Species Interactions

Species interactions can enhance or preclude persistence of some species, as discussed in Chapter 8. As noted in the preceding text, species populations cannot persist where their host species are absent. However, the presence of competitors, predators, and mutualists also affects persistence of associated species, both directly and indirectly. In the past, species interactions often were viewed as evidence of co-evolution. However, species colonizing new areas can occupy niches through preconditioning or left vacant by extinction (Diamond 1990, Janzen and Martin 1982, Janzen 1985). For example, introduced species often become successfully established in their new habitats without benefit of co-evolution. Nevertheless, their presence in the community shapes environmental conditions that affect subsequent adaptations. S. Gould and Vrba (1982) proposed the term "exaptation" to describe characters evolved for other purposes but that "preadapt" an organism for current conditions.

Some species can have particularly profound effects on community structure. Their presence in a community leads to a different community structure than occurs in their absence. A top predator that preferentially preys on the most abundant of several competing prey species can prevent any single species from competitively suppressing others. R. Paine (1966, 1969a, b) considered such species to be keystone species. Bond (1993) and Power et al. (1996) applied this term to any species that has effects on ecosystem structure or function that are disproportionate to their abundance or biomass.

Some insect species could be considered to be keystone species to the extent that their abundance greatly alters diversity, productivity, rates of energy or nutrient flux, etc. Many herbivorous insects increase the diversity of plant species by selectively reducing the density of abundant host species and providing space and resources for nonhost plants (Lawton and Brown 1993, Schowalter and Lowman 1999). The southern pine beetle, Dendroctonus frontalis, is capable, at high population densities, of killing pine trees and increasing the availability of woody resources that maintain populations of other xylophagous species (Flamm et al. 1993). Naiads of the large dragonfly, Tramea lacerata, prey on other dragonflies as well as on various other taxa that also are prey of other dragonfly and dam-selfly naiads. Wissinger and McGrady (1993) found that addition of T. lacerata to wetland communities had a direct negative effect on damselfly prey but also an indirect positive effect through reduced numbers of other predaceous dragon-flies. Termites and ants affect soil structure and fertility in ways that determine vegetation development (see Chapters 13 and 14).

As discussed in Chapter 6, the combination of bottom-up (resource supply) and top-down (trophic cascades) factors tends to stabilize population levels. Changes in abundance of any trophic level, however, affect abundances at other trophic levels. Generally, increased abundance at one trophic level increases resources available to the next higher trophic level, increasing abundance at that level but reducing abundance at the next lower level. Reduced abundance at the lower trophic level reduces its control over the second lower trophic level, which increases in abundance and reduces abundance at the third lower trophic level.

Such trophic cascades are commonly observed in aquatic ecosystems (Batzer et al. 2000b, Carpenter and Kitchell 1984, 1987, 1988, Vanni and Layne 1997). Fewer examples of trophic cascades controlled by top predators have been observed in terrestrial ecosystems. M. Hunter et al. (2003) reported that exclusion of litter predators in litterbags increased Collembola abundances and litter decomposition rate.

Letourneau and Dyer (1998) and L. Dyer and Letourneau (1999a, b) described a trophic cascade in a neotropical rainforest community. Clerid beetle, Tar-sobaenus letourneauae, predation on ants, especially Pheidole bicornis, reduced ant abundance and increased herbivore abundance and herbivory on Piper cenocladum ant plants. Where this beetle was absent and spiders were a less effective top predator, ant abundance was higher and reduced herbivore abundance (Fig. 9.12). Manipulation of top-down and bottom-up effects indicated that increased resources (light and nutrients) directly increased plant biomass but had no indirect effect on predators or top predators, but ant exclusion indirectly affected plant biomass by increasing herbivory (Dyer and Letourneau 1999a).

Top Down Trophic Cascade

Months After Experiment Initiated

| Mean ant abundance (percentage of occupied petiole chambers per plant), mean folivory (leaf area eaten per Piper cenocladum shrub), and mean leaf area per shrub before (0-2 months) and after (7-18 months) addition of a top predator, Tarsobaenus beetles, to half the shrubs. Vertical bars represent 1 SE. From L. Dyer and Letourneau (1999) with permission from the National Academy of Sciences.

Linkages between communities can affect trophic cascades. Terrestrial arthropod inputs to aquatic systems provide a resource subsidy that influences predator-prey interactions. Nakano et al. (1999) experimentally manipulated terrestrial arthropod inputs and predatory fish presence in forest headwater streams in northern Japan. When terrestrial arthropod inputs were reduced, predatory fish switched from terrestrial to aquatic arthropod prey, reducing aquatic arthropod abundance and increasing periphyton biomass. By contrast, removal of predatory fish did not significantly alter the food web, indicating that the trophic cascade was controlled by terrestrial arthropod inputs. Such linkages control fluxes of energy and nutrients between ecosystems.

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