The living dead

How can you tell when a plant is going extinct? Afterall, I have a heck of a time eradicating certain plants from my garden because the number of weed seeds in the soil is far greater than the number of weeds sprouting each year. If a plant’s mechanism for dispersing seeds is disrupted it may live on for years or decades because of the soil’s seed bank, but it is biding its time. If seeds aren’t dispersed, eventually the plant will cease to exist.

Some researchers think we are in the midst of a global seed dispersal crisis, that many plants have lost the ability to add their seeds to next year’s stock and are living on the borrowed time of previous dispersals. McConkey et al.’s (2012) review of seed dispersal research describes four mechanisms threatening seed survival: habitat fragmentation, overharvesting, biological invasion, and climate change. Each challenges plant survival and together, may be the perfect storm.

Habitat fragmentation: Human development of landscapes breaks up native habitat. The remaining spaces conducive to plants and animals are often islands between which it is difficult for animals and particularly the less mobile plants to migrate. If a plant’s habitat becomes unsuitable it must find a way transport seeds into the next fragment or acquiesce to its immobile fate.

Climate change: I.e. the thing that makes a plant’s habitat suddenly unsuitable. As climate fluctuations rearrange latitudes at which a seed will survive, dispersal patterns need to track with the newly suitable latitudes. Connectivity of habitat will be important. So will hitching a ride.

Overharvesting: In this case, the threat is in overharvesting animals more than plants. A heavy removal of seeds by human harvest may be affecting a plant’s struggle to survive, but the effects are slower to materialize on the landscape than overhunting. Human removal of fruit-eating animals from the landscape, somewhat ironically, reduces the fruit-bearing plant’s ability to survive. Hunting has removed many of the larger mammals and birds who, by eating fruit, dispersed a plant’s seeds. Those animals, many of which are now gone, would’ve been crucial in transporting seeds to new latitudes as climates shift.

Biological invasionThe plants and animals that do find their way around the globe, either with humans and by other means, are now competing with native seeds. It is difficult to generalize across invasive specie scenarios but it can definitely be said that the increased competition, in combination with the other threats to a seed’s survival, offers a stiff challenge.

What is a seed to do? Saving our seeds and enabling their ability to persist on the landscape is not an overlooked challenge, ahem, biodiversity conservation. The Svalbard Global Seed Vault  holds seeds from over 400,000 plants safe in the Arctic permafrost. Small seed banks also exist and some, like Seed Savers Exchange in Iowa, are even participatory. The Global Crop Diversity Trust is trying conserve as much of the world’s genetic pool for crops as possible. Conservationists are building habitat corridors and implementing hunting controls to soften the effects of fragmentation.

Which plants are silently counting down to extinction? Which seeds are losing the struggle to disperse? Only time will tell. The more hands and heads involved, the better.

Reference:

McConkey, Kim R., Soumya Prasad, Richard T. Corlett, Ahimsa Campos-Arceiz, Jedediah F. Brodie, Haldre Rogers, Luis Santamaria. 2012. Seed dispersal in changing landscapes. Biological Conservation 146:1-13.

Image credit:

The Grange Biology Forum, http://grangebiology.forumotion.com/t44-seed-dispersal

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