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Wednesday, 13 March 2013

Do fragment size, connectivity and the composition of the landscape surrounding a grassland fragment influence insect species richness?


Calcareous grasslands are among the most species rich habitats in Europe, harbouring an extraordinary diversity of plants and invertebrates (Figure 1). Unfortunately, since the onset of agricultural intensification these grasslands have become increasingly threatened due to conversion to arable land and fertilisation or abandonment and subsequent shrub encroachment. The remaining grasslands are increasingly fragmented and frequently of small size. However, little is known about how the surrounding landscape influences fragmentation effects. In this study we focussed on the interaction between habitat fragmentation and landscape composition on leafhoppers. They are a highly diverse group of herbivorous insects and include many species that are likely to be vulnerable to changes in their environment.

We selected 14 small (0.1–0.6 ha) and 14 large (1.2–8.6 ha) fragments of calcareous grassland in central Germany near the city of Göttingen, differing in isolation from other calcareous grasslands (measured by a connectivity index) and in the composition of the surrounding landscape, i.e. the percentage of arable land surrounding each fragment within a 500 m-radius (ranging from 27 to 77%). Leafhoppers were sampled by sweep netting three times per fragment in summer 2010. The species were subdivided into specialists that depended on calcareous grasslands and generalists that could use the landscape matrix, but most of them still required low-productivity habitats.

Figure 1. An impression of a typical calcareous grassland in the vicinity of Göttingen in July. The leafhopper Batracomorphus irroratus (Cicadellidae, Iassinae) is confined to calcareous grasslands since its host plant Helianthemum nummularium only occurs there. Picture of B. irroratus by Gernot Kunz.

We found that increasing habitat isolation reduced leafhopper species richness in simple (dominated by arable crops), but not in complex landscapes (see Figure 2). Surprisingly, this effect was driven by those species that we had classified as generalists. We assume that in simple landscapes, leafhoppers may find it more difficult to reach the next suitable fragment due to a lack of alternative resources during dispersal and that generalist species use calcareous grasslands as a refuge, since many species depend on low-productivity habitats.  Moreover we found that generalist species richness increased with increasing connectivity on small fragments, whereas it remained stable with increasing connectivity on large fragments. We think that in small, isolated fragments, a higher extinction rate combined with a lower probability of recolonisation may cause the reduced species richness. Finally, we found a steeper increase in generalist species richness with increasing plant species richness on connected than on isolated fragments, assuming that more colonisers reach connected fragments and can subsequently use the available plant resources.

Figure 2. Effect of habitat isolation (measured by a connectivity index) on a) overall leafhopper species richness and b) generalist species richness in conjunction with landscape composition. c) Effect of habitat isolation on generalist leafhopper species richness in conjunction with fragment size. d) Effect of plant species richness on generalist leafhopper species richness in conjunction with habitat isolation (modified from Rösch et al. 2013 - see reference).

Our results are the first to show that insect biodiversity on fragmented calcareous grasslands not only depends on habitat connectivity but that it is interactively affected by the four factors habitat connectivity, landscape composition, habitat area and plant species richness. Isolated fragments that are either small or located in simple landscapes are less likely to receive immigrants after extinction events, leading to a gradual reduction in species richness over time. These patterns should not only apply to leafhoppers but also to other insect groups (e.g. true bugs).

Mitigating the negative effects of habitat fragmentation therefore needs to take the surrounding landscape into account. Management should be prioritised towards increasing the connectivity (i) of small, isolated fragments, (ii) of fragments in simple landscapes and (iii) towards management efforts that enhance dispersal by increasing heterogeneity of both landscape composition and configuration. Moreover, extensive management of fragments by grazing or mowing, both relatively late in the season, to increase habitat quality for leafhoppers would benefit other insect groups as well.


Reference:
Rösch, V., Tscharntke, T., Scherber, C. & Batáry, P. Landscape composition, connectivity and fragment size drive effects of grassland fragmentation on insect communities. Journal of Applied Ecology in press (doi: 10.1111/1365-2664.12056).


Correspondence
Verena Rösch and Péter Batáry, PhD
Agroecology, Department of Crop Sciences
Georg-August University, Göttingen, Germany
verena.roesch@agr.uni-goettingen.de, pbatary@gmail.com
http://www.uni-goettingen.de/en/74726.html

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