February 19, 2010

Island biogeography: State and case of spider diversity in Macaronesia


ResearchBlogging.orgSome of the first organisms found on a newly risen or recently destroyed island are spiders. On mainlands, spiderlings of smaller species weave a tiny drag chute, perched atop the highest point in their immediate area – the leaf of an herb or the very tip of a blooming meadow grass – and let the breeze, even the slightest one, carry them away. Most only travel short distances, remaining in the ecosystem in which they were born, but some are spun upwards in varying winds, and swept into jet streams carrying millions of aerial plankton that sometimes happen upon bare, isolated earth of an island. Insects, birds and driftwood carry seeds and other organisms and their larvae which chances upon the island. The fertile soil provides a foundation for the burgeoning community of explorers and hitchhikers. The original inhabitants evolve and diversify, adapting to the alien and continually changing young environment. Immigration slows, mitigated by extinction, reaching an equilibrium point. Creative forces converge, proceed and decline on a tiny speck of land beaten by endless currents, forces described by island biogeography.

E.O. Wilson and Robert MacArthur first outlined the theory of island biogeography, publishing the dynamic model of equilibrium in 1967, which basically says that immigration, speciation and extinction will determine the number of species present in relation to area (more area, less extinction) and isolation (greater isolation, less immigration). As the number of species present increases, immigration rates decrease and extinction rates increase. There is a “dynamic” theoretical equilibrium point where these rates meet. Wilson and MacArthur’s theory is as influential and essentially applicable as any in ecology. The model is not only used for actual islands, but also in other isolated systems.

Ecologists and biogeographers have been trying to build on the theory for some time. A recent study published in the Journal of Biogeography tests a recent synthesis of island biogeography theory based on the equilibrium model, the general dynamic model of oceanic island biogeography (or GDM), along with other factors like area, elevation and distances between islands in the Canary and Azores archipelagos and the mainland in an attempt to find an adequate predictor of spider species diversity and endemism in those chains, diversity that was initiated by ballooning spiders from the mainland millions of years before.

Macaronesia* the “islands of the fortunate” is a general term applied to five chains of volcanic islands off the western coasts of Europe and Africa: Azores, Canary Islands, Cape Verde, Madeira and the Savage Islands. The results of the study focused on the Canary Islands and the Azores.

The Canary Islands exhibited the greatest richness with the highest numbers on Tenerife: 84 single-island endemics, 145 archipelago endemics and 230 indigenous spider species (Macaronesian total was 255, 370, 584 respectively). The numbers in the Azores were significantly lower (from 0% – 9% of the total); on Corvo, for example, the researchers found no single island endemics, only one archipelago endemic and five indigenous. There are different factors at work in these numbers and the researchers found that this wasn’t a one size fits all situation.

From Whittaker et al. 2008 doi: 10.1111/j.1365-2699.2008.01892.x

The GDM seemed to apply to main islands according to the authors, producing the notable hump-shaped curve pictured above. The GDM was proposed by Whittaker et al. in 2008 and incorporates geologic succession and time into determining species diversity on islands. Like Wilson and MacArthur’s model, immigration (I) decreases and extinction (E) rates increase through over the course of island development (time performs the same function as species number here). But notice the extra parameters. As succession proceeds, speciation (S and therefore richness (R) increases as the environment diversifies (niche opportunity/allopatry) and decreases as the island ages and becomes smaller and more homogenous, eventually disappearing into the sea, marking the end of its lifespan. The decreasing curve of richness in the chart is not as steady as its increase; that represents the progression of niche adaptations along the rise of environmental diversification, followed by interactions among and between trophic levels on the decline.

One genus in particular displayed this “deceleration” of speciation:

Maximum likelihood-based analysis of patterns of diversification in the Dysdera species endemic to Lanzarote and Fuerteventura revealed that diversification in this lineage has decelerated through time, which is consistent with an increase in extinction rates due to ecosystem transformation driven by island ageing.

The Dysdera species began this decline between 5 and 2 mya.

The Azores have very different environmental circumstances than the Canary Islands. First of all , they’re more isolated, farther west off the coast. Wind and sea currents in the area seem to be a barrier to all but the most mobile species, circulating north and south in the area, which favors deposition in the islands closer to the mainland more or less along that axis. The researchers believe that highly mobile spiders colonized the Azores and spread across the archipelago, which would allow for a high level of gene flow and ultimately low species richness. This contradicts the idea that more isolated archipelagos will always have more endemic species.

Habitat loss seems to be a more powerful predictor here. The Azores archipelago has retained only 2% of its natural forests after 400 years of deforestation. Anthropogenic changes have prematurely homogenized the environments, removing mass amounts of vegetation useful to spiders for web building and providing transportation for invasives. It’s been shown that habitat loss tends to more greatly affect organisms at higher trophic levels, and spiders in Macaronesia are historically the main terrestrial predator on those islands. This situation seems to fall in line with that observation. Since spiders exhibit this level of sensitivity, the authors suggest that these animals may be a good indicator species for future study.

As a side note, the laurel forests (laurisilva) are exceedingly beautiful. The trees are relicts of the Pliocene era, representational of the forests that once covered a warmer, more humid Mediterranean. I have that scrawled in the notebook in the hopes that I have time to write a series about relict and perhaps even prehistoric ecosystems.

*I didn’t know this before researching this article, but Macaronesia is the true spelling of the area, not Macronesia, which is how it’s spelled in the paper.

Whittaker, R., Triantis, K., & Ladle, R. (2008). A general dynamic theory of oceanic island biogeography Journal of Biogeography, 35 (6), 977-994 DOI: 10.1111/j.1365-2699.2008.01892.x

Cardoso, P., Arnedo, M., Triantis, K., & Borges, P. (2010). Drivers of diversity in Macaronesian spiders and the role of species extinctions Journal of Biogeography DOI: 10.1111/j.1365-2699.2009.02264.x

1 comment:

  1. Really interesting. Amazing to think of tiny spiders as the first inhabitants of a newly formed island.

    ReplyDelete