Unravelling abiotic and biotic drivers of biodiversity change in local plant and invertebrate communities after 80 years - a re-visitation study on the Studland peninsula.

Abstract

How ecological communities respond to long-term environmental change via changes in species richness and composition is an urgent question in the 21st century as anthropogenic forces drive biodiversity declines across taxa and regions. As environmental conditions change over time, effects may cascade through co-occurring taxa, directly disturbing some species and altering the structure of ecological networks to disturb others However, a paucity of data has meant that studies investigating abiotic and biotic drivers of biodiversity change over periods of decades are rare, particularly those spanning multiple co-occurring taxa. The aim of this thesis is to investigate how effects of long-term (ca. 80 years) environmental change propagate through co- occurring plant and insect communities, driving changes in species richness and composition across taxa. To achieve this aim I utilised a uniquely rich re-visitation study of species occurrence data on the Studland peninsula in the south of England. Data collection was led in the 1930s by the naturalist Cyril Diver, and in the 2010s by the National Trust in collaboration with a team of citizen scientists. I asked: 1) How have vascular plant assemblages of Studland changed in response to a changing abiotic environment between the 1930s and the present day?; 2) Have plant and insect taxa undergone congruent biodiversity changes under shifting environmental conditions?; 3) What abiotic and biotic factors relating to adult and larval ecological requirements have influenced long-term biodiversity change in Studland’s hoverflies?; 4) How are the plant and insect communities of Studland likely to change in coming decades under proposed management interventions? 1) Using multilevel models of differences in Ellenberg indicator values (EIVs) between assemblages of vascular plant species occurring in each time-period, I found that the most prominent driver of plant species compositional change was changing hydrological conditions, followed by successional processes. 2) Using hierarchical modelling of species loss/gains and a range of multivariate techniques, I found that species richness and compositional changes in plant and insect communities displayed cross-taxon congruence – correlated patterns of biodiversity change – over the ca. 80 year time-period, likely driven by a combination of abiotic and biotic change. 3) Hierarchical modelling of species loss/gains in the hoverfly community suggested that species richness in adult assemblages is limited by adult resource availability (plants) at highly localised scales, while compositional change is strongly affected by the availability of suitable larval microhabitat at the wider scale of a few kilometres. As with the plants, hydrological change was the main abiotic driver of change in the hoverfly community. 4) Bayesian Belief Network (BBN) models predicted that local management interventions to increase drainage and control ecological succession could benefit the taxa and habitats of Studland through increased species richness. However, BBN models also predicted that the health of the regional metacommunity is of utmost importance in maintaining a healthy local system. I have shown that congruent biodiversity change occurs across diverse plant and insect taxa, and is driven by changes in both abiotic and biotic conditions. These drivers do not act independently from one another, as demonstrated by effects of wetter winter conditions on hoverfly composition, coupled with effects of species richness change in the plant community, while the plant community was itself also responding to hydrological change via changing species composition. Crucially, my results suggest that the health of the regional metacommunity is of the utmost importance in maintaining a healthy local system when faced with environmental changes as seen at Studland; a reservoir of species available to take advantage of new conditions is vital. Therefore this work suggest a joint emphasis placed on local and regional conservation practices to mitigate effects of the anticipated acceleration in environmental change.