Disturbances, biodiversity changes and ecosystem stability
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Species abundance and ecosystem functioning
Many species are being reduced in abundance or driven to local extinction by human activities. Although changing biodiversity clearly has consequences for the functioning of ecosystems, better understanding is needed of the relative importance of different kinds of changes.
Scientists working on the MarBEF BIOFUSE project used an experimental system of intertidal communities of algae and invertebrates to assess the effects of changes in key species on the functioning of the selected ecosystem. The results showed that changes in the abundance of certain species were more important than changes in the diversity of species. Although the effects of changes in diversity vary according to the habitat and location, the effects of changes in species abundance are more consistent. Therefore alteration of key species abundances affects ecosystem functioning more than changes in species diversity. This outcome emphasises the importance of preserving not only particular species but also the relative abundances with which species populate our marine coastal environments.
Human disturbance and the stability of rocky shore assemblages
The BIOFUSE project focused in particular on the effects of human disturbances on the stability of rocky shore assemblages. The structure and functioning of marine ecosystems is threatened by a range of human activities.
The results suggest that only some types of human disturbance have strong effects on the stability of rocky shore assemblages. Some types of disturbance, such as loss of large seaweeds or nutrient enrichment, did not influence stability. Other sources of disturbance (such as removal of certain organisms) can reduce the stability of intertidal assemblages.
An increase in the severity of disturbances in intertidal habitats is predicted as a consequence of increased frequency of extreme meteorological events (i.e. sea-storms and hurricanes). Similarly, the introduction of exotic species is increasing rapidly with the intensification of global trading. Management initiatives should focus their attention on responses to climate change and on reducing the impact of invasive species on rocky shore assemblages.
Impacts of biodiversity change on ecosystem stabilityThere is concern about the potential effect of biodiversity loss on the functioning of ecosystems and their services to society. A key consideration is to what extent biodiversity can improve the stability of ecosystems. More stable ecosystems are more reliable providers of ecosystem services such as fish catches and stabilisation of coastal habitats.
The relationship between biodiversity and stability of marine benthic assemblages was investigated through meta-analyses using existing data sets (n = 28) covering various spatial (m–km) and temporal (1973–2006; ranging from 5 to >250 months) scales in different benthic habitats (emergent rock, rock pools and sedimentary habitats) over different European marine systems (North Atlantic and western Mediterranean). Little variability in time was a measure of stability, and variability was estimated as temporal variance of species richness, total abundance (density or % cover) and community structure. This study provides one of the few comprehensive assessments of large spatial scale variation in the relationship between diversity and temporal stability across different marine systems.
When data sets were analyzed separately, a large proportion of the observed relationships between stability and biodiversity were weak or not significant. The results suggest that diverse assemblages exhibit variability in species richness without strong variability in species abundance and community composition.
Using multivariate analyses, it appeared that stability (measured with Bray–Curtis dissimilarities) of species abundance and composition within communities are generally not correlated with diversity indices. (Diversity indices are broadly defined as: 'richness'=number of different species, 'abundance'=number of individuals per species, 'evenness'=abundance of all major species, see the article Measurements of biodiversity.) The results suggest that relationships between diversity and community stability may be governed by evenness rather than species richness. Moreover, contrasting results among habitats exist, with sediment communities with high evenness being less stable, perhaps from prevalence of positive species interactions in this habitat. The overall outcome of the analysis of the existing datasets indicated a negative (albeit weak) relationship between diversity and stability. These relationships were observed at small and large scales, but there was variation in the outcome depending on which habitats and locations were considered.
Temporal variability in species richness increased with the number of species at both quadrat ([math]\sim 1 m^2[/math]) and site ([math]\sim 100 m^2[/math]) scales, while no relationship was observed by multivariate analyses. The relationship between species richness or evenness and species richness variability was slightly positive and dependent on the scale of observation. Thus, species richness did not stabilize temporal fluctuations in species number, rather species-rich assemblages were most likely to undergo the largest fluctuations in species numbers and abundance from time to time. Overall, the results emphasized that the relation between species richness and species-level measures of temporal variability depends on scale of measurements, type of habitats and the marine system (North Atlantic and Mediterranean) considered.
Role of functional diversity
Fargione and Tilman (2005) found evidence that resistance to disturbance by invading species depends more on the functional diversity than on the number of taxa per functional group. This would explain why at elevated functional richness (high complementarity) the resistance to change is not strongly related to species richness. However, there is also strong evidence that ecosystems better resist to disturbance when multiple species carry out similar functional roles (Hooper et al., 2005). Loss of functional groups should therefore be more severe when functional richness is already low from the start. This would explain why at low functional richness higher species richness (more functional redundancy) makes communities less vulnerable to environmental change. In summary, the persistence of communities is strongly related to species richness at low functional richness and little related to species richness at high functional richness. The interplay between functional redundancy (reducing the consequences of species loss) and functional complementarity (reducing the risk of invasion) seem to explain the observed interactive effects of species and functional richness with regard to community level impacts of environmental change.
Combined effect of species loss and disturbances
Climate change scenarios predict a global increase in stresses, e.g. by extreme weather, species invasions, etc. Local human activities may cause the loss of some of the key species in the ecosystems such as large seaweeds, seagrasses and burrowing worms. It is not yet known how these different impacts might combine to affect ecosystem processes.
In the BIOFUSE project simple experiments were used to compare the effect of loss of a key species on a number of marine ecosystems, which were also subjected to an experimental disturbance. The goal was to find out whether the effects of biodiversity loss are the same across different habitats and locations.
The loss of key species affected many, but not all, ecosystems. The influence of loss of species and disturbance varied among habitats and locations. Complex combined effects of these two impacts occurred in a few cases. A major impact on ecosystem functioning was rarely observed, which suggests a widespread capacity of ecosystems to compensate for the loss of a single species, even key species. This is good news with respect to these habitats, but the results showed variation between locations, something which is reflected in the EU Marine Strategy Framework Directive where regional focus is emphasized.
- Resilience and resistance
- Measurements of biodiversity
- Ecosystem functioning
- Resilience and resistance
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