Ecology 101

What is ecology?

Ecology is the study of the interactions and relationships among living things and the physical world. These interactions largely define where species live and how many can live there. Since no living thing can exist in isolation, ecology is in many ways the study of the big picture in nature.  Every living thing must have water, material to build its body, a place to live, some sort of usable energy, and the ability to reproduce. In nature, all living things compete for these resources. Living things usually live where they do based on how well they can compete for the available resources. Under certain conditions, certain species have an advantage. If the conditions change, other species might have an advantage.  

What are some of the major concepts in ecology?

Habitats and Ecosystems

A habitat is where the needs of a living thing are met. For example, trout habitat consists of water that is clean, cold, and free-flowing; a river bed that has rocks and cobble; and food consisting of a good supply of aquatic insects and insect larvae. Wood turtle habitat consists of slowly moving water; a patchwork of floodplain shrubs and grasses; dry, sandy areas for laying eggs; and a mixed supply of food.

An ecosystem is a collection of habitats whose members are dependent on one another.  The wetlands and mixed forests of southern New Hampshire are one ecosystem, the Great Bay Estuary is another.  

Connectivity is the degree to which similar or useful parts of a landscape are connected. Living things can rarely succeed in isolated pockets. A vast, undeveloped forest with areas of different age trees and abundant wetlands is clearly of higher value to wildlife than a few scattered patches of trees and water in a human development. As an example, vernal pools are temporary wet pockets on the forest floor that are important breeding areas for certain frogs and salamanders. If the animals can safely migrate from one vernal pool to another without having to cross roads or active development, their habitat has high connectivity. Frogs from one pool can mate with frogs from another pool and the frog population stays healthy. If frogs can only mate with frogs from the same pool all the time, the health of that frog population is at risk.  

One of the primary reasons for threatened or endangered species is habitat loss. Habitat can be lost outright when people do the following activities:

  • clear land for development
  • make key areas inaccessible by building roads or dams
  • destroy or impair key functions of the habitat, such as through pollution.

Clearing a forest to build a big development is habitat destruction. The plants and animals that lived there must find a new home or somehow adjust to a very different, human landscape. Weeds and common rodents can often do well with such change. Animals that need old trees, quiet, and secrecy do not do well with that kind of change.  

When a road is cut through a forest, the road can become a major barrier, and this too, is habitat loss, also referred to as habitat fragmentation. Even though the size of the forest did not change significantly, access to the forest did. Animals that try to cross roads often have deadly encounters with vehicles. Even where cars do not kill animals outright, curbs can present an insurmountable obstacle to turtles, for example, none of which can climb vertical walls. Like roads, dams prevent animals from reaching parts of their habitat. Fish swimming up a natural stream can make use of small steps and rest areas. For a fish to jump over a ten foot high dam in a single leap, however, is impossible. The river is still there, but the dam creates an impassable barrier to the rest of the river.

Habitat loss can also be an invisible burden. A well-shaded stream that is perfect for trout can be rendered unfit if run-off from a nearby parking lot deposits grit, dissolved salt, and/or leaked oil into the stream following a rain storm. Rain that falls on a hot parking lot often runs off into a nearby stream, making the stream water warmer. Habitat loss can also occur due to activity far from a specific site. If an upstream town takes too much water for its own uses, habitats down stream can be left with too little water. If an upstream farm, housing development, or golf course fertilizes its land heavily, the nutrient-rich run-off can cause a harmful growth of algae that results in polluted water downstream.  

Energy Flow and Food Webs

All living things rely on other living things. In its simplest form, a food web is how energy and the chemicals of life pass from one living thing to other living things. Plants make food using the energy of the sun and nutrients absorbed from the soil. Many animals eat plants directly and other animals eat animals that have eaten plants, animals, or fungi. The plants, animals, and fungi that are not eaten ultimately die. All dead material is broken down to basic materials by fungi and bacteria and this material is returned to the soil or water. Matter and energy are neither created nor destroyed… they just change form and are recycled.  Living things survive if they can find enough material and energy and can avoid being eaten. They reproduce if they can find other members of their species.

If one species of the food web disappears or an extra one is added, the rest of the food web must adjust. As on a spider’s web, if a connection is broken or strengthened on one part of the web, the whole web feels the change. Some strings might be broken without major damage, but if a key string is lost, the whole web can be rendered broken. Scientists are working hard to understand all the connections, but even they do not always know which strings are the key ones. When in doubt, it is best not to tamper with something that works.


Where things live and thrive often is defined by factors that limit growth and reproduction. Plants are limited by nutrients and other chemicals in the soil, space for their roots, sunlight hitting their leaves, moisture in the soil, competition for resources with other plants, how often and how much animals eat them, and the ability of their seeds to be fertilized and dispersed to suitable sites. When a living thing can meet all its needs, it usually thrives. When a needed resource is limited, growth is also limited. A tree growing in an open site with good soil and moisture usually grows fast. A similar tree growing in the shade often grows more slowly. 


In common, human terms, adaptation is seen as a sometimes quick process, such as adapting to a new technology or diet. This is not the kind of adaptation that is used in ecology. In nature, adaptation is usually a very slow process, resulting in a change to inherited traits (genes) over time. A tree that grows in well-drained soil cannot suddenly adapt to growing in soil that is very wet all the time, and neither can its offspring. An animal that specializes in hunting frogs cannot simply switch to hunting rodents. Some species, such as rats, crows, or coyotes, are flexible and can adjust to new conditions. These species have an inherited range of behaviors that allows them to succeed in many new situations. For species that are hard-wired to live a certain way, they continue to be successful only if conditions do not change dramatically or persistently. If conditions do change and a species cannot adapt to those new conditions, individuals of the species must find a suitable place to live elsewhere or they die. 


Plants and animals that have existed together in the same conditions and area often have adaptations that developed as a result of their interaction. Sometimes the relationship is a battle to outdo the other, but sometimes it is a beneficial relationship. Plants that are tasty and easily digested by animals might not be able to reproduce successfully, because too much of their tissue is eaten. If certain individuals of that plant species are not quite so palatable, or have irritating thorns, they might reproduce more successfully.

In order to find enough food, the animals that can tolerate the more distasteful or thorny plants might have an advantage over other animals that can eat only the unarmed plants. As the plants develop new defenses, some of the animals develop ways to eat the plant without getting hurt.

Most of the plants that humans eat are tasty not only to people, but also to a wide variety of insects. Humans protect their plants by giving their plants all the advantages: good soil, good sun, fertilizers, lots of water, and protection against pests, either through pesticides, screens, or biological controls, such as lady bugs. Were it not for all these advantages, most of the fruits and vegetables that people eat would taste like almost all other plants: bitter.  

An interesting, local example of co-evolution is the relationship among cherry trees, tent caterpillars, and ants. Cherry leaves are bitter to most insects, but not when they first open in spring. New cherry leaves are tender and are not bitter. As a result, they are very susceptible to heavy and harmful attacks by tent caterpillars; howver, new cherry leaves are not defenseless. At the base of cherry leaves are two small knobs. These knobs excrete a tiny bit of sweet, sugary liquid that is irresistible to ants. Ants love sugar and they will fiercely defend a good source of food. When the leaves first open, the sweet liquid is present, and so are the ants. Any tent caterpillar that encounters ants will be attacked and carried away. As the leaves mature and acquire their normal bitter defense, the production of sweet liquid stops, and the ants stop actively patrolling the cherry.

An ant inspects the sugary bulbs at the base of a fresh cherry tree  photo by Africa Gomez

Another example of co-evolution can be found in the Lamprey River itself. Fresh water mussels have a unique adaptation that has co-evolved with certain species of fresh water fish. When mussels reproduce, the males release their sperm into the water and the females filter the water through their bodies. The females capture the sperm and then release their fertilized eggs into the water. The eggs hatch into larvae which attach themselves to specific fish. The larvae travel with the fish for a time, and then drop to the bottom where they will continue to mature into adults. They will spend the rest of their lives, usually 10-15 years but up to 100 years, living in the sediment. The larvae do not seem to harm the fish, so they are not parasitic, but the mussels derive a dispersal benefit from the fish. The fish derive a possible respiratory benefit from the mussels as they filter the water, cleaning it of debris.

Invasive Plants and Animals

Plants and animals that are introduced to an area where they historically have not been present often have two responses to the new habitat: 1. they do not do well or 2. they do extremely well. Invasive plants and animals are the ones that do very well.

If a plant or animal develops quickly, has a high number of offspring, and has few or no predators, it can and often does take over a site. For these species, a new habitat essentially has no limits. The resources they need are plentiful, they might face little or no competition from native species, and they have no natural predators. Southern New Hampshire is fighting against many invasive plants, such a purple loosestrife, Asian bittersweet, and Japanese knotweed, and insect species such as hemlock woolly adelgid, emerald ash borer. and Asian longhorned beetle.

Invasive plants not only take up valuable space on the landscape that would normally be used by local plants, but they do not support the associated communities of animals. Most local plants have a community of insects that pollinate, inhabit, or eat those plants. As an example, common reed (Phragmites australis) in its native Europe is eaten by over 170 species of insects. The same plant here in the United States has only 5 native species of associated insects. (Tewsbury et al, Potential for Biological Control of Phragmites australis in North America, Biological Control 23, 191–212 (2002)) In turn, those insects are important food sources for many birds. Recent studies have found that 96% of terrestrial song birds feed insects to their young. (Stoleson and Finch 2001, Tallamy 2004, Burghardt et al 2008) Because they lack the associated insects, invasive plants affect bird populations.  Also, mammals may not recognize the new plant as food or have the digestive chemicals to detoxify harmful chemicals within the plant. This not only leaves mammals hungry, but it puts extra pressure on native plants while leaving the invasive plants to proliferate.  

The New Hampshire Dept. of Agriculture has published a helpful “Guide to Invasive Upland Plant Species”. The guide has excellent photos to identify these plants and provides advice on how to remove them. The NH Lakes Association has a quick visual guide for identifying freshwater invasive aquatic plants and animals LRAC is particularly interested in Japanese knotweed. To learn more about efforts in removing this invasive plant and how you can help report patches of Japanese knotweed, click here KNOTWEED ERADICATION FULL REPORT PDF