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

“More generally, we thus ask whether the seeming contradiction between theory and data regarding IGP (i.e. Polis 1991; Diehl 1995; Polis & Strong 1996) may partly be a result of that interaction strengths in empirical systems are generally poorly known quantitatively.”

“Recently, Polis et al . ( Polis 1991; Polis & Hurd 1996; Huxel & McCann 1998; Huxel, McCann & Polis 2002) generated the theoretical framework and generalizations for these phenomena, although the exchanges have been noted in both terrestrial and aquatic communities since the work of Elton (1927).”

“The spread of an alien arthropod can both affect and be affected by native species as resources, competitors, or enemies (Polis 1991; Schmitz 2008).”

“…depicted as a set of nodes, typically species in food webs or habitat patches in spatial networks, connected by links that represent some kind of association among nodes, such as interspecific interactions (May 1974; Polis 1991) or dispersal movements (Hanski and Gilpin 1991; Minor and Urban 2008).”

“Concerns about lumping species into functional groups (Polis, 1991; Polis & Strong, 1996; Solow & Beet, 1998) have merit; however, since a major objective of ALFISH is to provide assessments of the resource base for predators such as wading birds, details of the species structure within a…”

“Additional resources are provided by high-quality plant products, such as fruits and tubers (Emmons 1987, Polis 1991, Doolan and MacDonald 1996, Polis and Strong 1996, Vernes et al.”

“…the web, as information on, e.g., taxonomy or feeding relationships may be more readily available for species high in the food web, such as vertebrates and other top-level consumers, whereas taxa low in the web (e.g., algae, small invertebrates) often are highly aggregated (Polis 1991, Link 2002).”

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“However, most prey face multiple predators (Polis 1991; Sih et al. 1998) and therefore must nevertheless respond to some level of predation risk at all times.”

“This is especially true for small, difficult to study organisms like zooplankton, where trophic lumping of taxonomic groups is common (Polis 1991).”

“Most prey species are predated upon by multiple predators [1], and the interactions between different predator species drive the spatial and temporal distribution of both prey and predators, playing a key role in structuring ecosystems [2–4].”

“However, web studies have been criticized for using data of poor quality, based on insufficient sampling of ecosystems (Bersier et al., 1999; Cohen et al., 1993; Hall and Raffaelli, 1991; Martinez, 1991; Polis, 1991; Polis and Strong, 1996; Winemiller, 1990).”

“All species engage in multiple interactions with one another, which can be represented by ecological networks (Elton 1927, Polis 1991, Bascompte 2009).”

“We determined the trophic connectivity (Polis 1991, Duffy 2002) to be the relative change in the producer population given a change in the herbivore population.”

“A focus on strong interactions, traditionally presumed to be the most important (Power et al. 1996), ignores weak linkages that may be critical to community function (Polis 1991; McCann 2012).”

“F(k) F(k) A 10-6 10-4 10-2 2 3 4 5 6 7 8 9 k Coachella Valley 10-4 10-3 10-2 10-1 2 3 4 5 6 7 8 9 k Caribbean Reef 10-4 10-3 10-2 10-1 2 3 4 5 6 7 8 9 k El Verde Rainforest 10-8 10-6 10-4 10-2 2 3 4 5 6 7 8 9 k N.E. Shelf 10-6 10-4 10-2 2 3 4 5 6 7 8 9 k Little Rock Lake 10-2 10-1 2 3 4 5 6 7 8 9 k F.A.A 10-4 10-3 10-…”

“There are numerous other examples of such ontogenetic role reversals (Aoki, Kuroso, & Usuba, 1984; Eaton, 1979; Girault, 1908; Palomares & Caro, 1999; Polis, 1991; Polis, Myers, & Holt, 1989).”

“…2007, Coll 2009), but most animals consume foods at more basal trophic levels (i.e., nonprey foods, nonanimal foods) and are often best described as true omnivores (Pimm and Lawton 1978, Polis 1991, Havens et al. 1996, Coll and Guershon 2002, Thompson et al. 2007, Hunter 2009, Lundgren 2009).”

“…the idea that populations aggregate into discrete homogeneous trophic levels each of which receives energy solely from its adjacent level nearer the ultimate source of energy (the outside or abiotic environment) and passes it to the next (Vadas, 1990; Winemiller, 1990; Polis, 1991, 1994).”

“The key role of trophic interactions and biomass fluxes in our understanding of the functioning of ecosystems (e.g., Hairston & Hairston 1993, Paine 1988, Polis 1991, Polis & Strong 1996) explains the wide interest food-web studies encounter among ecologists.”

“Paine, 1988; Polis, 1991; Cohen et al., 1993 .”

“Natural food webs are defined by countless reticulate connections between consumers and resources that link energy channels and regulate ecosystem functions (Polis, 1991; Polis & Strong, 1996; Thompson et al., 2012).”

“…result in increased animal abundance and diversity within the community, then the likelihood for more direct and indirect interactions between animals also increases (Adams and Tschinkel 1995); thus, potentially generating more complex trophic structures (Polis 1991, Schneider et al. 2016).”

“Empirical studies over the past few decades have shown, however, that species in most natural food webs cannot be neatly classified into static trophic levels; they usually are reticulately connected to various species at different levels of the food web [4,17,20,22,29,31,41].”

“Polis (1991) described the challenges associated with compilation of a food web for the Coachella Valle, California.”

“…the upper and lower thresholds of many species and precipitation is unpredictable, resulting in fluctuations between scarce or plentiful vegetation cover (Hillel & Tadmor, 1962; Rosenzweig, 1968), which determines the availability of resources to other species further up the food web (Polis, 1991).”

“Primary consumers, such as herbivorous insects, are often attacked by a diverse complex of natural enemies (Polis 1991).”

“Widespread trophic-level omnivory has been cited as one factor causing trophic cascades to dissipate in terrestrial food webs (Polis 1991; Polis & Strong 1996).”

“…and John Wiley & Sons Australia, Ltd hemipterans make up a large part of the total available arthropod biomass after rare desert precipitation events (Polis 1991), and many small wasps feed on C3 flower pollen or are predators on insects that feed on C3 plant production (Scholtz & Holm 1985;…”

“Generalism The dietary breadth of a consumer, often measured as dietary species richness [11, 76].”

“…by some benthic macroinvertebrates; 2. a certain degree of omnivory, including cannibalism (Hunte and Myers 1984; Dick 1995; Wissinger et al. 1996; Polis 1991), and intra-guild predation (Wissinger and McGrady 1993; Williams and Martinez 2000), affecting many species; 3. niche ontogeny (Werner…”

“Empirical studies surpport that omnivory is common in the natural food webs (Polis 1991; Diehl 1993; Winemiller 1996), and the debate on omnivory and food web properties has persisted for more than 40 years.”

“Despite some intriguing and important theoretical results, this approach is hardly applicable to studying real communities with multiple and often unobvious, not merely trophic, interspecific relations (Begon et al., 1986; Polis, 1991).”

“How and when the effects of predation, resource availability and trophic heterogeneity propagate through food webs remains an open question (Hairston, Smith & Slobodkin, 1960; DeAngelis, 1975; Power, 1990; Polis, 1991; Strong, 1992; Brett & Goldman, 1996; Polis & Winemiller, 1996; Fox, 2002).”

“Most terrestrial ecosystems possess numerous species of arthropod generalist predators that share prey species on lower trophic levels and that also prey upon each other (Polis, 1991, 1994).”

“The cannibalism in burbot and diet overlap with other piscivores appears consistent with a size-structured intraguild predation system (IGP), where a predator consumes prey it also competes with (Polis, 1991; Holt & Polis, 1997).”

“Recently, the characterization of predation has been expanded to include the diversity of predators and the diversity of their potential interactions (e.g., Polis, 1991; Sih et al., 1998; Rosenheim, 1998; Schmitz, 2007; Letourneau et al., 2009).”

“…two terrestrial systems: the Coachella Valley Desert located in California, USA (166 370W, 33 540N; area = *740 km2; number of trophic species = 29; Polis 1991) and the Caribbean island of St. Martin in the northern Lesser Antilles (18 040N, 63 030W; number of trophic species = 42; Goldwasser and…”

“Although specialized digestive physiology is required to digest and assimilate both plant and animal tissues, omnivory is relatively common in both terrestrial and aquatic systems, especially among vertebrates (Martinez, 1991; Polis, 1991; Winemiller, 1990).”

“This tradeoff means that if a prey responds to a predator by modifying its foraging rate or habitat preference, then any interactions between the prey species and its resources are also affected.”

“We use the term ‘‘trophic level’’ with caution as there are many problems of interpreting real food webs using the notion of trophic levels (Polis 1991, Polis and Strong 1996).”

“Prior to two important publications in the early 1990s (Cohen, Beaver, Cousins, & DeAngelis, 1993; Polis, 1991), ecologists had no collectively agreed upon methods of data collection.”

“We acknowledge that while large predatory arthropods are the “top predators” in the context of the community of smaller arthropods that serve as their prey, they can be preyed upon by other predatory vertebrates (e.g., Polis, 1991) and thus are not true top predators (sensu Wallach et al., 2015).”

“This study also confirms the scale dependence of all links and link fractions, as reported in previous studies of aquatic (Martinez 1992, 1993; Deb 1995), desert (Polis 1991) and forest communities (Reagan et al. 1996).”

“, 1990), but doubts have also existed in the patterns on account of the insufficient data and variable methodology (Pimm et al., 1991; Polis, 1991).”

“Most empirical food webs that have been analyzed are very simple depictions of what actually are large, complex systems (Polis, 1991).”

“Previous work has documented extensive individual specialization within predator populations, but the consequences for food web diversity are still unclear (Bolnick et al., 2007; Polis, 1991; Polis and Winemiller, 1996; Tinker et al., 2012; Winemiller, 1990; Winemiller and Layman, 2005).”

“The greatest current challenge is to understand the diversity of trophic mechanisms within and among ecosystems (Sih et al. , 1985; Paine, 1992; Polis, 1991; Hairston and Hairston, 1993).”

“Email: marco.ferrante@live.it K E Y W O R D S acacia, biotic interactions, ecological functions, Evrona Nature Reserve, oil pollution, sentinel prey high biodiversity and numbers of trophic links (Durant et al., 2012; Polis, 1991a; Ward, 2016).”