Ecology might not be so complicated after all

Submitted by editor on 13 February 2015.

Ecosystem-shaping interactions between consumers and plants are notoriously variable. Indeed, even within a single system (and a single pair of interacting species) enormous variability can be seen -- the same consumer might increase plant biomass at one place and time, but obliterate plant biomass at another. But what mediates variation within particular consumer-plant interactions? Properties of the consumer population, particularly body size and density, likely determine whether individuals can access certain resources (e.g., break into tough leaves) and how much they engage in competitive or facilitative intraspecific interactions. But there’s possibly a simpler explanation for why consumer size and density matter: if we take into account the allometric scaling of metabolic demand, body size and density determine the amount of energy consumers collectively need to function. So far, very few experiments have explored how consumer body size and density affect consumer-plant interaction strength, or parsed out the influences of these factors and that of metabolic demand.


In the early view paper, “Consumer-plant interaction strength: importance of body size, density and metabolic biomass” we addressed this gap in knowledge by manipulating the body size and density of a salt marsh consumer, Littoraria irrorata. These snails, known to feed on both living and dead plant material, are capable of exerting strong top down control over smooth cordgrass, Spartina alterniflora --  which dominates southeastern US marshes. In conjunction with environmental stressors (e.g., drought), high snail densities (i.e., “snail fronts”) can generate expansive cordgrass die-off areas. Nevertheless, little is known about how body size factors in to this critical consumer-plant relationship.

Left: Salt marsh periwinkle (Littoraria irrorata),Right: Littoraria grazing on cordgrass at low tide.  

We used cages, installed in a salt marsh on Sapelo Island, Georgia (USA), to manipulate the body size (small and large) and population density (low to very high) of Littoraria over 4 months. We quantified consumer effects in two ways: by measuring the length of snail grazing scars (radulations) on cordgrass leaves and by harvesting the aboveground plant biomass remaining in the cages at the end of the experiment.

Cage used for experimental manipulations.

Our results revealed that snail size became a relatively more important driver of consumer-plant interaction strength as snail density increased – a trend that reflects the stronger per capita effects of large snails. Although these initial analyses suggested that consumer body size and density both play an important role in regulating consumer-plant interactions, the total metabolic demand of each consumer population (estimated using the proxy of ‘metabolic biomass’, which integrates both snail size and density; i.e., the sum of individuals’ mass.75), could explain much of the variation in snail effects on cordgrass across our experimental plots. In other words, despite differences in their size structure, metabolically equivalent snail populations had similar effects on plant biomass. Interestingly, as snail metabolic demand increased, the effect of consumer populations on plants shifted from facilitation (cordgrass biomass was enhanced relative to control plots with no snails) to suppression.

These results suggest that consumer size and density can be integrated into a single value, metabolic biomass, to more parsimoniously explain consumer effects and provide valuable insight into how energetic demand drives species interactions. This study also suggests that anthropogenic activities, such as hunting and fishing, that target and remove large consumers, may be having a  more potent effect on ecosystems than would be expected by their impacts on consumer population density, as large consumers have disproportionately strong effects on consumer metabolic demand. We now look forward to further exploration of consumer-plant interactions using a metabolic approach in this - and other - systems.   

Rebecca Atkins, John Griffin,  Christine Angelini, Mary O’Connor and Brian Silliman

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