Bangor University and CEH: frosty spiders and rooty soils

The Bangor University and CEH-Bangor team have had a great summer campaign, sampling salt marsh vegetation, spiders and beetles, and sediment cores for our erosion studies.

We are interested in understanding whether biodiversity of insect and spider populations are boosted by vegetation structural complexity (height, cover, weight per area) and vegetation biodiversity. Structural complexity, such as variation in plant height, and vegetation diversity can boost invertebrate diversity in other systems, but the principle has not been tested for marshes. Using high-tech equipment, an inverted leaf blower that sucks rather than blows, the Bangor team collected insects from 132 plots in Morecambe and Essex marshes. The taxonomy of marsh invertebrates is a highly specialist area and our postdoc, Hilary Ford, will go to the Netherlands by the end of the year to hone her beetle and spider identification skills. Our preliminary results indicate that invertebrate communities in Morecambe marshes are less abundant and relatively species poor, compared to those of Essex marshes. Morecambe marshes generally had shorter and less structurally complex vegetation than Essex marshes, so our complexity-diversity test prediction might be confirmed for marsh invertebrates. We expected to find hardly any spiders and beetles in winter. In fact, we caught more spiders on frosty winter days in Essex than on summer days in Morecambe. Exactly how tiny money spiders manage to survive weeks of deep snow and sub-zero temperatures is momentarily beyond us.

Figure 1: Salt marsh core in PVC pipe before and after exposure to water force in hydrological flume. This core from Morecambe Bay lost practically all of its soil in the flume, leaving only the vegetation root-net remaining. Other cores were more resistant to erosion.

The Bangor-CEH team examines the role of salt marsh vegetation to protecting our shorelines against erosion. It is known that vegetation roots bind soils and prevent sediment from washing away with waves and tidal currents. Resistance to erode is also related to the physical nature of the soil; organically enriched, fine-grained soils are more erosion resistant than organically poor and coarse-grained soils. We ask: how much of the resistance to erode is due to the vegetation and how much is due to the soil physical conditions? In particular, could diversity of vegetation enhance root binding of soils? After all, diverse vegetation will have a structurally more varied rootnet than species-poor vegetation, and this root variation might give a more holistic binding of soils.  To test this prediction, the team has collected 264 ‘cores’ of natural marsh soils with vegetation (16 cm diameter pipes, hammered 30 cm into soil). In the laboratory, we are still in the process of exposing the cores to jets of water pressure in a hydrological flume, to imitate erosion forces on coastlines (Fig 1). We record how quickly cores erode. So far, we have not detected a significant effect of vegetation diversity on erosion rates. However, we have found that species, such as Agrostis and Festuca grasses, bind soils more effectively than other species. One of our most marked results is that soils with plenty of roots are far more resistant to erosion than soils with few roots (Fig 2). Clearly, the biology of marshes matters to coastal protection.

Figure 2: The rate of erosion of salt marsh soils is dramatically reduced by an increase in vegetation root mass. Erosion rate is expressed as a percentage of original soil mass lost per hour in a hydrological flume. Marsh vegetation-soils were collected in Morecambe Bay and Essex salt marshes.