Bio-Core

Interspecific Competition

Introduction

Our planet is covered with life. Almost everywhere you look, you are staring directly or indirectly at a living organism (or the effects of one!). No species lives alone; every moment of every day members of one species bump into members of another species either amicably or with hostile intent. There is always interaction, friendly or unfriendly, between species.

Each species has a need for resources; food, light, air, liquids, a place to live, a place to mate, shelter for raising young, sources of minerals, places to hide. Because these resources are limited there is never enough for everyone. Hundreds of weed seeds try to germinate every year in the one sunny spot in my garden. At first they grow quickly, putting down roots into the rich, moist soil, and shoots up into the warming sunlight, but after a few days it is noticeable that only one or two will make it (if I don't pull them out first!). Each seedling started with what seemed like an even chance of making it, but only a tiny number succeeded. Why?

The use of limited resources by two different species is called interspecific competition. In this situation each species is trying to gain an advantage over the other one. Two different weed seeds germinating in my garden may start off with an equal chance of drawing limited water supplies into their roots, but if one grows its roots faster than another, the faster grower will soon be getting a larger share of the available liquids and will begin to starve out the slower grower. The successful competitor is having a negative effect on the survival chances of the second, weaker, competitor.

Even without competition, a population grows in size only until it reaches a natural carrying capacity, where the availability of resources just balances the reproductive capacity of that population. The number of individuals of a species in a particular community that can be supported depends on this carrying capacity. It follows, therefore, that, if two species are competing for a resource, this carrying capacity is going to be affected; but how?

Also, the degree of competition may vary from one population to another. A fast growing weed may not be affected very much by a slower growing oak tree seedling, but the oak seedling may be very badly affected by the competition from the weeds. The amount of this competition is called the 'overlap' between the two species. The weed may only have a small overlap (0.01) with the oak seedling, but the oak seedling may have a much larger overlap (0.5) with the weed. The degree of this overlap will change the nature of the growth curves of these two competitive species.

Mathematical bio-simulations help us understand what happens when two species compete.

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