Seed cards. As a basic unit, we will use seed cards. Instructions for making seed cards are included (Preparing seed cards). Seed cards work fine for birds, rodents, carabid beetles, slugs and crickets. They do not work for (harvester) ants. Ants tend to be important in Mediterranean countries. Therefore, Stephano and Jordi may have to use petri-dishes (instructions will follow). Nevertheless, it would be good to observe the seed cards for a while if you have no idea what might be around. Keep in mind though that most granivores are nocturnal.
Seed choice. We will use seed cards with Echinochloa crus-galli seeds and Chenopodium album seeds. Motivation for choosing these species were a) relevant weed species and available in all countries, b) can be glued to seed cards, c) are consumed by all sorts of granivores. After the meeting on the joint trials in Montpellier, Lars approached me because he wanted to participate. His main problem is blackgrass, Alopecurus myosuroides, same as Sarah’s. I can imagine that Lars and Sarah would be interested in blackgrass as a test species. I therefore suggest that they either add A. myosuroides to the spectrum of weed species or replace E. crus-galli with A. myosuroides. I suggest to use only one species per card and 50 seeds per card.
Exclusion cages. To distinguish between vertebrates and invertebrates, we will use exclusion cages with a mesh size of 9-12 mm. This mesh size allows access to (almost) all invertebrates. Please do not use plastic mesh, as rodents are able to bite through the plastic. I will send you pictures of different designs of exclusion cages. The cages are places over the seed cards and fixed to the ground using tent pegs or metal wire pins. Cages and pins are relatively easy to make. Feel free to contact me if you run into problems or require advice.
Exposure time. It is very difficult for me to recommending an exposure time, i.e. the time between begin and end of the trial. A too short exposure and predation rate will be 0%; a too long exposure and predation rate will be 100%. I suggest that each of you determines the best exposure time in a small preliminary trial, where some seed cards are placed in a field and you check every two days to see how quickly seeds disappear. Usually, the optimal exposure time is somewhere between 3 and 14 days.
General experimental set-up. We will place a minimum of 10 seed cards (5 with C. album and 5 with E. crus-galli) with exclusion cage and 10 seed cards (5 with C. album and 5 with E. crus-galli) without exclusion cage in each field. A good spacing is one seed card per 10 m2. The most convenient way of placing the seed cards in a row crop is in transects. For example, depending on the size of your field, you could use 2 transects with 10 cards each, or 4 transects with 5 cards each, using a 10 m spacing between cards. Be creative. If possible, keep a minimum distance of 20 m from the field edge. Most granivores also move along rows, so make sure to ‘hide’ your seed cards on either side of the transect row. I usually use spray paint (blue and pink work fine) or long sticks to be able to recover the seed cards.
Repetition over time. For those of you working in annual crops, we will repeat the trial at least three times over the course of a growing season; 1) at full crop closure, 2) shortly before crop harvest, 3) 4-6 weeks after crop harvest. Reasons for choosing these periods were; a) different activity patterns of granivores (e.g., spring vs. autumn breeding carabids), b) response to canopy density, disturbances, and temperature, c) timing of weed seed shed. For example, all granivores, except ants (and perhaps birds), respond strongly to the presence of a canopy and will avoid open spaces.
For those of you working in orchards and vineyards, I suggest you repeat the trial three times as well, for example, in spring, mid-summer and autumn. Several of the reasons for choosing different periods (i.e. activity patterns granivores, canopy density, temperature, timing of seed shed) are still valid.
Main type of granivore.
The main type of granivore can be determined from the proportion of seed removal inside (invertebrate) vs. outside the exclusions (invertebrate plus vertebrate). Observations on feeding patterns, bite marks, slime trails, foot prints and faeces on the cards can provide additional information about the exact granivore identity.
Next, we need to figure out what is responsible for the presence/absence of particular groups of granivores. For example, I found out that densities of granivorous rodents in arable fields in NE Germany are very low. A wide range of factors could be involved, for example, climatic factors, soil type, crop type, type of tillage, distance to open water, distance to coast, distance to suitable non-crop habitat, field size, yield potential, presence of predators, etc. To identify the most important factor(s), we need to document these factors and correlate these to our data. Unfortunately, we do not exactly know what we are looking for, so I suggest that we start by documenting the main environmental and management variables (in as far as possible):
Let me know of any factor that you think might be influencing granivores identity, numbers or activity in your field!