but bar-tailed godwit and curlew were affected at disturbance rates as low as two per hour.
  • Poole harbour shorebirds. Five shorebird species were modelled; dunlin, redshank, black-tailed godwit, oystercatcher and curlew. The model predicted that dunlin, redshank, black-tailed godwit and oystercatcher survival would start to decline when prey biomass densities dropped below 3 g AFDM m-2. Prey biomass densities for dunlin, redshank and oystercatchers were well above this, indicating that these three species would be less vulnerable to declines in their food supply. However, black-tailed godwit prey biomass densities were very close to this level. Curlew survival was not predicted to decline until prey biomass densities dropped below 2 g AFDM m-2, probably because curlew fed extensively in surrounding fields on terrestrial prey. However, mean prey biomass densities measured for curlew were close to this value, and the 95% confidence limits below it. Shorebird survival was most strongly influenced by the biomass densities of Arenicolaand other annelid worms, earthworms and the bivalve molluscs Cerastoderma eduleand Macoma balthica. It was unaffected by the biomass densities of Corophiumand Hydrobia.
  • Exe estuary shorebirds. Six shorebird species were modelled; dunlin, grey plover, black-tailed godwit, bar-tailed godwit, oystercatcher and curlew. Two models were developed. The simplified model (A) was comparable to those developed for the Humber estuary, Wash and Poole harbour. The more detailed model (B) incorporated more aspects of the behaviour and ecology of the shorebirds. In Model A, the response of shorebird survival to reductions in prey biomass densities were very similar to those obtained for the Humber, the Wash and Poole Harbour. In Model B, however, shorebird survival started to decrease at higher prey biomass densities and the decline in survival with prey biomass was much more gradual. We believe that this is likely to be the more realistic scenario and that threshold prey biomass densities for shorebird survival should not be regarded as around 3-4 g AFDM m-2as suggested in previous models, but as around 8-10 g AFDM m-2. All prey biomass densities on the Exe were as high, or higher, than those measured on other estuaries in this Report. Using Model A, no shorebird species appeared to be vulnerable to declines in their prey biomass densities. With Model B, however, curlew and bar-tailed godwit survival started to decline with prey biomass densities below the 95% confidence interval. It is likely, therefore, that curlew and bar-tailed godwit are the species most likely to be vulnerable to declines in their food supply in the Exe estuary.
  • Exe estuary brent geese. The time at which Brent geese switch from feeding on Zosterato feeding on pastures is affected by the abundance of Zostera. A reduction in Zosteraabundance will bring forward the switch date and an increase will delay it. Accordingly, the number of goose days that Zosterasupports before the switch is complete will be reduced if Zosteraabundance declines and will increase if it rises. This general pattern is little affected either by the assumed efficiency with which geese exploit the Zosteraor by the percentage of time lost to disturbance on the Zosterabeds during daylight (subject to the proviso that access at night is undisturbed). Moreover, the birds end of winter body condition and over-winter survival is unaffected by any changes to the abundance or availability of Zostera. Rather, the well-being of the Brent goose population on the Exe is much more dependent upon the availability of grass pastures on which they feed for the bulk of their winter stay on the estuary.

The following general recommendations (in italics) can be derived from the site and system-specific predictions detailed above.

  • Monitor bird food resources as well as bird numbers. The conservation importance of an estuary is often measured in terms of bird numbers using the estuary, but monitoring numbers is not necessarily a reliable way of assessing changes in site quality. In particular, this is because the numbers of birds using a site depend not only on the conditions at the site, but also the conditions at other sites both within the non-breeding and breeding seasons. The models developed in this project predict site quality as the survival rate of birds on a site rather than simply the numbers of birds on the site. When food is abundant, survival rates are high, but survival decreases when food abundance declines below a threshold value. From this is it possible to derive critical amounts of food required to maintain high body mass and survival. A recommendation derived from these predictions would be to establish a monitoring programme to record the abundance of food on sites at the start of winter as well as continuing the usual procedure of monitoring bird numbers.If resources are limited, surveys should be restricted to Annelid worms, earthworms and bivalves (the key species predicted to affect shorebird survival in this study). Provided that bird densities are comparable to those on the Humber estuary, Wash, Poole harbour and Exe estuary, an initial food supply in excess of 8-10 g afdm m-2, would be predicted to maintain high shorebird survival.
  • Monitor the use of marginal habitats and feeding times. The models developed during this project all predicted that birds fed in the most profitable and safest places and times when feeding conditions were good and survival rates high, behaviour which mimicked that of real birds. In contrast, birds were predicted to feed more in marginal habitats or at more risky times when feeding conditions were poorer, again behaviour which mimicked that of real birds. A recommendation would be to establish a monitoring programme to detect such changes in the behaviour of bird populations as an early warning that survival rates are likely to be falling. This approach would pick up possible detrimental changes on a site before increases in mortality rate could be detected through traditional approaches based on bird ringing programmes, increasing the chance that management can be implemented to improve conditions before bird survival declines greatly.
  • Include terrestrial habitats in conservation areas. The models predicted that terrestrial habitats were often critical for the survival of waders and geese, even though these habitats are often considered as marginal habitats. These habitats are often excluded from the designation of Special Protection Areas, but this means that vital habitat is not being protected and as a result may be lost to building developments, suffer high disturbance levels or simply be subject to changes in the way in which it is managed. A simple recommendation derived from these predictions is that wherever possible conservation areas should include the terrestrial habitats around estuaries as well as the intertidal habitats of the estuary itself.This would ensure that the full range of habitats required by birds is protected.
  • Further understand the response to disturbance. The disturbance experiments conducted during the study show considerable flexibility in the response to disturbance. Birds were more tolerant of human presence in parts of the Wash in which they more frequently encountered humans. Oystercatchers and brent goose feeding on recreational grassland in Poole harbour, in which they frequently encountered people and dogs, were very tolerant, usually only walking away from people rather than flying. The response to disturbance can be viewed as a trade-off between avoiding the perceived threat of the disturbance and the cost of moving. Increased tolerance of human presence (i.e. a reduced response to disturbance) will occur either if birds perceive the disturbance of less of a threat and / or if the detrimental effects of avoiding the disturbance are greater. In conservation terms the second situation is what needs to be avoided. It is less serious if birds can simply adapt to increased disturbance by learning that the disturbance source is less of a threat. Unfortunately, it is difficult or impossible to separate these two alternatives using the data collected in many disturbance studies. A recommendation is that future disturbance studies should aim to determine why birds respond to disturbance sources in the way they do, rather than simply measuring the response to disturbance alone.

This project used individual-based models to predict how site quality depends on the food density and levels of disturbance in the site. The survival rates of most shorebirds and brent goose were predicted to be high with the current biomass densities of food and current rates of disturbance. Target food biomass can be determined as biomass per bird in situations in which a resource is mainly consumed by one species, such as oystercatchers consuming large bivalves. However, this is not possible in multi-species situations and so the more simple measure of food biomass at the start of winter was used in the multi-species models developed during this project. Provided that bird densities are comparable to those on the Humber estuary, Wash, Poole harbour and Exe estuary, an initial food supply in excess of 8-10 g afdm m-2, is predicted to maintain high shorebird survival.

Durell et al. 2006 Reproduced with permission from Elservier:

An individuals-based model, MORPH, was used to assess the quality of Poole Harbour, UK,for five overwintering shorebirds: dunlin Calidris alpina, redshank Tringa totanus, black tailed godwit Limosa limosa, oystercatcher Haematopus ostralegus and curlew Numenius arquata. Site quality, and the effect of environmental change, was measured as predicted over winter survival. Dunlin had the highest prey biomass densities and were the least likely to be affected by reductions in their food supply, lower temperatures or loss of terrestrial habitats. Black-tailed godwits and curlew had the lowest prey biomass densities and were the most likely to be affected by reductions in their food supply, lower temperatures and loss of terrestrial habitats. All five shorebird species were seriously affected by simulated sea-level rise. Conservation issues identified for the Poole Harbour SPA were the relatively low densities of larger size classes of polychaete worms, the importance of maintaining and managing surrounding terrestrial habitats and the effect of sea-level rise on the length of time for which intertidal food supplies are available

Funding and Collaboration

English Nature

Study B

Study B

Birds

Oystercatcher, Haematopus ostralegus

Modelling

Non-MORPH

Abstract

Introductions of non-native species are seen as major threats to ecosystem function and biodiversity. However, invasions of aquatic habitats by non-native species are known to benefit generalist consumers that exhibit dietary switches and prey upon the exotic species in addition to or in preference to native ones. There is, however, little knowledge concerning the population-level implications of such dietary changes. Here, we show that the introduction of the Manila clam Tapes philippinarum into European coastal waters has presented the Eurasian oystercatcher Haematopus ostralegus ostralegus with a new food resource and resulted in a previously unknown predator–prey interaction between these species. We demonstrate, with an individuals-based simulation model, that the presence of this non-native shellfish, even at the current low density, has reduced the predicted over-winter mortality of oystercatchers at one recently invaded site. Further increases in clam population density are predicted to have even more pronounced effects on the density dependence of oystercatcher over-winter mortality. These results suggest that if the Manila clam were to spread around European coastal waters, a process which is likely to be facilitated by global warming, this could have considerable benefits for many shellfish-eating shorebird populations.

Funding and Collaboration

English Nature, ABP

Study C

Study C

Birds

Avocet, Recurvirostra avosetta

Modelling

MORPH

Abstract

Coastal ecosystems are undergoing unprecedented rates of environmental change. Many of these changes are anthropogenically-driven and linked to long-term, climate-related phenomena. This thesis focusses on ecological change in soft sediment intertidal habitats. One of the largest harbours in Europe, Poole Harbour, is used as a case study. It contains a variety of important habitats including intertidal mudflat and non-tidal saline lagoon.

The two main themes of the thesis are 1) assessing the physical and ecological factors that determine benthic invertebrate abundance, distribution and community structure, which is examined at the scale of the whole harbour, and at the scale of individual habitats: an intertidal mudflat and a saline lagoon; and 2) predicting the response of an overwintering shorebird population, the pied avocet (Recurvirostra avosetta), to future environmental changes, such as sea-level rise and habitat loss. This is achieved by development of an individual-based model (IBM) and consideration of the species’ unique foraging behaviour.

This study contributes to the understanding of the factors structuring soft sediment benthic communities, including the use of data from fine-scale hydrodynamic models. It offers a unique comparison of the spatial and temporal variables driving community structure of a saline lagoon and an intertidal mudflat. It also provides insight into the foraging ecology of the pied avocet at a level of detail that has not previously been considered, including a comparison of foraging behaviour in a tidal and non-tidal habitat, the importance of social foraging, and the novel application of an IBM to this species.

Funding and Collaboration

Bournemouth University and HR Wallingford

Study D

Study D

Birds

Grey plover, Pluvialis squatarola

Oystercatcher, Haematopus ostralegus

Curlew, Numenius arquata

Bar-tailed godwit, Limosa lapponica

Black-tailed godwit, Limosa limosa

Dunlin, Calidris alpine

Redshank, Tringa tetanus

Modelling

MORPH

Abstract

Bowgen et al. 2015 Reproduced with permission from Elservier:

Regime shifts in benthic invertebrates within coastal ecosystems threaten the survival of wading birds (Charadrii). Predicting how invertebrate regime shifts will affect wading birds allows conservation management and mitigation measures to be implemented, including protection of terrestrial feeding areas. An individual-based model was used to investigate the impact of regime shifts on wading birds through their prey (marine worms and bivalves) in the estuarine system Poole Harbour, (UK). The model predicted the number of curlew (Numenius arquata), oystercatcher (Haematopus ostralegus), black-tailed godwit (Limosa limosa), redshank (Tringa totanus) and dunlin (Calidris alpina) supported in the Harbour during the non-breeding season (autumn and winter months). The most dramatic declines in bird numbers were for regime shifts that reduced the abundance of the largest invertebrates, particularly marine worms. The least adaptable bird species (those with the most restrictive diets) were unable to compensate by consuming other prey. Generally, as birds adapt to changes by switching to alternative prey species and size classes, changes in invertebrate size and species distribution do not necessarily affect the number of birds that the Harbour can support. Our predictions reveal a weakness in using birds as indicators of site health and invertebrate regime shifts. Differences in bird populations would not necessarily be detected by standard survey methods until extreme changes in invertebrate communities had occurred, potentially beyond the point at which these changes could be reversed. Therefore, population size of wading birds should not be used in isolation when assessing the conservation status of coastal sites.

Bowgen 2016:

With the pressures that today’s ecosystems are being placed under, from both environmental change and anthropogenic developments, the speed at which management decisions need to be made has increased. Coastal development means that estuaries are particularly affected and their characteristic species, like wading birds (Charadrii), are now experiencing worldwide declines. In such situations there is a need for predictive ecology to understand in advance how species might react to future changes.

This thesis looks into how we can use individual-based models (IBM) to make accurate predictions of how wading birds are affected by environmental change. Starting with previously validated models I show the importance of measuring size of invertebrates though an IBM investigation into regime shifts and wading birds responses. The models show that by altering their diet preferences, birds adapt to regime shifts in their prey but that this maintenance of population size masks the true changes in the system and limits the use of waders as direct bio-indicators of ecosystem health. Using the current literature, an analysis on empirical responses of wader populations to environmental change revealed the lack of comparability between studies and the scarcity of studies on small scale events.

Data from literature and fieldwork was used to develop a comparable suite of individual-based models for five UK estuaries with up to eleven wading bird species. These models were validated using current BTO Wetland Bird Surveys data to increase confidence in final results. Using these new models, investigations of population thresholds and environmental change were carried out. Increases to current populations revealed that several estuaries are no longer able to support the number of birds around the time of Special Protection Area designation. This, alongside higher populations currently seen since the years of designation, indicates the need for re-assessment of SPA species numbers. When looking at the impacts of two types of environmental change, habitat loss and sea-level rise, certain species declined predictably across sites whilst the individual make up of each estuary had particular impacts on some waders more than others. The work of this thesis further indicates the great potential of using individual-based models to predict the effects of a wide range of environmental changes. With the new models and a quicker and systematic way of developing IBMs for additional areas, wecan aid the conservation and management of estuarine systems for wading birds.

Funding and Collaboration

Bournemouth University and HR Wallingford

Study E

Study E

Birds

Oystercatcher, Haematopus ostralegus

Modelling

MORPH

Abstract

Intertidal harvesting of marine invertebrates has significant potential to come into conflict with the interests of nature conservation. This is particularly so for overwintering shorebirds that rely heavily on invertebrate prey to maintain body condition throughout the winter and to fuel migration towards breeding grounds. Harvesting activities in these areas therefore require careful management to achieve sustainability and to maintain healthy ecosystem functioning. This thesis investigates impacts of intertidal harvesting on benthic habitats and invertebrate communities as well as the potential impacts of harvesting on shorebird populations. Implications for management of inshore and intertidal fisheries are discussed. A meta-analysis investigated the response of key invertebrate prey groups to different gear types used in different intertidal habitats. Hand gathering most severely reduces prey abundance, which is likely to be due to the accuracy of harvesting with these gear types, while recovery trends vary between different combinations of gear and habitat and taxonomic groups. Results suggest that impacts may persist for longer in sandy habitats than in muddy habitats.

In some cases fishermen may develop gears in response to local circumstance and the development of harvestable populations of new and introduced species. Extensive fieldwork was carried out to assess benthic impacts of ‘pump-scoop’ dredging in Poole Harbour, UK, a designated Special Protection Area under the European Union Birds Directive. The pump-scoop dredge is a novel gear type developed by local fishermen following the introduction of the manila clam Ruditapes philippinarum in the 1980s. The use of this gear type elicits significant changes to macrobenthic community structure and a loss of fine sediments, while reductions in abundance of the target species of up to 95% occur in some areas throughout the open season. Although population dynamics of R. philippinarum vary across a gradient of fishing pressure, determining cause and effect is prevented by a lack of environmental data that could help isolate fishing impacts more confidently.

Data on fishing effort is often lacking, particularly in inshore fisheries where Vessel Monitoring Systems (VMS) data are not collected. The analysis of aerial imagery collected by an unmanned aerial system (UAS) was used as an alternative measure of fishing effort in intertidal areas. Results indicate that the physical scarring of the sediment (quantified through image classification methods and calculation of a measure of image texture) is a reliable proxy for the distribution and intensity of fishing effort in intertidal areas. Remote sensing techniques offer an alternative source of data, useful to inform management of inshore fisheries, where no log book program or VMS data exists.

A combination of fieldwork and individual-based modelling (IBM) was used to investigate the effect of shellfish dredging on shorebird populations in Poole Harbour. Field surveys showed no significant effect of dredging on shorebird feeding or intake rates, nor species distribution across the site, although continued monitoring is recommended. IBM results indicate that increased shellfish landings in Poole Harbour elicit a behavioural response in the Eurasian oystercatcher Haematopus ostralegus population, characterised by an increase in the time spent feeding and the amount of marine worms consumed. These shifts in behaviour and diet represent compensatory measures in response to a loss of preferred shellfish prey.

The work presented in this thesis can directly contribute to ecosystem-based management of inshore fisheries. Results from the meta-analysis will assist managers in predicting the effects of harvesting on benthic ecosystems and provide useful evidence of recovery patterns, while survey data provide information on the impacts of pump-scoop dredging in Poole Harbour, directly contributing to management. Other work provides demonstration of how tools such as remote sensing and IBMs can be applied to accurately quantify disturbance and predict the responses of shorebird populations to harvesting. The work presented will help ensure sustainable fishing, productive benthic habitats and healthy shorebird populations into the future.

Funding and Collaboration

Bournemouth University, Natural England and the Southern Inshore Fisheries and Conservation Authority (SIFCA)

Study F

Study F

Birds

Grey plover, Pluvialis squatarola

Oystercatcher, Haematopus ostralegus

Curlew, Numenius arquata

Bar-tailed godwit, Limosa lapponica

Black-tailed godwit, Limosa limosa

Dunlin, Calidris alpine

Redshank, Tringa tetanus

Sanderling, Calidris alba

Modelling

MORPH

Abstract

Disturbance of wildlife is inevitable in a world with a rapidly increasing human population. Whilst improving engagement with the natural world can have the benefit of encouraging people to help protect it, there is also the issue of increased potential for damaging effects of disturbance. A better understanding is needed of the circumstances under which disturbance would be expected to be a conservation problem, either alone or in combination with the impacts of other human activities. The aim of this thesis is therefore to address these questions: using wintering waders in estuarine habitats as the study system and taking a joint fieldwork and simulation modelling approach. Fieldwork was centred on Poole Harbour; an estuary and wetland of international importance located on the south coast of the UK. Disturbance experiments and observations showed that bird responses to disturbance are highly variable and related to factors including body mass, environmental conditions, site quality, and disturbance type. The energetic and lost-feeding-opportunity costs of responding to individual disturbance events were relatively small and therefore considered unlikely to cause major reductions in individual body condition or significantly limit overwintering population size, given observations of present-day spatial and temporal patterns of human activities. Simulation modelling using two types of individual-based model (IBM) supported this conclusion. Although high levels of disturbance can have a significant impact on wintering bird populations, current frequencies of human activities in Poole Harbour were not found to be reducing the carrying capacity of the site. Increased disturbance frequencies were predicted to be problematic, however, in combination with environmental change that reduced bird ability to meet their daily energy requirements: such as loss of foraging habitat through sea level rise, or reductions in prey availability due to over-exploitation. This has important implications for identifying the most effective conservation management methods. As well as site-specific management recommendations, this research contributes to understanding of the mechanisms by which disturbance may or may not have a significant impact on wintering wader populations; along with applications to other systems; and tools and general principles that conservation managers and decision makers can use to prioritise further investigation and action.

Funding and Collaboration

British Association for Shooting and Conservation, and Bournemouth University

 

Related Paper:

Study A
Study A

Stillman, R.A., West, A.D., le V dit Durell, S.E.A., Caldow, W.R.G., McGrorty, S., Yates, M.G., Garbutt, R.A., Yates, T.J., Rispin, W.E. and Frost, N.J., 2005. Estuary Special Protection Areas – Establishing baseline targets for shorebirds, Centre for Ecology and Hydrology, Dorchester, Dorset.

Durell, S.E.A.L.V.d., Stillman, R.A., Caldow, R.W.G., McGrorty, S., West, A.D. and Humphreys, J., 2006. Modelling the effect of environmental change on shorebirds: A case study on Poole Harbour, UK. Biological Conservation, 131(3): 459-473.

 

Study B
Study B

Caldow, R. W. G, Stillman, R. A., Durell, S. E. A. le V. dit, West, A. D., McGrorty, S., Goss-Custard, J. D., Wood, P. J. & Humphreys, J. (2007) Benefits to shorebirds from invasion of a non-native shellfish. Proceedings of the Royal Society, London, Series B, 274, 1449-1455.

 

Study C
Study C

Ross, K.E., 2013. Investigating the physical and ecological drivers of change in a coastal ecosystem: From indivdual-to population-scale impacts. PhD thesis, Bournemouth University.

 

Study D
Study D

Bowgen, K.M., Stillman, R.A. and Herbert, R.J.H., 2015. Predicting the effect of invertebrate regime shifts on wading birds: Insights from Poole Harbour, UK. Biological Conservation, 186: 60-68.

Bowgen, K.M., 2016. Predicting the effect of environmental change on wading birds: insights from individual-based models. PhD thesis, Bournemouth University in collaboration with HR Wallingford.

 

Study E
Study E

Clarke, L.J., 2018. Ecosystem impacts of intertidal invertebrate harvesting: from benthic habitats to bird predators. PhD thesis, Bournemouth University.

 

Study F
Study F

Collop, C., 2016. Impact of human disturbance on coastal birds: Population consequences derived from behavioural responses. PhD thesis, Bournemouth University.