Ecology: Organisms and Their Environments
Topics in Depth
The Theme of Common Mistakes in Ecology: Organisms and Their Environments
Exponential vs. Logistic Growth. Perhaps you’re still trying to figure out the difference between exponential growth and logistic growth. The best way to understand these different growth patterns is to look at the graphs we presented earlier.
When a population is experiencing exponential growth, it is growing faster and faster and faster as time goes on. On the graph, the more vertical the line, the faster the population is growing.
Now, look at the logistic growth graph. What are some similarities and differences between the two graphs? Hopefully, you noticed that the first part of the logistic growth graph looks just like the first part of the exponential growth graph. The biggest difference, however, is that the line in the logistic growth graph changes direction and begins to level off as it nears the carrying capacity. That means that the main difference between exponential and logistic growth is that logistic growth takes into account carrying capacity.
- Predation vs. Parasitism. Sometimes the difference between parasitism and predation can cause some frustration. The first thing to remember is that humans like to classify things into neat and orderly categories even if things don’t fit those categories perfectly. This is likely the case with predation and parasitism. In both cases, one organism is harmed and the other is benefited. On the other hand, predators almost always kill and eat their prey, while not all parasites kill their hosts. In fact, the most successful parasites are those that keep themselves on the down-low, only taking what they need when they need it while minimizing damage to their hosts. Few prey, if any, are unaware of a feeding predator. While there are always exceptions, for the most part, predation and parasitism are different enough to warrant separate categories, if only for the sake of communication and learning. And we love communicating and learning.
- Interspecific vs. Intraspecific Interactions. As you read this section, you may have been asking yourself, "Hey, can’t organisms of the same species interact with one another?" If you did, good for you. We didn’t cover this topic specifically, but it is worth mentioning. Most of the interactions we discussed above are called interspecific interactions, or interactions between organisms from different species. Predation and symbiosis are almost always interspecific interactions. There are rare exceptions, like when parents eat their young. But, competition can occur at both interspecific and intraspecific levels. Intraspecific competition occurs when members of the same species compete for food, mates, or space. Because members of the same species have exactly overlapping niches, this type of competition is always more fierce than that which occurs between different species. Indeed, this type of competition is so important and so common that some community ecologists devote their entire careers to studying just intraspecific interactions.
Habitats vs. Ecosystems. Some people think habitats and ecosystems are synonyms. In reality, a habitat is the nonliving part of an ecosystem, or the physical environment in which organisms live. The living part of an ecosystem is the community, or interacting group of populations of organisms, that occupy the habitat.
Energy Pyramids vs. Food Chains (or Webs). While both energy pyramids and food chains (or webs) display producers and consumers in an ecosystem, they vary in the type of information they provide. Energy pyramids are primarily used to display trophic groups in general, grouping all of the producers into one segment, all of the primary consumers into another, and so on. Food chains and food webs, on the other hand, are primarily used to show trophic relationships between individual species within a community. All of the producers and consumers might be represented in a food web, if they are known, but aren’t necessarily grouped together in a particular area of the diagram. Moreover, a food web is actually just an assemblage of multiple food chains. For this reason, few food chains will ever contain all of the producers and consumers in an ecosystem. This will only be the case in extremely simply—and often artificial—ecosystems.
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