Restoration of habitats impacted by human activities

The term 'restoration ecology' can be used, rather unhelpfully, to encompass almost every aspect of applied ecology (recovery of overexploited fisheries, removal of invaders, revegetation of habitat corridors to assist endangered species, etc.) (Ormerod, 2003). We restrict our consideration here to restoration of landscapes and waterscapes whose physical nature has been affected by human activities, dealing specifically with mining, intensive agriculture and water abstraction from rivers.

Land that has been damaged by mining is usually unstable, liable to erosion and devoid of vegetation. Tony Bradshaw, the father of restoration ecology, noted that the simple solution to land reclamation is the reestablishment of vegetation cover, because this will stabilize the surface, be visually attractive and self-sustaining, and provide the basis for natural or assisted succession to a more complex community (Bradshaw, 2002). Candidate plants for reclamation are those that are tolerant of the toxic heavy metals present; such species are characteristic of naturally metalliferous soils (e.g. the Italian serpentine endemic Alyssum bertolonii) and have fundamental niches that incorporate the extreme conditions. Moreover, of particular value are ecotypes (genotypes within a species having different fundamental niches

... and the dynamics of small populations the challenge of global climate change using knowledge of species niches...

- see Section 1.2.1) that have evolved resistance in mined areas. Antonovics and Bradshaw (1970) were the first to note that the intensity of selection against intolerant genotypes changes abruptly at the edge of contaminated areas, and populations on contaminated areas may differ sharply in their tolerance of heavy metals over distances as small as 1.5 m (e.g. sweet vernal grass, Anthoxanthum odoratum). Subsequently, metaltolerant grass cul-tivars were selected for commercial production in the UK for use on neutral and alkaline soils contaminated by lead or zinc (Festuca rubra cv 'Merlin'), acidic lead and zinc wastes (Agrostis capillaris cv 'Goginan') and acidic copper wastes (A. capillaris cv 'Parys') (Baker, 2002).

Since plants lack the ability to move, many species that are characteristic of metalliferous soils have evolved biochemical systems for nutrient acquisition, detoxification and the control of local geochemical conditions (in effect, they help create the conditions appropriate to their fundamental niche). Phytoremediation involves placing such plants in contaminated soil with the aim of reducing the concentrations of heavy metals and other toxic chemicals. It can take a variety of forms (Susarla et al., 2002). Phytoaccumulation occurs when the contaminant is taken up by the plants but is not degraded rapidly or completely; these plants, such as the herb Thlaspi caerulescens that hyperaccumulates zinc, are harvested to remove the contaminant and then replaced. Phytostabilization, on the other hand, takes advantage of the ability of root exudates to precipitate heavy metals and thus reduce bioavailability. Finally, phytotransformation involves elimination of a contaminant by the action of plant enzymes; for example, hybrid poplar trees Populus deltoides x nigra have the remarkable ability to degrade TNT (2,4,6-trinitrotoluene) and show promise in the restoration of munition dump areas. Note that microorganisms are also used for remediation in polluted situations.

Sometimes the aim of land managers is to restore the landscape for the benefit of a particular species. The European hare Lepus europaeus provides a case in point. The hare's fundamental niche includes landscapes created over the centuries by human activity. Hares are most common in farmed areas but populations have declined where agriculture has become too intensive and the species is now protected. Vaughan et al. (2003) used a farm postal survey (1050 farmers responded) to investigate the relationships between hare abundance and current land management. Their aim was to establish key features of the two most significant niche dimensions for hares, namely resource availability (crops eaten by hares) and habitat availability, and then to propose management action to maintain and restore landscapes beneficial to the species. Hares were more common on arable farms, especially on those growing wheat or beet, and where fallow land was present (areas not currently used for crops). They were less common on pasture farms, but the abundance of hares increased if 'improved' grass (ploughed, sown with a grass mixture and fertilized), some arable crops or woodland were present (Table 7.1). To increase the distribution and abundance of hares, Vaughan et al.'s (2003) recommendations include the provision on all farms of forage and year-round cover (from foxes Vulpes vulpes), the provision of woodland, improved grass and arable crops on pasture farms, and of wheat, beet and fallow land on arable farms.

One of the most pervasive of human influences on river ecosystems has been

... to restore landscape for a declining mammal . . .

... and to restore river flow for native fish

Variable

Variable description

Arable farms

Pasture farms

Wheat

Wheat Triticum aestivum (no, yes)

***

-

Barley

Barley (no, yes)

**

-

Cereal

Other cereals (no, yes)

NS

-

Spring

Any cereal grown in spring? (no, yes)

*

-

Maize

Maize (no, yes)

NS

-

Rape

Oilseed rape Brassica napus (no, yes)

**

-

Legume

Peas/beans/clover Trifolium sp. (no, yes)

**

-

Linseed

Flax Linum usitatissimum (no, yes)

NS

-

Horticulture

Horticultural crops (no, yes)

NS

-

Beet

Beet Beta vulgaris (no, yes)

***

-

Arable

Arable crops present (see above; no, yes)

-

**

Grass

Grass (including ley, nonpermanent) (no, yes)

NS

-

Type grass

Ley, improved, semi-improved, unimproved

NS

***

Fallow

Set aside/fallow (no, yes)

***

-

Woods

Woodland/orchard (no, yes)

NS

*

Table 7.1 Habitat variables potentially determining the abundance of hares (estimated from the frequency of hare sightings), analyzed separately for arable and pasture farms. Analysis was not performed for variables where fewer than 10% of farmers responded (-). For those variables that were significantly related to whether or not hares were seen by farmers (*, P < 0.05; **, P < 0.01; ***, P < 0.001), the variable descriptor associated with most frequent sightings are shown in bold. (After Vaughan et al., 2003.)

NS, not significant.

Table 7.1 Habitat variables potentially determining the abundance of hares (estimated from the frequency of hare sightings), analyzed separately for arable and pasture farms. Analysis was not performed for variables where fewer than 10% of farmers responded (-). For those variables that were significantly related to whether or not hares were seen by farmers (*, P < 0.05; **, P < 0.01; ***, P < 0.001), the variable descriptor associated with most frequent sightings are shown in bold. (After Vaughan et al., 2003.)

NS, not significant.

Figure 7.2 Interrelationships among biological parameters measured in a number of reaches of the Colorado River in order to determine the ultimate causes of the declining distribution of Colorado pikeminnows. (a) Invertebrate biomass versus algal biomass (chlorophyll a). (b) Prey fish biomass versus algal biomass. (c) Pikeminnow density versus prey fish biomass (from catch rate per minute of electrofishing). (d) Mean recurrence intervals in six reaches of the Colorado River (for which historic data were available) of discharges necessary to produce widespread stream bed mobilization and to remove silt and sand that would otherwise accumulate, during recent (1966-2000) and preregulation periods (1908-42). Lines above the histograms show maximum recurrence intervals. (After Osmundson et al., 2002.)

Ecology Histogram

Figure 7.2 Interrelationships among biological parameters measured in a number of reaches of the Colorado River in order to determine the ultimate causes of the declining distribution of Colorado pikeminnows. (a) Invertebrate biomass versus algal biomass (chlorophyll a). (b) Prey fish biomass versus algal biomass. (c) Pikeminnow density versus prey fish biomass (from catch rate per minute of electrofishing). (d) Mean recurrence intervals in six reaches of the Colorado River (for which historic data were available) of discharges necessary to produce widespread stream bed mobilization and to remove silt and sand that would otherwise accumulate, during recent (1966-2000) and preregulation periods (1908-42). Lines above the histograms show maximum recurrence intervals. (After Osmundson et al., 2002.)

the regulation of discharge, and river restoration often involves reestablishing aspects of the natural flow regime. Water abstraction for agricultural, industrial and domestic use has changed the hydrographs (discharge patterns) of rivers both by reducing discharge (volume per unit time) and altering daily and seasonal patterns of flow. The rare Colorado pikeminnow, Ptychocheilus lucius, is a piscivore (fish-eater) that is now restricted to the upper reaches of the Colorado River. Its present distribution is positively correlated with prey fish biomass, which in turn depends on the biomass of invertebrates upon which the prey fish depend, and this, in its turn, is positively correlated with algal biomass, the basis of the food web (Figure 7.2a-c). Osmundson et al. (2002) argue that the rarity of pikeminnows can be traced to the accumulation of fine sediment (reducing algal productivity) in downstream regions of the river. Fine sediment is not part of the fundamental niche of pikeminnows. Historically, spring snowmelt often produced flushing discharges with the power to mobilize the bed of the stream and remove much of the silt and sand that would otherwise accumulate. As a result of river regulation, however, the mean recurrence interval of such discharges has increased from once every 1.3-2.7 years to only once every 2.7-13.5 years (Figure 7.2d), extending the period of silt accumulation.

High discharges can influence fish in other ways too by, for example, maintaining side channels and other elements of habitat heterogeneity, and by improving substrate conditions for spawning (all elements of the fundamental niche of particular species). Managers must aim to incorporate ecologically influential aspects of the natural hydrograph of a river into river restoration efforts, but this is easier said than done. Jowett (1997) describes three approaches commonly used to define minimum discharges: historic flow, hydraulic geometry and habitat assessment. The first of these assumes that some percentage of the mean discharge is needed to maintain a 'healthy' river ecosystem: 30% is often used as a rule of thumb. Hydraulic methods relate discharge to the hydraulic geometry of stream channels (based on multiple measurements of river cross-sections); river depth and width begin to decline sharply at discharges less than a certain percentage of mean discharge (10% in some rivers) and this inflection point is sometimes used as a basis for setting a minimum discharge. Finally, habitat assessment methods are based on discharges that meet specified ecological criteria, such as a critical amount of food-producing habitat for particular fish species. Managers need to beware the simplified assumptions inherent in these various approaches because, as we saw with the pikemin-nows, the integrity of a river ecosystem may require something other than setting a minimum discharge, such as infrequent but high flushing discharges.

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