Diversity of both grazers and habitats is key for healthy ecosystems

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Me working on the vertical rock wall that was the stage for this experiment. Behind me is a rock bench covered with living organisms and a retreating Pacific Ocean.

In order to maintain healthy ecosystems, we need to consider how environments change in relation to the organisms living in those environments. My colleagues and I recently published a paper showing how the varieties of both habitats and animals interact to speed the recovery of seaweeds on a rocky shore. Habitat and animal diversity were important on their own, but having a range of habitats was essential to promoting recovery of seaweeds when an important grazing animal species was removed from the community. Thus, a mix of habitats for organisms to utilize may provide a buffer against the loss of species. Maybe variety really is the spice of life.

Understanding the causes and consequences of biodiversity is a major motivation for ecologists, and these causes and effects may be related in important ways. The aspects of an environment that allow diverse communities to develop may also help explain how biodiversity influences essential processes in ecosystems, such as the ability of communities to recover after being disturbed. In our study, recovery meant that seaweeds grew back quickly after we removed them from small areas.

The stage for our study was a vertical rock wall high up in the intertidal zone at Bodega Marine Reserve. This location features a wide variety of life in very small areas, and much of this life is slow-moving or does not move at all. These aspects, along with steep environmental gradients where land becomes sea, have made rocky shores ideal systems for conducting experiments in the rough and tumble of nature for many decades.

Our cast of characters included stalwart barnacles, several varieties of snails (periwinkles and limpets), and a mélange of green and red seaweeds. These creatures interact with one another in a number of ways: seaweeds and barnacles compete for space on rocks, snails eat seaweeds, barnacles protect small seaweeds from being eaten by snails (they can’t reach between the barnacles), limpets can bulldoze young barnacles from rocks, and tiny periwinkles live inside dead barnacle shells. Given all of these interactions, it can be difficult to predict what will happen when we change something in the system, but this is exactly what excites me about ecology.

Here’s how we designed our experiment: we manipulated the cover of barnacles and the number of species of snails after removing seaweeds from small areas on the shore, and we tracked the recovery of seaweeds over the course of one year. We first set up areas in which we 1) left barnacles completely intact, 2) removed all barnacles, or 3) removed barnacles from only one half of the area. This last “half barnacle” treatment we considered to be more diverse because it contained two distinct habitat types. For every habitat type we then manipulated the number of snail species that were present: an intact snail community with periwinkles and two types of limpets, and three communities each with only one type of these snails (we removed the other snail types).

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The figure (above) summarizes the results for the seaweeds that grow slowly and tend to stay on the shore for long periods of time, so-called “perennial” seaweeds. The panel on the left shows the final percent cover of perennial seaweed in each barnacle and herbivore treatment, while the panel on the right shows cover of perennial seaweeds on each side of the areas in the half barnacle treatment. When the ribbed limpet was present, seaweeds recovered fastest in areas completely covered with barnacles likely because barnacles provided predation refuge from the ribbed limpet, which is the largest of the snails and a habitat generalist. However, when the ribbed limpet was removed (the rough limpet and periwinkle treatments) seaweeds recovered fastest in areas in which both barnacles and bare rock habitats were present. This happened because of the characteristics of the other snails that were present. The rough limpet tends to avoid barnacle areas (its shell actually grows to fit the shape of the rock surface!) so seaweeds were able to recover on the side with barnacles where it did not graze (see photograph). Tiny periwinkles, on the other hand, hang out near barnacles, but seaweeds recover faster there, too, because the barnacle-free side became covered with weedy seaweeds that choke out the perennials.

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One of the plots from our experiment. The top half has no barnacles and features several visible rough limpets, but the bottom half in full of barnacles and tiny periwinkles. Recovery of perennial seaweed was faster on the side of plots with barnacles.

The results of our experiment were complex and not easy to predict ahead of time based on our natural history knowledge, even though we worked in a relatively small and simple ecosystem. For me, this is much like changing your look when you only have a few articles of clothing at your disposal. A typical suit can look very different if you add a cowboy hat or a bolo tie. What if you threw some spandex into the mix? Chaos?

[NOTE: Originally published in The Aggie Brickyard in 2016]

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