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

“Some of the questions addressed by systems ecology include information transfer (Margalef 1968), the structure of food webs (Polis 1991), the hierarchical changes in ecosystem controls at different temporal and spatial scales (O’Neill et al. 1986), and the resilience of ecosystem properties after…”

“…Bridge Brook Lake: pelagic species from the largest of a set of 50 New York Adirondack lake food webs (Havens 1992); (15) Coachella Valley: a wide range of highly aggregated taxa in a southern California desert (Polis 1991); and (16) Skipwith Pond: invertebrates in an English pond (Warren 1989).”

“The similarities that exist across systems in the structure of trophic networks (Pimm 1982; Cohen 1989; Polls 1991) represent commonalities that emerge because all ecosystems are complex adaptive systems, governed by similar thermodynamic principles and local selection.”

“, predicting that food webs should not amass the large numbers of interacting species that are in fact found in natur”

“Movements of nutrients and matter through food webs and ecosystems are often difficult to quantify, being transient, variable and difficult to observe ( Polis 1991 ).”

“For example, recently compiled data tend to more accurately represent the levels of cannibalism, omnivory and intraguild predation seen in natural systems than were available in earlier datasets (e.g. Martinez 1991; Polis 1991).”

“However, food web theory based on species graphs bears little empirical significance (Paine 1980; Polis 1991): ecological interaction networks should be quantified based on what individuals do, rather than just who eats whom among species .”

“We can make a simple division of the trophic nature of the species for each community into three groups: top predators, intermediate species and basal species (Polis, 1991).”

“Although food web studies recognize and quantify important attributes such as omnivory, cannibalism, and reciprocal predation (Sprules and Bowerman 1988; Polis 1991), designation of “trophic linkages” is a subjective process that fails to consider the energetic importance of the represented trophic connections (Paine…”

“Much of my consideration of consumer-prey interactions presupposes that trophic levels are sufficiently real to be useful constructs, which is not universally accepted (Polis 1991, Polis and Strong 1996, Persson 1999).”

“Those interested in predicting species population dynamics have suggested that the structure of unweighted links alone tells little about the outcome of a species perturbation (Levine 1976; Holt 1977; Paine 1980; Vandermeer 1980; Abrams 1987; Paine 1988; Polis 1991), while others claim that generalities in unweighted…”

“To this end, correlation coefficients were first normalized by applying Fisher’s z transform, z ¼ 0.5 ln [(1 + r)/(1 ) r)] (Zar 1999), and then weighted by multiplying them by the inverse of the sampling variance, w ¼ 1/var(r) ¼ N ) 3 (Rosenthal 1991; Zar 1999; Gurevitch et al. 2001).”

“Omnivory appears to be common in food webs (Polis 1991; Sprules and Bowerman 1988).”

“…type Qualitative changes in species interactions over ontogeny are ubiquitous in natural communities and well-established in most taxa, including a diversity of plants, invertebrates and vertebrates (Wilbur 1980, 1988; Werner & Gilliam 1984; Polis 1991; Boege & Marquis 2005; Rudolf 2008b).”

“…raised questions regarding the validity of data on ecological networks, emphasizing problems of sampling effort also in systems other than plant–pollinator systems (e.g. Paine 1988; Polis 1991; Cohen et al. 1993; Martinez et al. 1999; Banasek-Richter et al. 2004; but see Nielsen & Bascompte 2007).”

“…documenting the feeding relationships in communities, did not uphold the initially found regularities (Warren 1989, Winemiller 1990, Martinez 1991, Polis 1991, Deb 1995, Carney et al. 1997), and new hypotheses about the structure of communities have been proposed (Martinez 1992, Polis and Strong…”

“Complex trophic interactions characterize many food webs (e.g., Polis 1991, Polis and Strong 1996), and indirect behavioral effects on habitat use, like those described for dolphins, may be common.”

“Earlier, food-web ecology experienced similar shortcomings regarding temporal resolution, and the methodology of this research field received serious critique (e.g., Polis 1991).”

“Topological descriptions of food webs focus on connection patterns without considering either the magnitude (i.e. interaction strength), or the population dynamics of the interacting species; an aspect that has generated doubts regarding their validity (e.g. Paine 1988; Polis 1991, 1994).”

“One criticism of the concept of trophic levels and simplified food-web diagrams is that omnivory blurs the distinction between trophic levels, affects the vertical flow of energy and materials, and dampens top-down control (Polis 1991).”

“The best predicted web was ‘‘Coachella Valley’’ (31), a food web of a desert ecosystem that includes plants, herbivores, and predators; here, 149 of 229 links were correctly predicted.”

“5%) is consistent with values reported previously by Reynolds et al. (1985). Can dilution by gut material account for the difference in total N content between predators and herbivores (average 1.”

“…strengths [1,2], and thus any motivation for measuring interaction strengths, such as understanding the stability of populations and communities [3–6], the flows of energy in ecosystems [7] and the functional consequences of biodiversity loss [8], applies equally to estimating feeding rates.”

“…prey density (Walsh & Riley 1868; Pimentel 1961; Root 1973; Snyder et al. 2004), in the last 15 years there has been an explosion of interest in the complexities that arise from interactions within diverse predator–prey assemblages (Polis 1991; Polis & Holt 1992; Rosenheim 1998; Sih et al. 1998).”

“Polis and others (e.g. Polis et al. 1989; Polis 1991, 1994; Polis & Strong 1996; Winemiller 1996) have argued cogently that the early food webs are simply not detailed enough—they lack taxonomic resolution.”

“Similar points about the community- or ecosystem-level consequences of speciesspecific ecology have been made for terrestrial (Polis 1991, Polis and Strong 1996), benthic marine (Paine 1969, 1980, 1992), and pelagic marine systems (Lehman 1988).”

“Although these links can be strong, with AGP exerting “keystone” effects that dictate community structure (Paine 1966), in many other cases AGP have numerous, relatively weak but pervasive interconnections with other community members (Polis 1991).”

“Shortweighted trophic level estimates, like estimates based on flow-weighted link information, produce lower trophic levels than those reported by studies using the more common ‘chain-averaged trophic level’ algorithm (Martinez 1991, Polis 1991, Fussman & Heber 2002).”

“…for our focus on arthropods also stems from the fact that these taxa constitute the most abundant and diverse group of terrestrial animals (Southwood 1978; Erwin 1982; Wilson 1987; Samways 1994) and are significant components of most terrestrial food webs (Polis 1991; Schoenly et al. 1991).”

“Intraguild predation might limit the ability of generalist predators to control herbivore populations (Sih et al. 1985, Polis 1991, Strong 1992, Rosenheim et al. 1993, Polis and Strong 1996).”

“…to trophic levels provides direct evidence that omnivory is prevalent in these systems, supporting recent microcosm, observational, and modelling studies showing that omnivory is prevalent and can stabilize food webs (Polis 1991; Lawler and Morin 1993; Holyoak and Sachdev 1998; McCann et al. 1998).”

“More recently, ecologists have addressed interactions of greater complexity in three-species community modules (e.g., Polis, 1991; Polis and Holt, 1992; Coll and Guershon, 2002; Finke and Denno, 2004).”

“However the accumulation of plant� pollinator links observed over the entire three year period is significantly greater than what was observed in a single summer, indicating significant annual variation in the identity of plant� pollinator links (Fig. 1). Given that the cumulative rarefaction curve did not show clear…”

“The importance of trophic omnivory for the structure and dynamics of food webs is a long-standing controversy in ecology (Cousins 1987, Burns 1989, Polis 1991).”

“For example, many predatory species grow during their juvenile period through size ranges that are similar to their future prey (Werner 1988; Wilbur 1988; Polis 1991).”

“Results by Sprules and Bowerman (1988), Hall and Rafaelli (1991), and Polis (1991) indicate that omnivory is actually quite common in natural Dean Billheimer is Assistant Professor of Biostatistics at Vanderbilt University, Nashville, TN 37205 (E-mail: dean.billheimer@mcmail.vanderbilt.edu).”

“However, we still lack a good understanding of how prey respond to combined predators, even though experiencing combined predators is the more natural condition (Schoener 1989, Polis 1991, 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).”

“This seems to suggest that predation could be a wider spread feeding strategy in soils than previously assumed, also complicated by the coexistence of cannibalism and intra-guild predation (Polis, 1991).”

“However, there are numerous examples of omnivory in natural ecosystems (Pimm and Lawton 1978; Polis and Holt 1992; Diehl 1993, 1995), and the current view is that omnivory is widespread (Winemiller 1990; Polis 1991; Polis and Strong 1996; Persson et al. 1996).”

“…approach, these studies highlight 546 the potential variability in food webs, such that higher-order consumers do not occupy a single TP. 547 Indeed, the capacity for intraspecific variation in TP has long been known based on theory and 548 empirical work (Polis, 1991; Polis and Strong, 1996).”

“However, to know all food web links in an ecosystem is likely impossible; indeed, identification of a large fraction of the links in a food web is a formidable task (Polis 1991).”

“and omnivory is common (8), as in Figure 5 c.”

“Past comparative food web studies have used connectance webs an approach that has been the subject of criticism due to the poor quality of the original data (Paine 1988, Polis 1991, Polis and Strong 1996).”

“It has also been argued that the prevalence of omnivory increases with food web size (Polis, 1991).”

“…of observational effort may be needed to go beyond the limitations imposed by poorly sampled webs that misrepresent the trophic structure of ecological systems (May 1983, Paine 1988, Lawton 1989, Winemiller 1990, Martinez 1991, Polis 1991, Cohen et al. 1993, Goldwasser and Roughgarden 1993, 1997).”