Does density dependence matter in foraging?Submitted by editor on 3 March 2017.Get the paper!
When explaining animal distributions in ecosystems, ecologists have long debated the relative importance of food availability on the one hand, and density-dependence on the other. A high availability of food typically attracts and concentrates animals. Foragers may also experience negative effects of nearby competitors, forcing them to leave to a location with less food, but also less competition. This is a manifestation of density-dependence: the suitability of a foraging location not only depends on the amount of food, but also on the number of competitors.
Oystercatchers have become a model system for studying such competitive distributions. In winter, they forage in large groups on intertidal mudflats, concentrating on shellfish beds, where shellfish are highly aggregated.
High tide roost at Balgzand, 8 Feb 2015, the Netherlands (Bruno Ens)
They frequently have conflicts over food items. Opening bivalves like Cockles can be difficult, and birds may even risk damaging their strong bill in the process of opening them. It may therefore be tempting to try to steal an already opened bivalve from a neighbour. Density-dependent effects, also called conspecific interference, are therefore strong in this species.
In our study area, we studied Oystercatchers in the presence of two prey items which strongly differ in the likelihood of causing interference among foraging Oystercatchers: Cockles (Cerastoderma edule) and American Razor Clams (Ensis directus). Cockles contain much flesh and opening them takes considerable effort and time, so competitors may attempt to steal the flesh content if they get the chance. Cockles are therefore considered an interference-sensitive prey. Razor Clams, on the other hand, contain little flesh and are very easy to open and consume, and are therefore considered an interference-insensitive prey.
Mistnets on the Balgzand study area at the site where Oystercatchers were caught at night time.
If you were to observe all the foraging shorebirds with a bird’s eye view, you would see an intricate pattern of foraging animals continuously in motion: foragers are continuously adjusting their position in response to the changing tides, the food, and their neighbouring competitors. Traditionally it has been very hard to study foraging distributions on mudflats (because of difficult access and the large areas that shorebirds cover within a tidal cycle). But this bird’s eye view has becoming a reality through novel tracking technologies: in this study, we equipped a cohort of birds with GPS tags, that monitor how individuals are moving in space and time continuously (see movie 1).
Oystercatcher being tagged with an Uva-BiTS GPS tag.
Nature helped us repeat an experiment on this complex food landscape twice a day: food resources are being covered and exposed by the tides, and we may observe how animals respond to this dynamic landscape. This provides an opportunity to disentangle different mechanisms underlying the distributional patterns we observe, like the ‘attractive effects’ of food and ‘repulsive effects’ of density-dependence.
We found that in our study year, Oystercatchers avoided the large Cockles on which they could achieve a high-energy intake, and foraged preferably on Razor Clams instead, a much less profitable prey. Using a density-dependent analysis, we could show birds prefer Razor Clams not for their low sensitivity to interference, but to avoid a risk of bill damage when foraging on Cockles. Most of the variation in Oystercatcher’s habitat use was thus explained by density-independent mechanisms, and we found only limited evidence for variation in density-dependence within our study area, at least as far as space use was concerned.
Foraging Oystercatchers in a gully at the Balgzand study site, 19 Jan 2011 (Adriaan Dokter)
From a practical perspective this result may be good news for many modellers of animal distributions: density-dependent effects are usually ignored in statistical models, and it has been an open question how valid this approximation is.
From a fundamental ecological perspective, our results put density-dependence in a different perspective. There is no doubt that density-dependence can affect foragers, for example in the case of Oystercatchers it has been shown that winter survival can be density-dependent. The subtle but important point however is that the presence of density-dependence not immediately implies that it necessarily affects landscape-scale distributions. At least in our study year, we found that density-dependent interactions only had a limited effect on spatial distribution, even for a highly interference-prone species like Oystercatchers.
Oystercatcher GPS positions (red dots) of 10 individuals in the period 2011-10-15 to 2011-11-15 split out in panels by tidal height (at the time and position of each GPS fix) during falling tide. Blue indicates areas where the bathymetric height is below the indi- cated tidal height at each panel. Birds associate with the tide line and have a preference for the northerly deeper tidal zone, rich in Ensis directus prey.
Installing data downloading stations for the UvA-BiTS GPS trackers at high vantage points: both on the mudflats (left, next to observation hide) and at a high-rise building on the shore (right) (Adriaan Dokter)
Benthos sampling in teams of two people: one person taking the samples and navigating the mudflats by GPS, a second person storing the collected bivalves in plastic bags for later analysis in the lab. 28 Oct 2011 (Jelle Treep and Kees Oosterbeek, photo Adriaan Dokter)
The authors through Adriaan Dokter