Tracking predators in seagrass beds

Journal source: Reynolds et al. 2018. Latitude, temperature, and habitat complexity predict predation pressure in eelgrass beds across the Northern Hemisphere. Ecology 99:29-35. 10.1002/ecy.2064/suppinfo

Delving into organism interactions

For decades, ecologists have been fascinated with identifying global patterns in species abundances and organism interactions. Understanding patterns in species interactions, such as predator-prey dynamics, are important, as they can have far-reaching effects on biodiversity and ecosystem function. For instance, predators can control the numbers of herbivorous prey that would otherwise over-exploit primary resources such as plants and algae. The importance of predators can be seen in kelp forests, highly productive ecosystems that support highly diverse and abundance fish and invertebrates. Sea urchins, which love to eat kelp, are kept in check by sea otters that have an appetite for sea urchins; when otters are near, sea urchins hide away, only eating scraps of kelp, which allows the kelp forests to maintain their high productivity.

(Featured image) An up-close view of seagrass beds, and a resident juvenile ray enjoying the protection provided by these habitats. Photo via Wikimedia Commons.

A striking pattern that scientists have recognized for a long time is that the influence of predators appears to decrease as we move away from the tropics and into higher latitudes. One hypothesis suggests that temperature is the major driver of predators; temperature directly influences organism metabolisms, body size, and ecological interactions. With warmer temperatures at lower latitudes compared to more northern environments, predators are more abundant. Sure, but why do we care about such questions, and what is the connection to marine ecosystems?

Why do we care about organism interactions in marine ecosystems?

Let’s envision ourselves on the coast, venturing out into dense eelgrass beds, similar to the kelp forests. Eelgrass is a key foundation species in estuaries and coastal areas across the northern hemisphere. As a foundation species, eelgrass provides many important functions and services in coastal ecosystems, forming productive habitats for abundant and diverse fish and invertebrate species. In these ecosystems, small invertebrate grazers, such as snails and amphipods (affectionately called scuds or side-swimmers), are abundant residents that play a key role as trophic links because they are a major prey item of marine fish. In addition, invertebrate grazers are beneficial to eelgrass because they feed on other algae that could, in the absence of these herbivores, outcompete the eelgrass. Imagine if a predatory fish with a voracious appetite for scuds were to invade eelgrass beds: the loss of the scuds could translate into major changes for seagrass productivity and ecological functioning.

Map showing distribution of eelgrass beds in the northern hemisphere. Photo via Wikimedia Commons.

Given the above scenario, and the occurrence of eelgrass beds throughout a wide latitudinal range of the Northern Hemisphere, these ecosystems provide an ideal study to test how predation intensity is different from lower to higher latitudes. The authors in the present study investigated predation pressure on scuds and snails using field surveys in 48 eelgrass communities in the Northern Hemisphere.

The study

The researchers measured the intensity of predation on snails and scuds by tethering one individual of each species within several experimental plots at each of the 48 eelgrass sites. After 24 hours, the researchers recorded whether the snails or scuds were still tethered in their plot, and if the scud or snail was absent, then the researchers attributed the absence to predation.

An amphipod (scud), a common invertebrate grazer in coastal seagrass beds. Photo via Wikimedia Commons.

In combination with the predation surveys, the researchers also surveyed the characteristics of the seagrass community. To investigate the influence of seagrass density on predation, the researchers measured seagrass shoot density and biomass within each plot where the scuds and snails were tethered. The researchers used minnow traps to survey the potential predators of scuds and snails.

The major findings

As predicted, the authors found that predation intensity on baited scuds declined with increasing latitude-so that seagrass beds located at higher latitudes had lower rate of attack on scuds. In addition, the authors observed lower predation rates in seagrass beds with higher stem shoot density-likely due to scuds finding good hiding spots from predators.

Temperature is a major driver of predator influence on seagrass beds

The authors found that annual mean water temperature was an important driver of predation rates on amphipods. However, coastal water temperature gradients of northern continents can be drastically different at similar latitudes, and the rate of change in temperature can be more dramatic on western compared to eastern coasts. In this study, the authors attributed steeper temperature gradients on the west coast of North America compared to the east coast to differences in ocean currents. On the west coast, boundary currents move warm tropical water offshore as they flow poleward and deliver it to the higher latitudes of eastern margins. Thus, patterns in predation may be more sensitive to temperature on western coasts, and this could carry important implications as climate change is likely to alter oceanic currents.

Global patterns of surface water currents. Photo via Wikimedia Commons.

What ever happened to those snails? The authors noted that while scuds were preyed upon more heavily at lower latitudes, snails were hardly touched at each of the 48 study plots. Sure, snails are slow, and may seem an easy meal for many hungry fish, but snails have hardened shells that protect them against predators. Thus, not all prey are created equal, and it is important to consider differences in prey that could alter expected patterns in predation.

The bigger picture

Marine seagrass ecosystems reflect a balance among temperature, predator abundance, and herbivores. In this study, predators appeared to be more effective in controlling amphipod populations at lower latitudes, but local habitat characteristics, such as seagrass density, became important in providing scuds with a prey refuge.

Predator-prey relationships are critical components to how marine ecosystems function. Many of these seagrass beds are highly productive, providing food resources, habitats, and small niches to accommodate a wide diversity of fish and invertebrates. These ecosystems also are a huge sink for carbon dioxide- they absorb CO2 for photosynthesis-making them an important component of the global carbon cycle. Predator-prey interactions, which are largely driven by water temperatures, are a major component of seagrass and kelp forest productivity. It will be important for future studies to examine predator-prey interactions and seagrass productivity under the influence of climate change. We have yet to see how warming water temperatures at higher latitudes will influence predator-prey relationships and seagrass productivity.



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