Posted by Hester Williams @HesterW123
When a population is small, or at low density, the classical view of population dynamics used to be that the major ecological force at work is the release from intraspecific competition – the fewer we are, the more we all have, and the better each will fare…
But in the 1930s, an ecologist named Warder Clyde Allee used goldfish in tanks to demonstrate experimentally that conspecifics had a beneficial influence on each other and survived better in larger groups. This led him to conclude that a certain degree of aggregation (and consequently higher population density/size) can improve the survival rate of individuals, and that cooperation may be crucial in the overall survival of the population. This is basically because larger group sizes provide individuals with more opportunities to mate, defend themselves, feed themselves, and/or can work together to alter their environment in a beneficial manner to the whole group – too few and we might not fare so well!
Allee’s idea on the unsustainability of small populations is today known as the Allee effect and is formally defined as: ‘an increase in individual fitness and/or per capita growth rate, caused by an increase in population size or density’.
The Allee effect can be generated through several mechanisms in small populations including: difficulty in finding a mate, pollen limitation, inability to satiate predators, cooperative anti-predator behaviour, cooperative breeding, foraging efficiency, habitat fragmentation and habitat loss.
Cooperative living: The Southern African meerkat (Suricata suricatta) lives in groups of up to 40 individuals and is a prime example of how cooperation can improve survival. Responsibilities such as baby-sitting and raising the young, foraging, burrow maintenance and standing guard are shared. They also huddle together for warmth, and band together against rivals and predators. If group sizes fall too low, local population crashes can ensue.
Habitat fragmentation and loss: Small and more fragmented patches of woodland habitat decreased the resilience and survival of populations of the ringlet butterfly Aphantopus hyperantus by reducing successful dispersal between patches and build-up of sufficient population sizes.
Why does the Allee Effect matter?
The implications of the Allee effect are potentially very important in many areas of ecology and the practical management of population numbers, whether aiming to increase or reduce them, is strongly affected by this effect.
In Conservation the prevention of population collapses is a priority, and it is widely acknowledged that populations of small size are often at greater risk of extinction.
With only around 100 individuals scattered in the wild (some experts believe only 30!), the Sumatran rhino, Dicerorhinus sumatrensis, is on the verge of extinction. This species is clearly in the grips of an extreme Allee effect – as numbers of individuals decline, factors associated with low numbers (e.g. narrow genetic base, skewed sex ratio, mate-finding, reproductive pathology associated with long non-reproductive periods) combine to drive numbers ever lower, even with adequate habitat and zero poaching. In a 2017 WWF Report experts urge that the days of “conserving” Sumatran rhinos are gone and that efforts to save this species should be in advanced crisis mode to prevent extinction.
Another area of ecological studies where the Allee effect plays an important role is in Invasion Biology. It can inhibit the establishment of newly arrived species or in other cases delay or prevent range expansion of established pest species. This is the case for the gypsy moth (Lymantria dispar) where some of the isolated low-density colonies founded by long-distance dispersal go extinct without any management interventions, simply because of the Allee effect.
The Allee effect also plays a critical role in Biological Control programmes (the introduction of a natural enemy species to control a pest species) where success often depends on releasing sufficient numbers of individuals to ensure establishment of the natural enemy species.
The Allee effect is a key focus in my PhD studies. I am studying the establishment success of small, isolated populations of Neolema ogloblini, a beetle introduced as a biocontrol agent for Tradescantia fluminensis in NZ. The aim is to determine whether an Allee effect plays a role in the population dynamics of this beetle and to identify the mechanism driving the Allee effect. This project will generate a better understanding of the key factors that affect biocontrol agent establishment and also invasion success of pest species.
Hester Williams is a PhD candidate in the School of Biological Sciences, University of Auckland and is stationed with the Landcare Research Biocontrol team in Lincoln, Canterbury. She is interested in invasion processes of both insect and plant species. Hester is supervised by Darren Ward (Landcare Research/University of Auckland) and Eckehard Brockerhoff (Scion), with Mandy Barron (Landcare Research) as advisor. Her studies are supported by a joint Ministry for Primary Industries – University of Auckland scholarship. The project is an integral part of an MBIE program “A Toolkit for the Urban Battlefield” led by Scion.