Ecologists describe plants and algae as producers because, with the help of solar energy, they produce sugar and other complex carbohydrates from carbon dioxide in the process we call photosynthesis. Consumers, such as horses, chironomids and lions, eat producers or other consumers, using the chemical energy in their food’s complex molecules to fuel their own metabolic processes. Without photosynthesis there would be almost no life, as consumers would need to scavenge the organic molecules that happened to be lurking about in the environment.
You’ll notice I slipped in chironomids as consumers, because they are the diminutive heroes of this tale. There are actually more than 10,000 chironomid species, but the ones we are discussing lay their fertilized eggs into lake water where they sink into the sediment and hatch out as larvae. These larvae go through several growth stages, building tubes which serve as their houses during growth and development. Larvae then transform into pupae, which after a few days swim to the surface and metamorphose into winged adults, which mate and produce more fertilized eggs.
We usually think about consumers as having a negative effect on their food and/or prey. If we introduce a herd of sheep into a small pasture, we are confident that the grass will quickly disappear inside of sheep bellies. Similarly, if we introduce a few lions into a pen of sheep, we can be fairly confident that the sheep population will decline. But consumer effects on populations can act in strange and wonderful ways.
The subarctic Lake Mývatn in Iceland is host to an amazingly huge number of chironomid larvae; over 100,000 of these insects can squeeze into 1 m2 of lake bottom!
As Cristina Herren describes, Lake Mývatn has long been studied because of the dramatic changes in chironomid densities. Chironomid populations can increase 10-fold in each of four or five consecutive generations during the subarctic spring and summer. So if you began with a density of 10 chironomids/m2 in April, you might find 100/m2 in May, 1000/m2 in June, 10,000/m2 in July and 100,000/m2 in August. Herren was intrigued by this exponential growth, as it suggested that somehow chironomid food resources (primarily algae) were keeping pace with the exploding chironomid population, despite being eaten by this ever-expanding population. How could the algae keep pace with the furious chironomid growth rate? Herren wondered whether the chironomids were acting as farmers and somehow stimulating algal production.
Herren worked with several other researchers to figure out the answer to this puzzle. First, they wondered whether the larval tubes provided an awesome place for algae to live. Second, perhaps chironomids were pooping-out vast quantities of nutrient-rich waste products, which algae used to build their bodies. Either or both of these positive effects could more than compensate for the direct negative effect from chironomids eating the algae.
To investigate both hypotheses, the researchers set up 1-liter clear plastic containers with lake sediment and either 0, 50, 100, 200, 400, 600, 800 or 1200 chironomids (that must have been fun to count!). They placed six or seven containers with each population size into racks at the bottom of the lake.
The first task was to discover whether production actually increased with chironomid abundance. Oxygen is a waste product of photosynthesis, so scientists use oxygen production to measure photosynthetic rate, and also to infer algal abundance. They found that oxygen production actually increased with chironomid abundance. Note that chironomids (and the algae too) actually consume oxygen (just like you and I do) in the process of respiration, so this is a truly remarkable result. It indicates that production increased so much that it more than compensated for the oxygen used up by the chironomids and algae from respiration.
The algae use chlorophyll a in their cells as the molecule for photosynthesis, so algal abundance can be estimated by measuring how much chlorophyll a is present in each bottle. Again, they found more chlorophyll a in the bottles with more chironomids, despite the fact that chironomids were eating all the algae they could find.
In addition, the chironomids from more crowded conditions weighed more than those from sparsely-populated bottles. The researchers concluded that by eating algae, the chironomids were actually increasing algal abundance, thereby creating more food for themselves.
These findings fly in the face of our usual assumptions about consumers reducing the population size of the species they are consuming. The researchers confirmed two important indirect effects of chironomids on algae. First, the chironomid larvae do excrete vast quantities of concentrated nutrients (nitrogen and phosphorus) that are consumed by the algae. In addition, the chironomids tubes are indeed ideal surfaces for algae to hang-out on, as evidenced by finding much more chlorophyll a on the tubes than in the nearby sediment.
We citizens need to recognize that both direct and indirect effects operate within ecosystems. In this case, algae are clearly directly benefitting chironomids by providing them with food. Less obvious, the chironomid effects on algae are in two directions. Chironomids have a direct negative effect on algal populations by consuming vast quantities of algae. However, they have a stronger indirect positive effect on algal populations, by providing algae with high quality housing and abundant resources, so that the algal population grows dramatically over the year. The net effect of chironomids on algal populations (dashed arrow) is positive.
Most ecosystems have many different direct and indirect effects operating between a large number of species. It’s a real challenge to identify and understand these diverse interactions, which is why we need to be cautious about changing ecosystems in significant ways. For example, when we dump our waste products into waterways or soils, the added nutrients can have pronounced direct and indirect effects on the species that live there. In many cases, we won’t be able to identify the unintended consequences of our actions on an ecosystem until the native species have gone extinct.