Evaluating the ecosystem function of selected dipteran families and their role as indicators of farm habitat quality in Ireland
Date
2021-09-27Author
Ahmed, Karzan Sabah D.
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Abstract
Biodiversity has undergone significant declines worldwide as a result of
human activities. This trend is particularly apparent in agricultural ecosystems where
farming intensification is recognised as one of the primary causes of rapid decline in
farmland biodiversity particularly in insects with multiple ecosystem services
including pollination, biological pest control and nutrient cycling. While the positive
role of results-based agri-environment payment schemes in protecting and enhancing
farmland biodiversity and increasing habitat quality has been recognised in recent
years, there are still gaps in knowledge regarding how farmland habitats should be
assessed particularly in relation to non-iconic invertebrates.
Two dipteran taxa selected for this study, Syrphidae and Sciomyzidae, have a
range of ecosystem functions. These include pollination, biocontrol and nutrient
cycling (Syrphidae) and biocontrol of molluscs (Sciomyzidae), Both families co-exist
within many of the same habitats but have markedly different ecological requirements
and ecosystem functions, making them good indicators of farm habitat quality. This
study comprises three major elements conducted at different scales as follows: 1) an
investigation of the ecosystem function/biocontrol potential of Tetanocera elata
(Diptera: Sciomyzidae) which feeds on pestiferous slugs; 2) an examination of the
contribution of linear habitats across a range of farming intensities to both Syrphidae
and Sciomyzidae diversity and abundance; and 3) an exploration of how variables (at
local and landscape level) could be used to incorporate hoverfly functional groups in
rapid farmland biodiversity assessments.
The results of laboratory experiments investigating the biocontrol potential of
T. elata demonstrated that larval survival outcomes depend on prey slug species with
neonate and third instar larvae appearing to prioritise different predatory strategies. Of
particular note is that successful pupariation in T. elata is reduced for larvae fed on
slugs exposed to Phasmarhabditis hermaphrodita (Rhabditida: Rhabditidae), a slug
parasitic nematode. This potentially negative effect of P. hermaphrodita (which can
be purchased in Europe for the control of pestiferous slugs) on T. elata survival has
implications for protecting wild populations of T. elata and for how biological control
of slugs is undertaken in the future.
The contribution of farm linear habitats across a gradient of farming intensities
to Syrphidae and Sciomyzidae abundance and diversity was investigated in Co. Sligo
(Ireland) using bi-directional malaise traps. Overall, Syrphidae and Sciomyzidae
species richness and community composition reflected a farming intensity gradient
with significantly greater species richness for both families on extensively managed
farms. In addition, although their abundance and species richness were greatest in
dense hedgerows with adjacent watercourses, Syrphidae showed no significant
difference overall between linear habitat types. This is in contrast to Sciomyzidae
where open hedgerows with adjacent watercourses had significantly more
Sciomyzidae species and individuals than dense or open hedgerows without adjacent
watercourses. Syrphidae species richness was also significantly correlated with the
flowering plant species richness of linear habitats while Sciomyzidae species richness
was correlated with a habitat quality score for grasslands adjacent to the linear habitats.
The results suggest that while low intensity farming appears to support species
richness of both taxa, universal management prescriptions for a farmland linear habitat
(such as a hedgerow) may not benefit all dipteran taxa equally. This has implications
for future agri-environmental schemes and is discussed in the context of how the
conservation value of linear habitats on farmlands is currently assessed.
Given that detailed field surveys of invertebrates at landscape scale are
extensive and time-consuming, determining the best habitat features (reflecting
Syrphidae ecosystem services) that can be rapidly measured through high resolution
satellite imagery rather than field survey, is crucial for national agri-environmental
payment schemes. With this in mind, selected habitat features that best reflect hoverfly
requirements and their associated ecosystem services were investigated at local and
landscape scale using pan traps in two study regions with contrasting farming
conditions (Cos. Sligo and Wexford, Ireland). Predatory and non-predatory species
dominated the Syrphidae in Cos. Wexford and Sligo respectively. In addition, results
indicate that at landscape scale, hedgerows and tree lines (irrespective of county and
farming conditions) are the most important driving factors positively affecting
hoverfly abundance/richness, with other linear habitat features showing variable
results within and between each study region. The implications of these results in the
context of selecting the best explanatory variables that can be incorporated in rapid
assessments for agri-environmental payment schemes are discussed with particular
reference to total lengths of hedgerows/tree lines on farms.