A Buzz in the Meadow (28 page)

Of course habitat islands are not quite the same as oceanic islands; for most animals, crossing an arable field is much easier and less daunting than crossing an expanse of sea, so – all else being equal – we might expect habitat islands to have more colonisers than oceanic islands. Nonetheless the broad principles remain and have profound consequences for conservation, for they make predictions as to how many species a nature reserve might be likely to support, and how we might prioritise conservation efforts and expenditure.

Conservation efforts are limited by funds, and are constrained by other pressures on the land. In our modern, crowded world we can only have so much in the way of woodlands and meadows, for we need to grow food and build houses, out-of-town supermarkets, industrial estates and roads.
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Supposing we are faced with a situation in which we can only save, say, 1,000 hectares of woodland, would we be best to save one big wood or dozens of little ones? In 1975 Jared Diamond used Island Biogeography Theory to argue that the former would be the best strategy, for the larger patch would support more species. I'm sure he didn't realise it at the time, but in doing so Diamond helped to spark one of the longest-running debates in ecological history, one that has rumbled on in various forms till today. It became known as the SLOSS debate: ‘Single Large Or Several Small'.

Perhaps surprisingly, given that Diamond was essentially building on Dan Simberloff's work, Simberloff himself was one of the first to disagree with Diamond, pointing out that the logic only worked if small habitat patches simply contained subsets of the species found in a larger habitat patch. In reality, this is rarely likely to be the case. Suppose, for example, that we had the job of prioritising woodland conservation in the UK – all but 1,000 hectares was to be swept away to provide extra parking. Which bit(s) should we save? One huge chunk of, say, the New Forest, or lots of little patches, from the twisted, lichen-encrusted oaks of Wistman's Wood on Dartmoor, to a bit of ancient Caledonian pine forest at Abernethy in the Cairngorms and a patch of Breckland forest on the sandy soils of Norfolk? In this example, one would certainly save many more species by having lots of little patches, for these woods all have different characters and each supports a different range of species.

Maybe this example is a little unfair, and it is certainly not a very plausible scenario. So let's suppose instead that we had a single chunk of reasonably homogeneous habitat – let us say one large flower-rich meadow – and that much of it was to be lost to make way for a housing development. Would we be best to save one large piece, or lots of little ones scattered among the houses? This seems like the sort of scenario that planners might regularly face in the real world. David Quammen writes beautifully about the SLOSS debate in the
The Song of the Dodo
, in which he coins the Persian Rug analogy. Imagine we take a ten-foot-by-ten-foot Persian rug and cut it into 100 pieces. Do we get 100 perfect replicas of the original? Of course we do not; instead, we get 100 worthless, frayed scraps of carpet. The same argument could be applied to the meadow: the little patches would be trampled by the residents, invaded by garden weeds and would be unlikely to support much more than a handful of the original diversity of life in the meadow.

These two arguments lead to opposite conclusions: the first that we should save lots of small fragments; the second that we should save large, intact tracts. Resolving this difference occupied quite a number of the world's ecologists for several decades in the later twentieth century, and there is still no clear agreement, although interest in the debate has finally waned.

Of course to some extent all of this misses the point. In the real world we save what we can where we can, and generally make do with what is possible, rather than worrying about what would be ideal. The housing planners are likely to be much more driven by the practicalities of road access, drainage and so on, when deciding where to put the houses, than they are with worrying about biodiversity. Where a habitat has been largely lost we might try to re-create patches of it, but the size and location of such patches are rarely determined by any ecological theory, but rather by what is available. When I bought Chez Nauche I did not have unlimited choice as to where it would be, or how large the meadow. As I had very limited funds, its location was largely determined by the cheapness of property and land in the Charente compared to other parts of France, and by what was for sale. Nonetheless, many of the ideas that emerged from the arguments over Island Biogeography Theory are useful. For example, the distance from a source of colonists plays a role in deciding how to manage my meadow. If there were a lovely flower-rich grassland in one of the neighbouring fields, there would be a ready source of wild-flower seeds and the recovery of my meadow to a flower-rich state would have been rapid, just as Simberloff's fumigated islands were colonised quickly when they were close to shore. Unfortunately there was not, so I have had to collect seeds from some flowers – particularly those with heavy seeds and no long-distance mechanism for dispersal – and sprinkle them in the meadow (avoiding, of course, my experimental plots, which would mess up my experiments).

There is another way in which we can increase colonisation of nature reserves and so boost the number of species they are likely to have, and this is by providing habitat corridors along which animals can move. I have already mentioned that many animals are likely to be unwilling to cross water, but may be much more likely to cross land, even if it is inhospitable. However, this varies enormously between species. Some birds and butterflies will happily zoom across cereal fields, towns or busy roads to get from one meadow or woodland to another, but many will not. For example, some woodland bird species, such as long-tailed tits and dunnocks, are unwilling to leave their preferred habitat and fly out into the open, and it is these species that are likely to be most badly affected by habitat fragmentation. We can make life much easier for such creatures if we provide links between patches of habitats; in the case of woodland, hedges often provide routes for dispersal for creatures that shun open fields. Tunnels under roads have been used with some success to help creatures as diverse as elephants and badgers to get from one habitat patch to another.

Other creatures are harder to accommodate. For instance, some butterflies found in flower-rich meadows are enormously sedentary – particularly blue butterflies such as the Adonis and small blue, both of which will often happily live out their entire life in a few square metres of downland, and will rarely (if ever) choose to head off into the unknown on a perilous adventure. One can hardly blame them these days; their chances of finding another suitable patch of habitat are remote, so they are probably wise to stay put. On the other hand, their sedentary lifestyle cuts them off from gene flow – each small population in a habitat fragment becomes entirely cut off from all the others, and this can doom it to eventual extinction.

The problem with isolation is that it leads to inbreeding. Small populations tend to have less genetic variation than large ones to start with, simply because there are fewer copies of each gene. They also tend to lose genetic diversity over time, through a process known as ‘genetic drift', whereby rare forms of genes drop out of the population by chance. With less genetic diversity, the population is less able to adapt to any change in the environment. What is more, inbreeding can lead to the expression of rare, harmful, recessive genes. Every individual has a few dud genes; humans have somewhere in the region of 50,000 genes (though probably far fewer that are absolutely vital to our well-being), of which on average about four are likely to be faulty, but because we have two copies of every gene, and as long as at least one copy is okay, we are fine. Fortunately your partner is unlikely to have the same faulty genes that you do, so there is little chance of your offspring getting two non-functioning copies of the same gene, although it does occasionally happen. If they do, they may be severely ill, disabled or even die early in development. However, if you are related to your partner, then you are likely to have some of the same faulty genes. This is not good, for then, for every faulty gene you have in common, there is a one-in-four chance that your offspring will inherit both non-functioning copies. As I have mentioned, in a small population it takes only a few generations before everybody becomes cousins of one another, sharing grandparents or great-grandparents. In lab studies using insects, forced inbreeding over a few generations quickly results in a population of feeble individuals with low fertility and reduced life expectancy, a phenomenon known as ‘inbreeding depression'. Even without this, small populations are much more likely to go extinct than large ones just through bad luck, but when these small populations are also subjected to inbreeding they are unlikely to persist for long.

Although the effects of inbreeding are well established from lab studies, until fairly recently there were few good examples of inbreeding hampering the survival of insect populations in the wild. The first – and still one of the most elegant – demonstration of the negative effects of inbreeding on insects was provided by Finnish ecologist Ilkka Hanski's work on populations of the Glanville fritillary, one of the most common butterfly species in my meadow. In Finland, Glanville fritillaries are found only in the Åland archipelago, a cluster of more than 6,000 small islands in the Baltic Sea, halfway between mainland Finland and Sweden. On these islands the Glanville fritillary is widespread and fairly common, occupying dry, grassy meadow areas where its food plant, ribwort plantain, is to be found. The lovely adult butterflies with their orange-and-black chequerboard wings emerge in May and June and lay batches of eggs which, when they hatch into small black spiny caterpillars, live in gregarious clusters within protective webs that they spin among the plantains.

Hanski's team (which consists largely of undergraduate students) has repeatedly surveyed about 4,000 of these meadows every year since 1991, recording whether or not the butterfly is present in each patch – an astonishing effort, but presumably involving many happy hours of splashing about in boats to get between the mostly uninhabited islands. Their work has become a classic study in what is known as ‘metapopulation dynamics', and their findings have a lovely symmetry with MacArthur and Wilson's theory.

Hanski's group has been studying just one species while MacArthur and Wilson were trying to understand what determined the number of species in entire communities, but the processes involved are exactly the same. Hanski found that the majority of the 4,000 meadows had no Glanville fritillaries in any particular year, with only 300–500 occupied ones. Some of these populations are tiny – just a few butterflies. Populations in any particular patch go extinct quite frequently, but these extinctions are roughly balanced by colonisation events whereby butterflies discovered and established a new population in an empty patch. A large regional population composed of lots of small fragmented populations, each of which blink in and out of existence from time to time, is what is known as a ‘metapopulation'.

Just as small and remote islands have fewer species in them in part because they are less likely to be bumped into by wandering creatures, so Hanski's habitat patches were less likely to be colonised by butterflies (and so tended to remain unoccupied for longer) if they were small, or a long way from other patches. Similarly, he found that small populations were more likely to go extinct. Extinctions were caused by a variety of events – outbreaks of parasitoid wasps, overgrazing and trampling by cattle, human disturbance, and so on, but all of these were more likely to prove fatal to a local population if it was small. What is more, populations were found to be more likely to go extinct if they lacked genetic diversity – the first time that inbreeding had been shown to be harmful to wild populations of any insect. Populations with low genetic diversity were found to have reduced survival of both caterpillars and adults, and a low hatching rate of eggs, factors that are obviously likely to push a small population over the edge to extinction.

This finding was followed up by one of Hanski's students named Marko Nieminen, who created new wild populations from either the offspring of unrelated individuals or a mating between a brother and sister. The latter proved much more likely to die out, mainly because the low egg-hatching rate led to smaller groups of the gregarious caterpillars, and smaller groups are known to have a much-reduced chance of surviving through the winter.

There are reasons to believe that social insects such as bumblebees might be particularly prone to inbreeding, and this might partly explain why some bumblebee species have fared so poorly in recent years. In butterflies such as the Glanville fritillary – indeed, in most creatures – every adult individual tries to breed, and although quite a few fail, many pass on their genes to the next generation. In bumblebees this is far from the case. The large majority of the population are workers, most of which will not produce any offspring. Save for a few sneaky workers that try (largely unsuccessfully) to produce sons, reproduction is the preserve of the queen. This means that the breeding population – the gene pool – is much smaller than you might at first think. Each nest has just one breeding individual (who also carries inside her the sperm of her mate). So ten nests, which might collectively contain several thousand bumblebees at their peak, actually represent just twenty breeding individuals. A viable population, one that will be big enough that it does not suffer from inbreeding, needs to contain several hundred breeding individuals – let's say at least 100 nests. We don't know for sure how big an area of habitat would be needed to support 100 bumblebee nests of any particular species – it would depend on how good the habitat was, and would probably vary between bumblebee species – but it is likely to be in the region of hundreds of hectares. In the UK, indeed in most of western Europe, most nature reserves are smaller than this. For example, most of the patches of flower-rich grassland that survive consist of single meadows, often of no more than a dozen hectares or so, surrounded by arable crops. In other words, the habitat islands that remain are likely to be too small to support a viable bumblebee population. Lots of such small islands, close together, could support a metapopulation just like that of the Glanville fritillaries in the Åland archipelago. But we don't have lots of patches of flower-rich grassland, so it is likely that fragmented populations of our rarer bees are suffering from inbreeding.