Artificial control pats

4.1 Comparison of Shannon's diversity index (H'j of dung beetles in artificial control pats 1 month and 2 months after treatment of cattle with a single standard injection of the insecticide/acaricide ivermectin; control paddocks (C1 and C2) and paddocks with treated cattle (I1 and I2). Recovery is virtually complete after 2 months. (Redrawn from Scholtz and Kruger, 1995, with kind permission of Elsevier.)

4.3.3 Other impacts

It is not only insecticides that have an impact on insect diversity. The herbicides atrazine and pentachlorophenol can reduce soil collembolan populations by 80%, as well as reducing staphylinid beetles and spiders (Curry, 1994). Such effects are likely to change community structure, albeit temporarily and locally (Ellsbury et al., 1998).

The question that now arises is that with a projected increase in pesticide usage of 270% compared with present levels by the year 2050 (Tilman et al., 2001), the environmental impacts need to be considered in more detail, especially as these impacts are synergistic with other impacts, from increased fertilizer input to landscape fragmentation and invasive alien organisms.

4.4 Agriculture and afforestation

4.4.1 Scale of the challenge

Demand for food by a wealthier and 50% larger global population will be a major driver of global environmental impacts. Should past dependencies on agriculture continue, 109 ha of natural ecosystems would be converted to agriculture by 2050, with a 2.4- to 2.7-fold increase in nitrogen- and phosphorus-driven eutrophication of terrestrial, freshwater and near-shore marine ecosystems (Tilman et al., 2001). If the global human population stabilizes at 8.5-20 billion individuals, the next 50 years may be the final episode of rapid

4.2 (Left) Patch-scale study. Each observation represents the information from a single patch. Only one landscape is studied, so sample size for landscape-scale study inferences is one. (Right) Landscape-scale study. Each observation represents the information from a single landscape. Multiple landscapes, with different structures, are studied. Here, sample size for landscape-scale inferences is four. (From Fahrig, 2003.)

4.2 (Left) Patch-scale study. Each observation represents the information from a single patch. Only one landscape is studied, so sample size for landscape-scale study inferences is one. (Right) Landscape-scale study. Each observation represents the information from a single landscape. Multiple landscapes, with different structures, are studied. Here, sample size for landscape-scale inferences is four. (From Fahrig, 2003.)

global agricultural expansion. These impacts are likely to be devastating for many insect species, with synergistic effects of pollution, pesticides, fragmentation, invasive aliens and global warming taking an immediate toll, followed in time by the effects of ecological relaxation as populations are gradually lost from the small remaining natural fragments.

The results of Leon-Cortes et al. (2000) suggest that some common species may decline at the same rates as ecological specialists. In the case of smaller indigenous patches, there is likely to be greater threat from invasive alien plants as well as from proportionately less interior habitat. This decline in patch quality may matter as much as patch size for some of these species (Dennis and Eales, 1997) (see Chapter 10). The debate as to the relative importance of patch quality relative to patch size is an intense one. Fahrig (2003) provides some valuable insights emphasizing that empirical studies of habitat fragmention are often difficult to interpret because (a) many researchers measure fragmentation at the patch scale, not the landscape scale, and (b) most researchers measure fragmentation in ways that do not distinguish between habitat loss and habitat fragmentation per se, i.e. the breaking apart of habitat after controlling for habitat loss (Figures 4.2 and 4.3). Evidence suggests that habitat loss has large, consistently negative effects on biodiversity, whereas this might not be the case with fragmentation.

4.3 Both habitat loss and habitat fragmentation per se (independent of habitat loss) result in smaller patches. Therefore, patch size itself is ambiguous as a measure of either habitat amount or habitat fragmentation per se. Note also that habitat fragmention per se leads to reduced patch isolation. (From Fahrig, 2003.)

4.3 Both habitat loss and habitat fragmentation per se (independent of habitat loss) result in smaller patches. Therefore, patch size itself is ambiguous as a measure of either habitat amount or habitat fragmentation per se. Note also that habitat fragmention per se leads to reduced patch isolation. (From Fahrig, 2003.)

However, the situation is complex and depends very much on the species concerned and the spatial and historical context of habitat loss and landscape fragmentation.

4.4.2 Some comparative issues

Duelli et al. (1990) suggest that not all species will suffer declines as the landscape mosaic changes. In Switzerland, 'hard-edge' species tend to be specialists for undisturbed perennial habitats while 'soft-edge' species with a diffuse distribution are mainly associated with annual crops. It seems that in cultivated areas, a mosaic landscape of small-sized crop fields and semi-natural habits maximizes arthropod diversity and decreases the probability for overall extinction, even of rare species. This apparent contradiction between these results and some others may be because the Swiss landscape has been highly anthropogenically disturbed over many centuries, even millennia in places, meaning that the original palaeobiodiversity was ameliorated a long time ago. Nevertheless, the highly modified European landscape is still losing species. Among carabid beetles, large-bodied, habitat specialists are declining the most, seemingly because of their lower reproductive output and lower powers of dispersal (Kotze and O'Hara, 2003). Size, per se, is not the important factor, with weak dispersers and habitat specialists (which can be small), being the most prone (Kotze et al., 2003).

Fragmentation and its synergistic consequences may be more severe in the tropics where, as Soule (1989) puts it 'the demographic winter will be more severe and longer'. In Costa Rica, forest fragments have more moth species than surrounding agricultural habitats with many of these species utilizing both natural and transformed habitats, such that the forest fragments have halos of high species diversity 1.0-1.4 km from the forest edge (Ricketts et al., 2001). Conversely, alien pine-tree patches have a 30-50 m halo of reduced indigenous grasshopper populations (Samways and Moore, 1991). Similarly, alien pines can reduce macroinvertebrate diversity in streams formerly subject to litter input from Nothofagus (Albarino and Balseiro, 2002).

Generalizations however, are likely to be difficult, especially as agricultural habitats differ greatly in vegetal and litter structural and compositional diversity, so resulting in differential effects on different insect species (Samways et al., 1996). Afforested plots in Cameroon with the tree Terminalia ivorensis encouraged a rich butterfly assemblage, although they did not provide habitat for some of the indigenous forest species (Stork et al., 2003). Overall interpretation of research results must also be done carefully. Insect species richness declined dramatically in an afforested African grassland landscape, but then it also did in natural forest, indicating that it is essential to compare like with like in terms of habitat characteristics in transformed versus untransformed patches (Kinvig and Samways, 2000). Sun coffee plantations, for example, are less suitable for foraging army ants than shade coffee plantations (Roberts et al., 2000).

Yet some agricultural features enhance insect survival. Indeed, it is difficult to generalize because some insect species decrease in abundance with land-use intensity while others increase, as does species richness, even among taxonom-ically close groups (Klein et al., 2002) (Figure 4.4).

The damselfly Lestes barbarus uses hedges for maturation (Hill et al., 1999), while structural features of the habitat mosaic such as trees, can alter dispersal distance of scarabaeid beetles (Conradi et al., 1999).

Using data for butterfly species in the fragmented European landscape, Thomas (2000) showed that species of intermediate mobility have declined most, followed by those of low mobility, whereas high-mobility species have survived well (Figure 4.5). Compared with the more sedentary species, those of intermediate mobility require relatively large areas where they can breed at slightly lower local densities. Intermediate mobility species have probably fared badly through a combination of metapopulation (extinction and colonization) dynamics and the mortality of migrating individuals which fail to find new habitats in fragmented landscapes. Habitat fragmentation is likely to result in the non-random extinction of populations and species characterized by different levels of dispersal, although the details are likely to depend on the taxa, habitats and regions considered.

4.5 Urbanization and impact of structures

4.5.1 Cities and insect diversity

Urbanization is a high-impact activity characterized by high human densities (> 620 individuals per km2) and an urban-rural gradient (McDonnell and

Solitary bees al SB

Was this article helpful?

0 0
Relaxation Audio Sounds Babbling Brook

Relaxation Audio Sounds Babbling Brook

This is an audio all about guiding you to relaxation. This is a Relaxation Audio Sounds with sounds from the Babbling Brooks.

Get My Free MP3 Audio


Post a comment