The right conditions

Becky Seeley

It is not just holidaymakers that like sandy beaches, sand hoppers (Talitrus saltator , Montagu, 1808) are also a common sight on them around the British Isles. But why are they only found on sand, not on rocky or muddy shores? Scientists call the pattern of habitats where a species is found its distribution. To understand distributions scientists have to investigate both how the species relates to its environment and to the other organisms that share this environment.

Investigating the environment

One step in understanding the distributions of a species is to look at the non-biological, or abiotic, parts of the environment. There are an enormous variety of environmental factors to consider, including: the climate (temperature, moisture), the substrate (sand, rocks, soil, mud) and the location (how high is the area, what direction does it face?)

Tolerance and adaptation

Every individual has a range of environmental conditions that it can survive in, mostly determined by the genes it inherits. Different individuals will have slightly different tolerances to conditions because they differ genetically. However, every population, or group of individuals, has a set of optimum conditions, where the majority of the population grow most and produce the most young.

To find out how an organism is affected by changes in the environment you have to investigate its survival rate in a variety of conditions. This could be how well a species of plant survives with different levels of rainfall, how well it re-grows after forest fires, or how it survives in soils of different pH (how acid or alkali the soil is).

You can tell a lot from the effects of a single environmental condition, but in nature living things encounter a combination of environmental conditions at any one time. The sand hopper is affected by day length, humidity, temperature, the availability of oxygen, and the timing and height of high tide – all at the same time. So it also is important to investigate how species are affected by lots of different conditions and how the combination of all the resources affects them.

Different horses for different courses - habitat specificity within species.

When a species has a wide distribution there are often differences in the conditions it experiences. The sand hopper is found across Europe, but the sandy beaches it inhabits can have very different conditions. The hot and dry Mediterranean beaches with little tide, provide a very different place to live from the cooler, highly tidal shores of the British Isles. These are different again from the cool and non-tidal shores of the Baltic Sea in Sweden.

The different populations of this species are adapted to their local conditions. Mediterranean hoppers burrow near the waterline to escape drying out. In Britain it is cooler, more humid and the sandhoppers would be washed away by the strong tides if they stayed near the waters edge, so they head for the base of sand dunes to burrow. These different populations can show genetic differences and are known as ecotypes of a species.

How much does the environment determine distributions?

The physical environment is important in determining where an organism can live, but the distributions of many organisms cannot be explained by physical factors alone. Living things interact in a variety of ways and may influence each other’s distributions, so to understand distributions it is also important to understand the relationships between living things.

Next: Predation and Parasitism