Controls on engineering

Because shelter-builders have cascading impacts on arthropod communities through both their engineering effects and their resulting trophic interactions, it is salient to consider what factors regulate their abundance on different plant species or in different habitats. Leaf age appears to be a limiting factor for some shelter-builders, due to changes in leaf toughness or foliage size, both of which can affect the ability of shelter-builders to build and maintain their constructs (Damman 1987, Mueller and Dearing 1994, Dorn et al. 2001). For example, many leaf-rollers are restricted to using young foliage, which may be available only at a certain time of year in seasonal environments with synchronously flushing plant species (Coley and Barone 1996). Similarly, the creation of other shelter types (e.g., leaf ties) may be possible only after leaf expansion is near completion and leaf overlap occurs (Marquis et al. 2002).

Marquis et al. (2002) found that plant architecture, specifically the number of touching leaves, determines the density of leaf ties formed on white oak (Quercus alba) saplings. Bailey and Whitham (2003) found that elk browsing of aspen (Populus tremuloides) decreased leaf fold galls formed by the sawfly Phyllocolpa bozemanii, an important aspen microhabitat used by a variety of arthropods. Hunter (1987) found that early-season damage by two caterpillar species (Tortrix viridana and

Operophtera brumata) increased the leaf-rolling ability of late-season Diurnea fagella caterpillars. Finally, Seyffarth et al. (1996) found that fire in the cerrado vegetation of Brazil increased the abundance of leaf-rollers (unidentified species) on the common host plant Ouratea hexa-sperma, as a result of refoliation following the burn. The growing recognition of the importance of such "interaction webs" requires that engineering be integrated into more traditional models involving trophic interactions alone in formulating predictive models of the network of factors influencing community and ecosystem dynamics (Proulx et al. 2005).

6.5 - PROSPECTUS

Looking forward, there are a number of open paths of inquiry into the ecology of shelter-builders. Studies that quantify the engineering impacts of different shelter types on arthropod abundance, diversity, and community structure are needed for a much wider variety of plant species, growth forms, and climatic regimes. Most studies conducted to date (and all of those examining community-level responses) have focused on leaf shelters constructed on temperate tree species. Studies documenting engineering effects for both herbaceous and woody plants growing in other parts of the world (especially in the tropics) are needed to test the generality of the results obtained thus far. Replicated experiments across taxa or sites are imminently feasible because most shelter types can be created by investigators. Within a particular habitat, it would be prudent to compare the effects of leaf shelters on plant species with well-developed vs. poorly developed shelter-building faunas; such a comparison would shed light on whether arthropod responses to these constructs are driven by evolved, canalized habitat selection behaviors or are more serendipitous and behaviorally plastic.

Because modification of the abiotic environments inside of shelters is thought to have played an important role in the evolution of the shelter-building habit, faunal studies examining the incidence of shelter building along an abiotic stress gradient, or that examine the phylogeography of shelter-builders, would help to test this assertion and perhaps illuminate which environmental variable(s) are the most commonly altered by the construction of the shelter. In addition, more detailed autecological studies of the physical and chemical properties of different shelter types would help circumscribe the niche space produced and occupied by shelter-builders and their associates (Odling-Smee et al. 2003). These include more detailed studies of temperature and humidity fluctuations inside of shelters, further examination of how and when shelter-building influences leaf quality, and how the microclimate of a shelter influences arthropod physiology and fitness.

The biotic interactions that occur within leaf shelters also are likely to be important drivers of subsequent engineering effects. For example, the importance of inter- and intra-specific competition for food resources by shelter-inhabiting herbivores can determine how frequently shelter-builders move and thus the quantity of sheltered habitat available for use by other animals. The relatively small size of most shelters and the large amount of damage they incur is compelling evidence that direct competition for limiting food resources is a common occurrence, but this requires confirmation through carefully designed field and/or laboratory experiments. Moreover, because leaf shelters are frequently occupied by more than one arthropod at a time, studies of species interactions occurring within these shelters are likely to provide much-needed insights into how engineering and trophic effects interact to influence the size and diversity of arthropod assemblages. For any particular shelter type, are the resident arthropods random collections of potential colonists, or do certain species tend to co-occur? Are shelter-dwelling predators important sources of mortality for cohabitants? Can residents behaviorally exclude potential colonists? Do arthropods sharing a shelter partition the space to reduce interactions? How frequently do shelters provide enemy-free space (Jeffries and Lawton 1984) and against what types of predators? To address many of these questions, "windows" into the dynamics of within-shelter interactions are needed and may require some creativity. For leaf ties, we have had some success with using artificial leaves of clear acetate that can be clipped to leaves to provide one surface to visualize interactions occurring with the ties.

The engineering impact of multiple shelter types constructed on the same plant is another area that warrants investigation. Many plants host a variety of different shelter types as well as other types of plant modifiers (e.g., concealed feeders and internal feeders: miners, borers, and gallers). All of these structures have the potential to increase habitat heterogeneity (Lawton 1983), but it is not known whether their engineering effects are additive or nonadditive. In addition, the relationships between shelter density and community responses are totally unknown (e.g., are they linear, saturating, or unimodal, and over what scales?; Marquis and Lill 2006). We argue that the ease of manipulation of these constructs relative to other types of ecosystem engineers holds great promise for addressing both system-specific questions and more general questions posed by theoreticians and those seeking to integrate ecosystem engineering more fully into ecological and evolutionary studies (Wright and Jones 2006).

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