Complex Trophic Interactions in Deserts: An Empirical Critique of Food-Web Theory

“Food webs have become a theoretical field based on an oversimplified and often inaccurate understanding of trophic interactions within communities (Goldwasser & Roughgarden 1993, 1997; Polis 1991).”

“birds included as basal species) (Hall and Raffaelli, 1993; Ings et al., 2009; Paine, 1988; Polis, 1991).”

“Also, the results presented here may be relevant to studies addressing whether the pattern of subweb structure may affect the likelihood of trophic cascades (Polis 1991, Strong 1992, Berlow 1999, Pace et al. 1999, Yodzis 2000, Shurin et al. 2002).”

“Structural differences among food webs like heterogeneity within trophic levels, omnivory, non-equilibrium dynamics and individual behaviour are all likely to be important for determining the magnitude of cascades ( Polis 1991; Power 1992; Strong 1992; Polis & Strong 1996; Peacor & Werner 2001).”

“Figure 5 shows a removal/extinction plot obtained by removing species in the Coachella Valley food web (Polis 1991) in a random order.”

“This type of models has been reported in the past two decades [10, 20,21,23,24].”

“…structure in which omnivory is widespread and higher-order predators (predators that consume other predators) may be dynamically significant (Polis 1991, 1994, Wise 1993, Moran et al. 1996, Polis and Strong 1996, Schoener and Spiller 1996, Fagan 1997, Janssen et al. 1998, Letourneau and…”

“Intraguild interactions between predators have resulted in diverse eVects on prey population dynamics and plant productivity (Polis et al., 1989; Polis, 1991; Prasad and Snyder, 2004; Rosenheim et al., 1995; Rosenheim, 1998; Snyder et al., 2004a,b).”

“It is important to understand the complexities of diet selection in omnivores because they are integral species in most natural communities (Polis, Myers & Holt 1989; Polis 1991; Polis & Strong 1996; Polis & Winemiller 1996).”

“From a view that emphasized functionally discrete trophic levels, as most forcefully enunciated by Hairston et al. (1960; see also Hairston and Hairston 1993, 1997), we have now moved to a new understanding of animal communities as often being replete with omnivory, defined as feeding at a variety of trophic levels (H…”

“Coachella Valley (N ¼ 30, E ¼ 241) is formed by a wide range of highly aggregated taxa from the Coachella Valley desert in southern California (Polis, 1991).”

“For example, classic food webs are depicted with species as nodes and feeding links determined with mean trophic data from species or solely from adults (Cohen 1978; Polis 1991).”

“However, ecosystem structure can rarely be reduced to single food chains (Polis and Strong 1996), and predators, prey and resources often cross habitat and ecosystem boundaries (Polis 1991).”

“Scavengers eat dead meat but also invertebrates within the carrion (Polis 1991).”

“Following accepted protocol, original food-webs were modified such that any species or functional groups with identical predators and prey were grouped as a ‘‘trophic species’’ or trophospecies to reduce methodological bias in the data (Cohen et al., 1990; Pimm Table 1 Topological properties of 17 empirical food-webs…”

“Analyses of links and connectedness between species in natural ecosystems have shown that idealized food chains often do not adequately capture the complexity found in nature (Pimm 1982; Polis 1991).”

“Underground food webs are often characterized by intraguild predation, sometimes supporting five to 10 trophic levels of similarly sized, relatively immobile organisms (Polis 1991; De Ruiter et al. 1995).”

“Trophic and taxonomic similarity is computed for five foodwebs, Chesapeake Bay (Baird and Ulanowicz, 1989), Coachella Valley (Polis, 1991), Skipwith Pond (Warren, 1989), St-Martin Island (Goldwasser and Roughgarden, 1993), and Ythan estuary (Hall and Raffaelli, 1991).”

“These findings suggested that observed food web structure should be simple and affected by external factors at small spatiotemporal scales, while observed food web size, chain length, and number of interactions observed should be large and less affected by external processes at large spatiotemporal scales (e.g., Polis…”

“Published food webs vary considerably in quality and detail, but nearly all exclude consumers that are not readily detectable, such as endoparasites and other infectious agents (Polis 1991; Cohen et al. 1993).”

“Such controversies over the concept of trophic levels and their role in food webs will only be resolved by experimental tests of the various hypotheses[3,26,31,32].”

“A common criticism of early food-web data was the failure to include this type of feeding (Polis, 1991), although this was perhaps least true of freshwater food webs (Thompson et al.”

“Key words: bioaccumulation; Clear Lake, California, USA; diet analysis; fish; food webs; foraging habitat; mercury; stable isotopes; Sulphur Bank Mercury Mine; trophic transfer.”

“78% in [28]), so it is again tempting to list omnivory as another attribute which has been underestimated in the older web data, and in fact it has been found to be sensitive to sampling effort [31].”

“Moreover, this technique for establishing trophic linkages can be used to examine relationships at other levels of the soil food chain to help resolve the tangle often associated with complex food webs (Polis 1991).”

“Most organisms Polis 1991; Murtaugh & Kollath 1997).”

“Like many other studies on diet (Werner and Gilliam 1984; Polis 1991; Polis and Strong 1996), our results cast considerable doubt on the validity of the assumption of clas- sic food-chain models that species recognize single, discrete trophic levels in nature (Hairston et al. 1960; Oksanen et al.…”

“…density-mediated indirect interactions; Eurycea cirrigera; foraging behavior; predator-dependent functional response; Gyrinophilus porphyriticus; ontogenetic niche shift; population dynamics; Pseudotriton ruber; size/stage-structured interaction; trait-mediated interactions; twoprey system.”

“…cascades, alter the stability of food webs, and profoundly influence the movement of energy and nutrients through ecosystems (Polis et al. 1989, Polis 1991, Polis and Strong 1996, Fagan 1997, Holt and Polis 1997, Ostrum et al. 1997, Holyoak and Sachdev 1998, McCann et al. 1998, Rosenheim 1998,…”

“…primary predator (i.e. consuming herbivores) and as a secondary predator (i.e. consuming primary predators) (Menge & Sutherland, 1976, 1987; Polis & McCormick, 1987; Warren & Lawton, 1987; Mooreet al., 1988; Poliset al., 1989; Polis, 1991; Polis & Holt, 1992; Strong, 1992; Hall & Raffaelli, 1993).”

“…Bay 33 (33) 36 (72) 13.65 0.79 0.083 1.683 0.092 Baird and Ulanowicz (1989) Coachella Valley 30 (30) 101 (290) 52.44 0.6 0.142* 2.875 0.004 Polis (1991) El Verde 146 (156) 491 (1510) 22.75** ,0.001 0.208** 4.319 ,0.001 Waide and Reagen (1996) Little Rock Lake 170 (181) 724 (2375) 24.51**…”

“They have also provided information on the real composition of the soil fauna in wet tropical forests (Lave `lle et al. 1981) and on soil food web complexity in deserts ecosystems ( Polis 1991 ).”

“display a community cascade, sensu Polis (1999), in which the indirect effects of top predators are manifested among both resource species that increase (Fig.”

“These findings support theories and results from other reviews of top-down processes (e.g., Sih 1985; Strong 1992; Polis 1991; Hunter and Price 1992; Shurin et al. 2002; Shurin et al. 2006; except see Schmitz et al. 2000).”

“…(Polis and Strong 1996, Lundvall et al. 1999), movement and spatial dynamics (Kareiva 1987, Walde 1991, Ellner et al. 2001), intraguild predation (Polis 1991, Rosenheim et al. 1995), indirect effects (Strauss 1991, Wootton 1994), and emergent impacts of multiple predators (Sih et al. 1998)…”

“A similar system with intraguild predation but without the trade-o between predation and competition was analyzed by Polis et al. (1989). Model (1) comprises three di erent food web topologies.”

“In particular the latter prediction was not found to be consistent with empirical data as more detailed food webs were compiled (e.g. Martinez 1991; Polis 1991).”

“…omnivorous feeding habits are increasingly being recognized, including species that feed on both plants and arthropod prey (Coll and Guershon 2002) and species that prey on both herbivorous and predatory arthropods (‘‘intraguild predation’’; Polis et al. 1989, Polis 1991, Rosenheim et al. 1995).”

“Terrestrial food webs often are rich in species of arthropod generalist predators, which together comprise a guild of predators that share non-predacious food species and also prey on each other ( Polis 1991, 1994 ).”

“These and other critiques suggest that natural food webs are too reticulate to be described by linear food chains (Winemiller 1990; Polis 1991; Polis and Holt 1992; Polis 1994; Polis and Strong 1996; McCann et al. 1998b; Polis 1999).”

“Omnivory, feeding on more than one trophic level, is reported to be common in food webs (Winemiller 1990; Hall and Raffaelli 1991; Polis 1991; Havens 1993; Holyoak and Sachdev 1998).”

“Although this pattern is possible under the scenarios outlined by Abrams (1993), we believe that these findings support the arguments of Polis (1991) and Polis and Strong (1996).”

“…from the fourth trophic level, suppressing populations of intermediate predators and thereby potentially releasing herbivore populations from ‘‘top down’’ control (Hurd and Eisenberg 1990, Polis 1991, 1999, Wise 1993, Polis and Strong 1996, Janssen et al. 1998, Rosenheim 1998, Halaj and Wise 2001).”

“Spatial subsidies - Gary Polis Gary Polis also grounded his research in the empirical study of trophic relationships and called for more direct, detailed, assessments of real-world webs (Polis, 1991).”

“Given that most prey face more than one predator species (Schoener 1989; Polis 1991), understanding when and where such non-additive multi-predator effects (MPE) arise is a pre-requisite for understanding community dynamics.”

“Polis (1991), for example, found that 76% of the species in the Coachella Valley food web showed evidence of omnivory, while Sprules and Bowerman (1988) showed the prevalence of omnivory in pelagic aquatic ecosystems.”

“1987, Polis 1991, 1994, Diehl 1992, 1993) has led Polis and Strong (1996) to question the validity of the trophic level concept and the generality of trophic cascades.”

“These characteristics appear to be common in aquatic ecosystems (Sprules and Bowerman 1988) as well as in habitats such as deserts (Polis 1991) and probably in other webs if high-resolution data are obtained.”

“nton [17] 15 Stink Lake [4] 45 Dempster [17] 16 Little Rock Lake [4] 47 Healy [17] 17 Sierra Lakes [5] 50 Stony [17] 18 Skipwith Pond [6] 51 Grand Caricaie Cl C1 [18] 19 Tuesday Lake [7] 52 Coachella [19] 21 Lough Hyne [8] 53 EcoWeb 59 [20] 22 Mondego Zostera Meadows [9] 54 EcoWeb 60 [20] 23 Caribbean Reef, small [10]…”

“…for predator–predator interference (Polis and Strong 1996) or predator– predator facilitation (Losey and Denno 1998), and multiple prey species (Polis 1991, Sih et al. 1998), providing opportunities for prey switching by predators (Siddon and Witman 2004) and apparent competition among prey…”

“But there is a dearth of such food webs in the literature (Cohen, et al., 1993, Polis, 1991), and even the best available webs intentionally omit mutualisms and other non-feeding interactions (Cohen, et al., 1993).”