Galapagos Giant Tortoise Expert
Oh The Places You’ll Go! Dispersal of Cacti in the Galapagos Islands
I picked the chicken sandwich and orange juice at the plane. As I devoured the airplane delicacy, I looked outside of the airplane’s window and simply saw the sky meeting the ocean. I couldn’t distinguish between sea foam and clouds. As the plane shook down to the runway, I was shaking as well as I anticipated taking my first step on the Galapagos Islands.
I had just eaten a chicken sandwich and all of a sudden I prepared myself to look at the original sites where the ideas of evolution by natural selection were formulated (1). Modern biology, in a way, was one of the original ideas that Darwin collected and spread throughout the scientific community.
Coming to the Galapagos Islands, I was interested in learning about the vast amounts of wildlife present in the islands. Getting off the plane, I stepped foot on Baltra Island and felt the warmth of the ground rise up my legs. The air in front of me was waving like the ocean waves that moved parallel to each other at the distance. I wiped my forehead clean and was later told to hurry because I was blocking the way. Some of my classmates passed me and although I wanted to catch up to the group, I stopped. A cactus that reached up to my knees had its finger-like pads poking out towards the sidewalk. A person with any sense would simply walk around it. But, I pretended to tie my shoe just to look at it closer. Looking at the cactus, I began to look around it and where it was. The ground that surrounded the plants was made up of red sediment that contrasted to the white sidewalks. I looked at it and I saw that the island across had tree-like cacti structures. As I stared at the cacti, my professor remarked on the lava lizard that was next to it. I disregarded the lizard after taking a look at it because it was simply doing pushups on a rock, and continued to gaze around my surroundings. I found that the cacti in the island did not extend to the coasts of the island, or to the tips of the hills. Something was going on. But, I did not want to ask my professors questions already, minutes after getting to the Galapagos Islands. At that time, I still did not know how to observe and ask questions, but I caught a pattern of differences in shapes and sizes of cacti. I looked at my sketchbook and began to draw what I saw.
I went back to the boat and reviewed the notes that I took on that first day on Baltra Island. At first, I thought that the patterns of cacti location were results of speciation in individual islands. I came to the Galapagos with the idea of natural selection and how the Galapagos Islands are an excellent example of adaptive radiation. Adaptive radiation occurs in an area where an ancestor adapts to different niches.
One of the prime examples of adaptive radiation in the Galapagos Islands was the diverse speciation of finches in the islands. Adaptive radiations create the variation present in each island, which is essential for the evolution of the animals on each island. As the environment plays more of a role in selecting traits that are available in the gene pool. My first hypothesis in the cacti puzzle was that each cactus was a different species on each island (2).
On the next day of my trip to the Galapagos Islands, my whole class visited the different birds that were going mating at Genovesa Island. Birds like the great frigatebirds were present at the island. The male great frigatebirds extended their throat and showcased it to females. If the females like the display, they will stay with the male frigatebird. This method of courtship is different to those of the red-footed boobies. The male red-footed boobies grab tree branches and show their mates.
As exciting as nesting birds are, the plants in that island were exciting as well. There were different kinds of vegetation that red-footed boobies would rip apart to make nests out of them. However, they would avoid the cacti.
I noticed that the cacti were growing on top of each other rather than the small plants that were in Baltra. The cacti in Baltra were on the ground because there were not large sized herbivores that would threaten the cacti populations, therefore, they stayed growing in the middle of the island, where rain would fall. However, in Genovesa, animals are bigger and are adapted to live in vegetation. I noticed that the cacti were nowhere near the frigatebird mating and nesting sites. Cacti resided on lava rocks and at the peaks of cliffs, where they can get water, and stay away from animals.
This idea of natural selection came to mind again because it was the only reason I can come up with to solve this phenomenon. The Genovesa Island’s cacti were long and grew on top of cliffs because it was away from the other organisms. On the other hand, Baltra Island’s cacti were small and on the ground because those cacti have minimal predators. I was quick to express my hypothesis about cacti speciation and everyone looked at me. My professor then replied by saying that the cacti are actually subspecies, which have high variation due to their ability to disperse around the archipelago, but still remain the same species (3).
My professor was right, speciation would only happen if the plants could not breed with each other based on the biological species theory (4). Before he concluded with the idea of natural selection, I had mistaken speciation with variation. The cacti were still related to each other because they were the same plants that had simply invade the new land. Cacti radiated subspecies across the archipelago because the Galapagos Islands is a place where each island can be a different ecosystem. These cacti were able to have different forms because of their ability to disperse around the islands for hundreds of years allowed them to adapt to the different niches in the islands (5).
Just like animals can migrate, plants’ seeds can float to other islands and seeds can be eaten and dropped at other locations as well. Imagine running around your local park and getting those spiky seeds stuck on your shoes. If you drop those seeds anywhere and allow it to grow, that is dispersal. For that reason, every day at the Galapagos Islands, I would have to wash my legs and leave my shoes on the boat’s deck to avoid dispersing sand, seeds, or pollen to other islands.
Most of the cacti species disperse through animals (6). Animals like tortoises and land iguanas eat the fruits and flowers of the cacti. Their diet includes the seeds of the cacti, but their bodies do not digest the seeds and then are later released to the habitat once again. Those seeds are usually large (7). However, small round seeds are pushed by the waves of beaches end up becoming the first plants in deserted islands (8).
Baltra Island was going through a crisis in species extinction because the island has been struggling with invasive species. During the 1940’s, the U.S. military brought animals like goats and pigs. These animals deteriorated the vegetation in Baltra Island and left little vegetation to the endemic and native species (9). However, the Galapagos Conservancy’s conservation initiatives have regulated the introduction of invasive species by human travel. Since conservation efforts have been successful, cacti are now thriving in a less competitive environment. Environments where there are more herbivores, the cacti spread through land animals. This explains why Genovese's cacti are identical to Española’s cacti because either an iguana or a tortoise migrated to those islands with the seeds. Any migration of tortoises or other land animals push the dispersal efforts of cacti.
Cacti are usually the first plants on an island because they can thrive in acidic conditions. These first arriving cacti are usually small and have many thorns because it does not have many predators. Other islands have a bush like cacti plants on top of high cliffs because it is away from other competing plants, and animals that would use it for shelter (10).
What started as an idea of speciation over time, it ended to be a learning experience of subspecies and the dispersal of plants. I observed and I learned that evolutionary concepts are present not only in a laboratory but nature. Not everything is theoretical because places like the Galapagos Islands show how dispersal can cause variation within a species.
In retrospect, I am glad I was able to visit the Galapagos Islands because I was able to experience what it is like to be a field scientist and draw conclusions from mere observations. Oh, the places we'll go. For cacti, they will disperse to continue to thrive as a species. For me, I will continue following my passion of biology, by simply observing my surroundings.
- Bregman, R. (1988) "Forms of seed dispersal in Cactaceae." Acta Botanica Neerlandica 37.3: 395-402.
- Browne, Robert A., et al. (2003): "Evidence for low genetic divergence among Galápagos’ Opuntia cactus species." Noticias de Galápagos 62, 11-13.
- Cox, A. L. L. A. N. (1983) "Ages of the Galápagos islands." Patterns of evolution in Galapagos organisms 1: 1-23.
- Gibbs, James P., Cruz Marquez, and Eleanor J. Sterling. (2008): "The role of endangered species reintroduction in ecosystem restoration: tortoise–cactus interactions on Española Island, Galápagos." Restoration Ecology 16.1, 88-93.
- Hart, Gavin (2005):"The Cacti and Fauna of the Galapagos Islands: Interactions and Interdependence." Cactus and Succulent Journal 77.4 164-169.
- Meier, Rudolf. 2000 Species concepts and phylogenetic theory: a debate. Columbia University Press.
- Phillips, R. B., et al. (2005) "Eradicating feral cats to protect Galapagos land iguanas: methods and strategies." Pacific Conservation Biology 11.4, 257-267
- Schluter, Dolph. 2000 The ecology of adaptive radiation. OUP Oxford.
- Thornton, Ian WB.1971, Darwin's islands: a natural history of the Galápagos. American Museum of Natural History.