An ecosystem, short for ecological system, is defined as a collection of interacting organisms within a biophysical environment. Ecosystems are never static, and are continually subject to both stabilizing and destabilizing processes. Stabilizing processes allow ecosystems to adequately respond to destabilizing changes, or perturbations, in ecological conditions, or to recover from degradation induced by them. Yet, if destabilizing processes become strong enough or fast enough to cross a critical threshold within that ecosystem, often described as an ecological 'tipping point', then an ecosystem collapse (sometimes also termed ecological collapse) occurs.

Ecosystem collapse does not mean total disappearance of life from the area, but it does result in the loss of the original ecosystem's defining characteristics, typically including the ecosystem services it may have provided. Collapse of an ecosystem is effectively irreversible more often than not, and even if the reversal is possible, it tends to be slow and difficult. Ecosystems with low resilience may collapse even during a comparatively stable time, which then typically leads to their replacement with a more resilient system in the biosphere. However, even resilient ecosystems may disappear during the times of rapid environmental change, and study of the fossil record was able to identify how certain ecosystems went through a collapse, such as with the Carboniferous rainforest collapse or the collapse of Lake Baikal and Lake Hovsgol ecosystems during the Last Glacial Maximum.

Today, the ongoing Holocene extinction is caused primarily by human impact on the environment, and the greatest biodiversity loss so far had been due to habitat degradation and fragmentation, which eventually destroys entire ecosystems if left unchecked. There have been multiple notable examples of such an ecosystem collapse in the recent past, such as the collapse of the Atlantic northwest cod fishery. More are likely to occur without a change in course, since estimates show that 87% of oceans and 77% of the land surface have been altered by humanity, with 30% of global land area is degraded and a global decline in ecosystem resilience. Deforestation of the Amazon rainforest is the most dramatic example of a massive, continuous ecosystem and a biodiversity hotspot being under the immediate threat from habitat destruction through logging, and the less-visible, yet ever-growing and persistent threat from climate change.

Biological conservation can help to preserve threatened species and threatened ecosystems alike. However, time is of the essence. Just as interventions to preserve a species have to occur before it falls below viable population limits, at which point an extinction debt occurs regardless of what comes after, efforts to protect ecosystems must occur in response to early warning signals, before the tipping point to a regime shift is crossed. Further, there is a substantial gap between the extent of scientific knowledge how extinctions occur, and the knowledge about how ecosystems collapse. While there have been efforts to create objective criteria used to determine when an ecosystem is at risk of collapsing, they are comparatively recent, and are not yet as comprehensive. While the IUCN Red List of threatened species has existed for decades, the IUCN Red List of Ecosystems has only been in development since 2008.

Ecosystem collapse has been defined as a "transformation of identity, loss of defining features, and replacement by a novel ecosystem", and involves the loss of "defining biotic or abiotic features", including the ability to sustain the species which used to be associated with that ecosystem. According to another definition, it is "a change from a baseline state beyond the point where an ecosystem has lost key defining features and functions, and is characterised by declining spatial extent, increased environmental degradation, decreases in, or loss of, key species, disruption of biotic processes, and ultimately loss of ecosystem services and functions". Ecosystem collapse has also been described as "an analogue of species extinction", and in many cases, it is irreversible, with a new ecosystem appearing instead, which may retain some characteristics of the previous ecosystem, yet has a greatly altered structure and function. There are exceptions where an ecosystem can be recovered past the point of a collapse, but by definition, will always be far more difficult to reverse than allowing a disturbed yet functioning ecosystem to recover, requiring active intervention and/or a prolonged period of time even if it can be reversed.

While collapse events can occur naturally with disturbances to an ecosystem—through fires, landslides, flooding, severe weather events, disease, or species invasion—there has been a noticeable increase in human-caused disturbances over the past fifty years. The combination of environmental change and the presence of human activity is increasingly detrimental to ecosystems of all types, as our unrestricted actions often increase the risk of abrupt (and potentially irreversible) changes post-disturbance; when a system would otherwise have been able to recover.

Some behaviors that induce transformation are: human intervention in the balance of local diversity (through introduction of new species or overexploitation), alterations in the chemical balance of environments through pollution, modifications of local climate or weather with anthropogenic climate change, and habitat destruction or fragmentation in terrestrial/marine systems. For instance, overgrazing was found to cause land degradation, specifically in Southern Europe, which is another driver of ecological collapse and natural landscape loss. Proper management of pastoral landscapes can mitigate risk of desertification.

Despite the strong empirical evidence and highly visible collapse-inducing disturbances, anticipating collapse is a complex problem. The collapse can happen when the ecosystem's distribution decreases below a minimal sustainable size, or when key biotic processes and features disappear due to environmental degradation or disruption of biotic interactions. These different pathways to collapse can be used as criteria for estimating the risk of ecosystem collapse. Although states of ecosystem collapse are often defined quantitatively, few studies adequately describe transitions from pristine or original state towards collapse.