Leaf shelters increase the habitat heterogeneity that exists on plants (Lawton 1983) and thus would be expected to have positive effects on species richness and abundance if the engineered habitats are used by a different set of species than use corresponding nonengineered habitats (Jones et al. 1997). Secondary occupants include other shelter-builders (Carroll and Kearby 1978; Carroll et al. 1979; Cappuccino 1993; Cappuccino and Martin 1994; Bailey and Whitham 2003; Lill 2004; Lill and Marquis 2003, 2004) and non-shelter-building arthropods (Morris 1972; Carroll and Kearby 1978; Carroll et al. 1979; Hajek and Dahlsten 1986; Martinsen et al. 2000; Bailey and Whitham 2003; Fournier et al. 2003; Nakamura and Ohgushi 2003; Lill and Marquis 2003, 2004; Lill 2004; Crutsinger and Sanders 2005) from a wide variety of taxa (reviewed in Fukui 2001, Marquis and Lill 2006). Secondary occupation of existing leaf shelters can occur while the original shelter-builder is still present or following the abandonment of the shelter by the engineer (Cappuccino 1993, Lill 2004).
The resources provided to secondary occupants of shelters are even more varied than those created for use by the primary occupant; these may include each of the habitat features detailed in preceding text, as well as prey items for predators and parasitoids, detritus (e.g., frass, exuvia, uneaten dead portions of leaves, and accompanying fungi) fed upon by scavengers, and honeydew produced by aphids or other sucking insects occupying (or creating) leaf shelters (Fukui 2001, Nakamura and Ohgushi 2003). Some secondary occupants take up permanent residence in leaf shelters, while others use them temporarily as resting sites, ovi-position sites (e.g., many adult moths and beetles; Lill and Marquis 2004), or protected sites in which to molt (Fukui 2001). In systems in which shelters are occupied by a succession of shelter-builders (of the same or different species), the engineering effects are likely to be magnified because the structural integrity of the shelters can be extended well beyond the residency of the initial constructor(s). Such positive feedback loops between successive generations of shelter-builders may increase local densities of these species, potentially resulting in outbreaks (e.g., of some agricultural and forestry pests; Table 6.1).
Increased habitat heterogeneity created by shelter-builders can alter arthropod diversity in a variety of ways. Shelters can provide recruitment sites for species with specialized microhabitat requirements that otherwise may not be found on the plant. For example, in our study of the effects of leaf ties on the arthropod communities of white oak saplings, we found that several species of low-density shelter-builders occurred only on trees containing preexisting shelters (Lill and Marquis 2003). The habitats provided by shelter-builders also can increase the abundance and diversity of habitat generalists capable of occupying both engineered and nonengineered habitats. For example, spiders often use leaf constructs when building webs or selecting nesting sites. As a result, plants with high densities of leaf shelters may have higher densities of spiders than plants with low densities of shelters due to the increased structural complexity offered by these plants (Fournier et al. 2003). Specialist predators and parasitoids also would be predicted to increase, tracking the increased diversity of their prey or hosts. Non-shelter-using arthropods (shelter-avoiders) could be negatively impacted by shelter-constructors if the availability of non-shelter habitats becomes limited (Marquis and Lill 2006). Because there is no a priori reason to expect that the habitats produced by shelter-builders will support a greater or lesser diversity of arthropods than similar-sized patches of nonengineered foliage (Jones et al. 1997), any net positive effects of engineering on diversity measures should stem from the availability of multiple habitat types, each with its own set of associated arthropod species.
Recruitment of arthropods to engineered habitat patches frequently occurs in conjunction with reproduction, whereby arthropods of various types lay eggs or give birth (e.g., many parthenogenetic aphids) on foliage incorporated into a shelter (Cappuccino and Martin 1994, Lill and Marquis 2004) or lay eggs into host arthropods residing within shelters (Pasek and Kearby 1984, Lill 1999). Shelters used temporarily by more mobile arthropods (e.g., many adult beetles) may increase the "residence time" of these arthropods on the plant by providing concentrated, high-quality resources. The fidelity, survivorship, and residence time of arthropods secondarily occupying leaf shelters of different types require further study so that we can move beyond simply documenting shelter use to gaining insights into the ecological factors determining arthropod responses to habitat creation.
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