Hi everyone, today in ecology class a question came up What does the size of the seeds depend on? Why cacti and several desert plants have small seeds? Why most plants with large seeds are distributed in the tropics and warm areas? I think it could be because of the availability of resources but I can’t be sure, I hope you can help me solve these questions. Thanks in advance.
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In a general sense, seed size and morphology is a function of the ancestral form (starting point), the habitat (minimum energy requirements), and the ecology (dispersal mechanisms and possibly defense against predation) of the plant. For instance, most Euphorbia seeds are quite small but generally get larger near sand dunes where more energy is required to germinate through blow sand that inevitably piles over them. Some seeds are altered to float in the are through wind dispersal and have to be light and small. Plants adapting water dispersal may become larger than ancestral forms that have seeds that disperse by different means.
As for larger seeds being more common in the tropics, I’m not sure but I have a couple ideas. Think about how much energy it would take to make a plant grow tall enough to get any light in a tropical rainforest. Tropical areas (rainforest or otherwise) are typically more competitive with a denser canopy that the plant has to overcome to survive. It also helps to think in terms of what the dispersal mechanism is. I seem to remember there being some fruits that evolved to be dispersed by large mammals that are now extinct.
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Avocado. I believe osage orange and pawpaw fall in the category, tho not necessarily due to size. There is a really interesting popular science book called Ghosts of Evolution that explores this topic. Made me think about a lot of the organism relationships from quite a different perspective by adding the dimension of time to geography, climate, etc. Author Connie Barlow, based on an idea published in 1982 by Dan Janzen and Paul Martin.
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It’s one of the classic tradeoffs in ecology. Organisms can make lots of cheap offspring most of which won’t survive (ie many small, cheap seeds like orchids) or a few expensive offspring with a better chance of surviving. Large seeds contain more energy to give the seedling a head start to help it get established.
Typically you can find a mixture of species along this continuum in any given plant community.
Certainly the biggest seeds are found in the tropics, but I’m not sure that there’s any trend of seeds being larger on average in the tropics. Most plant diversity is found in the tropics, so it’s where you would also expect the most diversity of such traits. Plants with tiny seeds like orchids are certainly very abundant in the tropics.
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Ugh, I wish people would stop with the avocado nonsense. The avocado we eat with the gigantic seed is a product of cultivation. Wild avocados (which are still quite common; when working with Spectacled Bears in the Andes we would keep a close eye on them as they are a favorite food of the bears) are far smaller and have seeds about the size of a date, large, but not immense. These avocados are doing fine, widespread, and are dispersed by a number of currently living animals, including birds.
There are quite a few variations in wild avocados, depending on the species, some with larger seeds, but they all share a pretty much inedible, bitter, oily flesh that is thin, not that nice meaty thing we eat now.
Yes, these wild avocados were also, and maybe more effectively, distributed by extinct megafaua, but they are very different from the avocados we eat now, which humans bred to be that way.
Most wild, not escaped or feral, avocados look like this, or sometimes this, and a few do have a larger seed like this, but the norm is the first image posted.
What we eat is a cultivar, not an example of what megafauna was chomping on.
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Perhaps the most successful dispersal strategy… be palatable or aesthetically pleasing to humans :)
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Plants did a good job of domesticating us.
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Um, I’m not quite sure where I stated that I was referring to the domesticated/cultivated version… My recollection was that the info I read was about avocados in Latin America, but I may have been mistaken. Regardless, thank you for the further insight.
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My grandmother, neighbor, and great aunt all have wild avocado trees and with all of them the seeds are extremely large. I had always been told the cultivated ones’ seeds had been on-purposely shrunk to have more meat on the inside. This makes more sense because why would the gov want less food for a bigger seed that can’t even be planted?
Our avocados from there are very good, organic, and are not bitter. I am sure they were not cultivated because they were thrown out by another wild avocado tree.
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Once upon a long time ago when I was in grad school I thought a lot about seed size and seed dispersal. I haven’t at all kept up with the field, but I can share a couple of thoughts and links to freely accessible resources that go into more depth.
One factor influencing seed size is plant size: in general, a bigger plant has the potential to make a bigger seed. In the tropics, most of the plant species diversity is in woody plants (trees and vines) whereas there is more herbaceous diversity in temperate areas. (Unfortunately, I am completely failing to find the article that I recall presented the evidence so clearly.)
The wikipedia article on Evolution of Seed Size is a good overview of the selective pressures we understand so far.
It’s pretty interesting to think about the different paths that animal-dispersed seeds may take, and how it favors different seed and fruit characteristics. Again, wikipedia has a decent overview, though it doesn’t get into the complicated relationship between seed predation, dispersal, and caching that occurs with scatterhoarding rodents (think squirrels and acorns, if you have those). One of my personal favorite seed dispersal strategies is the poop-mimic seeds that are dispersed by dung beetles.
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The cultivation of avocados predates “the gov” by a few thousand years. They were cultivated by native people in Meso and northern South America. Even with a larger seed they have immensely more flesh than their wild progenitors, and the seeds grow just fine. I currently have a 4 meter tall one that’s growing ridiculously quickly that was planted from seed not long ago, and there are two more growing from seed in its shade.
Where I worked in Ecuador we’d find cultivars that had sprouted from discarded seeds all the time and even from fallen fruit on the edges of orchards.
What you have is almost certainly an escaped or feral avocado, essentially an escaped cultivar (a very common occurrence where the climate is right), not a truly wild one.
Unfortunately there is a lot of incorrect and misunderstood information about avocados and both their wild and cultivation history, and that misinformation is repeated ad-nauseam across the board. It’s immensely frustrating that it happens.
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Another important piece is growing season. Plants in the tropics have a far longer growing season than temperate and arctic plants, so they have a lot longer to grow larger fruits/seeds.
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Huh. Thanks for the info.
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https://onlinelibrary.wiley.com/doi/full/10.1111/j.1466-8238.2006.00259.x
If you can’t access above, here’s a link to a shared pdf on ResearchGate ( https://www.researchgate.net/publication/227679724_Global_patterns_in_seed_size_Glob_Ecol_Biogeogr )
Good question:
If you want to dig into this in more detail the paper above is a global global analysis on variation in seed size. Nothing that probably hasn’t been said above.
There are a lot of physical constraints on growing big seeds. The constraints on the max seed size may explain much of the pattern we notice. In the tropics there are species with very large seeds, and species with tiny seeds (orchids for example). But in temperate regions seeds generally don’t get as big as the biggest seeds in the tropics. As others have pointed out, you can’t get a big seed without a big plant to produce it. It also takes time to produce big seeds so it helps to have a long growing season. Thus in a place with big trees and long growing season–tropical forest–you find the largest seeds. In tropical forests there is also a big advantage to a big seed: very very little light reaches the forest floor in tropical forests so it can be too dark for effective photosynthesis. Seedlings use the carbohydrates stored in seeds for growth for their first few weeks or months until they can reach more light. Larger seeds allow for more carbohydrate storage and thus increase seedling survival as was pointed out by @paulexcoff.
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Two other plants that have domesticated us.
Coffee apparently has a lot of discarded flesh, which can be harvested, dried, powdered, and eaten.
Cashew nuts (the bit we eat is an appendix dangling at the bottom of a much larger fruit)
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The proverbial carrot!
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Both are tasty too.
Cashew fruit is easily bruised, so you generally only find it in places where the trees are grown, but in those areas it’s common to find them in the market during the season. Technically it’s not a fruit, it’s an inflated stem.
The coffee cherries are sweet, I’ve eaten them in South America. One of the places I was working had an abandoned coffee plantation on it.
Those coffee cherries are the basis for the disgusting kopi luwak (aka “weasel coffee” outside of Indonesia). Civets eat the fruit and poop out the seeds, which are then washed, roasted, and made into coffee. Unfortunately this has become such a lucrative industry that the vast majority of this product is produced from captive civets that are force-fed large amounts of the coffee cherries then, at least here in Vietnam, sold to restaurants when their health fails from the poor diet.
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Our old boss was a millionaire and she bought the kopi luwak and liked it. We thought that was gross. But, hey, she likes it and can afford it, I’m not gonna stop her.
In all honesty she was probably cheated. 99.99% of what’s on the market is farmed (using very abusive practices - tiny cages, force feeding, etc), but when they export it they pretend it’s wild collected and charge insane prices for it.
Here in Vietnam every single coffee retailer (you’ll have a bunch of them all in a row often times) has tons of it for sale cheap.
The history of it is interesting, and kinda sad. The Dutch brought coffee to Indonesia, and because it was a valuable cash crop the locals they used for labor weren’t allowed to ever take any or try it, under threat of corporal punishment. Eventually they noticed that civets were eating the cherries and pooping out the beans, which the Dutch didn’t touch. The being extremely curious what the fuss over these beans the locals collected the civet poop, washed it, etc and made coffee.
It was the only coffee they were ever allowed to have and it was never a luxury item, contrary to how it’s marketed now.
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She used to live in mexico and that is where she got it.