The Ecology Book

by DK

  Brown had tried and tested this methodology in the 1970s while studying the potential effects of global warming on species in cool, moist forest and meadow habitats on 19 isolated ridges of the Great Basin, in California and Utah. He realized it would take years of fresh fieldwork to collect enough data. Instead, he used existing findings to draw new conclusions. First, he predicted how much shrinkage would occur in the area of ridge-top habitat with an assumed increase in temperature. Using known data on the minimum area required to support a population of each small mammal species, Brown was able to work out the extinction risk on each ridge as temperatures rose, and suggest conservation priorities.

  Enhancing fieldwork

  Macroecology often supplements fieldwork and can lead to surprising discoveries. In Madagascar, satellite data was used to develop models for chameleon species and predict them in areas beyond their known ranges. As a result, scientists investigating these areas found several new sister species.

  By comparing community studies made in deserts around the world, macroecologists can determine the greatest threats to a desert species such as this banner-tailed kangaroo rat.

  See also: Field experiments • Animal ecology • Island biogeography • Big ecology • Endangered habitats

  IN CONTEXT

  KEY FIGURE

  Ilkka Hanski (1953–2016)

  BEFORE

  1931 In the US, geneticist Sewall Wright explores the influence of genetic factors on species populations.

  1933 In Australia, ecologist Alexander Nicholson and physicist Victor Bailey develop their model of population dynamics to describe the host–parasite relationship.

  1954 In The Distribution and Abundance of Animals, Australian ecologists Herbert Andrewartha and Charles Birch challenge the idea that species populations are controlled by density alone.

  AFTER

  2007 American ecologist James Petranka links metapopulation theory to the metamorphosis stages of amphibians.

  A metapopulation is a combination of separate, local populations of the same species. The term was coined by American ecologist Richard Levins in 1969 to describe how insect pest populations rise and fall on farm fields. Since then, its use has expanded to cover any species broken up into local populations in fragmented habitats, both on land and in the oceans.

  A particular species of bird, for instance, may be found in separate populations in a lowland forest, in mountain woodlands, and various other places. The species is like a family whose members have moved to different cities yet are still related. The combined effect of many populations may boost the long-term survival of the species.

  Apart but together

  A crucial aspect of metapopulation theory is the level of interaction between the separate local populations. If the level is high, it is not considered to be a metapopulation—all the local groups are part of one big population. In a metapopulation contact between the various local groups is limited, and they remain partly cut off in their own local habitat or “patch.” Yet there has to be at least some interaction. It may be just a single brave or outcast member of one group that enters another patch and mates with the local population there. Isolation for too long pushes local populations apart to the point where they can no longer mate with one another, and in time they become separate species or subspecies.

  In the 1990s, Finnish ecologist Ilkka Hanski showed that at the core of metapopulation theory is the notion that local populations are unstable. The metapopulation as a whole may well be stable, but the local populations are likely to rise and fall in their individual patches in response to inside and outside influences. Some patch members may emigrate and join a much reduced population in danger of extinction, giving it renewed strength—a metapopulation feature known as the “rescue effect.” Other groups may completely vanish, leaving vacant patches for another population to recolonize. Hansk argued that there is persistent balance between “deaths” (local extinctions) and “births” (the establishment of new populations at unoccupied sites). He likened this balance to the spread of disease, with the susceptible and the infected representing in turn empty and occupied “patches” for disease-carrying parasites.

  Ecologists see the concept of metapopulations as increasingly important in understanding how species will survive, particularly in the face of human influence on habitats. The theory helps them analyze the way populations rise and fall, using mathematical models to play out interactions, and enables them to predict how much habitat fragmentation a species can endure before it is driven to extinction.

  The Glanville fritillary butterfly metapopulation, in its fragmented habitats on Finland’s Åland Islands, provided the ideal subject for Ilkka Hanski’s studies into species patches.

  ILKKA HANSKI

  Widely seen as the father of metapopulation theory, Ilkka Hanski was born in Lempäälä, Finland, in 1953. As a child, he collected butterflies, and after finding a rare species, he devoted his life to ecology, studying at the universities of Helsinki and Oxford.

  Ecologists at the time paid little attention to the distribution of local species populations, but Hanski realized this was crucial, and spent much of his career testing his metapopulation theory by mapping out and recording more than 4,000 habitat patches for the Glanville fritillary butterfly on the Åland Islands. This work earned Hanski global fame, and enabled him to establish the Metapopulation Research Centre in Helsinki, which became one the world’s leading focuses of ecological research. Hanski died of cancer in May 2016.

  Key works

  1991 Metapopulation Dynamics

  1999 Metapopulation Ecology

  2016 Messages from Islands

  See also: Animal ecology • Clutch control • Island biogeography • Metacommunities

  IN CONTEXT

  KEY FIGURE

  F. John Odling-Smee (1935–)

  BEFORE

  1969 British biologist Conrad Waddington writes about ways in which animals change their environments, calling this “the exploitive system.”

  1983 Richard Lewontin, an American biologist, argues that organisms are active constructors of their own environments, in Gene, Organism, and Environment.

  AFTER

  2014 Canadian ecologist Blake Matthews outlines criteria for deciding whether an organism is a niche constructor.

  All organisms alter the environment to cater to their own needs. Animals dig burrows, build nests, create shade from the sun, and create shelter from the wind to provide a more secure environment, while plants alter soil chemistry and cycle nutrients. When organisms modify their own and each other’s place in the environment, this is “niche construction”—a term coined by British evolutionary biologist F. John Odling-Smee in 1988.

  American evolutionary biologist Richard Lewontin had previously suggested that animals are not passive victims of natural selection. He argued that they actively construct and modify their environment, and affect their own evolution in the process: the lynx and the hare, for example, shape each other's evolution and shared environment by striving to outrun each other. Odling-Smee similarly argued that niche construction and “ecological inheritance”—when inherited resources and conditions such as altered soil chemistry are passed on to descendants—should be seen as evolutionary processes.

  “Hares do not sit around constructing lynxes! But in the most important sense, they do.”

  Richard Lewontin

  Levels of construction

  Some common examples of niche construction are obvious, while others operate at a microscopic scale. Beavers build impressive dams across rivers, creating lakes and altering river courses. This alters the composition of the water and materials carried downstream, creates new habitats for other organisms to take advantage of, and also changes the composition of the river’s plant and animal communities. British biologist Kevin Laland has suggested that, while a beaver’s dam is clearly of great evolutionary and ecological importance, the impact of its dung may also be significant.

&n
bsp; Earthworms are highly effective niche constructors, constantly transforming the soil in which they live. They break down vegetable and mineral matter into particles small enough for plants to ingest. The worm casts they secrete are five times richer in usable nitrogen, have seven times the concentration of phosphates, and are about 11 times richer in potassium than the surrounding soil.

  Similarly, microscopic diatoms living in seafloor sediments secrete chemicals that bind and stabilize the sand. In Canada’s Bay of Fundy, for example, the changes diatoms make to the physical state of the seabed allow other organisms, such as mud shrimp, to colonize it.

  British biologists Nancy Harrison and Michael Whitehouse have also suggested that when birds form mixed-species flocks—as many do outside of the breeding season—they are altering their relationship with competitors to find more food resources and gain more protection from predators. The complex social environment they create modifies their own ecology and behavior.

  In his explanation of niche construction, Odling-Smee pointed to ancient cyanobacteria, which produced oxygen as a by-product of photosynthesis more than 2 billion years ago. This was a key factor in the Great Oxygenation Event, which changed the composition of Earth’s atmosphere and oceans, massively modifying our planet's environment. The oxygen boost helped create the conditions for the evolution of much more complex life forms—including humans.

  Earthworms leave castings that make them valuable natural fertilizers. They not only transform the soil for themselves but also help plants to grow.

  Ecosystem engineers

  A European Starling in Arizona, US, takes advantage of a hole abandoned by a Gila Woodpecker to make its own nest.

  Niche constructors have been described as “ecosystem engineers,” a term coined in 1994 by scientists Clive Jones, John Lawton, and Moshe Shachak. They outlined two kinds of ecosystem engineers. The first, allogenic ecosystem engineers, change physical materials. Take, for example, beavers building dams, woodpeckers excavating nest holes, and people mining for gravel; these activities modify the availability of resources for other species. When woodpeckers abandon their holes, smaller birds and other animals move in. If water floods a gravel pit, ducks and dragonflies can colonize it.

  Other ecosystem engineers are autogenic, which means that simply by growing, they provide new habitats for other plants and animals. A mature oak tree, for example, is a suitable environment for a broader range of insects, birds, and small mammals than an oak sapling. Likewise, a coral reef provides homes for more fish and crustaceans as it grows larger.

  See also: Ecological niches • The ecosystem • Organisms and their environment • The ecological guild

  IN CONTEXT

  KEY FIGURE

  Mathew Leibold (1956–)

  BEFORE

  1917 Arthur Tansley observes that two species of Galium plants grow differently in different soil patches.

  1934 Georgy Gause develops the competitive exclusion principle stating that two species competing for the same key resource cannot coexist for long.

  2001 Stephen Hubbell’s “neutral theory” argues that biodiversity arises at random.

  AFTER

  2006 Mathew Leibold and fellow American ecologist Marcel Holyoak refine and develop the theory of metacommunities.

  One of the limitations of traditional community ecology was that it tended to look at communities purely locally and take little account of what happens at different scales or across different places. Therefore, over the last few decades, ecologists have been developing theories of “meta” communities; the concept was summed up in 2004 in a key paper led by American ecologist Mathew Leibold.

  The idea of metacommunities is linked to that of metapopulations. While studies of metapopulations examine the different patches where populations of the same species coexist, in metacommunity theory the different patches consist of entire communities that include a number of interacting species.

  Mountain goats in Colorado live in a metacommunity of species in a mountain range—the Rocky Mountains—but within a population of goats on one single peak.

  What is a metacommunity?

  Metacommunities are essentially groups or sets of communities. The communities making up a metacommunity are separated in space, but they are not completely isolated and independent. They interact as various species move between them. For example, a metacommunity might consist of a set of separate forest communities, spread across a region. The various species within each patch of forest habitat interact as an independent community. However, certain species, including deer or rabbits, may migrate or disperse to another community in the metacommunity, moving to a different patch of forest in search of better opportunities to feed, shelter, or breed. Differing types of habitat will influence this balance between interlinked and independent development. The theory of metacommunities provides a framework for studying how and why variations develop and their impact on biodiversity and population fluctuations.

  Local versus regional

  A major advantage of looking at communities in this spatial way is that it may help resolve a number of seemingly contradictory observations. One ecologist’s study, for instance, might look at the way species live and interact together in a small local community. This narrowly focused study finds that competition between species for resources is a crucial factor in the workings of the community. Another study might look at the picture across a larger community. This macro-study discovers that competition plays virtually no part. So which result is correct? The answer may be that both are right, and the difference simply depends on scale. The benefit of metacommunity theory is that it allows ecologists to reconcile these differences. It enables them to look for explanations on both a local and regional scale.

  A metacommunity might be a set of half a dozen deciduous trees within a park, with each tree an individual community. However, it could equally be all the deciduous forests in temperate zones all around the world. What metacommunity theory does is allow ecologists to work at any scale, at least in theory.

  Umbrella framework

  According to Mathew Leibold, the study of metacommunities brings together many seemingly disparate branches of ecology and apparently conflicting theories. It may make it easier, for example, to resolve the century-old debate between the “deterministic,” niche-based theory of community ecology, in which species diversity is determined by each species’ ecological niche, and “stochastic” (random) theory, which emphasizes the importance of chance colonization and ecological drift (random fluctuations in population sizes).

  Metacommunity theory provides an umbrella framework for seeing how deterministic and stochastic processes can interact to form natural communities. It allows ecologists to state that patterns of biodiversity are determined both by local biological features, such as the balance of sun and shade in rock pools or variations in water quality in streams, and by regional stochastic processes, such as the spread of a species by freak storms or a die-off due to an epidemic. It also acknowledges that regional changes can be caused by the combined effect of local ones.

  In this example of a metacommunity, arrows show how species move between lakes to feed or breed. Seeds and the spores of algae are dispersed by the wind.

  Finding metacommunities

  One of the problems with Leibold’s concept is that in practice it is not so straightforward to identify the separate components of a metacommunity. For the fish and other water creatures in different lakes within a lake district, for instance, each lake may clearly be a distinct community. However, for those birds able to fly between the lakes in minutes, the different lakes are all part of the same single community. This may explain why much of the continuing work and research on metacommunities has been theoretical and abstract rather than rooted in fieldwork. Some metacommunities are easy to identify, such as islands in an island group, or rockpools that are separate between tides but joined when the tide comes in. In their 2004 paper, Leibold and his colleagues acknowl
edged that local communities, or patches, do not always have clear boundaries that make them recognizably separate, and that different species may respond to things happening at a different scale. They identified three kinds of metacommunity: markedly separate patches; short-lived but distinct patches that appear in a habitat from time to time at varying size; and permanent patches with vague or “blurred” boundaries.

  Distinct patches

  The most obvious markedly separate patches are islands in the ocean. These are a convenient subject to study and there is a vast literature on island biogeography, reaching back to Charles Darwin’s famous study of variations between finches in the Galapagos Islands in the Pacific Ocean. Neatly separate patches make good subjects for study, which is why they have been popular with community ecologists. But, of course, birds and many other organisms blown across by the wind or washed in by the sea ensure that even island communities are never completely isolated. This is why some metacommunity studies focus on the space between the communities even where the patches are distinct, as they are with ponds and lakes, and analyse how species move between them.

  Short-lived but distinct patches may be much harder to identify, simply because of their ephemeral nature. Nonetheless, ecologists have made metacommunity studies of holes in trees that fill with water for a period of time following a storm, patches of fungal fruiting bodies that live just a few days or weeks, and even pitcher plants that, after dew or rain, provide a short-lived aquatic home for both bacteria and insects.