Linking individuals with ecosystems: Experimentally identifying the relevant organizational scale for predicting trophic abundances

Abstract

Ecosystems are complex owing to the fact that emergent properties like trophic structure and productivity depend on details related to lower-scale interactions among individuals. A key challenge is identifying how much individual-level detail is needed to predict patterns at the ecosystem level. We tested for the effect of individual herbivore body size on trophic interactions and consequent abundances of plant and herbivore trophic levels in a New England meadow ecosystem. Body size is an important determinant of vulnerability to predation and thus should influence the way individuals tradeoff time spent foraging against time spent avoiding contact with predators. Such tradeoffs can then influence the degree of damage herbivores inflict on their plant resources. We experimentally assigned field-caught grasshoppers to three distinct body size treatment groups (small, normal, and large) and crossed them with two spider predator treatments (spider present and absent) in a fully replicated design. We observed size-dependent differences in grasshopper survival and development. Moreover, predators caused grasshoppers to inflict greater damage to herbs and lesser damage to grasses relative to treatments without predators. However, there were no size-dependent differences in net damage level on grasses and herbs in either predator or no predator treatments owing to size-dependent compensation in grasshopper foraging effort. We thus conclude that in this ecosystem the foraging-predation risk tradeoff displayed by typical or average-sized herbivore is a sufficient amount of individual-level detail needed to explain ecosystem patterns.size-dependent predation risk ͉ herbivore-mediated trophic effects ͉ old-field ecosystem ͉ grasshoppers ͉ trait-mediated indirect effects E cosystems are paradigmatically complex. They contain many different components that interact directly and indirectly in integrated networks (1, 2). In such complex networks, higher scale system properties like trophic structure, nutrient fluxes, and productivity emerge from lower scale interactions and selection among components (1, 2). Furthermore, feedback loops in which higher scale properties modify lower-scale interactions cause new emergent properties to arise over time (1, 2). A central problem is identifying which lower scale processes should be included in theory aiming to predict higher-scale properties of ecosystems.Classical ecology (e.g., refs. 3-6) has approached this problem by assuming that it is sufficient to abstract lower scale details, such as interactions among individuals in populations, and characterize ecosystem function simply in terms of net changes in numbers or densities of individuals at the level of whole populations. Abstracting such individual-scale detail is reasonable if the effects of individual-level interactions attenuate on the time scale of changes in population density. However, the assumption that individual-scale detail can be safely abstracted is increasingly being called into question (7). Populations are effec...

Toward a predictive theory of risk effects: hypotheses for prey attributes and compensatory mortality

“…As with grasshoppers preyed upon by spiders, vertebrate prey face different risks from predators with different tactics, and their antipredator responses vary accordingly (e.g., antelope responding to stalkers vs. coursers, or solitary vs. pack hunters; Fitzgibbon and Fanshawe 1989, Scheel 1993, Creel and Creel 2002, Caro 2005, Stankowich and Coss 2006, 2007. Because the strength and type of antipredator responses ultimately determine the importance of risk effects, these studies support the argument that consideration of hunting mode will help to frame theory about the strength of risk (Elgar 1989, Hunter andSkinner 1998), age and sex (Childress andLung 2003, Winnie and, body condition (Ovadia andSchmitz 2002, Heithaus et al 2007), position within the group (Hamilton 1971, Keys and Dugatkin 1990, Hunter and Skinner 1998, Stankowich 2003, habitat type (Lima 1987, Scheel 1993, time of day (Elgar 1989, Scheel 1993, and local environmental conditions (Elgar 1989, Lima andDill 1990). These studies clearly demonstrate that animals adjust their level of vigilance in response to their own condition, the size and composition of the group that they occupy, and in response to the local environment.…”

Individual Size Variation and Population Stability in a Seasonal Environment: A Discrete‐Time Model and Its Calibration Using Grasshoppers

“…We calibrated the model based on field observations and experiments performed in an old-field system, in which the generalist grasshopper Melanoplus femurrubrum is a dominant consumer (e.g., Schmitz et al 1997;Schmitz and Suttle 2001). Using data on density-and size-dependent survival and growth (Beckerman 2000(Beckerman , 2002Ovadia and Schmitz 2002;appendix) we quantified the effects of size variation on M. femurrubrum population dynamics. We found that in the case of deterministic dynamics (i.e., a fixed season length), size variation tends to destabilize population dynamics, that is, causes slower return to equilibrium density after a perturbation ( fig.…”

“…Body size is a fundamental physiological trait that structures populations and influences all facets of an individual's function and performance, such as foraging (e.g., Belovsky 1997), growth rate (e.g., Pfister and Stevens 2002), and survival (e.g., Ovadia and Schmitz 2002). Invariably, besides variation linked to ontogenetic stage, phenotypic variation in body size exists within any natural population (e.g., Uchmanski 1985).…”

“…Such variation has important consequences for grasshopper population dynamics (Joern and Gaines 1990). Ovadia and Schmitz (2002) found that the survival rates of the grasshopper Melanoplus femurrubrum change as a function of body size. In addition, theoretical work at the community level showed that size variation markedly influences the strengths of trophic interactions as mediated through mean herbivore survival (Ovadia et al 2007).…”

“…A major issue in our study is the effect of size variation on the stability properties of population dynamics. We calibrate our analytical model with results of field observations and experiments on the generalist grasshopper M. femurrubrum inhabiting old fields in Connecticut (e.g., Schmitz et al 1997;Schmitz and Suttle 2001;Ovadia and Schmitz 2002).…”