The Ontogenetically Variable Trophic Niche of a Praying
Mantid Revealed by Stable Isotope Analysis
LAWRENCE E. HURD,1,2 PIETER A. P. DEHART,3 JOSEPH M. TAYLOR,1
MEREDITH C. CAMPBELL,1 AND MEGAN M. SHEARER1
Environ. Entomol. 1–7 (2015); DOI: 10.1093/ee/nvv004
ABSTRACT Praying mantids have been shown to exert strong influences on arthropod community composition. However, they may not occupy the same trophic level throughout their lives. Trophic shifting over a life cycle could explain the documented variation in results from field studies, but specific interactions of predators within food webs have been difficult to determine simply by comparing control and treatment assemblages in field experiments. We examined the trophic position of the Chinese praying mantid, Tenodera aridifolia sinensis (Saussure), using stable isotope analysis (SIA). We measured the d13C and d15N of field-collected arthropods, and of laboratory groups of mantids fed known diets of these arthropods chosen from the most abundant trophic guilds: herbivores (sap feeders and plant chewers), and carnivores. We also collected mantids from the field over a growing season and compared their SIA values to those of the laboratory groups. Both d13C and d15N of mantids fed carnivorous prey (spiders or other mantids) were higher than those fed herbivores (grasshoppers). SIA values from fieldcollected mantids were highly variable, and indicated that they did not take prey from trophic guilds in proportion to their abundances, i.e., were not frequency-dependent predators. Further, d15N decreased from a high at egg hatch to a low at the third instar as early nymphs fed mainly on lower trophic levels, and increased steadily thereafter as they shifted to feeding on higher levels. We suggest that the community impact of generalist predators can be strongly influenced by ontogenetic shifts in diet.
KEY WORDS praying mantid, predator ontogeny, stable isotope, Tenodera aridifolia sinensis, trophic guild
Among arthropods, apex predators such as spiders and praying mantids historically have been of interest to experimental ecologists because of their influence on food webs (Riechert and Bishop 1990, Fagan and Hurd 1994). The specific interactions of predators within food webs often are complex and difficult to trace, exhibiting a combination of direct reductions in prey populations and indirect enhancements of other species.
These interactions are manifested via predatormediated facilitation through a reduction in competition or release from other predators among species occupying lower trophic levels (Sih et al. 1985, Diehl 1993). Such effects are common because most predators are generalists, feeding on many different prey species on multiple trophic levels, leading to potentially simultaneous interactions such as intraguild predation and competition (Polis et al. 1989). In addition, ontogenetic shifts in diet can result from changes in foraging tactics throughout a predator’s life that can complicate or obscure its effects on the rest of the community (Newman et al. 2012). As a result of this complexity, many questions about the trophic niche of apex arthropod predators remain, including how their diets may change over a life cycle.
Studies on arthropod dietary habits in nature often have been difficult to perform given the challenge in directly observing their elusive and rapid feeding habits (Martinez et al. 1999). Stable isotope analysis (SIA) offers a more direct measure of predator diet than simple control–treatment differences in abundance from field experiments (Post 2002, Wise et al. 2006). Organisms contain in their tissues a unique ratio of heavy to light isotopes for carbon (d13C) and nitrogen (d15N) acquired from their food sources (Hobson 1999, Bennett and Hobson 2009), which are detectable through SIA.
As carbon and nitrogen isotopes are incorporated into predator tissues, they tend to undergo fractionation, or a predictable stepwise trophic enrichment from prey to consumer. Thus, SIA can yield insight into how an organism’s diet changes over time, and many studies have shown that it holds great promise for a more detailed and direct measurement of the trophic niche of predators (DeNiro and Epstein 1978, Ostrom et al. 1997,
McNabb et al. 2001, Bearhop et al. 2004, deHart 2006,
Hood-Nowotny and Knols 2007, deHart and Strand 2012). Carbon and nitrogen isotopes are enriched at roughly 1 and 3.4% per trophic level, respectively (DeNiro and Epstein 1981). While there are some caveats associated with using these generalized 1 Biology Department, Washington and Lee University, Lexington,
VA 24450. 2 Corresponding author, e-mail: email@example.com. 3 Department of Biology, Virginia Military Institute, Lexington, VA 24450.
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Environmental Entomology Advance Access published February 26, 2015 fractionation values for all systems (Karasov and del
Rio 2007), similar values have been deemed appropriate for arthropod assemblages (Oelbermann and Scheu 2002, Gratton and Forbes 2006, Bennett and Hobson 2009, Hyodo et al. 2011).
Praying mantids are extreme generalists in terms of breadth of diet, consuming mainly other arthropods, but also the occasional small reptile, bird, or mammal (Prete et al. 1999). They also are famous cannibals, particularly during their mating season, but the actual frequency and cause of sexual cannibalism have been subject to various interpretations (Gould 1984, Hurd et al. 1994, Maxwell et al. 2010). Mantids generally have been assumed to be frequency-dependent opportunistic predators that would feed mainly on the most abundant prey available, including each other, throughout their lives. They are at least bitrophic, preying on herbivores and carnivores (Hurd and Eisenberg 1990a), but may in fact be tritrophic because in laboratory studies, feeding on pollen enhanced survival, growth, and egg production (Beckman and Hurd 2003). Field experiments have demonstrated that mantid predation has substantial effects on arthropod assemblages, and can instigate top-down trophic cascades to the primary producer level (Moran et al. 1996,