Gamelon et al. aim to answer this question in their recently published Commentary.
The harvest-interaction hypothesis
Natural populations in marine to terrestrial ecosystems are experiencing a climate that is rapidly changing. These changes can induce marked fluctuations in population size over years and lead to higher extinction risk. In addition to human-induced changes in climate, many natural populations are affected by harvest, with removal of individuals through fishing or hunting. In our Commentary, we address a new question – can harvest amplify the effects of environmental changes on the variability in population size over time and increase extinction risk (the ’harvest-interaction’ hypothesis”)? Alternatively, can harvest stabilise population dynamics? The objective of our Commentary was to identify the key mechanisms causing harvest to amplify or decrease environmentally-induced fluctuations in population numbers over time. We also reviewed selected case studies that have tested the harvest-interaction hypothesis for exploited populations of marine and terrestrial organisms.
Harvest changes population structure
Our case studies show that harvest can impact age structure of natural populations. In populations of northern cod Gadus morhua, the age structure contained 18 age classes in the 1960s, but after a half century of intensive harvest, the older fish disappeared, and the age structure was truncated to nine age classes. Age truncation is pervasive in exploited populations where the proportion of individuals in the oldest age classes decreases. Empirical evidence shows that age truncation might amplify climate sensitivity and generate fluctuations in population size over years in several fish species. Conversely, dampened environmentally-induced population fluctuations are expected if harvest targets young individuals in long-lived species that are the most sensitive to climate variation.
Harvest modifies the spatial structure of populations
Harvesting may favour the displacement of individuals and enhance spatial heterogeneity of animal distributions. Heterogeneous distributions can weaken a population’s ability to buffer the effects of unfavourable environmental conditions, thereby amplifying fluctuations in population numbers. Moreover, spatially structured populations that exhibit synchronised fluctuations in population size over years are more prone to extinction because local patches can go extinct without being rescued by immigration from other populations. Theoretical studies have shown that proportional harvest quotas tend to induce spatial synchrony. In accordance with theoretical work, a growing body of empirical evidence in fisheries indicates that synchronised harvest at a large spatial scale can affect the degree of spatial synchrony in population fluctuations.
Harvest modifies community dynamics
How does harvest affect the fluctuations of population size in a community of interacting species? Trophic interactions imply that harvest of a given species may affect the dynamics of entire food webs, also affecting non-harvested species. Interactions among species are essential to the resilience of the community, and when some species decline or are removed from the system, interactions are broken and the ability of the entire community to recover from disturbances such as changing climate conditions decreases. If harvest quotas are proportional to population size at different locations, harvest will synchronise fluctuations in population size of competing harvested species in space and increase the probability of extinction. In contrast, threshold harvest strategies may act to desynchronise local dynamics and thereby reduce extinction risk.
Empirical and theoretical evidence for the harvest-interaction hypothesis
A diverse array of field studies and theoretical models provide preliminary evidence for the harvest-interaction hypothesis. In marine species of fish, harvesting might magnify environmentally-induced fluctuations in population sizes. A comparison of the temporal variability of 13 exploited vs. 16 unexploited fish populations living in a common environment and subject to the same environmental fluctuations showed that the addition of mortality from fishing resulted in amplified temporal variability. In bird species, we found some theoretical studies that investigated the effects of contrasting harvest strategies on the variability in the annual yield. But to date, the harvest-interaction hypothesis has not been tested for any population of terrestrial vertebrates. Few field experiments have been conducted and little is known about the combined effect of hunting and climate on population fluctuations in terrestrial species. A promising avenue of research is to explore how hunting and climate interact and evaluate whether, like observed in marine species, hunting enhances environmentally-induced fluctuations in population sizes for terrestrial exploited species. In the future, a better understanding of the synergetic effects of environmental change and harvest is of major importance to core goals of ecology, management and conservation.
Read the full Commentary, Does harvesting amplify environmentally induced population fluctuations over time in marine and terrestrial species? in Journal of Applied Ecology.