For decades, ecologists and geneticists have tried to understand the population dynamics of different animal species, establishing models to understand the changes that occur in the number of individuals of a population, the composition of populations and the causes of such variations. Nature isn’t homogeneous or stable, so it’s not easy to set patterns to model these transformations.
Nature doesn’t work as a mathematical formula, in which populations inhabit a single area and can easily interact. For example, it’s unlikely for five males and five females of a species to be in the same area at the same time. In reality, landscapes are fragmented and these populations might live in different areas, isolated from each other.
In any case, population dynamics have tried to model the movements, changes and interactions that occur between groups of individuals, in order to understand the behaviour of different populations. In this article, we’re going to explain the foundations of some of these population models, which have been studied by different authors.
Metapopulations: a great resource in conservation biology
In order to understand the different models of population dynamics, it’s important to first understand the concept of ‘metapopulation‘, which is key in this branch of ecology. A metapopulation is a group of populations (or a combination of subpopulations) that occupy a series of areas and patches of fragmented habitats, interacting with each other and allowing for gene flow. In metapopulation dynamics, all of these occupied fragments (subpopulations) are considered vulnerable to extinction, as their life span is finite.
However, this doesn’t mean that these populations face imminent extinction. In fragmented spaces, the landscape matrix has patches (habitats) that are more suitable for life than others, so there are population movements between fragments, even when they’re not well communicated. While empty patches are slowly occupied, the inhabited ones will experiment local extinctions, thus balancing the populations.
Fragments can disappear, but they can be quickly replaced, even in the same spot. Local extinctions are fairly common and are compensated through the colonisation of empty neighbouring fragments.
Levins’ model: the classic metapopulation model
Richard Levins was the first ecologist to speak about metapopulations in genetic terms. He believed that the variation in occupied patches could be used as a model; the rate of occupied patches in an ecosystem is higher when a species is able to migrate from one fragment to another. In contrast, the higher the extinction rate, the higher the number of unoccupied patches. Thus, the occupation level of the patches will depend on the characteristics of the species: is it good at colonising? Does it have a great dispersal capacity? Is it an r-strategist or a K-strategist?
Levins’ model assumes that all habitat fragments are the same and that they all provide populations with the same resources for survival. But the reality is that each fragment has its own size and shape, and the distance that separates the patches is variable. Therefore, each fragment has different characteristics and will provide different solutions (or problems) to subpopulations.
The source-sink model: achieving balance in a fragmented landscape
A population is called ‘source’ when its habitat is of enough quality to guarantee that extinction will not take place in the short term and where birth rates are higher than death rates. ‘Sink’ populations, by contrast, live in an inadequate patch, where mortality rates exceed birth rates. Sinks are not sustainable without sources.
Even though sources have the appropriate quality and size to survive, fragmented landscapes also need at least one sink, where populations can also remain for a certain period. The idea isn’t for populations to constantly “jump” from one fragment to another or from an appropriate habitat to a worse one, but to have an escape area in case of danger. Although sinks face local extinction, its individuals can reproduce, which can be an advantage for a population at critical times.
This model is important when drawing conclusions: as sources are not at risk of extinction, it’s important to focus all conservation efforts towards them. Conservation actions have often taken the wrong approach, setting conservation objectives in inadequate habitats, like sinks. The priority should be to conserve sources, where adaptation is more efficient.
Final conclusions and Ilkka Hanski’s studies
Applying these theoretical models to nature is a complex task, as nature is not always predictable. It’s important to study properly how population dynamics influence genetic diversity, as local extinctions can lead, for example, to loss of heterozygosity or of genetic variability. It’s key to establish patch management models, ensuring connectivity between them (through biological corridors) and thus allowing any type of use in the landscape matrix.
Ecologist Ilkka Hanski studied metapopulations in depth, reaching various conclusions. Through his studies, he revealed that an ecosystem needs a minimum of 15 to 20 fragments to achieve balance. Below that number, the system is doomed to regional extinction.
In order to avoid this, we should ideally preserve as much territory as possible (including unoccupied fragments, which, realistically, is very difficult). In addition, it’s important to take into account the design of the matrix: the patches must be close enough to each other for populations to migrate easily, but far enough to avoid the entire network from being destroyed in the event of a catastrophe. In short, the distance separating fragments should be balanced and all fragments should present the same regional stochasticity (the same risks).
Translated by Carlos Heras