The who, when, where, how and why of partial migration

Submitted by editor on 21 April 2017.Get the paper!

We were amazed when we first heard rumours that giant tortoises migrated up and down the volcanoes of the Galapagos Islands. Darwin had first mentioned the possibility after discussing with local farmers why tortoise trails went uphill! Later, several researchers found seasonal shifts in the abundance of tortoises on Santa Cruz Island in the heart of the archipelago; during the wet season it seemed that tortoise abundance was highest in the arid lowlands while in the dry season, tortoises seemed to prefer the highlands.

Figure 1. Gluing GPS tags onto a giant Galapagos tortoise on Santa Cruz

We investigated by fitting GPS tags (Figure 1) onto the shells of 10 adult giant tortoises of both sexes and followed then for a year. Lo and behold, most of the tagged tortoises underwent seasonal long, linear and stereotypical migrations, driven it seemed by seasonal variation in the distribution of high quality foods. In the dry season, the Galapagos lowlands are arid and hot, with few or no feeding opportunities for tortoises except cacti and scattered unpalatable leaves. In contrast, at this time, (June-Dec), large areas of the Galapagos highlands are (paradoxically) bathed in thick cloud and persistent drizzle. This allows plants to thrive, which provides a dependable source of food for the tortoises (Figure 2). The arrival of the rainy season sees the lowlands green up with abundant new plant growth, which fills the landscape with nutritious and easily digestible herbaceous vegetation. At this time, tortoises leave their highland dry season refuge and migrate to lower elevations, where they spend the next few months feeding, and in the case of females nesting. As the rains stop and the lowlands become dry once again, most of the tortoises return to their drizzly, damp but dependable highland range.

Figure 2. The very different quantities of available food for tortoises in the lowlands versus the highlands of Santa Cruz.

We then became intrigued by what tortoises might be doing on different islands in the archipelago. Each Galapagos island has a unique suite of environmental conditions driven by interactions of seasonal changes in ocean currents, island size and topography. Since many islands also contain different, yet closely related species of giant tortoises, we envisioned a comparative study that might reveal how environmental conditions shape the evolution of movement strategies.

Figure 3. Change in the seasonal distribution of vegetation productivity across the Galapagos Archipelago (darker colours indicate higher productivity)

Rising to the challenge, in 2010 our engineering friends at e-obs GmbH build us 32 new GPS tags, sold them at cost, and helped us deploy them onto tortoises on three Galapagos Islands – Espanola, Santa Cruz and Isabela (on Alcedo volcano). We selected these study sites because each island has very different environmental conditions. Santa Cruz has both extensive highlands and lowlands and also has a large permanent human population. Espanola is a small low lying arid island where we suspected that there would be little spatial variability in the distribution of resources for tortoises. Alcedo Volcano on the other hand, rises to some 1100m above sea level and would, we thought, have wildly different cloud, rainfall and insolation patterns over its surface. On each island then tortoises would be influenced by very different patterns of environmental conditions and resource distributions.

 

The GPS tags covered data on animal movement, but we also needed to simultaneously collect data on the environmental variables from remote, rugged and highly variable terrain at spatial and temporal scales relevant to tortoise movement. Ideally we would have deployed weather stations throughout our study sites, but this was unfeasible. Luckily NASA provides such data free of charge from the MODIS Terra Satellite, which has been monitoring numerous environmental variables across most of the globe since 2001. We were able to download biweekly (more or less) data on vegetation productivity and land surface temperature across the entire Galapagos archipelago. From these data we could quantify the spatial and temporal variability in these variables we considered could be important drivers of tortoise movements (e.g. Figure 3).

We found that environmental predictability is a strong determinant of tortoise movement strategies. On Espanola Island, there is little spatial variation in resource distributions, however conditions are temporally unpredictable. Here tortoises are either residents or nomads, presumably because they cannot predict where and when high quality resources will be available. In contrast, Alcedo volcano has considerable spatial variation in resource distributions, but these distributions are relatively predictable. Here, all the tortoises we fitted with GPS tags were migratory, again likely because patterns of resource distribution are reliable. On Santa Cruz Island we found four movement strategies – residency, dispersal, nomadism and migration. Thus there seems to be a continuum of movement strategies from resident to nomadism which appear to be governed by the predictability of resources. Migration will only be advantageous if the probability of finding valuable resources at the end of the journey is high. A nomad will be at an advantage if there is little pattern in where and when high value resources can be found.. How these energetic costs and benefits drive the evolution of behavioural adaptations such as migration, we cannot say, but the selective pressures for optimal movement strategies are likely to be high.

Figure 4. Migrating tortoises on Santa Cruz Island

Long distance migrations are disappearing around the world due to habitat degradation, blockages to migration routes and over-exploitation. Galapagos is not immune to these anthropogenic pressures and tortoise migration faces an uncertain future on some islands. We hope our work will highlight the importance of maintaining high quality habitats and access to them for giant tortoises and improve the long term conservation of these iconic species.

Stephen Blake

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