Invertebrate diversity in groundwater filled lava caves is influenced by both neutral and niche-based processes

Aim: Understanding which factors shape and maintain biodiversity is essential to understand how ecosystems respond to crises. Biodiversity in ecological communities is a result of the interaction of various factors which can be classified as neutral or niche-based. The importance of these processes has been debated, but many scientists believe that both processes are important. Here we examined the importance of neutral vs. niche-based factors for shaping invertebrate communities. We hypothesized that if neutral processes are the main drivers of community structure we would not see any clear relationship between the structure of community and ecological factors. If niche-based processes are important we should see clear relationships between community structure and variation in ecological variables.

Location: Groundwater-filled lava caves near Lake Mývatn, Iceland.

Methods: We collected various ecological variables from these caves. Invertebrate communities were collected on the hard bottom using stone scrubbing and from epibenthic traps. Results: Both communities were species-poor, with low densities of invertebrates, showing the resource-limited and oligotrophic nature of these systems. Unusually for Icelandic freshwater ecosystems, the benthic communities were not dominated by Chironomidae (Diptera) larvae, but rather by crustaceans, mainly Cladocera. The epibenthic communities were not shaped by environmental variables, suggesting that they may be structured primarily by neutral processes. The benthic communities were shaped by the availability of energy, and to some extent pH, suggesting that niche-based processes were important drivers of community structure, although neutral processes may still be relevant. 

Main conclusions: The results suggest that both processes are important for invertebrate communities in freshwater, and research should focus on understanding both of these processes. The ponds we studied are representative of a number of freshwater ecosystems that are extremely vulnerable to human disturbance, making it even more important to understand how their biodiversity is shaped and maintained.

Methods
The following environmental variables in the caves were measured in June and August from 2013 – 2019: temperature (°C), oxygen saturation (%), pH, and specific conductivity (µS/cm), using a multiprobe sonde (Hydrolab DS5 (46711) Water Quality Probes). From a digital map we estimated the minimal distance of each cave from the lake (in meters) and key physical characteristics of the caves, such as the combined vertical area of all openings and the size of the cave pond exposed to air (both in m2), in some caves the roof was overhanging, so the pond exposed to air value was negative. Based on this, we estimated the area available for photosynthesis by adding half the vertical area of openings to the size of the cave pond exposed to air. In each cave, we placed fall-in traps within 20 cm of the shore to collect external inputs into the pond. The main objective of the traps was to estimate food availability for Arctic charr for another study. The traps were built of clear buckets (Unipak model 5141, 2300 ml, 196 cm2 surface area) placed in a bracket and tied fast with a zip-tag about 10 cm above the water surface. In each bucket, we mixed propylene glycol (30%) with water and a few drops of aroma free soap to reduce water tension. The number of traps differed among the caves, depending on the size of the cave and the number of cave openings, but the minimum number of fall-in traps per cave was two. However, not all traps survived the conditions during the winter, so on some occasions no trap data exist for a given sampling event. The traps were collected in June and August each year, by removing the old trap and replacing it with a new one. The content of the fall-in trap was sieved through a 125 µm sieve and preserved in 70% ethanol for at least 6 months. The fixed organic material of animal origin was then separated from obvious plant material, placed in a measuring cup, and allowed to sink for 15 – 30 min, and measured to the nearest mL. We thus obtained an estimation of gross volume of external organic animal material per cave.

All invertebrate samples were collected in June 2014. Benthic invertebrates were collected from two to three stones along the shore of 16 caves, at a depth of 10 – 30 cm. Stones were carefully removed from the bottom and placed immediately in a 10L bucket containing water from the cave. They were then brushed, using a soft dishwasher brush. The stone was rinsed with clean filtered (125 µm sieve) water, placed as they were lying in the cave and photographed with a mm scale for size reference. This allowed us to estimate standardised densities (#/100 cm2) of invertebrates across stones and caves. The water and invertebrates retained in the bucket were sieved through a 125 µm sieve and stored in 70% ethanol until further analysis.

Epibenthic invertebrates were collected from 18 caves using crustacean traps modified from Örnólfsdóttir & Einarsson (2004). The traps were haphazardly laid within 2 m from the shore at the opening of each cave. The number of traps per cave varied from two to six depending on the size of the cave and the size and number of openings.  The traps were collected after 12 hours, and their content filtered through a 63 µm sieve and stored in 70% ethanol until further analysis.