Ground invertebrate monitoring

Under the Soles of our Shoes

Most of our long-term monitoring has con­centrated on groups of species which are the more obvious and conspicuous. Over the years we have built up a knowledge of the plants, butterflies, birds, dragonflies and small mammals which inhabit our fields, but have known very little about the ground invertebrates.

And yet ground invertebrates, which are mostly very small and therefore the invisible hordes, are a major component in the ecosystem of our fields and are vital for the survival of any community. By far the largest number of invertebrates are active in the breakdown of plant material, so they play an essential role in creating the humus component of soil. Without invertebrates most plant communities will not develop any degree of diversity.

These species which feed on plants or detritus are also essential as food for larger predatory invertebrates and as hosts for parasitic species. Then all the invertebrates provide food for vertebrate predators – amphibians, shrews and mice, birds, larger mammals such as fox and badger.

Thus the invertebrates are the animal foundation to a food pyramid which ends in the higher predators, often those species we do notice and about which we are often so concerned.

Without a good variety of invertebrates, the whole ecosystem will be im­poverished.

Neil Taylor's work in studying the small mammals in our pastures and meadows demonstrated that we had high populations of shrew, and as they are a carnivore that spends up to 22 hours a day hunting and eating, it was reasonable to feel that we must have lots of ground invertebrates. But how many was 'lots'? And what groups and what species and in what proportions? And were some of our grasslands more suitable for invertebrates than others? And how did the fields of Denmark Farm compare with the neighbouring rye-grass fields?

For years we have been advocating holistic management of our fields and arguing about the equal rights of all species, and yet our monitoring was continuing to concentrate on the big and the relatively obvious. So in 1998, Richard Williams embarked on a pilot scheme to investigate the ground invertebrates in our fields. We reported on some of the species he encountered in the article "A Myriad of Connections" (Newsletter 18, October 1998).

An Ethical Dilemma

Up until 1998 we had set our faces against killing creatures as a deliberate part of our monitoring programme. “If you can't identify it alive, then it doesn't get identified” was the basic rule for ourselves and visitors.

We had to look at the whole issue again once Richard embarked on his pilot, as it became clear that it was impossible to carry out the research without a lot of invertebrates dying, because very many species of invertebrates are only possible to identify through dissection and examination under a microscope. So we had to accept a measure of deliberate killing.

The problem became more acute as Richard designed his research programme for 1999. He decided to collect 10 cm² (4 inch square) samples of vegetation regularly throughout the year from four very different types of grassland on Denmark Farm, and then from a fifth site in one of the neighbouring rye-grass fields. The grass samples would be put into Tullgren Funnels, in which the light and increasing dryness would 'drive' all the invertebrates living in the grass into a killing and preserving solution. It was obvious that a huge number of invertebrates would have to die.

After much soul-searching, in the end we decided to embark on this research, feeling that the Trust needed to have a lot more authoritative data on the ground invertebrates in our fields, so that we could argue for the needs of these groups and species to be taken into account in con­servation management. There was no way we could collect this data without the killing.

There are times when it has been hard to live with that decision, especially when we looked at a particular specimen Richard placed in front of us to admire its beauty or intricacy. What are we humans doing when we decide what is sacred and what is not? – or what is dispensable in our greater scheme? And is it easier to justify the killing of those species which are less obviously beautiful or intricate?

Looking at One Sample

In the photograph you can see part of a dish containing some of the creatures extracted from a typical 10 cm² (4 inch square) of the grasses in the section of Cae Pond (Pond Field) which is only managed by grazing animals.

Over the twelve months, the average of 28 samples shows the following creatures living in this small piece of grassland:

Mites	194.2	Bees, Ants, Wasps (adult)	0.4
Pot Worms	2.8	Woodlice	0.2
Earth Worms	0.2	Butterflies, Moths (larvae)	0.9
Spiders	0.5	Millipedes	0.2
Beetles (larvae)	1.0	Centipedes	0.2
Beetles (adult)	2.1	Nematode Worms	0.2
Springtails	18.4	Harvestmen	0.2
Flies (larvae)	6.5	Thrips (wingless)	3.7
Flies (adult)	1.5	Thrips (winged)	0.1
Slugs	0.4	Proturans	0.1
Snails	0.3	Booklice	0.1
Bugs (Heteroptera)	0.1
Bugs (Homoptera)	4.1
			238.5

These numbers also equal the extrapolated population per hectare in millions. In other words, in a hectare of the grazed grassland of Cae Pond, there will be a minimum of 238 million invertebrates; and in an acre there will be at least 96 million.

And these are only minimum figures, because Richard's research method inevitably meant that he caught very few of those invertebrates which can fly, jump or run out of harm's way. This explains the absence of grasshoppers, for example. And the sampling of grass is only taken to ground level, so the figures do not include all the myriad of creatures living just below the surface of the soil. (The Pot Worms and Earth Worms recorded would be those feeding on the surface.)

So we can look at Pond Field with new eyes, knowing that it is literally crawling with life, and of a variety of species and a complexity of inter­connections which make this truly living field an unbelievably complicated web.

Making Comparisons

Below you can see the average results per sample throughout the year for the seven sites Richard studied.

Effectively we were studying three types of field, each with very different plant communities and structures resulting from the system of agri­cultural management. So we can further summarise Richard's work:

Field Type	Management	Average Population over the year in millions per:		Ratio
		Acre	Hectare
Rough pasture	Non-intensive grazing	87	207	13
Hay Meadow	Cut annually, usually in August	31	75	5
Rye-grass	Fertilisers, silage, intensive grazing	7	16	1

We can look at this another way: if you stand in a field wearing size 9 shoes, then you may well be standing on the following numbers of invertebrates in the grass:

Rough Pasture	1,106
Hay Meadow	400
Rye-grass	86

It gives a depth of meaning to “treading lightly on the earth”!

Hay Fields Compared to Pastures

Current agri-environment schemes are encouraging farmers to re-create 'traditional hay meadows' as a key objective.

Whilst it's clear that hay meadows are an important habitat, the main beneficiaries are a range of specific plants and the insects which visit them to feed. In contrast, Neil's work has shown that the hay meadow compartment of Top Field supports far less small mammals over a year than the neighbouring rough pasture compartment, and now Richard's study reveals that this is also true for ground invertebrates.

The most noticeable consequence of repeated annual cutting for hay compared to the development of rough pasture, with its matted and decaying vegetation at ground level, is that invertebrate numbers are limited to about 36% of the numbers found in the pasture. Specifically, for the following groups of invertebrates, numbers in the hay meadow are less than 50% of those found in the rough pasture: Mites, Beetles (larvae and adults), Flies (larvae), Slugs, Snails, Bugs, Woodlice, Butterflies (larvae) and Moths (larvae).

Grass Tussocks Compared to Non-Tussocked Pasture

In Top Field Richard found that the tussocks of Cock's-foot Grass had an invertebrate population about 32% higher than the other grass habitat in the same rough pasture. But because these tussocks only form a small part of the pasture, they only contribute an additional 3% to overall invertebrate numbers.

However, overall numbers are only one way of evaluating the significance of each habitat for invertebrates. We found that several groups of invertebrates were present within the tussocks in more than double the numbers of those found in the rest of the grassland: Spiders, Beetles (adults), Woodlice, Millipedes and Thrips.

Tussocks of Rushes

Over the years we have resisted all advice that we should mow the rushes which seemingly dominate the wetter sections of Cae Eithin (Gorse Field), Cae Pond (Pond Field) and Waun Fach, advice based on the belief that it would produce a greater diversity of plants, and create a better breeding habitat for Lapwing.

We have discovered that the rushes support high populations of small mammals, especially in the winter; provide ideal nesting habitat for Pipits and Reed Bunting; and attract a wide range of mammals and birds to hunt. Our rushy pastures also support a great diversity of wetland plants, so we are glad we have let these fields evolve without our strict control.

We now know that although they support lower overall invertebrate numbers than the surrounding grassland, some groups of species benefit greatly from the rush tussocks, inside which, compared with the surround­ing grass­land, there were more than three times the number of Spiders and six times the number of Woodlice.

Intensive Rye-grass Ley

This area is notable for the paucity of both the overall numbers and the groups of invertebrates present. The majority of the total invertebrates recorded were Mites, Springtails, and Mosquito and Midge larvae. Most other groups were conspicuous by their near or complete absence. Predatory invertebrates, with the possible exception of some species of Mites, were virtually absent.

This desperately impoverished invertebrate fauna was present at overall populations of only about 22% of those found in the restored hay meadows and just 7% of those recorded in the restored pastures.

As with our studies of breeding birds, butterflies and odonata on this neighbouring intensively managed land, the impoverished invertebrate fauna reminds us how impoverished was Denmark Farm when we began our work of restoration. But the astonishing numbers and variety of invertebrates in pastures and meadows also reminds us yet again of the potential of the ordinary to be transformed into the special. Every piece of land has a similar potential.

The Price

Nearly 26,000 individual invertebrates have died as a result of this research work – the minute and the more obvious, the simple and the complex, the 'plain' and the 'beautiful' to our eyes. All these have value and have been sacrificed to the god of knowledge and under­standing and we find this hard. Yet perhaps we should also consider how many get crushed by the hooves of one grazing cow, or even by our own feet as we walk the land for work or to admire its beauties.