Stress frequency structures communities

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.


Yellowtail surgeonfish (Prionurus laticlavius) stand out as voracious herbivores that can feed even in the most wave-swept coastlines of the Galapagos Islands. Credit: Dr. Alejandro Perez-Matus.

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.


Robert Lamb bolts experimental cages to the rock as Eucidaris urchins stand guard at the sheltered side of Caamaño. Credit: Salome Buglass.

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.


Effect of wave action (exposed – dark bar, sheltered – light bar) on abundance of some of the important members of the subtidal community off of the 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.


An Ulva 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.


Percent of Ulva biomass remaining after five days of the Ulva sandwich experiment. Error bars are 1 SE.

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.


Effect of herbivores on macroalgal community assembly, as measured by amount of algae colonizing the polycarbonate plates after two weeks.

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.

Indirect effects of the lionfish invasion

I’m old enough to remember when ecological studies of invasive species were uncommon.  Early on, there was a debate within the ecological community whether they should be called “invasive” (which conveyed to some people an aggressive image akin to a military invasion) or more dispassionately “exotic” or “introduced.” Lionfish (Pterois volitans), however, fit this more aggressive moniker. Native to the south Pacific and Indian Oceans, lionfish were first sighted in south Florida in 1985, and became established along the east Atlantic coast and Caribbean Islands by the early 2000s. They are active and voracious predators, consuming over 50 different species of prey in their newly-adopted habitat. Many population ecologists study the direct consumptive effects of invasive species such as lionfish.  In some cases they find that an invasive species may deplete its prey population to very low levels, and even drive it to extinction.


A lionfish swims in a reef. Credit: Tye Kindinger

But things are not always that simple. Tye Kindinger realized that lionfish (or any predator that feeds on more than one species) could influence prey populations in several different ways.  For the present study, Kindinger considered two different prey species – the fairy basslet (Gramma loreto) and the blackcap basslet (Gramma melacara). Both species feed primarily on zooplankton, with larger individuals monopolizing prime feeding locations at the front of reef ledges, while smaller individuals are forced to feed at the back of ledges where plankton are less abundant, and predators are more common.  Thus there is intense competition both within and between these two species for food and habitat. Kindinger reasoned that if lionfish depleted one of these competing species more than the other, they could be indirectly benefiting the second species by releasing it from competition.


Fairy basslet (top) and blackcap basslet (bottom). Credit Tye Kindinger.

For her PhD research, Kindinger set up an experiment in which she manipulated both lionfish abundance and the abundance of each basslet species.  She created high density and low density lionfish reefs by capturing most of the lionfish from one reef and transferring them to another (a total of three reefs of each density).  She manipulated basslet density on each reef by removing either fairy or blackcap basslets from an isolated reef ledge within a particular reef.  This experimental design allowed her to separate out the effects of predation by lionfish from the effects of competition between the two basslet species.  Most of her results pertained to juveniles, which were about 2 cm long and favored by the lionfish.


Alex Davis

Alex Davis captures and removes basslets beneath a ledge. Credit Tye Kindinger.

Kindinger measured basslet abundance in grams of basslet biomass per m2 of ledge area.  When lionfish were abundant, juvenile fairy basslet abundance decreased over the eight weeks of the experiment (dashed line) but did not change when lionfish were rare (solid line).  In contrast, juvenile blackcap basslet populations remained steady over the course of the study, whether lionfish were abundant or rare. Kindinger concluded that lionfish were eating more fairy basslets.


Abundance of juvenile fairy basslets (left) and blackcap basslets (right) as measured as change in overall biomass. Triangles represent high lionfish reefs and circles are low lionfish reefs.

Competition is intense between the two basslet species, and can affect feeding position and growth rate.  Kindinger’s manipulations of lionfish density and basslet density demonstrate that fairy basslet foraging and growth depend primarily on the abundance of their blackcap competitors. When competitor blackcap basslets are common (approach a biomass value of 1.0 on the x-axis on the two graphs below), fairy basslets tend to move towards the back of the ledge, and grow more slowly.  This occurs at both high and low lionfish densities.


Change in feeding position (top) and growth rate (bottom) of fairy basslets in relation to competitor (blackcap basslet) abundance (x-axis) and lionfish abundance (triangles = high, circles = low)

In contrast, blackcap basslets had an interactive response to fairy basslet and lionfish abundance. Let’s look first at low lionfish densities (circles in the graphs below).  You can see that blackcap basslets tend to move towards the back of the ledge (poor feeding position) at high competitor (fairy basslet) biomass, and also grow very slowly.  But when lionfish are common (triangles in the graphs below), blackcap basslets retain a favorable feeding position and grow quickly, even at high fairy basslet abundance.


Change in feeding position (top) and growth rate (bottom) of blackcap basslets in relation to competitor (fairy basslet) abundance (x-axis) and lionfish abundance (triangles = high, circles = low)

By preying primarily on fairy basslets, lionfish are changing the dynamics of competition between the two species. The diagram below nicely summarizes the process.  Larger fish of both species forage near the front of the ledge, while smaller fish forage further back.  But there is an even distribution of both species.  Focusing on juveniles, they are relatively evenly distributed in the rear portion of the ledge (Figure B).  When fairy basslets are removed experimentally, the juvenile blackcap basslets move to the front of the rear portion of the ledge, as they are released from competition with fairy basslets (Figure D).  Finally, when lionfish are abundant, fairy basslets are eaten more frequently, and juvenile blackcaps benefit from the lack of competition (Figure F)


Kindinger was very surprised with the results of this study because she knew the lionfish were generalist predators that eat both basslet species, so she expected lionfish to have similar effects on both prey species.  But they didn’t, and she does not know why.  Do lionfish prefer to eat fairy basslets due to increased conspicuousness or higher activity levels, or are blackcap basslets better at escaping lionfish predators? Whatever the mechanism, this study highlights that indirect effects of predation by invasive species can influence prey populations in unexpected ways.

note: the paper that describes this research is from the journal Ecology. The reference is Kindinger, T. L. (2018). Invasive predator tips the balance of symmetrical competition between native coral‐reef fishes. Ecology99(4), 792-800. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.