COVID-19 has amplified our experience of stress, but even in a COVID-free world, we share with most other organisms a continuously stressful existence, highlighted by situations affecting our survival (e.g. getting food and not becoming someone else’s food) and our reproductive success. Today we will discuss organisms that live in a very stressful environment – the subtidal zone off of the Galapagos islands – located just below the line demarcating the furthest extent of low tide. One serious stress for subtidal organisms is coping with dramatically fluctuating ocean currents. The speedy surgeonfish uses its powerful pectoral fins and slender, disc-shaped body to minimize drag, permitting feeding in high flow conditions brought about by powerful ocean waves. In contrast, the broad-bodied torpedo-shaped parrotfish is unable to do so; for it, fast water is too much of a drag.
Waters near the Galapagos Islands are enriched by upwelling equatorial currents, which provide nutrients to a diverse community of plankton and benthic (attached to the ocean bottom) algae. These in turn support a high diversity of macroinvertebrates and herbivorous fish that feed on them, including the pencil urchin, Eucidaris galapagensis, a voracious feeder on algae, barnacles and coral. This species wedges itself among rocks and crevices during the day, and emerges to feed at night. It attaches itself (and moves very slowly) using its tube feet. Robert Lamb, Franz Smith and Jon Witman hypothesized that given the weak attachment strength of the pencil urchin’s tube feet, it might only be an effective feeder in locations where wave action was minimal.
To explore how wave action might affect the subtidal community, the researchers set up two research locations at Caamaño and Las Palmas – both off the Galapagos Island of Santa Cruz.
At each location, they chose an exposed site with strong wave action and a sheltered site that had much reduced wave action. Mean flow speed was more than twice as fast at exposed sites than in sheltered sites. As you can see in the figure to your left, site differences in mean flow speed corresponded to differences in the subtidal community. Crustose coralline algae (red algae firmly attached to corals) were more common in sheltered sites (Figure A), while a variety of red and green macroalgae were more common at exposed sites (Figure B). Surgeonfish (Figure C) and parrotfish (Figure D) were much more abundant in exposed areas, while pencil urchins were much more abundant in sheltered sites (Figure E).
Lamb and his colleagues wanted to know why these differences exist. They set up a series of exclosures within each of these sites using wire mesh cages to either allow fish, but not urchins (+ fish treatment), allow urchins but not fish (+ urchins), or exclude both groups of herbivores (- all). They also had a control treatment that allowed all herbivores (+ all).
In one experiment the researchers created sandwiches made up of the delectable green algae Ulva. For five days, they ran six replicates of each treatment at exposed and sheltered sites at Caamaño and Las Palmas. Lamb and his colleagues then harvested the sandwiches, weighed them, and calculated the percent remaining of each sandwich.
At exposed locations, urchins (without fish) consumed very little Ulva, while fish (without urchins) consumed about 2/3 of the Ulva (when compared to the –all controls). In contrast, at sheltered locations, urchins took some mighty significant bites from the Ulva sandwiches, while fish also ate substantial Ulva at Caamaño, but not at Las Palmas.
In a related experiment, the researchers used the same cages to explore how macroalgal communities assemble themselves in the presence or absence of urchin and fish herbiores under different flow rates. If this was not enough to consider, they also ran these experiments both during the cool season, when nutrient-rich ocean currents lead to high production, and during the warm season when production is usually lower. Lamb and his colleagues bolted two 13 X 13 cm polycarbonate plates to the bottom of each cage, and after two months measured the abundance and type of algae that colonized each plate.
Several trends emerge. First, macroalgae colonized much more effectively during the cool season. Second, urchins profoundly reduced macroalgal colonization at sheltered sites, but had little effect at exposed sites. In contrast, fish herbivory reduced macroalgal colonization at exposed sites at Caamaño but not Las Palmas, during the warm and cool season.
In addition, the researchers set up video cameras and were able to document herbivory by 17 fish species, with drastically higher herbivory rates at exposed sites.
Lamb and his colleagues conclude that the dominant herbivores switched between urchins in low flow sites and fish in exposed sites. Fish can leave the resource patch when stress (flow rate) is unusually high, and return when flow rate drops, while the slow-moving pencil urchins do not have that option. The researchers argue that in many ecosystems, consumer mobility in relation to the frequency of environmental stress can predict how consumers influence community structure and assembly. They point out that the coupling of mobility effects with environmental stress is common throughout the natural world. As examples, many shorebirds feed on marine organisms that become available during low tides, or also between crashing waves. Large mammals in Africa can migrate long distances to escape drought-stricken areas, while smaller animals cannot undertake such long journeys. In locally acidic regions of the Mediterranean Sea, many fish species can enter, feed and leave before experiencing toxic effects from the acid water, while slow-moving urchins are excluded from feeding in those habitats. Thus, while extreme environmental stress often decreases consumer activity, there are also times when it doesn’t. In these cases, we need to understand how particular species will behave and perform in the stressful environment to predict how stress influences community structure and functioning.
note: the paper that describes this research is from the journal Ecology. The reference is Lamb, R. W., Smith, F., and Witman, J. D.. 2020. Consumer mobility predicts impacts of herbivory across an environmental stress gradient. Ecology 101( 1):e02910. 10.1002/ecy.2910. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2020 by the Ecological Society of America. All rights reserved.