In the Canary Islands, a good seed disperser is hard to find

It may seem like yet another set of picture-perfect tropical islands: high cliffs and modest hills rolling into the brilliant blue Atlantic Ocean. Scrubby bushes and small rocks dominate the entire landscape, and everything acquires a lovely tinge of warm sunshine. These vistas draw millions of people to the Canary Islands every year.

But not everyone is here for the scenery, however lovely. Ecologists Néstor Pérez-Méndez from Río Negro National University in Argentina and Alfredo Valido from the Doñana Biological Station (EBD-CSIC) in Spain are more interested in the islands’ scrubby plants — and the plants’ surprising seed dispersers.

“Gallotia lizards consume fruits and efficiently disperse seeds of approximately 50 plant species” of the archipelago’s 80 native species, says Pérez-Méndez.

Just 100 kilometers (60 miles) off the northwestern coast of Africa, the Canary Islands, an archipelago of seven islands and some small islets, are a strange biological fusion of European and African species and even remnants of life before the two continents formed. With no fruit-eating native mammals and very few bird species, endemic lacertids, or wall-climbing lizards, in the genus Gallotia have become the primary seed dispersers on the island. But there is a potential caveat.

The lizards have to be large enough.

Those fruit-eating giants

Having arrived here about 17 million to 20 million years ago, this unusual lizard group is one of the oldest inhabitants of the Canary Islands. Eight species evolved on the seven different islands. Some of them were quite small, but five became giants.

Today four of these five giants remain, named after their resident islands and ranging in length from 13 to 23 centimeters (5 to 9 inches). Three of them — the El Hierro giant lizard (Gallotia simonyi machadoi), the Tenerife speckled lizard (Gallotia intermedia), and the La Gomera giant lizard (Gallotia bravoana) — are listed as critically endangered on the IUCN Red List. They are imperilled by feral cats and rats brought over by humans, as well as tourism and development that destroys the wild dry scrub that constitutes their habitat. These days, the species only survive on a few inaccessible cliffs and rocks.

The Gran Canaria giant lizard (Gallotia stehlini) is the least threatened of the four.

“Maybe because they are more aggressive, and they can defend [themselves] against feral cats, rats, and so on,” Valido says.

The remaining seed dispersers — the Atlantic lizard (Gallotia atlantica) of Fuerteventura and Lanzarote islands, the West Canaries lizard (Gallotia galloti) of Tenerife and La Palma, and the Boettger’s lizard (Gallotia caesaris gomerae) of La Gomera and El Hierro — are abundant but considerably smaller in size.

Researchers call this phenomenon ecological downsizing, when large animals disappear and small animals must fill their shoes — if they can.

Indeed, the Canary Islands’ largest ever lacertid, the Tenerife giant lizard (Gallotia goliath), is already extinct. Fossil records show it was found on Tenerife and La Gomera, and measured up to 48 centimeters (19 inches) in length excluding the tail, although some historical accounts suggest it may have been even larger.

The giant lizards of the Canaries are mostly herbivorous; smaller species eat a lot of insects in addition to plants, but the big guys tend to focus on plants. In fact, Valido says that based on paleo-diet studies, the extinct Tenerife giant lizard was literally and figuratively the biggest fruit lover of them all. After surviving for millions of years, the species was decimated when the first humans arrived on the islands around 2,500 years ago. Plants may have suffered after its loss.

Does size matter?

In 2015, when Pérez-Méndez was a graduate student co-advised by Valido, he set out to understand how downsizing might affect seed dispersal in the Canaries. He tested how three different-sized lizards — the small Boettger’s lizard from La Gomera, the medium-sized West Canaries lizard from Tenerife, and the giant Gran Canaria lizard — dispersed the seeds of Neochamelea pulverulenta, an endemic shrub that depends entirely on lizard dispersers.

The smallest lizards barely dispersed the seeds, because they weren’t large enough to swallow the fruit. All they could do was eat part of the pulp and potentially move seeds a few meters away.

Unexpectedly, the researchers found that the medium-sized Tenerife lizard dispersed more seeds than the bigger Gran Canaria lizard. But they believe this is due to the fact that the Tenerife lizard is more abundant today than the Gran Canaria species.

In terms of quality, however, the Gran Canaria giant lizard bested its smaller rivals. The researchers found that the Neochamelea seedlings tended to be thicker and of better quality on Gran Canaria Island than on Tenerife.

The giants were also dispersing seeds as far as 94 meters (308 feet) away, twice the distance of the medium-sized lizards and 20 times the distance of the small La Gomera lizards.

Such distances matter because seeds that grow close to the parent plants are much more likely to reproduce with genetically similar individuals, essentially their siblings or even parents. Seeds that are dispersed far away, however, have a better chance of being cross-pollinated by unrelated individuals, thus improving genetic diversity.

The incredible shrinking seed disperser

As their research progressed, Pérez-Méndez became curious about another aspect: he suspected that downsizing could be happening even within the same species.

Fossil records indicate that the Gran Canaria giant has gone from 37 to 23 centimeters (14.5 to 9 inches), while the El Hierro giant has shrunk from 43 to 20 centimeters (17 to 8 inches). Would this shrinkage impact seed dispersal?

In his latest study, Pérez-Méndez compared the germination of seeds of two plant species, Rubia fruticosa and Withania aristata, both commonly dispersed by the medium-sized Tenerife lizard. He fed fruits of both plants to eight Tenerife lizards: four large ones, ranging from 12.6 to 13.7 centimeters (4.9 to 5.3 inches), and four small ones, 8.2 to 9.4 centimeters (3 to 4 inches). He then planted the seeds they defecated.

The size difference of the dispersers didn’t affect Rubia fruticosa, but it did Withania aristata. Seeds that passed through the guts of the bigger lizards had a better chance of germinating, and did so faster.

The results show the importance of size even within a species, according to Sandra Bibiana Correa, an assistant professor at Mississippi State University in the U.S. Correa studies seed dispersal by another surprising disperser: freshwater fish in the Pantanal and the Amazon.

“Most studies look at species level. But all animals start small and then grow,” she says, adding that it matters how much an animal grows before it is removed from an ecosystem.

Downgrading seed dispersal

Shrinking dispersers may result in an ecological mismatch, according to Pérez-Méndez. There may be plenty of small lizards, but without the big lizards, the larger fruits remain uneaten. Only smaller seeds might survive.

“Remember that the seed is the energy storage to give the kick to the plant to get established,” Correa says. A large seed is likely to have a better chance of survival in soil with poor nutrients because it has more stored energy than a smaller seed.

Pérez-Méndez calls this the “cryptic loss of functionality” for an ecosystem.

“You can watch the species and think that everything is fine. However, if you look carefully you can appreciate that [the] ecosystem is not functioning as it should,” he says.

Correa cautions that this study was done with a small sample size, only eight lizards, but “it is one more level of information.”

Pérez-Méndez says he believes the results indicate that conservationists must go beyond just trying to save the Canarian lizards — they must focus on saving all the ways in which species interact with their environment.

How this could be done is less clear, as even decades-long reintroduction programs are not seeing much success for the El Hierro and La Gomera giant lizards.

“There are many feral cats in these islands and they practically kill [all] of them,” Valido says.

Even the bold and aggressive Gran Canaria giant lizards, which have weathered hunting, encroachment and killer cats, now face a formidable new threat: California kingsnakes, bred on the island for the exotic pet trade, have escaped and spread. Valido says the Canarian government has tried to eradicate the snakes, but with little success so far.

If the Canary Island lizards go extinct, like the Tenerife giant lizard before them, would the plants they disperse soon follow?

“One of the main characteristics of islands is that they show low species richness,” says Pérez-Méndez. That means there are few species that could potentially take the place of any of the lost giants. If plants lose a disperser, they probably don’t have another species to fall back on.

While birds also disperse plants like Rubia and Withania, they may not be as effective dispersers as the big lizards. Birds usually defecate seeds beneath trees and shrubs, while the sun-seeking lizards prefer open habitats with rocks, which are better habitats for plant colonization, Valido says.

Others plant species, like Neochamelea, which are dispersed exclusively by lizards, could survive by germinating close to the parent tree, but would lose out on genetic diversity.

Correa points to a final wrinkle: native island plants often compete with exotic, invasive plants. If native plants lose dispersers and face new competitors, “the potential for extinction is greater,” Correa says.

The extinction of that goliath of Canarian giants, the Tenerife giant lizard, may have been the beginning of the seed disperser downsizing, but it may not be the end.

Citations:
Pérez-Méndez, N., Rodríguez, A., & Nogales, M. (2018). Intra-specific downsizing of frugivores affects seed germination of fleshy-fruited plant species. Acta Oecologica, 86, 38-41.

Pérez‐Méndez, N., Jordano, P., & Valido, A. (2017). Persisting in defaunated landscapes: reduced plant population connectivity after seed dispersal collapse. Journal of Ecology.

Pérez-Méndez, N., Jordano, P., García, C., & Valido, A. (2016). The signatures of Anthropocene defaunation: cascading effects of the seed dispersal collapse. Scientific reports, 6, 24820.

Pérez-Méndez, N., Jordano, P., & Valido, A. (2015). Downsized mutualisms: consequences of seed dispersers’ body-size reduction for early plant recruitment. Perspectives in Plant Ecology, Evolution and Systematics, 17(2), 151-159.

Olesen, J. M., & Valido, A. (2003). Lizards as pollinators and seed dispersers: an island phenomenon. Trends in Ecology & Evolution, 18(4), 177-181.

Barahona, F., Evans, S. E., Mateo, J. A., García-Márquez, M., & López-Jurado, L. F. (2000). Endemism, gigantism and extinction in island lizards: the genus Gallotia on the Canary Islands. Journal of Zoology, 250(3), 373-388.

Valido, A., & Nogales, M. (1994). Frugivory and seed dispersal by the lizard Gallotia galloti (Lacertidae) in a xeric habitat of the Canary Islands. Oikos, 403-411.