Degradation and fragmentation of ecosystems

In the current cycle one last natural selection has begun to operate of which man is the delegated agent, and the demesnes of oak and heather, home of the David and Saxon, are fast ceding their primitive mysteries before the steady march of building, agriculture, horticulture, floriculture, and domestication of species; handmaidens of an era of civilization, the creative wonders of whose potent wand can scarcely compensate an entomologist for the loss of his breezy heath and sylvan shade where the Fritillaries once sunned, or the lonely classic fen-land where the Large Copper Butterfly, Chrysophanes Hippothoe, variety Dispar, once flew.

4.1 Introduction

A combination of human population increase and elevated consumerism is changing the character of the biosphere. Contamination, mostly from byproducts of manufacturing and from agricultural chemicals, are the major substances impacting on biological diversity. Pesticides are among these substances and require special focus with regard to insect diversity conservation.

In addition to contamination, landscapes worldwide are being fragmented. In turn, the fragments are gradually being made smaller through a process termed attrition. Among the causal factors are more extensive and intensive agroforestry systems.

Urbanization has become a major force on insect diversity. Quite simply, wildlands, and even agricultural land, are lost when buildings are erected. This is not to say that urbanization is unchallenged by insect diversity conservation. One of the current major opportunities is to ecologically landscape urban areas so as to maintain biological diversity.

Of general concern worldwide is the loss of wilderness. Some would say that there is no wilderness left, with no place being unsullied by human impact, immediate or distal. Unquestionably, for insect diversity conservation, the loss of forests, especially tropical ones, has been and continues to be devastating. Insect species extinctions are occurring on a daily basis as tropical forests are removed. Yet for insects this is not the only biome under severe pressure. Grasslands are also rich in insect life, and they too are being degraded or lost at a phenomenal rate. High domestic animal stocking rates are among the impacts on grasslands and savanna, and encourage desertification. Such simplification in the composition and structure of ecosystems changes insect assemblages and reduces diversity.

Deterioration and loss of aquatic systems is of great concern worldwide, mainly as the quality of human life also deteriorates. Insects have been particularly hard hit from these adverse changes. Many insect species have suffered geographic range retraction and even extinction as water systems have deteriorated. In fact, changes in insect diversity are often one of the first signs of water quality deterioration.

Overcollecting may not at first seem to fall easily into this chapter. But a closer look sees overzealous removal of insect specimens as targetted deterioration of ecological integrity. This can be especially important in those areas with small-range endemics that are highly sought after by collectors.

Many of the impacts listed above are essentially changes to the composition, structure and function of the land mosaic. Some of these impacts are multiplicative when acting together, with the resultant synergism being particularly harmful.

Insect responses to the changing land mosaic are being intensively researched, and are addressed in more detail in the next chapter. Other synergistic effects, such as arrival of new organisms and genes, are also important (Chapter 6), and all these impacts are overlain by the blanket effect of global climate change (Chapter 7).

4.2 Environmental contamination

4.2.1 Pollution

Contamination of ecosystems can come about from agriculture, industry and urbanization (Freedman, 1989). The combination of increased human population and increased consumption of resources and energy has, as measured by Gross Domestic Product, increased by 460% over the last century (Maddison, 1995), with current figures likely to increase by 240% by the year 2050 (National Research Council, 1999).

Among systems most affected in terms of changing insect diversity, as a result of environmental contamination, are riverine systems. Some species, such as Tobias' caddis-fly Hydropsyche tobiasi, may even be extinct as a result of industrial and urban contamination of the River Rhine (Wells et al., 1983). Indeed, it is well known that aquatic insect assemblages change in response to water pollutants. This has led to the development of water-monitoring programmes based on the relative abundances of certain taxa (Resh and Jackson, 1993). However, the aim of such programmes is not usually to monitor named endemic species but rather to reflect pollution levels. In other words, they do not measure biodiversity at the species level but rather they indicate environmental stress on the system. Thus, they monitor ecological health rather than finer aspects of ecological integrity.

Air pollution has frequently been suggested as a cause for the decline of some butterfly species. But there is little evidence as to whether this is in fact so. It is not clear what exactly is the causal mechanism between levels of air pollution and butterfly decline. If the pollutants affect the larva via ingestion, or any stage, via direct deposition, one would expect most species of Lepidoptera to be affected (Corke, 1999). But since larvae usually select the youngest food-plant leaves, direct ingestion of pollutant deposits should be minimal. Similarly, adult Lepidoptera feed mainly from nectar, which also rarely contains airborne deposits. In contrast, honeydew and sap feeders are much more exposed, and circumstantial evidence strongly suggests that they are indeed vulnerable (Corke, 1999). Interestingly, the most famous industrial melanic of all, the Peppered moth Biston betularia, which has survived in smoke-polluted habitats for many decades, as have other industrial melanic moths, is a species that does not feed in the adult stage.

Nevertheless, evidence so far indicates that insect diversity can be remarkably tolerant of air pollution. Although Russian noctuid moths were heterogeneous in their response to pollution from a smelter, neither species richness nor diversity were affected by the pollution (Kozlov et al., 1996). In a further Russian smelter study, studies on a geometrid moth Epirrita autumnata produced the surprising result that parasitism rates of this moth were not associated with pollution, indicating that parasitoids were no more sensitive to pollutants than their herbivorous host (Ruohomaki et al., 1996). In contrast, although the larvae of the butterfly Parnassius apollo in Finland can excrete metals, this appears to be insufficient to enable it to tolerate high levels of this pollution on its host plant. Relaxation of this heavy metal pollution has enabled this butterfly to widen its geographical range (Nieminen et al., 2001).

4.2.2 Synergistic effects

Still very little is known of the effects of pollutants on insect diversity. Although in some cases, pollution impacts may be relatively benign, as in the case of detergents on dragonflies on the island of Mayotte (Samways, 2003a), there may be in reality three other factors to consider. Firstly, pollution may have a long-term effect that is not detected in short-term studies. Secondly, pollution can pulse depending on intensity and frequency of emission activities, and may go undetected unless the monitoring is taking place at just the right time and in the right place. Thirdly, pollution is very often synergistic with other impacts, especially fragmentation and threats from invasive aliens, making it difficult at times to pinpoint the pollution threat and to make recommendations for sound conservation management. Pollution effects can also be synergistic with global climate change (see Chapter 7). Both gaseous pollutants and increased CO2 concentrations are likely to alter the amount of insect damage to trees. In addition to direct harm to trees by pollutants, damage is often increased through larger populations of herbivores, which cannot be controlled by predators and parasitoids (Docherty et al., 1997).

4.2.3 Lack of evidence for negative effects

Not all aspects of pollution are negative for insect diversity. Brandle et al. (2000) looked at plant and insect diversity along a pollution gradient in Germany. Their results relate to previous emissions from a smelter that increased the pH-values of the soil as a result of particulate deposition. The prior emissions increased both plant and hemipteran herbivore species richness close to source. In particular, the proportion of specialized herbivores increased with closeness to the smelter, which favours the 'specialization hypothesis' rather than the 'consumer rarity hypothesis' which purports more plant productivity and hence more herbivore individuals which, in turn, would lead to more species. Interestingly, the predatory bugs did not follow the pollution/plant/herbivore gradient, possibly because of (1) a combination of factors including energetic loss between trophic levels (so damping gradients) (Huston, 1994); (2) predators not being host specific (so leading to niche limitation and increased interspecific competition) (Dolling, 1991); and (3) larger home ranges of predators (so damping measurable differences across the landscape) (Brandle et al., 2000), as is the case with ladybirds (Magagula and Samways, 2001).

4.2.4 Long-term effects

We cannot leave the topic of environmental contamination without considering possible long-term effects, especially as some metal pollutants have extremely long residence times in soils. Although some insect species are able to survive because they are adapted to cope with a wide range of metal concentrations in their diet (e.g. polyphages), others are vulnerable to poisoning since the levels of metals in their food are normally within limits (e.g. sap suckers). When essential metabolic metals are present in high concentrations due to pollution (along with those not required, such as cadmium, lead and mercury), they disrupt normal biochemistry. Although certain sensitive insects may die from acute or chronic poisoning, others die from deficiency of an essential element through antagonism from a non-essential metal in the diet (Hopkin, 1995). Yet species that are tolerant to pollution, as some of the European examples above, may respond to the subsequent lack of competition and much higher population densities than in an uncontaminated area. These changes at the community level may lead to disruption of ecological processes such as plant litter decomposition (Hopkin, 1995). Similarly, the gaseous pollutants SO2 and NO2 increase the performance of herbivorous insects, while the situation with O3 is more complex, with a range of possible responses (Brown, 1995).

4.3 Pesticides

4.3.1 Threats posed by pesticides

Pesticides, especially insecticides and acaricides, would by their very name, appear to be the antithesis of insect diversity conservation (Pimentel, 1991), especially as 5 million tonnes are used annually. But what is the evidence? There has been much speculation but little concrete evidence has been forthcoming. Indeed, there is apparently no verified case of an insect species going extinct primarily from insecticide usage.

Most insecticides are used in the agricultural sector, while those used in urban pest control pose little threat to insect diversity conservation (Samways, 1996c). The problem with pesticides lies mostly in their impact on food chains through bioaccumulation (Moore, 1987). The important point is that it is not generally how poisonous per se a compound is, but rather the persistence of its toxic impact. This is why certain organochlorines, which have been used widely for mosquito and leaf-cutter ant suppression, are so environmentally threatening. Those environmental threats coupled with human health hazards and high costs, are reasons why pesticide usage is being reduced where possible (Pimentel, 1995).

In comparison with biological control, pesticides are generally much more spatially and temporarily explicit. They are sprayed in a particular area and last a particular length of time. This is why insect diversity has proportionately been little affected by insecticides compared with landscape fragmentation and habitat loss.

4.3.2 Impacts on insect populations

Longley and Sotherton (1997) have reviewed the effects of pesticides upon butterflies inhabiting arable farmland. Factors determining a species' exposure and susceptibility to particular compounds range from chemical properties of the insecticidal or herbicidal compound, intrinsic susceptibility of the species, exposure of the butterfly-related plants to drifting pesticides, and species-dependent ecological factors determining their within-boundary behaviour and dispersal. This is also why conservation headlands, which are the outer 6 m-wide edges of cereal fields, and which receive reduced and selective pesticide inputs, are a feasible option for maintaining hedgerow insects as well as other organisms (Dover, 1991, 2001), including insectivorous birds (de Snoo, 1999).

One of the problems with pesticides, and principally insecticides, relative to insect diversity conservation, is that insect natural enemies are often differentially killed, and/or their suppressing effect reduced, relative to the host. A manifestation of this is familiar to insect pest managers as secondary pest resurgence. Often, the natural enemies, being more mobile and less cryptic than the pest, are more exposed. The difference in susceptibility between natural enemy and host may also be magnified by the pest being partially chemically resistant. Even outside the crop environment, there may be similar effects. Collembolans, for example, are not affected by DDT and often increase in numbers in treated soils after depletion of their acarine predators, which are susceptible to it (Curry, 1994).

Ivermectin is used to control nematodes and parasitic arthropods in cattle, and has been speculated to be a risk to dung beetles. Although there is indeed an initial depression of dung beetle diversity, after 2 months, populations return to normal (Scholtz and Kriiger, 1995) (Figure 4.1). However, some results from Irish pastures have indicated that the use of chemical fertilizers and veterinary drugs such as ivermectin alongside removal of herbaceous field boundaries can be detrimental to dung beetle diversity (Hutton and Giller, 2003).

In the case of grasshoppers treated with deltamethrin, one day after treatment, numbers were significantly reduced. There was no local loss of species, although population levels, especially of flightless bushhoppers, were reduced, even after summer rain (Stewart, 1998). Nevertheless, caution is required. Not only may insecticides be adversely synergistic with other impacts but, as demonstrated by Cilgi and Jepson (1995), they can have subtle effects such as reduced fitness on larval and adult butterflies when deltamethrin is applied at only 1/640th of the field dose rate.

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