Effects of Habitat Fragmentation on Plant-Insect Communities. 55. 4.2 Using Plant-Insect Communities as Model Ecosystems. Since the studied insect ...
CHAPTER4
EFFECTS OF HABITAT FRAGMENTATION ON PLANT-INSECT COMMUNITIES ANDREAS KRUESS and TEJA TSCHARNTKE Agroecology, Georg -August University, Gottingen, Germany
4.1
Introduction
Changes in landscape structure due to human activities includes habitat destruction and the fragmentation of the remaining habitat patches (Harris, 1984). This process of habitat fragmentation has been perceived as "the principle threat to most species in the temperate zone" (Wilcove et al., 1986) or "the single greatest threat to biological diversity" (Noss, 1991). Although habitat fragmentation occurs naturally, it is mostly caused by the expansion and intensification of anthropogenic land use (Burgess & Sharpe, 1981). For example, in the Australian wheat belt region 93% of the native vegetation has been cleared, mostly during the last 50 years (Saunders et al., 1993). Estimating the current effects of fragmentation on species diversity is often difficult (Margules et al., 1994), and most investigations have studied the effects a posteriori (Villard & Taylor, 1994). As mentioned by Didham et al. (1998), little attention has been paid to alteration in the trophic structure of communities due to habitat fragmentation, because most studies have focused on single species or several species within one trophic level. Since habitat fragmentation does not affect all species equally, results from systems with such reduced levels of complexity cannot be extrapolated to explain responses of food-web or community interactions. The equilibrium theory of island biogeography (MacArthur & Wilson, 1967) predicts species numbers on islands as a function of island area and isolation. With regard to biological conservation it is important to know which kind of species will go extinct first or will be particularly negatively affected by habitat fragmentation. Greatly endangered species show population characteristics like rarity (reduced abundance and distribution), high population variability (enhanced fluctuations), high degrees of specialization, dependence on mutualists, or little dispersal ability (see also Lawton, 1995; Tschamtke & Kruess, 1999). Fragmentation of habitats is characterized by at least three important processes each affecting the diversity and the spatial distribution of species (Andren, 1994): (i) area reduction of the original habitat in the landscape due to habitat loss; (ii) area reduction of the emerging habitat fragments; and (iii) increasing distance between the fragments. As a consequence of the reduced island area, edge effects may have additional effects on diversity and species distribution pattern. These three major features of fragmentation processes are related in a non-linear way (Gustavson & Parker, 1992; Andren, 1994). B. Ekbom, M Irwin and Y. Robert (eds.), Interchanges of Insects, 53-70 10 2000 Kluwer Academic Publishers.
54
A. Kruess and T TJ,·charntke
When fragmentation affects critical proportions of habitat, rapid changes in size and isolation of the habitat fragments can crop up (Turner, 1989; Gustavson & Parker, 1992; Andren, 1994; Bascompte & Sole, 1996). Andren (1994, 1996) and Bascompte & Sole (1996) found from mathematical modeling that for low-level habitat loss the quantitative effects of area reduction is the dominating process. Dissociation of the original habitat into fragments becomes more significant when habitat loss reaches 40%. Further losses of habitat caused rapid increases in the number of habitat fragments. When habitat fragmentation reaches 80%, the number of habitats declined heavily (Bascompte & Sole, 1996), and isolation of the habitat fragments exponentially increased (Andren, 1994). In modern agriculture habitat loss on a landscape scale has often reached 80% or more (e.g. Saunders et al., 1993). At such a high level of fragmentation, isolation appears to be a major threat to biological diversity. In addition, areas of near-natural habitats still existing in the agricultural landscape often enclose a wide range, from small patches with only a few hundred square meters to large areas extending hundreds of hectares. This pattern does not meet expectations from the results of computer-simulated fragmentation processes mentioned above. An explanation for this may be that real fragmentation processes are not random, but powered by economic decisions due to landscape management or geographic conditions. For example road building causes a more regular (but not random) "cutting pattern" of the landscape. Significance of habitat area depends on the species or taxa under investigation. For example, invertebrates can cope with smaller islands than vertebrates. Moreover, the effects of fragmentation on a particular species depend on its ecological requirements (e.g. home range) or biological characteristics (e.g. mobility, body size). For example species with large home ranges like many birds are not affected by habitat isolation on a local scale because their territories may include several habitat patches (Tjernberg et al., 1993). For those species, the consequences of habitat fragmentation are only the quantitative effects of habitat loss, but not the qualitative effects of area or isolation of the remaining habitat islands. In this chapter, we present some empirical support for answers to the following questions: (I) Does habitat fragmentation negatively affect insect diversity? (2) Are the effects equal for different trophic levels? (3) What are the consequences of fragmentation for herbivoreparasitoid interactions? (4) How can the most affected species be characterized? (5) What are the consequences for biological conservation with respect to the SLOSS debate? To answer these questions we will focus on two well-known plant-insect communities, comprising both endophagous herbivores and their parasitoids (Kruess & Tschamtke, 1994, 1999; Kruess, 1996; Kruess, 1998). Each of the two insect communities were centered on a single plant species, red clover (Trifolium pratense) and bush vetch (Vicia sepium). We investigated the effects of habitat fragmentation in two different ways: (I) we analyzed both insect communities on near-natural old meadows, nearly equal in management regime and vegetation structure, but different in area and isolation (Kruess, 1996; Kruess & Tscharntke 1999). (2) we experimentally analyzed the colonization process of both insect communities on manually established, small and isolated plant plots in the agricultural landscape (Kruess & Tscharntke, 1994; Kruess, 1996).
Effects of Habitat Fragmentation on Plant-Insect Communities
4.2
55
Using Plant-Insect Communities as Model Ecosystems
Since the studied insect communities comprise both herbivores and parasitoids, changes in species diversity can be analyzed on different trophic levels. Moreover, effects on interactions among these trophic levels can be determined. Most studies on the effects of fragmentation on plant-insect systems have analyzed ectophagous insect communities (e.g. on nettle by Davis, 1975; Zabel & Tscharntke, 1998; on juniper by Ward & Lakhani, 1977; on bracken by Rigby & Lawton, 1981). Only few studies have included investigations on endophagous insects (MacGarvin, 1982; Davis & Jones, 1986). Endophagous insect communities are more likely to comprise high proportions of monophagous herbivores and parasitoids, so that isolation or habitat area can be easily defined. Polyphagous species are less sensitive to fragmentation processes than monophagous species (Zabel & Tscharntke, 1998).The inclusion of generalists in analyses of species composition of habitat islands can lead to an overestimation of species diversity in small habitats because species depending on surrounding habitats in the landscape are also included (Loman & von Schantz, 1991). For generalists, populations on habitat islands are not isolated, but closely connected with conspecific populations in the surrounding landscape. We focused on the endophagous insects in the flowerheads and stems of red clover (Trifolium pratense), and in the pods of vetch (Vicia sepium), comprising mostly monoor oligophagous herbivores and their parasitoids. The two plant species are abundant and typical representatives of the investigated meadows. Most studies that have included more than one trophic level of insect communities have analyzed herbivores and predators (Davis, 1975; Ward & Lakhani, 1977; Kareiva, 1987; Spiller & Schoener, 1988; Zabel & Tscharntke, 1998). Since predators are on average less specialized than parasitoids, investigations of multitrophic interactions based on host-parasitoid associations are more likely to show island effects. In the following we will briefly describe the two insect communities. 4.2.1
ENDOPHAGOUS INSECTS ON RED CLOVER (TRIFOLIUM PRATENSE)
Dissections of flower heads and stems of red clover revealed an insect community of 23 species, comprising 8 herbivores and 15 parasitoids (Table 1). The most abundant herbivorous species were the seed-feeding weevil Protapion apricans, the two stem-boring weevils Catapion seniculus and lschnopterapion virens, the seed beetle Bruchidius varius, and the seed-feeding chalcid wasp Bruchophagus gibbus. Parasitoids were associated only with the weevils of the family Apionidae and an undescribed gall midge species, Lasioptera sp. nov. The most abundant parasitoid species were the pteromalid wasps Spintherus dub ius, Trichoma/us campestris, T. fulvipes, two unidentified eulophid wasps of the genus Aprostocetus, and two braconid wasps of the genus Triaspis. All but one species (Spintherus dubius with 79% of all specimens) were relatively rare (less than 10% of all specimens). All but one of the herbivores feed on red clover only. The weevil Protapion assimile also feeds on white clover (Trifolium repens), a clover species occurring on only few of such meadows and in small populations. As far as we know, all but five of the parasitoids attack only hosts on red clover, but four species (Triaspis obscurellus, Spintherus dubius, Stenomalina gracilis, Trichoma/us campestris) were also known to
Braconidae (Hym.)
Braconidae (Hym.) Braconidae (Hym.)
Colasres sp.
Triaspis obscurellus (Nees) Triaspis jloricola (Wesmael) Spintherus dubius (Nees)
Trichoma[ us fulvipes (Walker) Trichoma/us helvipes (Walker) Aprosrocetus cf. tompanus Erdt>s Aprostocetus vassolensis Graham Aprostocetus sp. I Aprostocetus sp. 2 Entedon cf. procioni Erdtls Pseudotorymus apionis (Mayr) Eupelmus vesicularis (Retzius)
Stenomalina gracilis (Walker) Trichoma/us campestris (Walker)
Family
Parasitoids
Pteromalidae (Hym.) Pteromalidae (Hym.) Pteromalidae (Hym.) Pteromalidae (Hym.) Eulophidae (Hym.) Eulophidae (Hym.) Eulophidae (Hym.) Eulophidae (Hym.) Eulophidae (Hym.) Toryrnidae (Hym.) Eupelrnidae (Hym.)
Pteromalidae (Hym.)
Catapion, /schnopterapion Protapion Catapion, lschnopterapion Lasioptera Protapion Protapion Catapion, /schnopterapion Protapion Catapion, lschnopterapion, and others
Co.rapion, lschnopterapion, Protapion
Oligophag Oligophag Monophag Monophag Monophag? Monophag? ? ? Oligophag Monophag Polyphag
Oligophag
? Oligophag Monophag?
Monophag?
Monophag (oligophag?)
Monophag Monophag Monophag Oligophag Monophag Monophag (oligophag?)
Host range
= hosts within one family, Polyphag = hosts from
Catapion, /schnopterapion Calapion, Jschnopterapion, Protapion Catapion, Jschnopterapion Protapion Catapion, Jschnoprerapion
Host genus
stems
flower heads
Eurytom.idae (Hym.)
flower heads flower heads
Cecidomyiidae (Dipt.)
flower heads
Apionidae (Col.)
Apionidae (Col.) Bruchidae (Col.)
Lasioptera sp. nov.
stems
stems flower heads
Apionidae (Col.) Apionidae (Col.)
Site of attack
Apionidae (Col.)
Catapion seniculus (Kirby) lschnopterapion virens (Herbst) Protapion apricans Herbst Protapion assimile (Kirby) Protapion trifolii (Linnaeus) Bruchidius varius Olberg Bruchophagus gibbus Boheman
Family
more than one family, ? =unknown)
in the third column, host range of both herbivores and parasitoids is given in the right column (Oiigophag
Endophagous insect conununity in the flower heads and stems of Trifolium pratense. Site of attack of the herbivores and host genus of the parasitoids are listed
Herbivores
Table I.
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