Fishes are not only important sources of food for people, they are also key “engineers” in marine ecosystems, potentially reducing establishment of invasive species and maintaining desirable habitats like reefs and seagrass beds in the face of overgrowth by algae. Although the ecological importance of such “top-down control” by predators is widely appreciated, it has not been measured systematically across a range of sites in marine waters. The Bitemap project aims to do so.
The structure and function of ecosystems is largely dictated by the types and productivity of plants that populate them. These characteristics are in turn set by the supply of resources and the intensity of consumption by animals, commonly called bottom-up and top-down forcing, respectively. Bottom-up control is relatively well understood, with primary production following straightforward theoretical and empirical relationships with temperature and the availability of light, water, and mineral nutrients. Vegetation can be mapped relatively easily, and as a result, remote sensing has yielded a picture of the distribution and biomass of primary producers at a level of detail unimaginable a few decades ago.
The geography of top-down control is much less well understood. Characterizing this variation in consumer pressure is important because evidence indicates that top-down control by predators tends to have stronger impacts and penetrates farther through the food web than does bottom-up control by resources. Yet predators are declining around the world, with often profound but poorly documented implications for ecosystem structure and function.
Many important questions in both basic and applied ecology could be addressed with high-resolution spatial and temporal data on consumer activity. Examples include:
- How does consumer pressure vary across the globe with latitude, temperature, resource supply, and along stress gradients?
- To what extent are bottom-up and top-down control correlated with one another?
- How accurately can top-down control be predicted by the biomass, body size distribution, and/or diversity of consumers?
- How do management practices such as protected areas affect consumer pressure on native and introduced species?
- What effect does extinction or decline of predators have on ecosystems?
Mapping top-down control and answering such questions would be advanced by standardized methods that can be compared rigorously across ecosystem types and taxa. One promising approach to this goal involves exposure of standardized prey in different environments or conditions and comparing the rate of loss to predators. This is our approach in Bitemap. The assay and sampling method were designed to be simple, economical, and scientifically rigorous to ensure that it is doable to a wide range of users, including students and citizen scientists.
Bitemap uses a simple, standardized assay, the squidpop, for measuring the relative feeding intensity of generalist predators in aquatic systems. The assay consists of a 1.3-cm diameter disk of dried squid mantle tethered to a rod, which is either inserted in the sediment in soft-bottom habitats or secured to existing structure. Each replicate squidpop is scored as present or absent after 1 and 24 hours, and the data for analysis are proportions of replicate units consumed at each time. Tests in several habitats confirmed the assay’s utility for measuring variation in predation intensity among habitats, among seasons, and along environmental gradients. Quantitative visual surveys confirmed that assayed feeding intensity increased with abundance and species richness of fishes across sites,. Predation intensity varied seasonally, being highest during summer and declining in late autumn. Deployments in marsh habitats generally revealed a decline in mean predation intensity from fully marine to tidal freshwater sites..
(1) Obtain quantitative, standardized estimates of predator feeding intensity;
(2) Obtain quantitative, standardized estimates of fish abundance, size and species composition
In order to adequately test predation pressure and associated fish community composition, abundance, and biomass, it is necessary to conduct the following two protocols (squidpops, fish seine) in tandem at no fewer than three time points per habitat sampled. This will ensure variation associated with predation assays and sampling events is accounted for. Fish seines should be conducted immediately after collection of squidpops from 24 hour deployment no further than 100 meters from squidpops in the same habitat type the squidpops were deployed in (seagrass, sand). In order to capture a snapshot of the community and community processes it is important to complete all three squidpop deployments and fish seines in a period of no longer than three weeks. This will provide confidence in replication of observations in terms of having generally the same predator/fish communities present at each deployment.
These protocols are meant to be accomplished as pairs in one seagrass and one sandy/soft bottom habitat for a total of six squidpop/seines over the course of the experiment (seagrass × 3, sand × 3). The habitats should be geographically paired at a distance of no more than 500 meters apart and be sampled as close together in time as possible.