Keystones and connections

In Science, two biologists reported on the effects of fishing in South American rivers. Removing a large fish, Prochilodus mariae, from the river causes rapid changes in how carbon (stored energy) passes through the river, decreasing the cycling of carbon. In fact, they explain, "Impacts of removing Prochilodus on carbon flow equaled or exceeded effects of removing [in different studies] all fish, invertebrates, shrimps, and predatory fish in other streams and lakes."

The removal of one species from one side of a river reduced flow of organic material by 60%, and changed the sort of growth on the bottom. Previous studies showed that removing this species changed the biofilm growing on the riverbed from one dominated by nitrogen fixing bacteria to one dominated by bacteria that consume organic material that drifts by. That reduces the total production possible in the river, and the loss of nitrogen-fixing species removes nutrients necessary to the entire system.

The authors went on to examine museum specimens to examine historical trends in harvesting of the fish. Examining 28 years worth of specimens showed:

that the maximum body size of individuals has declined dramatically, from about 2.2 pounds to a half a pound, which is a hallmark of overharvested populations.

All told, the paper is a reminder that relatively small changes in an ecosystem, excessive pressure on one fish species in a river, can have dramatic and wide-ranging effects throughout the system. Understanding any part of the system requires a full appreciation of how all the parts fit together.

That's a lesson also evident in a more recent bit of research on what happens under leaves.

Like all early plants, moss have flagellated sperm, and it's traditionally taught that the sperm must swim from the antheridium of a male plant to the archegonium of a female, and that this helps explain why these plants must be in wet conditions. Cronberg, Natcheva and Hedlund show that springtails and mites, microscopic arthropods common in rotting leaves, play an unexpected role in fertilizing moss.

This figure, ripped shamelessly from the original paper, shows the experimental setup. Patches of unisex moss were placed at varying distances on a plaster of Paris substrate. When the two patches were in contact, reproduction was fine. Any distance between the patches prevented sperm from swimming to the female patch, but the arthropods allowed fertilization. Faster moving springtails (they are named that because they have structures that function like springs to propel them long distances) are better at fertilizing over long distances.

They also found that the arthropods prefer to frequent fertile mosses. It isn't clear why, though fertile mosses exude sugars and fats. This may represent a mutualism between plants and insects (or Proinsecta, if you prefer) that predates the evolution of flowering plants. The explosive radiation of flowering plants was thought to be the first evolutionary radiation driven by plant fertilizers, but this result suggests otherwise.

Understanding how plants spread across the landscape in the earliest days is a topic of obvious interest, and understanding how to preserve existing patches of plants is important, too. This shows that preserving mosses, liverworts and other basal plants means protecting the springtails and mites that fertilize them.