Why Is the Edge of the Wood the Best Bit? Two Habitats Meet There

Where two communities meet there is a zone of transition rather than a line, and that zone has properties of its own rather than simply being a mixture. Conditions change across it, so light, moisture, temperature and shelter all vary over a short distance, which means a great many different sets of conditions are available within a small area. The width varies enormously, from a metre between a lawn and a border to kilometres between forest and grassland, and the sharpness depends on what is driving the change, with an abrupt soil boundary producing a narrow one and a gradual climate gradient producing a broad one.

Several effects combine and the result is usually higher diversity:

• •Species from both neighbouring communities occur there
• •Some species require both, feeding in one and nesting in the other
• •A few live only in the transition and nowhere else
• •The range of conditions supports more specialists in a small area
• •Structure is complex, with layers that neither habitat has alone
• •The effect is called edge effect and is not always beneficial

Higher diversity at a boundary was treated for decades as straightforwardly good and management aimed at creating more edge, which is now understood to have been a serious mistake. Fragmenting a large habitat into pieces increases edge enormously while destroying the interior, and many species require interior conditions rather than boundaries, including birds that will not nest within a certain distance of an edge. Edges admit light, wind and drying into a forest interior, changing conditions well inside the remaining fragment. They also admit predators and brood parasites that hunt along boundaries, and invasive plants that establish in disturbed sunny conditions and spread inward.

Not every boundary is natural and the managed ones behave differently. A woodland edge cut straight and mown annually is abrupt and supports far less than a graded edge running from grassland through scrub to trees over several metres, which is why conservation management now aims at graded edges rather than clean ones. Hedgerows are essentially long narrow edges and derive most of their value from that. Road verges, field margins left uncultivated and the banks of ditches all function the same way in an otherwise uniform landscape. Where a landscape has been simplified, deliberately creating structure at the boundaries returns a disproportionate amount of what was lost.

Boundaries respond to environmental change faster than the middle of either habitat, which makes them useful for monitoring. A species at the limit of its tolerance sits at the boundary, so a small change in temperature or rainfall moves that boundary visibly while conditions in the core of each habitat remain adequate. Treelines at altitude and at latitude are watched for exactly this reason and have moved measurably. The same logic applies to the boundary between salt marsh and freshwater, which tracks sea level, and to the edges of deserts. Monitoring programmes therefore place transects across boundaries rather than within uniform ground.

A transition between habitats is a zone rather than a line, and conditions vary sharply across it, so species from both sides occur there along with some that need both and a few found nowhere else. Fragmenting habitat creates enormous quantities of edge while destroying the interior that many species actually require. Boundaries shift before either core habitat changes, which makes them early indicators.