Invasive engineers alter ecosystems

Ecosystem engineers  change the environment in a way that influences the availability of essential resources to organisms living within that environment.  Beavers are classic ecosystem engineers; they chop down trees and build dams that change water flow and provide habitat for many species, and alter nutrient and food availability within an ecosystem. Ecologists are particularly interested in understanding what happens when an invasive species also happens to be an ecosystem engineer; how are the many interactions between species influenced by the presence of a novel ecosystem engineer?

For her Ph.D research Linsey Haram studied the effects of the invasive red alga Gracilaria vermiculophylla on native estuarine food webs in the Southeast USA. She wanted to know how much biomass this ecosystem engineer contributed to the system, how it decomposed, and what marine invertebrates ate it. She was spending quite a lot of time in Georgia’s knee-deep mud at low tide, and became acquainted with the shorebirds that zipped around her as she worked. She knew that small marine invertebrates are attracted to the seaweed and are abundant on algae-colonized mudflats, and she wondered if the shorebirds were cueing into that. If so, the non-native alga could affect the food web both directly, by providing more food to invertebrate grazers, and indirectly, by providing habitat for marine invertebrates and thus boosting resources for shorebirds.

least-sandpiper-in-grac_signed.jpg

A least sandpiper forages on a red algae-colonized mudflat. Credit: Linsey Haram.

Since the early 2000’s, Gracilaria vermiculophylla has dramatically changed estuaries in southeast USA by creating novel habitat on mudflats that had previously been mostly bare, due to high turbidity and a lack of hard surface for algal attachment.  But this red alga has a symbiotic association with a native tubeworm, Diopatra cuprea, that attaches the seaweed to its tube so it can colonize the mudflats.  This creates a more hospitable environment to many different invertebrates, providing cover from heat, drying out, and predators, while also providing food to invertebrates that graze on the algae.

2015-01-20 14.39.58

Closeup view of the red alga Gracilaria vermiculophylla, an invasive ecosystem engineer.  Credit: Linsey Haram

Haram and her colleagues decided to investigate how algae presence might be influencing bird distribution and behavior.  They realized that this influence might be scale-dependent; on a large spatial scale birds may see the algae from afar and be drawn to an algae-rich mudflat, while on a smaller spatial scale, differences in foraging behavior may lead to differences in how a particular species uses the algal patches in comparison to bare patches.

To explore large scale effects, the researchers counted all shorebirds (as viewed from a boat) on 500 meter transects along six bare mudflats and six algal mudflats.  They also measured algal density (even algal mudflats have large patches without algae), and invertebrate distribution and abundance both on the surface and buried within the sediment. These surveys showed that shorebirds, in general, were much more common on algal mudflats. As you can see, this trend was stronger in some shorebird species than others, and one species (graph f below) showed no significant trend.

HaramFig1

Field surveys of shorebird density (#/ha) on six bare mudflats compared to six mudflats colonized by Gracilaria vermiculophylla. * indicates weak trend (0.05 < P < 0.10), ** indicates a stronger difference (P < 0.05).  Bold horizontal bars are median values. Common names of species are (b) dunlin, (c) small sandpipers, (d) ruddy turnstone, (e) black-bellied plover, (f) semipalmated plover, (g) willet, (h) short-billed dowitcher.

Algal mudflats had a much greater abundance and biomass of invertebrates living on the surface, particularly isopods and snails, which presumably attracted some of these birds.  However, below the surface, there were no significant differences in invertebrate abundance and biomass when comparing mudflats with and without algae.

Having shown that on a large spatial scale shorebirds tend to visit algal mudflats, Haram and her colleagues then turned their attention to bird preferences on a smaller spatial scale. First, they conducted experiments on an intermediate scale, observing bird foraging preferences on 10 X 20 plots with or without algae.  They then turned their attention to an even smaller scale, by observing the foraging behavior on a <1mscale.  On each sampling day, the researchers observed individuals of seven different shorebird species on a mudflat with algal patches, to see whether focal birds spent more time foraging on algal patches or bare mud.  During each 3-minute observation, researchers recorded the number of pecks made into algal patches vs. bare mud, and compared that to the expected peck distribution based on the observed ratio of algal-cover to bare mud (which was a ratio of 27:73).

On the smallest scale, two of the species, Calidras minutilla and Aranaria interpres, showed a very strong preferences for foraging in algae, while a third species, Calidris alpine, showed a weak algal preference. In contrast, Calidris species (several species of difficult-to-distinguish sandpipers) and Charadrius semipalmatus strongly preferred foraging in bare mud, while the remaining two species showed no preference.

HaramFig2

Small-scale foraging preferences  (x–axis) of shorebirds. Solid blue curve is the strength of population preference (in terms of probability – y-axis) for mudflats, while solid red curve is the strength of population preference for algae.  Dashed curves are individual preferences.  Red arrows at 0.27 indicates the proportion of the mudflat that is covered with algae, while the blue arrow at 0.73 represents the proportion of bare mudflat (and hence indicate random foraging decisions).  Filled arrows are significantly different from random, shaded arrows are slightly different from random, while unfilled arrows are random. Common names of species are: (a) dunlin, (b) least sandpiper, (c) small sandpipers, (d) ruddy turnstone, (e) semipalmated plover, (f) willet, (g) short-billed dowitcher.

If you compare the two sets of graphs above, you will note that in some cases shorebird preferences for algae are similar across large and small spatial scales, but for other species, these preferences may vary with spatial scale.  For example, Arenaria interpres was attracted to algal mudflats on a large scale, and once present, these birds foraged exclusively amongst the algae, shunning any mud that lacked algae.  Small sandpipers (Calidris species) also were attracted to algal mudflats on a large scale, but in contrast to Arenaria interpres, these sandpipers foraged exclusively in bare mud, rather than in the algae.

The researchers conclude that different species have different habitat preferences across spatial scales in response to Gracilaria vermiculophylla. Most, but not all, species were more attracted to mudflats that harbored the invasive ecosystem engineer.  But once there, shorebird small-scale preference varied in response to species-specific foraging strategy.  For example, the ruddy turnstone (Arenaria interpres) discussed in the previous paragraph, forages by turning over stones (hence its name) shells and clumps of vegetation, eating any invertebrates it uncovers.  Accordingly, it forages primarily in algal clumps.  In contrast, willets (Tringa semipalmata), short-billed dowitchers (Limnodromus griseus) and dunlins (Calidris alpine) were all attracted strongly to algal mudflats, but showed basically random foraging on a small spatial scale, showing little or no preference for algal clumps.  The researchers explain that these three species use their very long beaks to probe deeply beneath the surface, using tactile cues to grab prey. So unlike the ruddy turnstone and some other species that forage for surface invertebrates, they don’t use the algae as a cue that food is available below.  Thus species identity, and consequent morphology, behavior and foraging niche are all important parts of how a community responds to an invasive ecosystem engineer.

note: the paper that describes this research is from the journal Ecology. The reference is Haram, L. E., Kinney, K. A., Sotka, E. E. and Byers, J. E. (2018), Mixed effects of an introduced ecosystem engineer on the foraging behavior and habitat selection of predators. Ecology, 99: 2751-2762. doi:10.1002/ecy.2495. 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.

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