Insects and the conservation of ecosystem processes

. . . But in an African forest not a fallen branch is seen. One is struck at first at a certain clean look about the great forests of the interior, a novel and unaccountable cleanness, as if the forest-bed was carefully swept and dusted daily by unseen elves. And so, indeed, it is. Scavengers of a hundred kinds remove decaying animal matter - from the carcase of a fallen elephant to the broken wing of a gnat - eating it, or carrying it out of sight, and burying it on the deodorising earth.

Henry Drummond (1889)

3.1 Introduction

Insects, being good dispersers and exploiters of virtually all types of organic matter, can be found almost everywhere. That horrendous term 'bug splatter' on the front of moving vehicles, bears witness to the general abundance and omnipotence of insects and all those little lives lost. Insects do not occur in permanently frozen areas, nor at any depth in the oceans. Some, however, come close to these extremes. In North America, under snowfields, bittacid scorpion flies continue to hunt.

The ecological grandeur of insects is in their ability as a group to transfer vast amounts of energy. As such, they are determinants of community structure and shapers of habitats. Some, like termites, are such notable movers of physical materials, that they are known as ecological engineers.

One of the most beneficial attributes of insects is that many are pollinators. Not that insects always have the upper hand, as many are food items for vertebrates, as well as for each other. Conservation of insects, therefore, goes hand in hand with conservation of plants, vertebrates and other invertebrates. But insects can also be harmful to other organisms, and some are also vectors of plant or animal disease. These facts lead to the inevitable conclusion that understanding conservation of insect diversity is largely about appreciation of ecosystem processes.

Insect assemblages and communities are shaped by the compositional and structural land mosaic. In turn, the floral and faunal features of the landscape can be shaped by insects. This interaction between insects and the landscape may not always be linear, with weather and climate changing the direction and magnitude of the interactions. Conserving insect diversity is therefore about conservation of the integrity of ecological systems.

3.2 Insects as keystone organisms

A keystone species could be considered as one whose impact on its community or ecosystem is large and disproportionately large relative to its abundance (Power et al., 1996). This concept, however, has been criticized as it threatens to erode the utility of the keystone concept (Hunter, 2000b). Paine's (1969) original idea was that the species composition and physical appearance of an ecosystem are greatly modified by the activities of a single indigenous species high in the food web. Such a keystone species influences community structure and ecological integrity, with persistence through time. Mills et al. (1993) have pointed out that the term keystone has since been applied to a plethora of species, at different levels in food webs, and with very different effects, both qualitative and quantitative, in their communities.

In terms of conservation, it is not so much the keystone species per se that is significant but its keystone role (Mills et al., 1993). Following on from this, these authors advocate the study of interaction strengths and subsequent application of the results into management plans and policy decisions, instead of using the keystone/non-keystone dualism. Such an approach recognizes the complexity, as well as temporal and spatial variability of interactions. This complexity and the different effects in different places at different times has a familiar ring to practicing insect biocontrol workers. It is well known that any one insect host may well have 10 or more natural enemies (Miller, 1993). Some of these are generalist, and others specialist or host-specific. In turn, many of these natural enemies are in competition with each other, and with other species on other hosts. These interactions shift and change in strength with weather, season, climate and elevation, which, in turn, increases temporal and spatial complexity enormously.

Nevertheless, very strong competition sometimes occurs among keystone species. Risch and Carroll (1982), for example, observed an increase in abundance

Insect ecosystem engineers and soil modifiers 41

in 24 other insect species and a decline in three species when certain species of ant were excluded from agricultural fields. The dynamics of severe competition was emphasized among other ant species, where asymmetrical competition became so severe that there was complete amensalism and local extinction of nests of one species by another (Samways, 1983a).

Although interaction strengths is a scientifically more rigorous concept than keystone species, De Maynadier and Hunter (1994) argue that we must not too hastily discard the term 'keystone', as it can effectively rally public understanding and protection. The point is, so long as we use it and understand its ramifications and implications, then we do indeed have powerful imagery with which to work in practising conservation.

3.3 Insect ecosystem engineers and soil modifiers

Ants are well known to influence certain terrestrial ecosystem processes, at least in the tropics (Folgarait, 1998) and in arid areas (Whitford, 2000). They are able to do this because some species are major modifiers and controllers of the physical state of abiotic and biotic materials (Samways, 1983a). In this way, they may be regarded as ecosystem engineers (Jones et al., 1997). The Funnel ant Aphaenogaster longiceps in Australia, for example, single-handedly is responsible for moving some 80% of the soil that is moved to the surface by soil fauna (Humphreys, 1994).

Another major engineering taxon is the termites (Whitford, 2000). Nests of macrotermitines in West Africa can cover as much as 9% of the land area and have a volume of 300 m3 / ha (Abbadie et al., 1992). Such mounds have a higher organic carbon and nitrogen content than the surrounding soil. Termites also play a significant role in global carbon fluxes. Global gas production by termites in tropical forests represents 1.5% of carbon dioxide and 15% of all methane produced from all sources (Bignell et al., 1997).

These insect ecosystem engineers can locally influence structural, compositional and functional biodiversity. West African termites, by modifying water dynamics and organic matter status, increase local tree diversity (Abbadie et al., 1992), and some ants allow unique plants to exist (Folgarait, 1998). Besides these effects on vegetation diversity, engineers with distinct and/or large structures provide homes for many symbiotic organisms (Wilson, 1971).

The concept of ecosystem engineers is far from fully explored and there may be far more less explicit engineers than is fully realized. Grasshoppers in the arid region of South Africa produce frass which is finely divided and provides nutrients to plants far faster than that, say, of sheep (Milton and Dean, 1996), which in turn influences plant diversity (Stock and Allsopp, 1992). Although plant diversity inevitably increases insect diversity, there is not necessarily a direct relationship. Again, in the southern part of South Africa, it does not seem to be simply the plant variety that has generated the insect diversity, but that the insects have speciated through allopatric isolation, as did the plants themselves (Cottrell, 1985). The point being that although we generally think of ecosystem engineers working at the recent, ecological temporal scale, it is likely that they have had, and continue to have, a differential influence on the evolution of other organisms, depending on how dependent those organisms are on the controlling role played by the engineers. A very strong controlling role has thus generated the symbionts.

3.4 Insects as food for other animals

Conservation of invertebrates is intimately tied in with conservation of ecosystems. This is because many species are major components in those ecosystems (Coleman and Hendrix, 2000), although many others are not (New, 2000). Insects are major prey for many vertebrates, and of course for many invertebrates, including other insects. Insects provide a large food resource. Pimentel (1975) estimates that in the USA, their fresh biomass is about 450 kg/ha, about 30 times that of humans in the same country. In the Brazilian rainforest, ants and termites make up more than one-quarter of the faunal biomass, and ants alone have four times the biomass of all land vertebrates (Wilson, 1991). In terms of shear abundance, the record holders are probably the Collembola, which can occur at densities of between 104 and 105 per m2 (Hopkin, 1998). This biomass is inevitably a major foodbase for many dependent faunal elements. Even in freshwater, the role of insects is pivotal, with the fly-fishing industry, to name one, being built on the functional role of insects as food.

Certain alien predators are having a detrimental threatening impact on certain insects, such as poeciliid fish on endemic damselflies in Hawaii (Englund, 1999). It is not certain the extent to which many red-listed amphibia, reptiles, birds and mammals depend on specific insect prey items for their survival. Certainly, some are generalist predators, with the Seychelles endemic skinks Mabuya sechellensis and M. wrightii depending on local arthropods, while interestingly, the highly threatened Seychelles Magpie Robin Copsychus sechellarum has the introduced cockroach Pycnoscelus indicus as a main food item (LeMaitre, 2002). In Britain, agricultural change in recent decades has caused a decline in birds, which appears in part to be due to the decline in quality and quantity of their invertebrate food source (Benton et al., 2002).

3.5 Insect dispersal

3.5.1 Differential mobilities

Many insects are remarkably mobile. This appears to be part of the natural dynamics of many insect species as a survival strategy for dealing with

Insect dispersal 43

natural, local extinctions. Perhaps this diffusive dispersal is more common than generally realized, especially considering the discovery that there is longdistance gene flow in an apparently sedentary butterfly (Peterson, 1996). Certainly for an assemblage of British butterflies it was not dispersal ability limiting their overall abundance but lack of suitable habitat (Wood and Pullin, 2002).

Not all insects are so vagile. There are what Adsersen (1995) has described for plants, 'fugitive species'. These are species that instead of radiating and diversifying, originally dispersed and then remained localized. For fugitive Seychelles damselflies, they can be surprisingly tolerant of droughts, even maintaining territories when water is no longer present (Samways, 2003b). Similarly, populations of Maculinea alcon butterflies in Denmark are more isolated than counts of flying adults or eggs on foodplants indicate (Gadeberg and Boomsma, 1997). Nevertheless, in the long-term, some apparently low mobility species can disperse long distances and colonize remarkably remote oceanic islands (Figure 3.1) (Peck, 1994a,b; Samways and Osborn, 1998; Trewick, 2000).

3.5.2 Tracking resources

As the subtitle of Drake and Gatehouse's (1995) book on insect migration indicates, movement is about tracking resources through space and time. The critical point is that knowledge of a species' dispersal ability, and inhibitions to that dispersal, are fundamental to conservation of a species. To name one example, Appelt and Poethke (1997) illustrated that populations of the grasshopper Oedipoda caerulescens undergo local extinction when there are stray fluctuations in environmental conditions. This leads to the concept of metapopulations, the framework of which explicitly recognizes and provides a conceptual tool for dealing with the interactions of within- (e.g. birth, death, competition) and among-population processes (e.g. dispersal, gene flow, colonization and extinction) (Thrall et al., 2000). The within- and among-population interactions are well-illustrated in Prokelisia spp. planthoppers, where there is a negative relationship between dispersal capability (i.e. genetically determined per cent macroptery) and habitat persistence (Denno et al., 1996).

Whereas not all species whose populations have undergone fragmentation fit the definition of a metapopulation, the metapopulation paradigm (Harrison and Hastings, 1996) (see Figure 10.6) is nevertheless a useful management tool. As Thrall et al. (2000) point out, a metapopulation perspective ensures a process-oriented, scale-appropriate approach to conservation that focuses attention on among-population processes that are critical for persistence of many natural systems.

3.5.3 Myriad of mobilities

The problem is that we do not know what each insect species, each evolutionarily significant unit and each insect polymorphism needs under all a. \

3.1 The aptly named Globe skimmer Pantalaflavescens, the only dragonfly to have colonized the world's most remote island, Easter Island (Rapa Nui). After colonizing the island, its morphology and behaviour have changed. It has undergone natural selection and become adapted to surviving on this small speck of land, where to wander away from the island would mean almost certain death. On the island, it flies in small circles close to the ground, whereas on continents it flies high and wanders (see Samways and Osborn, 1998).

environmental conditions. This is illustrated by the various movement patterns in a butterfly assemblage at any one point. Each species, although seemingly simply flying by, is reacting sensitively to the various landscape vegetational structures (Wood and Samways, 1991). This serves to illustrate that conservation of insect diversity encompasses a vast complexity of interactions that in themselves vary over space and time. Against this background, the landscape may be considered as a continuously varying differential filter (Ingham and Samways, 1996), and try as we may, cannot always be managed to provide optimal conditions for all species all of the time. This argues strongly for the conservation of larger spatial scales (i.e. landscapes and larger) such that all aspects of an insect's behaviour, and all types of insects are supported. It also means a whole

Insect pollinators 45

3.2 Specialized pollinator systems are under threat. Here, a Convolvulus hawkmoth Agrius convolvuli inserts its long proboscis to the bottom of a long corolla to obtain nectar.

range of interaction types, interaction strengths, ecological processes, and biotic units are conserved, without us knowing the details. This returns us again to the familiar coarse filter approach (Hunter, 2000a), so critical for long-term insect diversity conservation, although nevertheless requiring critical evaluation, the subject of Chapters 8 and 10.

3.6 Insect pollinators

Flowers show a range of specialization to pollinators (Johnson and Steiner, 2000) (Figure 3.2). Some flowers like the Madagascan orchid Angraecum sesquipedale, which has a 30 cm long tubular nectary, is pollinated by a moth Xanthopan morgani praedicta with a correspondingly long proboscis. Another Madagascan orchid Angraecum 'longicalar' has been discovered, with an even longer nectar spur, 36-41 cm long, which presumably has a matching pollinator (Schatz, 1992).

Although certain flowers do have specialized pollinator systems, there are strong differences among plant families. Indeed, some plant specializations for pollinators may not be a determinant for an evolutionary 'trend' (Ollerton, 1996).

There is no doubt that certain pollinator systems are threatened and probably more so in the case of the most specialized systems (Bond, 1994), and for plants in small, isolated landscape remnants. But the main issue is that whereas plants occupy virtually every point on the continuum from extreme specialization to extreme generalization, in realistic terms, we know very little of the ecological dependency of plants on pollinators, both for seed production and for population viability (Johnson and Steiner, 2000). In particular, besides broad-scale community studies, we need more studies on pollinator effectiveness (Schemske and Horvitz, 1984), so as to determine which are the important pollinators and where the weak links lie in terms of ecological integrity of the system. This is particularly so as the break down of mutualisms can have long time delays. This arises from compensatory mechanisms such as clonality, longevity and self-pollination, which enable the plant species to survive only temporarily, albeit over many years (Bond, 1994). It is now timely to identify keystone species that sustain particular pollinator systems (Corbet, 2000).

Dependency on pollinators relates to crop plants as well as to wild plants (Bonaszak, 1992; Allen-Wardell et al., 1998). Human-transformed landscapes are likely to have complex, serious and largely unpredictable consequences for wide-scale, long-term conservation of both insects and plants (Kearns et al., 1998; Kremen and Ricketts, 2000). This serves to underscore again that conservation of insect diversity is not an isolated activity but is intimately linked with that of plants, although each group is not necessarily an exact surrogate for the other.

3.7 Insect herbivores

The world total biomass of plants to animals has the ratio 99.999: 0.001, whereas the total number of higher plant species to animal species may be 0.026: 99.974 (Samways, 1993b). In other words, there are few but bulky plants compared with many and varied, but not bulky, animals. And four-fifths of the animals are probably insects. This suggests that many insects are feeding on few plants. Yet it is not easy to feed on plants, where environmental conditions are harsh. Also, much of the plant material is non-nutritious and protected by noxious substances. Nevertheless, the throughput of energy of insects feeding on plants can be enormous, with some grasshopper species transferring 5-10 times the amount of energy as a bird or mammal species in the area. Even in the African savanna, where megaherbivores visually dominate the landscape (or used to), the grasshopper assemblage can ingest 16% of the grass cover (Gandar, 1982).

The insect--plant interaction is highly significant for maintaining biocycles. The Brown locust Locustana pardalina in the Karoo of South Africa produces 2.26 million tonnes of frass during an outbreak, which represents 14 700 tonnes of nitrogen (Samways, 2000a). The point is that the Karoo is an arid ecosystem, poor in organics and nitrogen. Large quantities of nutrients can become available only after rains and principally in the upper 30 cm of soil. As insect frass is fine grained, it means that locusts and termites convert plants to a nutrient-rich

Insect herbivores 47

Herbivore

Herbivore

Herbivore

Faster decomposed plants

Slower decomposed plants

Faster decomposed plants

Slower decomposed plants

Nutrient availability

Nutrient availability

Nutrient availability

Nutrient availability

Detritus

Fast cycle Slow cycle

3.3 The conditions for herbivores to decrease nutrient cycling and primary productivity (a) and to increase nutrient cycling and primary productivity (b) are shown. The solid lines reflect the trophic transfers within the ecosystem. The broken lines reflect the pathways by which nutrients are recycled in the ecosystem. Line thickness reflects the relative magnitude of the trophic transfer or recycling pathway. (Redrawn from Belovsky, 2000, with kind permission of Kluwer Academic Publishers.)

resource that is immediately available to the plants (Milton, 1995). Similarly, in the USA, it is the high abundance of grasshoppers that enhances grassland productivity (Belovsky, 2000) (Figure 3.3). This contrasts with the forest situation, where plant primary production is stimulated more by low to medium levels than by high intensities of phytophagy (Schowalter, 2000). These seemingly contradictory results are probably related to differing conditions and the extent to which nutrients are resupplied through the fast cycle of frass and herbivore corpses as opposed to the slow cycle of litter decomposition (Belovsky, 2000).

While it is well known that plant species and assemblages influence insect diversity, less well known is the fact that insects can determine the secondary succession of certain plant assemblages (Brown et al., 1987). Such interactions between herbivores, plants and ecosystem processes are now known to involve complex interactions with feedback loops (Schowalter, 2000), which vary according to the time span under consideration (Anderson, 2000). Furthermore, these interactions involve a host of biochemical and physiological reactions in addition to the ecological ones (Dyer, 2000) (Figure 3.4). Let us not forget too, that these interactions are also taking place on the roots as well as on the aerial parts.

The interaction between insects and plants must also consider seeds. Insects, especially ants, distribute and bury seeds, an essential survival strategy for many plants. This may even involve the production of special attractant seed structures

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