What Do Anoles Get Out of Seaweed Subsidies?

Adult male Anolis sagrei looking out over a pile of seaweed

Aloha, my name is Amber Wright and I’m a first-time poster here on Anole Annals. I did my dissertation on niche variation between native and introduced populations of brown anoles, with field sites in Hawaii, Florida, Little Cayman, and the Bahamas. I will be starting up a new lab at the University of Hawaii, Manoa in January 2014, so look forward to future posts on Anolis vs. Phelsuma, and get in touch if you’re interested in joining the lab!

As covered in previous posts on Anole Annals (e.g. 1, 2), our team has been studying the effects of seaweed subsidies on near-shore food webs in the Bahamas where Anolis sagrei is a key predator. While studies published to date have detailed the effects of seaweed on direct and indirect interactions among lizards, insects, and plants, our most recent paper focuses on how lizards are able to capitalize on seaweed-derived resources.

To briefly summarize the most relevant previously reported lizard results (Spiller et al. 2010), when we added seaweed to experimental plots we found that lizards switched from foraging on terrestrial prey to consuming seaweed detritivores, and that lizard density increased by about 60%. We saw an initial increase in density within the first three months, suggesting that lizards quickly moved into plots to take advantage of the seaweed. However, peak lizard abundance was observed a full year after the initial subsidy, which suggested that a lagged reproductive response could also be contributing to the overall increase in lizards.

We analyzed mark-recapture data from close to 500 individuals over the 20-month experiment to try and figure out how lizards could be turning resource input into reproductive output. We found that subsidized lizards did not survive better or have better body condition than unsubsidized lizards, but they did grow 30% faster.

A 30% faster growth rate may not seem like much of an advantage, but achieving reproductive size sooner could be a big deal in light of some key aspects of anole life history. While A. sagrei can reproduce over much of the year, there is a period of reproductive quiescence during the winter. Having a breeding season coupled with the fact that anoles can reproduce continuously (about an egg a week for A. sagrei) means that when you reach maturity during the breeding season constrains how many eggs you can produce.

We fit a model of individual growth to the mark-recapture data to quantify this effect, and proposed the following scenario shown in Figure 3 from the paper below. Lizards hatching very early in the season reach reproductive size before or near the start of their first breeding opportunity regardless of whether seaweed is present; the difference therefore lays in the lizards that hatch late. Late-hatching lizards without access to seaweed do not reach reproductive size in time to lay any eggs and must survive until the next breeding season to reproduce. Subsidized lizards that hatch late are able to catch up a bit, hitting reproductive size in time to take advantage of at least half of their first breeding season. Averaging egg production over all possible hatch-dates in a year, these growth differences translate into subsidized females laying an average of 16 eggs vs unsubsidized females laying an average of 8 eggs in year one. That’s a doubling in fecundity due to seaweed addition.

Model-estimated growth and reproductive phenology for females born during the study period. This scenario assumes a size at hatching of snout–vent length = 16.5 mm, a one-month incubation time, and that the peak egg-laying period is from May to September (shaded boxes). The dotted line indicates minimum reproductive size (snout–vent length = 34 mm). Growth trajectories follow lizards hatching early (June) and late (October) in the breeding season. Symbols (triangles and circles) mark when lizards reach minimum reproductive size depending on hatch date and whether seaweed was added (solid lines) or removed (dashed lines).

So life history characteristics and the timing of subsidy are critical in determining the effects of the added resources. For example, if anoles laid a single clutch a year, as long as you got to reproductive size at any point during the breeding season you would be able to lay your full complement of eggs. In that case there wouldn’t be this strong correlation between how long you’re big enough to breed and how many eggs you can lay. Similarly, if seaweed washed ashore during a different time of year then it would not coincide with juvenile development, and therefore would be unlikely to shorten time to maturity. Shifts in phenology, the timing of life cycle events, due to climate change have already been documented in several taxa, and climate is an important driver of resource pulse dynamics in many systems. Thus climate change has the potential to alter these kinds of resource-consumer interactions with implications for the entire food web. The study of seaweed subsidies also provides another example of how A. sagrei is able to respond rapidly to environmental variation at the individual level, and what the resulting consequences are for populations and communities.

We are continuing to study the effects of seaweed subsidies on terrestrial food webs, and are currently running a five-year experiment–hurricanes willing–manipulating 32 whole small islands and 20 plots on large islands. This time we are varying the presence/absence of lizards and the frequency and magnitude of seaweed deposition. On our upcoming September field trip we’re going to be collecting some behavioral data on lizards to see what the immediate short-term effect of seaweed addition is on time budgets. Is all the action really in juvenile growth, or do adults benefit indirectly from things like spending less time foraging when such abundant and easy prey is available? Stay tuned!

One of our +seaweed +lizards small islands from an ongoing experiment in Abaco, Bahamas. This picture is a little deceptive in that the island does extend outside of the frame for a few meters. But it does show all of the vegetated area (i.e. lizard habitat) on the island, so it’s still pretty small!

Amber N. Wright, Jonah Piovia-Scott, David A. Spiller, Gaku Takimoto, Louie H. Yang, Thomas W. Schoener. Pulses of marine subsidies amplify reproductive potential of lizards by increasing individual growth rate. Oikos Early View DOI: 10.1111/j.1600-0706.2013.00379.x

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Amber Wright

I am hapa-filipina and was born and raised in Honolulu, Hawaii. I grew up catching introduced anoles, which through a lot of luck on top of hard work became a career. I am a professor at the University of Hawaii at Manoa.