Paper: Heck, K.L., Jr., F.J. Fodrie, S. Madsen, C.J. Baillie, and D.A. Byron. 2015. Seagrass consumption by native and a tropically associated fish species: potential impacts of the tropicalization of the northern Gulf of Mexico. Marine Ecology Progress Series 520: 165-173. Doi: 10.3354/meps11104
Featured image source: Jim Garin
With global temperatures on the rise, many organisms are shifting their range toward the poles. In the Northern Hemisphere, that means many species are striking out from their historical homelands and heading north for the new frontier. In the Northern Gulf of Mexico (GOM), tropical species (fish, manatees, green sea turtles, corals, and black mangroves) are beginning to move into the northern warm-temperate zone. The temperate species can’t shift north in response. They’re blocked by North America! So what will happen as the tropical and temperate species are forced to interact and utilize the same habitat? Researchers wanted to know what a future seagrass habitat might look like under northward expansion. We all know from history that whenever people settled new areas, we changed the habitat. Therefore, they hypothesized that the increasing presence of the emerald parrotfish (Nicholsina usta) could affect the structure and function of the temperate seagrass habitat.
To begin to answer that question, the researchers determined and compared seagrass consumption rates of the emerald parrotfish to those of pinfish (Lagodon rhomboides) and the planehead filefish (Stephanolepis hispidus). The latter two species are natives and two of the northern GOM’s usual suspects of seagrass eaters.
Meet the cast of characters:
First, how much seagrass can one fish contain? To find out, the researchers performed stomach content analysis. They caught these fish (152 pinfish, 142 filefish, and 48 parrotfish), measured and weighed their catches and then dissected the stomachs and intestines. Then they weighed the partially digested gut contents.
How long does it take the food to become poop? (Or in scientific terms, what is the evacuation time for each fish species?) 25 fish of each species were starved, placed in individual tanks, then fed 3 seagrass shoots and 3 seagrass leaves. Parrotfish and filefish received turtlegrass (Thalassia testudinum) while pinfish received shoalgrass (Halodule wrightii). The fish was allowed to feed for 4 hours. The leaves and shoots were photographed before and after the trial and areas determined for later analysis. After the feeding trial, fecal materials were collected at regular intervals and weighed. These weights were used to calculate the time it takes to completely eliminate all the seagrass consumed.
How quickly do the fish consume seagrass? Two daily consumption rates were estimated. Remember those feeding trials and the photographs? The before/after differences in leaf and shoot area were used to calculate one consumption rate by scaling up how much was eaten in 4 hours to a 12 hour feeding day (just multiply by 3!). The other used the amount of material evacuated in a given time period and then also scaled up to a 12 hour feeding day.
Once they determined the consumption rates, they compared across the three species. They also examined previous scientific literature to extrapolate what these consumption rates would mean for the habitat.
What did they find?
1) Based on gut content analysis, parrotfish and pinfish consumed about 10 times the weight of seagrass eaten by filefish (Fig. 1).
2) Parrotfish consume more seagrass than either of the native species (Fig. 2).
3) Parrotfish also clear their guts faster than the pinfish and filefish. Their evacuation rate was 20-80% or 100-60% more rapid than pinfish and filefish, respectively (Table 1).
Filefish consume relatively little seagrass. And even though pinfish stomachs contained similar amounts of seagrass as the parrotfish, their impact on seagrasses is lower. Pinfish eat less and retain food in their gut for longer time periods. This means parrotfish consume far more seagrass than both study companions.
The authors suggest that a sustained increase in emerald parrotfish could be problematic. If parrotfish abundance parallels that of the pinfish, they could consume 16-25% of daily seagrass production. Pinfish by comparison only consume 2-9% of the seagrass’ daily production. Seagrass canopy height and density is thereby expected to decrease with increasing parrotfish abundance. More of the seagrass will be directly consumed rather than turned into detritus; therefore more of the energy will flow up to the grazers. Less detritus would support fewer detritivores, including microbes, worms, and clams, as well as predators that feed on them, like rays. In addition, by reducing seagrass habitat, fewer fish, especially juveniles that utilize seagrasses as a nursery habitat, can be sheltered and fed. That may potentially lead to a decline in the number of adults of those species.
Ultimately, the authors expect continued warming and northward range expansion by tropical herbivores to reduce seagrass cover, increase the movement of energy through the grazers, and diminish the nursery habitat and populations of species that rely on them. Combined, those habitat alterations could drastically alter the food web dynamics. Interestingly enough, such changes would actually reflect a state closer to that of historical seagrass habitats in which large herbivorous sea cows were still numerous.
There are still many other questions about the species interactions that may change and develop as the GOM becomes more tropicalized. Will new predator-prey relationships form between temperate and historically tropical species? How will potential competition for space and food factor in? But, this study moved the state of knowledge forward. This study integrated the results of a consumption experiment with knowledge from previous literature to project what may happen as more seagrass herbivores move north. Such studies may have interesting applications for future habitat management plans, especially for places like the GOM where there’s a huge barrier to range expansion leading to a melting pot of marine fauna.
Note: Figures and Tables used came from the original research paper.