what’s a population? It’s a group of individuals that all belong to the same species. Populations are geographically based; they live in a particular area. But the size or scale of that area can be variable – we can talk about the human population in a city, a state, a country or a hemisphere. Or we can talk about the population of palm trees on just one island in the Indian Ocean, or on all of the islands that make up the Republic of Seychelles, or all of the islands in the Indian Ocean. The person studying or writing about the population gets to decide what scale to use, what is most appropriate for what they want to study or explain. That’s one of the exciting things about science – there’s a lot of freedom in defining the scope and scale of your project, but that means it is also important to explain clearly what scale you are using.
Species are made up of populations. How many populations? It all depends. It depends on how widespread the species is and how small or large the geographic area is. Some species have very limited ranges or distributions, being restricted, for example, to a single island or the top of a single mountain in the whole world. The single population on the island or mountaintop makes up the entire species. From a conservation perspective, such populations are extremely vulnerable – if anything happens to that one population, the entire species will be lost; the species will go extinct. But many species are more widespread. There are populations of moose, for example, in Yellowstone National Park, Maine, Minnesota, Alberta, Manitoba and other U.S. states and Canadian provinces. If you want to know how many moose there are on Earth, you have to know the sizes of all the different populations in all the different locations.
Communities are made up of all the populations of different species in a given area. Why the vague term “in a given area?” Because once again the scale is flexible, determined by the person studying or writing about the community. We might be talking about the community of all the organisms living in the very top or canopy of a single rainforest tree or of all the trees in the forest. What’s most important about the community concept is that it involves multiple populations of all the different species in the given area and how these species interact with each other. Each of the populations is made up of individuals of a particular species, and the individuals interact with each other – with members of their own species (e.g., fighting, grooming, mating, pollinating each other) and with individuals of other species (e.g., hunting them for food, using them as a place to build a nest, growing on them). Community ecologists study the populations in a given area and their interactions. There’s another article in this tutorial about different types of ecological interactions.
That leaves us with the ecosystem level. What’s the difference between communities and ecosystems? When you’re talking about ecosystems, you’re not only looking at all the different populations and species in the given area, but you’re also looking at the physical environment, the non-living or abiotic conditions (language alert: the prefix “a” means “without” and the root word “bio” means life, so abiotic is literally “without life” or in other words, non-living), and not just what they are, but how they impact the organisms, and in some cases how the organisms impact the physical environment. For example, temperature and rainfall patterns influence where different terrestrial species of plants and animals live; some can survive dry desert conditions, others need the high rainfall found in rainforests. But the forests themselves also influence temperature and rainfall patterns. Have you ever noticed on a hot summer day how much cooler and moist it is in the shade of a forest than out in the open?  And worms change the structure and composition of soil as they churn through it.
What size is an ecosystem? Guess what – it depends on how big or small the scientist or author wants to define it to be. It could be as small as your backyard, or Walden Pond, or the entire Australian outback.  Different sizes or scales will be appropriate for different types of studies, reports and policies. The scientist or author just needs to explain what the size is and why it is appropriate.
here is the other ones as well as population
