What will be affected by fragmentation?

Before we can discuss the effects of fragmentation, we must identify the species likely to be affected.² That means identifying species in which fragmentation will alter

• the distribution of populations,

• the migration rates among populations, or

• the sizes of local populations.

Let's start with the last one first. For animals, the ones most severely affected are (obviously) those with large home ranges, because we expect these species to be entirely lost from small fragments.

• ivory-billed woodpeckers: ca. 7km$^2$ per breeding pair

• European goshawk: home range of 30-50km$^2$

• male mountain lion (puma): home range of 400km$^2$

  Suppose that 50 males are needed for a population to have a reasonable chance of persistence for a couple of centuries. This would require an area of about 20,000km$^2$, which is roughly equivalent to the area of Connecticut, Rhode Island, and Cape Cod combined. Fortunately, puma are learning to live in close proximity to development, so completely undeveloped habitat doesn't appear to be necessary to conserve them. Some of the habitat they use can be moderately altered, as in south Florida or southern California. You see occasional news reports of joggers being attacked by puma in southern California, and I've had people tell me that attacks also occur occasionally on trails near Boulder, Colorado.

• Forest birds--size of area needed for 50% probability of occurrence [7]
  • Great Crested Flycatcher: 0.3ha (a circle about 60m in diameter)

  • Cerulean Warbler: 1000ha (a circle about 3.5km in diameter)

  Consequence: species composition of small fragments will differ from that of large fragments. The species remaining in small fragments will not be a random subset of those originally present. Species with large area requirements will be preferentially lost.

Some years ago Deborah Rabinowitz [6] developed a typology of rarity that is both interesting and useful in thinking about the possible impacts of fragmentation (Table 1).

Table 1: Rabinowitz' typology of rarity

	Geographic range
	Large		Small
	Habitat specificity		Habitat specificity
Local population size	Wide	Narrow	Wide	Narrow
Large	locally abundant, large range, several habitats	locally abundant, large range, specific habitat	locally abundant, restricted range, several habitats	locally abundant, restricted range, specific habitat
Small	locally sparse, large range, several habitats	locally sparse, large range, specific habitat	locally sparse, restricted range, several habitats	locally sparse, restricted range, specific habitat

Notice that Rabinowitz identifies three axes of rarity:

1. limited geographic range,

2. specific habitat requirements, and

3. low population density.

Plants (or animals) that are rare in either of the first two senses seem unlikely to be greatly affected by habitat fragmentation, except to the extent that habitat fragmentation leaves them outside a fragment. To put it another way, they will be affected by habitat destruction, but habitat fragmentation may have little impact on either the migration rate among populations or the sizes of local populations.³

Furthermore, not all plants or animals that are widely distributed will suffer the effects of fragmentation. Those that are patchily distributed are already fragmented. Unless fragmentation disrupts patterns of migration among the populations that remain, it will have fewer effects than it will on those that are continuously distributed.

Think about what this means, though. The plants⁴ that are most likely to be affected by habitat fragmentation:

• are broadly and continuously distributed,

• have relatively non-specific habitat requirements, and

• are numerically abundant almost everywhere they occur.

These are roughly the characteristics we associate with plant species regarded as ecosystem dominants: douglas fir and hemlock in the Pacific northwest, mixed hardwoods in the Northeast. In short, whatever the effects of fragmentation are, they may be exhibited in a minority of species, but that minority of species will be the ones that have the largest impact on the structure and function of ecosystems in which they occur.